OzVMX Forum
Clubroom => Tech Talk => Topic started by: Ji Gantor on February 16, 2009, 09:56:30 am
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Okay no more fun, down to business.
After all I would feel a bit of a Goose if I was not posting technical stuff.
Inspired by Brent J's posting about Soft Forks, I have decided to try and work out a formula to estimate rear shock spring rates.
There are three pieces of the puzzle I need from a suspension expert member.
Please can you supply the following info.
a) What is the average distance that the spring is preloaded when the shock is assembled (The spring is always longer than the shock and has to be compressed during assembly). and
b) What is the average load loss due to friction and stiction in the shock and the swingarm bearings ( is it around 10% of the load). and
c) What is the weight distribution of bike and rider to the rear wheel ( is it 35% to front 65% to rear).
This formula will apply to twin shocks and I hope to give factors for shock angle.
I hope this will be technical and old school enough for all members.
I would like to hear from anyone that can add to this process in a technical way.
Readers of this topic should not rely upon any info until tested and proven by a suspension expert.
Ji
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Ji, here's some numbers for you to play with. I have weighed various parts of my XT500 over time.
The bike is 124kg, weight on the front wheel is 57kg and on the rear 67kg
With me seated this becomes front 97kg and the rear 124kg.
Unsprung weight on the front is 13.5kg. This is tyre, tube, wheel, brakes lower fork legs and internals and half the weight of the springs.
Rear unsprung weight is 20kg, Much the same things as the front but with half the swingarm and half the shocks.
The weight bias of this bike is totally different to a normal XT/TT500, they are generally more rear end heavy.
My front springs are 25lbs/in with about 2mm preload and I'll have to check the rears.
The position and angle of my rear shocks give 2:1 leverage ie the shocks have 115mm movement and the axle moves 230mm. Fork travel is 230mm.
The bike normally sits with about 25-30 of travel used and about 60-65mm with me on it. I'll have to recheck these measurements just to be sure.
Brent
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Thank you Brent J.
Where are all these members that want to see more technical postings?
Hang on there is oil dribbling out of my forks.
There we go I have cleaned up my mess.
Ji
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Ji,
There should be a correlation between front and rear spring rates. For example you take the Works performance formula and multiply it by X to give a rear spring rate, this would work where axle and shock travel are the same. Then you would multiply it by the leverage ratio to get the required spring rate.
The Works Performance formula gives no allowance for weight bias, by rights there should be one formula for the front and one for the back.
I tried some things last night.
I'm working in pounds here as my spring rates are in pounds and I have a pre metric brain, kiwi model.
There's no allowance for preload in this or changing air pressure inside the fork or shock.
The sprung weight on the front of my bike = 96lbs
I want 1" of sag so by rights two 48lb/in springs should do the job. 96lb/2/1=48
If I sit on the bike the sprung front weight becomes 184lb.
I want it to sag 3" so I work that out at 30.6lb/in 184lb/2/3=30.6
25lb/in works very well.
As for the back.
Sprung weight on the back = 103lbs
To get it to sag 1" with a 2:1 leverage ratio means two springs of 103lbs/in each. 103lb*2/2/1=103
With me seated the rear sprung weight is 220lb.
To sag 3" = 73lb/in and again with a 2:1 leverage ratio is two springs af 73lb/in each. 220*2/2/3=73
I currently run (from memory) 110 or 115lb/in rear springs but they have about 10mm of preload. I did have a borrowed set of 122lb/in and they were much better.
I need about a month off work and a hundred assorted springs.......................................................
Brent
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Hi Brent,
I have been working on the problem off and on.
I will need the following dimensions off your bike.
I will have to write this selector as a program as it has to many calcs for the average Joe.
Ji
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im still looking at the picture ..............
glad im no longer in school
all i can say is yeah for computers, calculators and the internet.
more power to you JI.
Brent totally noob question but how do i get the weights ? are you sitting the front on a scale with the back propped at the same level ? and then swapping or is this a calculation thing based on some method, or is it as simple as stealing the missus weighting scales when she isnt looking.
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Hi Freakshow,
The drawing is a side view of the rear section of a bike.
There is the pivot where the swingarm meets the frame.
The swingarm.
The axle of the rear wheel
The shock
and the sub frame above the shock.
I need all of these dimensions to be able to calculate the spring rate. I have used X, Y... so when Brent measures his bike he can give me those dimensions I need.
I hope that it makes more sense now.
I have just done a run on my CZ shocks and determined that they require a 126lb spring.
This will give a 12mm (10%) static or bike sag and a 30mm (30%) rider/bike sag for a 12mm preload. The CZ400 1973 only has between 86-100mm of wheel travel stock.
This spring rate for the CZ was the first calculation run and has not been checked. I will check to see if I have added in all the variables and let you know.
Ji
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Hi Freakshow,
The best way to weigh the bike is to use two scales, or as you said use one and have the other wheel packed to the same level.
The weight distribution is critical to the calculation as it is the first piece of data used in the calcs.
Other calcs are based on,
The swingarm is a second class lever (algabra),
The shock is on and angle (trigonometry) and
Spring preload (algabra).
This probably does not make this sound any simpler but I just thought I would show the thinking behind the madness.
Ji
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This calculation will only apply to linear rate springs not for dual rate springs.
Ji
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i though most rears where dual rate. like the works. so this needs another calculator then ?
I get what the picture represents and stuff.
I was kinda just referring to my school days when math lessons are even a blurr today. although i was really good at statistics, graphs and other stuff, just anything that needs a formulae that had BODMAS or other 'over numbers' kinda got lost in the translation.
But them again MR Aschberger wanst the most freindly guy and kinda lost me in the first week of class every year, unfortunatley i reackon i got him 3 years in a row so i just ended up whatching squirrells out the window, or trying to see down Sarah 'S top or at least smell her lovely blond hair......ahhhhh school. :-[
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Works Performance shocks come in single, dual and triple rate springs.
