spring rates

[Cement Legs]

Hey guys, I'm part of a new team just starting to get our ears a little wet. I'm looking for a general starting point for spring rates on FSAE cars. We are looking to use either 500# or 600# springs (mountain bike shocks) but will incorporate into our design a factor of adjustment. What would be a standard or average springrate found on a 425lb car with competition performance?

[Didier Beaudoin]

It depends a lot on your motion ratio. I've seen springs from about 100 lb/po all the way to 600 lb/po. I beleive 500 or 600 is a bit stiff though, because it would mean you do not use a big proportion of your dampers because of a too low motion ratio.

[RiNaZ]

hey didier ... for 100 lb/po ... what does po stands for? Maybe the answer is obvious but right now i cant think of any unit that would stands for po.

Thanks

[Matt Gignac]

pouces, french for inches

[Didier Beaudoin]

oops...

I've never realized French is such a bad habit.. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

I meant lb/in, sorry.

[Carlos Torres]

Hi man... I'm part of a new team too, we did some simulations at Working models and found that 400 lb/in in the front and 500 lb/in in the back because we expect a Weight distribution of (40/60) to (45/55)
We want to use Vanilla Rc from Fox Racing Shox
CArlos Torres
FAE-LUZ
Suspension Chief
www.fsae-luz.org (http://www.fsae-luz.org)

[Kevin Hayward]

I think it always needs to be said on these sorts of threads that wheel rate and spring rates are two completely different things. It should be the wheel rates that you talk about. The spring rate is then a function of your motion ratio (which will be affected by your dampers).

A wheel rate of 400 to 500 lb/in is way too high. You should be in the order of magnitude of around 100lb/in. Although I do know some teams that are higher than this. But I would be surprised if you are 4 to 5 times as high.

With the stock damping of the fox you would need to run a motion ratio of quite a bit less than one. i.e. the damper will move less than the wheel. The problem is that the rebound damping of the fox is quite large compared to the compression (excessively so). With a ballpark motion ratio of 0.5 you should get decent enough damping within the range. That would be a spring rate of around 400-500 lb/in.

Problem with this is that the damper doesn't move much. Which means there is not much fluid movement. The friction effects are amplified and the damping isn't as nice. However it is more important to be in the right range on this.

...

I would just add that some people may disagree with the numbers I've presented. However I doubt there would be too many disagreements that a wheel rate of 400-600lb/in is just too high.

Kev

[Carlos Torres]

I was talking about 2 springs of 400 lbs/in in the front and 2 springs of 500lbs/in in the back. And you are right that gives a wheel rate of about 100 to 125 lbs/in depending of the motion ratio (0.5-0.6)
How about natural Frequency?
Is 4 Hz a high frequency? I´ve seen people using 2-3.5 HZ
Should the front frequency be higher than the rear?

[Kevin Hayward]

4 Hz does seem a little high.

As for frequencies front and rear. You should be aware that given that the properties of these cars that the front and rear frequencies are not decoupled. Hence you should really be looking at separate pitch and bounce frequencies.

Furthermore the tuning of frequencies front and rear is generally done to provide flat ride tuning, which may or may not be one of your design goals. Possibly you are more concerned with tyre loading under extreme conditions. This will sort of set your wheel rates. Because of this the answer will also depend on whether you run anti-roll bars or not.

Without wanting to be too simplistic I would say that the main reason for looking at front and rear frequencies for these cars is to ascertain whether the magnitude of springing is right or not. At 4 Hz I would say that is a little too stiff ... but others may disagree.

Kev