Shock Spring Selection

[Ji Gantor]

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

[brent j]

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

[Ji Gantor]

Hi Brent,
Did you get a chance to measure and weigh your bike and shocks over the weekend.

Ji

[JC]

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.)

[Ji Gantor]

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

[Ji Gantor]

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

[Lozza]

Just buy THESE BOOKS they explain everything(and more) you ever wanted to know.

[Ji Gantor]

Hi Lozza,
Thanks mate, they look like a good read.

Ji

[brent j]

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

[Ji Gantor]

Hi Brent,
Take your time mate.

Ji

[JC]

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

[Ji Gantor]

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
 

[JC]

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.

[Ji Gantor]

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

[JC]

Ji,  Yes yr right.  #2 doesn't take into account ride height. Only #3 does