In fact their dual rate shocks have two single rate springs with different rates.
Check out Works Performance
http://www.worksperformance.com/html/multirate_desc.html
You can still get single rate springs for most period shocks.
If you have a required single rate spring figure it is easier to determine a dual rate spring rate.
I will look into it for you.
Ji
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Every shock has an assembly spring preload.
The S & W shock I just pulled apart has a 12mm assembly preload. This is important as it is part of your preloading of the spring. The adjustable preload on this shock is an additional 12mm bringing the total preload to 24mm or say 1 inch. The S & W D525-3 4 E B made in Mexico shock has a spring rate of 68lb (measured today, springs loose tension over time). This must have been very soft in the back end of a CZ400 and must have been designed for a lighter bike/rider combo.
Ji
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Just to give an idea of what is involved.
That S & W shock has 80mm of travel to the rubber bump stop, 115mm of travel if the bump stop was removed and the total measurement of spring compression before binding is 119mm. When the spring is preloaded the 1 inch or 25mm (including assembly and adjustable preload) the allowable spring compression is only 94mm before binding occures.
Ji
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I have a brand new works type shock spring for a YZ125g/h here for $100 if anybody wants one....
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Wow, glad I'm running a single shock (Linkage)bike ;)
If you are trying to determine the correct spring rate for a single shocker, the easiest way would to be check your manual or parts book for the original spring rate, that should also tell you what KG's rider this is designed for, most aftermarket manufacturers provide springs in .2 kg/?? increments, this represents and increase/decrease in rider weight by 5/10 kgs weight range.
So for instance if ytou are 20kgs heavier than the standard setting, you should require a spring rate increase of approx. .4 to .6 kg/?? rule of thumb would for slower riders go with the softer start point, fast guys go heavier.
:)
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Thanks evo555,
So what is your bike, what does your manual say the spring rate is, and for what weight rider?
Ji
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After reading some more details about springs and tapping out a few more calculations in theory this formula should work just as well for dual rate springs. After all the spring rate required to set the ride height is the same as I have calculated. The additional rate will be stiffer and help the shock from binding or bottoming out.
The problem with dual rate springs for short or less than 100mm travel shocks is that the spring pitch in the stiffer section is closer together and thus reduces the allowable compression of the spring. A dual rate spring must be part of the design at the time the shock is manufactured and not added later without checking all the facts that I have written about above.
Ji
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Walter my parcel has been given to my mother to hide until the end of next week :'( And I knew you would be watching this 8)
Ji, I've measured my bike. I have a drawing in "paint" but can't upload it and I need a new scanner so I'll write it down.
X Swingarm length 550
Y S/a pivot to lower shock mount 330
Height from axle to S/a pivot 140 (unladen)
T height from S/a to pivot top shock mount 330
Z distance from S/a pivot to top shock mount (rearward) 150
bottom shock mount above S/a beam 25
When I weighed my bike initially I had access to a set of certified aircraft scales. I used two of them and even turned the bike around. I've since found bathroom scales are fine but try to put the "other" wheel on a block or simillar to keep the bike level.
OH&S tip Freaky, DON'T LET THE MISSUS KNOW WHAT YOU ARE USING THE SCALES FOR ;D
Ji I'm not a fan of preloading springs. I had heavier (about 122lb/in) springs on the back with minimal preload and now run, I think 115lb/in. I had to add preload to hold the rear up but as the rate is low they are still too soft. Unfortunately the 122's weren't mine.
I will measure my rear springs this weekend and carefully measure the unladen and laden sag, front and rear.
I'll post up the spring rates and sag measurements as soon as I can.
Here's some links from another post to some spring rate calculators I have found invaluable
http://www.pontiacracing.net/js_coil_spring_rate.htm
http://www.proshocks.com/calcs/2spgrate.htm
If you are prepared to spend the $ I've found Tony Foales suspension software is great but I need about two weeks to learn to drive it!
And might I add I AM LOVING THIS THREAD 8)
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Hi Brent,
The 12mm preload I mentioned on my bike is the assembly preload, please pull your shock apart and check what yours is.
Ji
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Ji,
I'll check that when I measure the rest.
I'm keen to find out again as didn't note that down last time and I want to recalculate my rear end stuff.
The preload will be what ever I put in last time,
take notes Brent, take notes ::)
Brent
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Ji,
just picked up a '86 Husky 250xc, haven't even looked a suspension yet, BUT, refering to my earlier post, I had an '84 Maico 500 with rear suspension issues.
I was having alot of trouble getting anyone even interested in discussing it, until I rang Ballards.
They where able to determine the original spring rate by measuring the coil width, number of coils and distance between each coil. From that the correct spring rate for my weight was determined and problem solved.
Ask for Geoff mention Maico and gees he gets excited. ;D
Good thread guy's.........although it's way over my head.
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Thanks evo555,
That is great input.
I have always found Ballard's team to be very helpful and their gear well priced.
Ji
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Don't worry about being "over your head" Evo, I'm still feeling my way around. Believe me I've built some real kangaroos over time
I got interested in suspension for a few reasons.
I have a bad back (like most of us) and good suspension allows me to ride more often
When I started to build up my XT500 I could not find suspension advice. I'd been away from bikes for a lot of years so had to rely on local people. Basically if they couldn't find it on their charts they didn't know.
I also posess the two of the most dangerous qualities known. I'm inquisitive and I'm a tinkerer. Being a fitter and turner helps (or hinders?) too.
Brent
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so how are you confirming the spring rates of unkown or unmarked springs ?
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Freaky my post at 9.15pm has a link to a website (Pontiac of all things) with a really good spring rate calculator. Just enter the measurements and it does the rest.
I like this one as it gives the result in metric and imperial
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Ji,
126lb springs sounds too heavy for 73 CZ, even considering they were a fairly heavy bike.
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Hi JC,
Yes it is a bit high. What spring rate are you running in your CZ400 1973 ?
