Shock Spring Selection

[Ji Gantor]

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 

[brent j]

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

[Ji Gantor]

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

[brent j]

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

[Ji Gantor]

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

[Freakshow]

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.

[Ji Gantor]

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

[Ji Gantor]

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

[Ji Gantor]

This calculation will only apply to linear rate springs not for dual rate springs.

Ji

[Freakshow]

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.   

[Ji Gantor]

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

[Ji Gantor]

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

[Ji Gantor]

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

[E74]

I have a brand new works type shock spring for a YZ125g/h here for $100 if anybody wants one....

[evo550]

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.