Wheel rate not spring rate

[edmandsd]

We all tend to talk more about spring rates when in fact it's the rate the wheel experiences that's important.

 

As an example i'm using 250lb front springs but i've calculated the wheel rate to be only circa 100lb as a result of the shock angle and wheel/shock mounting points in relation to the chssis.

 

Does anyone recommend a specific wheel rate range for racing - although I obviously appreciate lighter engined cars will require lower rates ?

[edmandsd]

I've either touched on a sensitive issue here or one that isn't really understood - certainly by me in any event....hence the posting !

[Fred]

david i think the problem with springs and ride hight ect are all individual to the person driving the car you will find a lot of differntiation on this subject

but there is also a starting pointquestion.gif lots if you do a search

 

fredid=blue> smile.gif

[Chelspeed]

I agree with you thats it's wheel rate that's important but on this list, where we all drive the same design of car, it's just easier to compare spring rates as a measure of relative stiffness.

 

Unless you're comparing narrow track to wide track of course, or de dion rear to live axle rear....

[edmandsd]

I accept that on the whole suspension set up is to individual tastes but I was amazed to find that by going from std track to wide track i'd reduced my effective wheel rate by 50% (150 - 100) using the same 250 lb springs.

 

Without any specific direction i've decided to maintain the original wheel rate by fitting 350lb springs - However, i've no idea whether the original wheel rate was in the right ball park in the first instance !

[ian crocker]

Buy yourself a copy of one of Alan Stanniforth's books on suspension. Even the wheel rate is of little use. You should be looking at the suspension frequency. A commonly accepted value is around 120cpm for a racer. However some people who are very experienced in race car prep say you should run as soft as possible without hitting the track and while still maintaining a sensible roll angle (which is affected by anti-roll bars too). Many successfull Caterham racers have run around 150cpm. So, the only rule is, there are no rules!

 

Regards,

 

Ian

[edmandsd]

Thanks for the advice Ian.

 

Wheel rate, if nothing else, does go to show that things change quite dramtically when you move from standard track to wide track, and using the same spring rate is unlikely to be the recommended approach.

[Morls]

David,

 

I think with the shock unit being at an angle from the direction of wheel travel, the wheel rate will not be linear.

 

Assuming the lower wishbone to be parallel to the ground before deflection upwards; the effective (wheel) rate will be increased as the wishbone moves up.(With the wishbone finding it harder to compress the spring as the internal angle between them becomes nearer 90 degrees).

 

This theory used to be used on 70s and 80s scramblers to get a rising rate at the back wheel. The theory was that by using long suspension units, at about 45 degrees to the swing arm, the small bumps would only see a small spring and the big bumps a big spring.

Everyone has since gone for more sophisticated rising-rate rocker arms as per most modern race cars and (whisper it) Sylvas.

 

The other bit that I think complicates the issue is the speed of the damper rod.

 

As dampers are speed sensitive the distance moved by the damper piston (over a given time) will effect the wheel damper rate. Due to the inclination of the suspension unit these distances do not have a linear relationship with the wheel travel.

As I have a hangover I can't think whether this gives inceased or reduced damping force at high wheel deflections.

I'll have another Asprin and think about it.

 

Mark