As I said that was the first run and I was using a calculator. There are quite a few calculations involved and when I checked back I discovered that I had left the assembly preload out. I have now written an Excel program that works the spring rate out in the blink of an eye. It is great because you can perform what if functions, that really narrows down the spring rate you require. The spring rate wildly varies due to rider height and preload especially on bikes with only 100mm's of rear wheel travel.
Ji
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Anyone wanting more insight into this subject I suggest ringing Walter's old mate Greg from ASR in Melbourne pronto. I'll have my 100 pages of knowledge next week, will review when I put it down.
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Hi Lozza,
That is good news I will ring Greg next week.
Thanks Ji
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Ji,
Just had a bo peek at the Husky manual, factory shock spring is for a 75kg rider, preload is set at the factory for that weight at 95mm, Rear wheel travel with rear tyre off ground is approx 330mm the standard spring rate is given in sparts parts manual (I don't have one of those).
On the twin shock conundrum, does the spring rate required, need to halved, to split the load between two shocks?
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Hi evo550,
Yes, the load is placed on both shocks so the load is halved.
Pitty you don't have that spare parts manual.
There is lots more I have found out about this matter, but I am waiting to see what some members are going to do first. I will help any member that wishes to learn and help.
On old mono shocks the shock is at 45 degs and this makes them work hard, but the amount of sag (in your case 95mm) helps the spring out so much. New linkage systems are beyond my research at this point. The spring rate of 5kg is in units of 5kg/mm, just so you know.
Thanks Mate
Ji
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Walter I figure to work on the spring rate to suit the laden condition. And the damping to suit that also.
That's how the bike will be riden.
I'm just sorting through the measurements I took over the last few days then I'll have a few more ideas.
I've also got to re-measure the rear springs and preload. Once I get it all written down rather than trying to rmember what I did it will make more sense.
Our club are having a working bee tomorrow and their sign-on and fun day on Sunday so I'll actually be riding this thing for a change!
Brent
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Hi Brent,
Did you get a chance to measure and weigh your bike and shocks over the weekend.
Ji
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Hi again Ji,
To follow up my earlier post that you took exception to, I realize it was a 'first run' calc & this is a work in progress, w numerous factors to be considered (apart from ride height/sag - which aint hard to calculate).
eg weight distribution (esp 'live'), sprung/unsprung weight, rider weight/position, friction, damping force, leverage ratio, G-out forces etc - many of which have been mentioned already by others.
The problem for me (in trying to develop a useable formula) is accounting for dynamic loads. eg how do you measure/guestimate G-out forces like landing from jumps (what size jump?), bottom of downhills, bermshots etc??
There are so many variables, but don't let that stop you from trying.
Wrt 126lb spring for CZ400 I can only offer this: Its a long time since I rode one & I can't remember if it had stock springs/shocks or Konis w lighter springs (probably the latter), but the std CZ springs were said to be 100lb or 110lb (Dirt Bike Mag, Mar 74, test of a 73 model), & universally regarded as way too heavy even for such a heavy bike, unless the rider was "over 230lbs".
It was a common (almost universal) mod to replace std springs w 60-90 lbs progressive Girlings or 68lb straight-wound Koni, unless you were over 180lb when 78lb were suggested. The 68lb sounds a bit light to me, but thats what some ran then (when tracks were probably somewhat smoother).
Back in the day, tho my 250 was a bit lighter than a 400CZ & I was fly-weight, I ran 60-90lbs w about 3.5" stroke & it was fine. When I modified it for LTR, I ran 110-140lbs w 2:1 leverage ratio & 4" stroke & that was spot on.
Noted US CZ specialist Pete Maley used to run Boges w 90 or 100lb springs & 1.5:1 leverage ratio when his shop (CMC) did LTR mods to customer CZ400s.
Gil Vallencourt (owner/developer of Works Performance shocks) modified 400CZs w 6"-stroke WP shocks at 1.4:1 leverage ratio & ran a mere 60lb springs (on account of the long-stroke shocks & higher-than-average compression damping that WP shocks used).
So you see why I made the post that 126lb seems too hi for 73 CZ400 (unless you meant total for both shocks; then it seems too light). It was a simple observation based on my experience & understanding. But if I'm on a different wavelength, I'll happily apologise.
One other point that I trust is helpful: there is a much simpler way of measuring leverage ratio. Divide the length of the swingarm (pivot to axle) by the perpendicular distance from the shock to the s'arm pivot. (It changes a bit on most bikes from fully extended to fully compressed.)
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Hi JC,
Now we have some great info coming out.
Thanks mate.
This last posting of yours is much more interesting.
I was calculating the spring rate of only one spring. But as I said I left one factor out.
I am only trying at this stage to determine the starting point spring rate. The what if function allows me to try all sorts of different preload settings and ride heights.
My next adventure will be to determine the forces acting on the spring on the landing of a large jump. However I think that the spring rate is not the only influencing factor on dynamic loading. Dampening and valving will play their part in all of this and unless you know how your shock is set up it is pointless. I only started this to allow people like Brent to be able to select a spring rate that may suite their needs and then they would search through wrecking yards looking for springs that matched. It was not my intention to design shocks or tell some one like Walter what to do.
The dynamic loading is I believe imposable to determine. Even shock shops have telemetry that is set up on your bike or race car. This records all the forces as you drive around every track you will race and then they build the shocks to the data. If you read between the lines they can not determine it either.
May be they can but it seems unusual to me that they would go to this much trouble if they could just sit down and tap on the computer for ten minutes. This last comment was not made to upset any shock experts, it is just an observation.
You obviously have a great deal of knowledge on this subject, coupled with an interest. I now feel you would be an asset to this topic and I hope we can continue to discuss many more topics in the future.
Ji
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This is a sample of the print out of my program.
It is based on a
CZ400 1973 stock standard
Bike weight 105kg
Rider weight 100kg.
This data is for information only.
Please consult your shock expert for actual spring rates.
Ji
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Just buy THESE BOOKS (http://www.broadlandleisure.com/) they explain everything(and more) you ever wanted to know.
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Hi Lozza,
Thanks mate, they look like a good read.
Ji
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Ji I've only just unloaded my bike after yesterdays meeting. I hope to have the infl you need within a day or two.
Time is always my biggest problem
Brent
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Hi Brent,
Take your time mate.
Ji
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Ji,
Bradley's books that Lozza mentions are very expensive in Oz, but if you've got the $$ they're worth it. There is another (cheaper) one by Gaetano Cocco (sponsored by Aprilia) called Motorcycle Design & Technology that's pretty good too, but they're mostly about roadracing.
There were several formula used back in the day to get a starting point. Perhaps you have seen these:
1. Tim Witham (the 'W' in S&W) used a very simple formula where he made a lot of the assumptions for you: SR=MY/4
where SR is recommended spring rate,
M is 65% of bike wt + 50% of rider wt,
Y is swingarm length divided (pivot to axle) by distance from pivot to bot shock mt.
He doesn't say so, but that would only work where shocks are near verticle.
2. Bill Ocheltree (US 'guru' of the day) used a Load Factor (2.0 for rear; 1.2 for front) to try & account for dynamic loads, friction, damping force etc. w an educated guestimation, & a Balance Factor (60% rear, 40% front of combined rider + bike wt) to account for wt bias. His formula is
SR = BFxCWxLFxLR / 2xWT (for dual shock suspension)
where SR = spring rate
BF = Balance Factor as a decimal (not %)
CW = combined wt of rider + bike
LF = Load Factor
LR = leverage ratio on shocks
WT = wheel travel in vertical plane
Later Edit: This formula was found to be wrong & is corrected in Reply # 60
3. Another formula for rear suspension given in an Oz mag of the day was :
SR = FgxD1x100 / NxD2xTxP
where SR = spring rate
Fg = Wt on rear wheel w rider aboard
D1 = swingarm length (pivot to axle)
D2 = perpendicular dist from shock to swingarm pivot
N = no of springs (ie 2 for twinshock)
T = shock travel
P = percentage of static compression (sag)
They all have their limitations, but ea provides an approx starting point. They all assume no pre-load on the spring, & I always preferred to run little/no preload myself to keep initial travel over small bumps fairly soft, so long as there wasn't too much static sag
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Hi JC,
I have not seen any formula's until your last post.
They are very interesting.
As you say they are an approximation more than a detailed effort. They seem to be more like a quick rule of thumb. Interesting to see the evolution of thought.
They don't seem to take into account preload or shock angle. There is always some preload as the spring has to be compressed to fit the shock. The S & W's have a 12.7mm or 1/2 inch assembly preload and this has to be accounted for in the formula. If it is not you end up with a 126lb spring rate instead of 82lb on a CZ400 1973.
The load factor of 2 for rear shock dynamic loads is quite mind provoking.
I am sure in the right context that may prove useful but it does not seem to me to account for ride height. As we know your shocks must have some compression when in use or when you go over a pot hole you will loose contact with the ground. May be this was to determine the second figure for a progressive spring.
If I get time I will run these calculations and see how close they are to my own.
I did ask one of our shock expert members about loss due to friction in the swingarm bearings, stiction and dampening but he declined as he did not want the arguments. Pity !
I can remove the rear wheel and shocks of some of my bikes and with a set of spring scales measure what load it takes to move the swingarm. For the shock I can place it on a digital scale and compress it until the spring deflects 1 inch. Then all I have to do is subtract the spring's spring rate from the measured shock compression test and the diff should be the dampening, stiction.... Again this is not relevant to Brent and others because the internals of every shock is different.
Thanks for your post I really appreciate it.
Ji
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Ji,
Yes they were all 'rule-of-thumb' & they did put you in the ballpark of what we were all running then.
I too like the Load Factor of the 2nd one - at least trying to account for the variables we can't measure w a consant obtained to some degree empirically. Thats a common technique in engineering calculations where you have to make assumptions
The 3rd one does take into account ride ht with his factor 'P'. He pretty much calcs his spring rate to get the ride ht (sag) he wants. He measures it in % of total travel & in his eg in the article he used 33.3% sag for MX.
Both 2 & 3 do effectively take angle of shock into account when they measure leverage ratio on the shocks. No 3 formula did have a supplementary calc to account for preload, but I don't have the article in front of me at present. The others just assumed minimal preload, sufficient to keep spring retainers in.
None of them works on the sprung wt of the bike to start w tho. I like Brents approach of accounting for unsprung wt. After all, the springs don't have to support the unsprung wt, tho of course they do have to move it back in contact w the ground.
Yes, friction & damping force will be diff for every bike/shock combo. In practice, Load Factor would be diff for each rider too - eg how fast, whether he weights the front more than the back (which to some degree will depend on his height & riding style) etc. So many variables! I'll be interested in what (formula) you come up with.
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Hi JC,
The load factor of 2 for rear shock dynamic loads is quite mind provoking.
I am sure in the right context that may prove useful but it does not seem to me to account for ride height.
This comment was based on formula 2.
This is all just starting points anyway.
I read an article today from an MX magazine discussing suspension for super cross and it stated that Jeremy McGrath uses soft springs on his bike so his bike does not leap to high in the air over jumps. This allows him to get back on the gas quicker. Jeremy lets heavier valving do the work.
Ji
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Ji, Yes yr right. #2 doesn't take into account ride height. Only #3 does
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Hi JC,
I have also added in the unsprung weight into my calculations.
This is a very good point.
The springs do not support the swingarm or wheel but the sag I have calculated is the same as what happens on the bike, very interesting.
Ji
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Hi JC,
I seems to me the leverage ratio in their formula's is the resultant of the second class lever effect, not the shock angle.
Is that how you see it.
Ji
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Hi JC,
Now this is the mental stimulation I was looking for in a forum.
Ji
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Ji, in measuring the leverage ratio they (# 2 & 3 formulae) effectively convert the forces to torque/moment about the swingarm pivot & hence use perpendicular distance(s) measured from pivot to the line of force(s).
That automatically accounts for shock angle since the perpendicular distance (from spring/shock to s'arm pivot) is going to be less for a shock mounted at say 60deg than for one at say 90deg (when the bot shock mt is in the same pos'n). You can calculate it out by trigonometry using four measurments as in yr diag on pg1, or you can do it using just 2 measurements as they do.
#1 formula didn't take any account of shock angle, just simple lever, so it really only works for (near) vertical shocks & thus has limited use
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Ji, I found some more interesting things yesterday a'noon.
Jared mentioned in a thread in the Spanish Marque Remarks that the V75/VA Monty's rear suspension was hard to set-up. Make it soft for small bumps & it bottomed on jumps; stop the bottoming & it was very stiff over small bumps. That's fairly typical of falling rate suspensions. So I checked the geometry & sure enough, the V75 is falling rate from about half-stroke, while the VA is falling rate after about 1" (15%) travel!
I then checked other geometries of bikes w laid-down shocks from the early LTR era (74-75) & found the 74.5 Penton/KTM is falling rate from the start (w shocks in full laydown pos'n), GP2 Phantom & 74 Rokon are falling rate after only about 1" (about 15%) of travel, CanAm MX3 after about 35% stroke, GP1 Phantom after about 40% stroke, 76 MC5 Penton/KTM after about 60% stroke, & 75/76 GP Husky after about 75-80% stroke.
On the other extreme of shock pos'n, its quite obvious that the Mk8 Pursang/Mk9 Frontera rear end is falling rate right from the start too. Bultaco fixed that on all the later pursangs & fronteras.
Interesting isn't it that most of those bikes (w the exception of the Husky which at least was rising rate for about 70% of its travel) had a reputation for average rear suspensions
Then I checked the YZ250B. Its cantilever system is also falling rate after about 10-15% stroke (MXB & YZC are much the same). Now it all makes sense for these bikes - they've got the same problem! I've never been too convinced of the supposed virtues of the early cantilever Yams & long wondered why they ran such stiff springs when they already had apx 300psi in the shock. Now I know why (they had a fairly short stroke shock/spring too which of course exacerbates it). It all makes sense why they had a harsh ride too. (Later cantilever YZs did have more rising rate in the system, but its still limited. YZG/H is rising rate for about half-stroke while YZ D/E is somewhere in between B/C & G/H. From YZ250/400C I believe they used a progressive spring too)
As a general rule of thumb (it depends on more geometry/measurements than just shock angle), it seems that anything w the bot shock mt attached towards the rear of the swingarm & the shocks laid-down below about 50degs (from horizontal) goes falling rate sometime in the stroke; anything below about 45deg is falling rate for much of the stroke & anything around 55-60degs is safely in the rising rate thro'out its stroke. Hence the AW Maicos, RM Suzs, post77 Huskies, Mk 11 Pursangs etc had good rear suspension. By then the factories had 'cottoned on' to it. Tho the 79 CR250RZs & A5 KXs still appear borderline to me. (I believe the next model CR -RAs ran more upright shocks)
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Hi JC,
That is very interesting.
I have just "What Ifed" to see what the difference in spring rate would be if I laid down my CZ shock.
I have not changed any other factors just the shock angle.
Angles are to the horizontal
Standard CZ400 1973 shock angle 80deg spring rate as per my calculator 82lb/in
Improved angle 62deg spring rate as per my calculator 88lb/in
Improved angle 45deg spring rate as per my calculator 112lb/in
Ji
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Hi JC,
Here is an interesting "What If"
If for example I was to do a wheelie on my CZ, what spring rate would bottom out the shock. When you do a wheelie all the weight of the bike and rider except rear unsprung weight is applied to the rear shocks.
Answer 70lb/in
I would still like to have half of my rear shock travel available during a wheelie which would mean I would have to run a spring rate of 93lb/in.
Thus a 70lb/in spring would be no good for jumps as the force on the shocks would be higher.
Mass x acceleration.
In the first case we only have gravity 9.80665m/s but in the case of a jump we would have the speed of the bike. I know we also have to contend with the shocks sitting due to chain torque, but lets put that to one side for now.
Ji
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Hi JC,
Talking about jumps now and that #2 formula.
That chap uses a factor of 2 for dynamic loads applied to the rear shocks.
Now say we were travelling at 75km/h which is 21m/s (roughly twice the velocity of gravity) this may account for his factor. I know that impact loads are much more complicated to calculate than this but I just thought I would start the debate.
Thus, say I still want 1/2 an inch of travel left when I land after a jump when travelling at 75km/h. Assuming I land on the rear wheel first. What spring rate will I need in my CZ400 1973.
Answer 160lb/in
If I perform the same jump and allow the spring to completely compress or bottom and have no extra travel left what spring rate would I need.
Answer 140lb/in
It is obvious that the perfect spring rate on a bike with 4inches of travel is unattainable.
Thus for my CZ400 I would need a progressive spring with the rates of 82-140lb/in
This should give me my ride height and just bottom out over a good jump.
Of course the dynamics don't really apply only to the springs, there is dampening, oil, valving.........
Thus I would say a progressive spring of 82-110lb/in would be more right than wrong.
Ji
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Hi JC,
Could we say that his factor of 2 was based on an MX bike jumping say 2 metres above the ground.
PE = weight x height
PE = mass x gravity x height
PE = 205kg x 9.81 x 2
PE = 4022Nm
Which converts to
Wheel effective rate = 2 x 4022 / .1 squared
WER = 804400N/m
WER = 804N/mm
WER = 4572lb/in
Spring load per spring is WER/2
SL = 2286lb/in (when spring is bottomed)
Which is unattainable for a CZ 73 with 100mm of wheel travel.
Using my 160lb/in spring you can only soak up 640lb without any preload and 800lb with the full inch of preload.
And I think this is why damping plays a big part in landing a bike from a jump.
With out any kind of damping the spring would have to be huge.
Ji
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Hi JC,
A bike does not just drop 2 metres straight down after a jump.
It is more like a bullet that ricochets. It imparts only a portion of its energy not the full load. Also the down ramp is angled to minimise even more of the impact. The riders body collapses at knee and elbow like a crumple zone in a car.
All these factors make it impossible to calculate the dynamic load.
I like the idea of just doubling the weight and also the wheel stand calculation.
So from my findings I would work out the spring rate for ride height, then check the load of a wheel stand on the shocks and last punch in twice the rider/bike weight to see what is required when the spring is bottomed. If I was going to have Walter build me a pair of shocks I would bring this info to the table and ask his opinion on what dampening would be required.
Ji
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Hi Wasp,
I am sure of that now.
And may I add that if we set up a set of shocks for a track like Conondale with large jumps for VMX it will be terrible for a flat grass track.
Thus we really need our shocks set up for at least two types of tracks with different valving, oil and spring rates.
Walter I will be speaking to you soon about my CZ shocks that you will be building for me. Z302's .
Ji
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Ji,
I'm just about brain dead (got the flu) but there's surely something very wrong w yr calcs for a 2m jump.(Reply #56) No spoked wheel or VMX rear subframe would take those sort of loads, yet 2m jumps were hardly uncommon in 70's MX.
There's a few other 'gremlins' in the calcs above too. You can't compare a velocity of 21m/s to gravity & say its twice gravity cos 'g' is acceleration (9.8 m/sec-sq) not velocity. Its like comparing cucumbers to tennis balls.
However Ocheltree's Load Factor of 2 may be equivalent to accounting for G-out forces of 2g which would perhaps be reasonable for 70's MX.
But his formula (#2) doesn't work anyway (as it stands above in Reply #43). As far as I can tell, wheel travel should be shock travel in that formula, so it would read:
SR = BF x CW x LF x LR / 2 x ST
where ST = Shock Travel/stroke
LR = Leverage ratio of shocks
LF = Load Factor (2.0)
CW = Combined wt of bike + rider
BF = Balance Factor (0.4 for Front; 0.6 for Rear)
There's a few other things I'll comment on later too, perhaps tomorrow hopefully when my head doesn't hurt.
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Hi JC,
I agree, I did not spend much time on the stuff today so I don't think the calcs are right either.
I do how ever feel that impact load is a waste of time.
Set for ride height and be done with it.
Ji
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Hi JC,
I have to remember that my goal was to work out a way to determine a starting point spring rate. I have done that. Now we have moved onto an area that is beyond my knowledge base. I will study some more of this kind of stuff but from what I have seen so far there is no point as the dynamic forces are a moving target.
I have enjoyed our adventure so far.
Ji
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Ji,
Yes, accounting accurately for dynamic loading is all-but beyond me too these days. 30yrs ago I may have relished it, but I've long since (in a moment of madness) thrown out my dynamics text books & forgotten the formulae. And the 'grey matter' is more than a little sluggish these days.
The Load Factor in #2 formula does seem to offer some accounting for dynamic loads tho, even if thro a whole lot of assumptions/approximations. It's just a rule of thumb that seems to get you in the ballpark, but needs to be checked w time on the track(s).
Just reviewing the thread so far, there seems to be a few oversights in several of the calcs, eg:
Spring rate for doing a wheelstand is going to be diff for when the fr wheel is just off the deck to when the bike is at say 45deg, cos the movement of the rear axle is almost 90deg to ground in the former but around 45deg in the latter. (More variables!) The worst-case scenario would be the former, which is probably what you calc'd for. The figure you got for it sounds too high tho.
If I perform the same jump and allow the spring to completely compress or bottom and have no extra travel left what spring rate would I need.
Answer 140lb/in
It is obvious that the perfect spring rate on a bike with 4inches of travel is unattainable.
Thus for my CZ400 I would need a progressive spring with the rates of 82-140lb/in
This should give me my ride height and just bottom out over a good jump.
Unfortunately there's a slip-up there. Its not sound reasoning. Equivalent spring rate of an 82-140lb progressive spring (& hence total compression force at bottoming) is something considerably less than for 140lb straight-wound, so according to yr calcs, if it just bottoms w a 140lb staright-wound spring it would bottom terribly w an 82-140lb progressive.
I may well be wrong, but there also appears to be someting amiss in a 0.5" preload changing the required spring rate from 126lb to 82lb. That seems excessive. Working on a 4" shock stroke, I would have tho't an 82lb spring w 0.5" preload equates to a 92lb spring w'out preload when the shocks are just bottomed. But perhaps I'm still brain dead.
However I like yr Table in Reply #38. The results seem a bit hi (by about 15%) for what was widely used back in the day, but its reasonably in the ball park & perhaps accounts for us all being somewhat heavier these days. Or maybe we ran w more than 30% 'live' sag in the 70's. (We never checked/measured in those days. We just worked on what just bottomed occasionally on big-hits.) More likely tho, the diff also accounts for the amount of friction in the system &/or compression damping in the shocks. Perhaps you could use a factor like that (say 10-15%) & write it into yr formula to account for friction & comp damping in real-life situation. Or that could be another "what-if" variable.
I have reservations about working out spring rate just by sag/ride ht, but as has been said, its another way of arriving at a starting point & will have to be adjusted w time on the track(s). Y're perfectly right in saying dynamic forces are a "moving target". There are so many variables
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Hi JC,
All of these calcs can lead you to a place but when you get there you find that the place has moved.
The next variable is product availability.
I spoke to Walter today and the standard progressive springs he keeps on his shelf are
Newtons/mm Pounds/inch
10 15 57 85
13 18 74 102
17 25 97 142
20 30 114 171
25 35 142 199
25 40 142 228
25 45 142 256
30 50 171 285
45 60 256 342
I guess one of these is going to have to do
13 18 74 102
17 25 97 142
Ji
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Hi JC,
I think I have found my spring.
Units in Pounds Units in Millimetres
and Inches and Kgs
Progressive spring rate 97 143
Preload .5 12.7
Static sag 0.313707677 8 (8% of travel)
Dynamic sag 1.090685494 28 (28% of Travel)
load at bottoming 528.5 240 (480kg gross which is double bike/rider mass)
I know this is not the way to work out the load carring capacity of a progressive spring but it is a quick way and I will work it out properly when I have a hour spare.
Still it does show that my original calcs were spot on and the magic factor 2 may be real.
Ji
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Hi JC,
Spring rate for doing a wheelstand is going to be diff for when the fr wheel is just off the deck to when the bike is at say 45deg, cos the movement of the rear axle is almost 90deg to ground in the former but around 45deg in the latter. (More variables!) The worst-case scenario would be the former, which is probably what you calc'd for. The figure you got for it sounds too high tho
In engineering the worst case scenrio is what one has to design for. Once the front wheel leaves the ground by one inch the full mass is on the rear shocks. The other thing is when doing a wheelie the bike will be travelling and will hit bumps, these bumps will be perpendicular to the swingarm the more vertical the bike gets. This will, I feel, equate to the same loading on the shocks as the front wheel being only one inch off the ground. If you refer back to my table the mass of bike/rider on each shock is corrected for leverage and shock angle. This correction increases the load from the original mass and thus I feel the loads are correct.
I feel the load would actually be larger than my calcs. Wheelie, twist throttle, pull back on handle bars, push down with bottom to compress shocks. This transfer of weight backwards to raise the front wheel plus more chain torque would have to be more load on the shocks than what I have predicted but again a moving target.
Ji
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Hi JC,
Yep, your right.
Unfortunately there's a slip-up there. Its not sound reasoning. Equivalent spring rate of an 82-140lb progressive spring (& hence total compression force at bottoming) is something considerably less than for 140lb straight-wound, so according to yr calcs, if it just bottoms w a 140lb staright-wound spring it would bottom terribly w an 82-140lb progressive.
Ji
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Hi JC,
Yep an 82lb spring with a .5inch preload is equal to a 92lb spring without preload.
I did all those calcs by hand while still trying to work it out. I thought it was the missing preload but may be it was something else.
The table I posted is from the program I wrote it is correct.
Ji
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Hi JC,
I am going to leave the weight of the unsprung items in my calcs.
Reasoning is to combat stiction, friction, extra fuel, wet clothes and mud............
Also the centre of gravity of the rider changes from corner to corner, jump to jump. All the rider has to do is lean back a few inches and the weight distribution changes. For example have you seen how modern super cross riders seat bounce a jump!
I feel it will also help when the shock compresses due to chain torque.
Ji
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Hi Wasp,
Thanks for your kind words.
Ji
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In engineering the worst case scenrio is what one has to design for.
That's right Ji, thats why I mentioned it.
The reasoning for worst case scenario (for a wheelstand) being w the front wheel just off the deck is that the more altitude the front wheel has the more of the total combined weight of bike & rider is taken by the swingarm tubes (ie from the axle thro the tubes to the pivot) & less by the shock/springs until you get to near vertical when all the weight is taken (rigidly) thro the swingarm tubes & none by the shock/springs & the only effective suspension is the flexibility of the rear tire.
I'd have tho't the 97-143 progressive springs would be way too stiff if the shocks are in std pos'n unless yr very heavy. CZs came std w 100 or 110lb springs back in the day & they were way too heavy (& just about the first thing replaced) unless the rider was "over 230lb"
For my wt (71Kg/154lb) the 74-102s would be much closer to the mark, which is approx equiv to 88lb straights, & even that would be a bit stiff. Back in the day 60-90 progressives were widely used, or 68lb straights unless the rider was over 180lb when 78lb straights were used. The 57-85s would perhaps be the best for my wt as I like fairly soft suspension.
Using Ochletree's corrected formula (Reply # 60), for 240lb bike, 160lb rider, 0.6 balance factor, 2.0 load factor, 1.2 shock leverage ratio, & 4" spring/shock stroke, it gives 72lb springs - very close to what we ran back in the day. All calculations aside, thats what worked well on the track & in the end thats all that really matters. It'd be worth calc'ing the sag/ride ht for them.
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Hi JC
What If 74-103lb/inch
Units in Pounds Units in Millimetres
and Inches and Kgs
Progressive spring rate 74 103
Preload 0.5 12.7
Static sag 0.56661682 14
Dynamic sag 1.585087742 40
load at bottoming 391 178
Ji
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Holy Moly! I feel like I'm lurking on some quantam physics forum. As smart as you blokes are on this stuff, it's turning into a anorak pissing contest --- he with the longest, most technically obscure equation wins.
I'll never be able to look at simple a pair of Konis in the eye again. :'(
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No offense Red, but they're hardly either long or technically obscure.
Who was it said, "men are most likely to settle a matter rightly when they can discuss it freely" ? (or words to that effect)
That's all thats happening here. Its hardly rocket science. Or a contest.
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Hi Red Devil,
JC and I are not arguing who is the smartest or knows more.
I have found JC checks what I have done and passes is experienced eye over it which is a forum. I would like to see JC do some worked calcs so i could look over his work HaHa... but certainly I am enjoying the raking of each others minds.
Ji
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Agreed Ji,
I've enjoyed yr thread & the stimulus of yr posts.
There's an old proverb: "As iron sharpens iron, so one man sharpens another".
It's so true when done in the right spirit.
Thats all we're doing.
I'm no expert in the field - just trying to think things thro from 1st principles.
And Walter can probably show us both up!
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Hi All,
JC and Brent have pushed me to explore more about spring rates than I would have done on my own. After all this topic must have been interesting to a lot of members as it has been looked at over 700 times, so you could say that JC has made all of us look at our shocks just a bit more.
Another What If
I think this would be the ultimate set of springs
Do you have a pair of these Walter to put on my new shocks.......
Ji
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The forum is not letting me put this table up as a JPG so here I go again.
Units in Pounds Units in Millimetres
and Inches and Kgs
Progressive spring rate 91 114
Preload 0.5 12.7
Static sag 0.367358733 9 (9% of travel)
Dynamic sag 1.195565856 30 (30% of Travel)
load at bottoming 455.5 207 (414 Gross or twice bike/rider mass)
Ji
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I know that I started this topic to determine rear shock spring rates but now I have written a program to predict rear shocks, unless more data is discovered I feel we have covered it enough to be close to what is required.
It will not be very difficult to modify the program to allow front fork springs to be analysed as well.
When I get time I will copy and rearrange the program to predict fork springs. Once this is done may be we can figure out what Brent was getting at.
Brent's hypothesis;
The front and rear suspension should be balanced.
Both ends of the bike should be in harmony for the best ride and traction. To improve cornering, jumping, take off at the starting gate...........
We would have to determine frame geometry to be able to get anywhere near the answer but we have to start some where.
Ji
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I am back after an 8 day trade show.
I will start on Brents rear shocks this week.
Brent can you supply the front end geometry of your bike.
Ji
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I've really enjoyed this thread although I've not put much into it after the start.
And I've had to dust off my maths brain for some of it and that's been a shock to the system.
I think a starting spring rate can be determined and to my mind that would be static but fully laden (with rider) There are too many variables to try to work out landing from jumps etc unless you land from the same jump at the same speed etc etc.
I still think that getting the springs to hold the bike up and using the damping to control it is the way to go.
I've found with my 500's that not much of what I do with the rest of the bike affects the forks too much but the back is a different story.
My shocks are length adjustable and I tried different lengths over time to try different handling. With the bike low in the rear the swing arm will squat and the rear suspension will preload requiring more spring rate but then that's too harsh with no power. Conversely with the back jacked up the bike will rise and take the load off the rear springs. With changes in shock length and spring rates I also had to make changes in damping to compensate. At one stage I would be changing the damping to try and compensate for a different spring rate and also for a different suspension reaction. These two things could be opposing or compounding depending on what I had done.
I can say that I got it horribly wrong some times, quite a few times to be honest. Now I seem to get it right more often that wrong but that's not saying too much.
Thanks guys for for a lot of info and for making me think again!
Brent
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Hi Brent,
I should get some time this week to check out the data you sent me.
Ji
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(http://i382.photobucket.com/albums/oo263/ozmaico/YSS031.jpg)
(http://i382.photobucket.com/albums/oo263/ozmaico/YSS032.jpg)
(http://i382.photobucket.com/albums/oo263/ozmaico/YSS034.jpg)
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(http://i382.photobucket.com/albums/oo263/ozmaico/YSS037.jpg)
(http://i382.photobucket.com/albums/oo263/ozmaico/YSS064.jpg)
(http://i382.photobucket.com/albums/oo263/ozmaico/YSS074.jpg)
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walter you explain the pics ;)
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I want one of those spring rolling machines for my shed!
I used to build machinery like that way back when
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Pictures of the goings on inside the Sports Bar would be FAR more interesting ;D
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I have a PE250N with a set of piggy back Ohlins fitted. Taking a guess the rear springs have been set up for a 70-80 kg rider and probably not well set up given that there is an alloy spacer fitted between the spring and the top spring saddle/retainer. When i throw my 110kg frame on the bike i compress the the rear suspension better than 1/2 way through the stroke and obviously need to correct the situation.
A quick call to Cramers and i find out that a set of springs (rate and free length to be confirmed) will cost more than what i think is reasonable. The Ohlins run dual springs and i am told that the valving in the shock is set up to work with dual springs and that i should not fit a single spring. So the question, how do i get a spring that has the correct spring weight to suit my weight and the bike but also match the shock in relation free length that when compressed and fitted to the shock won't end up with coil bind before the bump stop get's to do it's job?
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(http://ozvmx.com/community/index.php?action=dlattach;topic=6430.0;attach=6690;image)
Using Ji's image i have the following specs.
Z = 60mm
Y = 335mm
X = 540mm
T = 270mm
The shock is an Ohlins Piggy Back that is;
395mm eye to eye
290-310 between the spring platforms
and if i am measuring correctly has a stroke (without bump stop) of 140mm
And with gear i'm around the 115 kg mark.
Suggestions as to the best spring?
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Any Suggestions? Need to get something sorted for the Harrow weekend.
Thanks in advance.
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Bahnsy i've got single ohlins progressive rate springs on the Ohlins from my RM250T and the same springs on my 490 Maico bought new from Cramer. They would be pretty close to what you need. The part number is 180-46. The spring length is 270mm, shock length is 385mm.
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Banshy,
You can determine your existing spring rate by using this calculator, then best ring a suspension shop (maybe Walter) with the details and weight existing springs where designed for, from that they can work out what rate suits your weight.
I have tried to get Cramers to help with Ohlins rates before, but they weren't interested unless I had a part number.
Your best bet is a suspension shop.
http://www.racingsuspensionproducts.com/spring%20rate.htm
p.s. the flat "half" spring at the very top and bottom of the spring can be counted as 1 active coil.
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To calculate if you will run into coil bind, first measure OD of the wire of the spring, then times the thickness of the wire X the number of coils. this gives you the solid/compressed length of the spring. if you have dual springs do this for the short and long spring and dont forget to add the thickness of the plastic spring divider. Then measure the compressed length of shock (with out bump stop) between the top and bottom spring retainers. That measurement will vary depending on what preload clip groove you have it on but then if you do it from maximum preload you know you will always be safe. Then if the compressed length of the spring/s is LESS than the distance between spring retainers/seats then you have no problems with coil bind. if it is more then that is not so good and the spring will bottom out on its self and before maximum shock travel and it will overload the top spring retainer and would most likely break.
Old Ohlins springs are identified by spring colour and coloured markings on the springs so if you know the paint is original the spring rate can be identifed by that.