Wheel Rate

[MercerFSAE C. Burch]

What wheel rates are you running in the competition? Do you bother to adjust your wheel rates for different events?

Chris
Mercer University - Drive!
Coming to an auto-x course near you, May 2004!

[MercerFSAE C. Burch]

Surely, somebody here knows what their effective wheel rates are for their car? Is this secret information that I'll only get through testing?

Chris

[Charlie]

I doubt anyone is willing to give that info in a public arena http://fsae.com/groupee_common/emoticons/icon_wink.gif

-Charlie Ping
Auburn University FSAE (http://eng.auburn.edu/organizations/SAE/AUFSAE)
5th Overall Detroit 2003
? Overall Aussie 2003. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Frank]

Yeah I doubt they would Charlie..

As an engine guy who's turned suspension hardware designer (another guy does the theory) , I've watched the calcs so I'll give it a stab..... but you'll have to deal with some metric numbers... and I'd warn that this might be wrong

OK?

our car weighs 240kg , unsprung mass is about 15 kg per corner , driver is a fat bastard (me) 80kg

therefore sprung mass is about 240 - 4*15 + 80 = 260 kg

weight distribution is 50:50

I'm shooting for natural frequency of 3.2 Hz in the rear , or w = 20.1 rad.s-1

So ¼ car model is m = 65 kg w=20.1 k= w^2.m = 26,260 N/m

K = 26,260 / 175 = 150 lbs/in

We use Hoosier 20*6*13 R25A tyres

http://www.hoosiertire.com/spring.htm#TIRE SIZE: 20.0 x 6.0 – 13

we run ˜em at 12 psi

load per tyre = 175 lbs

tyre rate is about 500 lbs / inch

therefore spring rate needed is 1 / (1/150-1/500) = 215 lbs / inch

say the pushrod is at included angle of 30 deg

then rate at pushrod = 215* 1 / sin 30 = 2 *215 = 430

and then maybe use a rocker of 2:1 ratio.. result = back to required spring rate of 215 lbs / inch

as for the front, i think we're going for natural frequency about 2.8 hz

correct ? wrong? abuse?

"Beware of internet engineers" http://fsae.com/groupee_common/emoticons/icon_smile.gif

[clausen]

How did you decide on a natural frequency?

Regards

Paul Clausen
Uni of Adelaide

[Frank]

asking around, and from experience with previous car

i guess its a start point.....

like i said, im not sure this is good / bad, correct / incorrect

[Jeff Gensler]

motion ratio=wheel travel/spring travel
wheel rate=spring rate/(motion ratio)^2

In general:
Passenger cars 1.0-1.5 Hz
Sports cars 1.5-2.0 Hz
Race cars 2.0-2.5 Hz
F1 with wings 2.5Hz+ to maintain ride height

[yofa]

hmmm...

i went with 3.2 front, 2.8 hz rear. i often read that front stiffness should always be greater than rear, for less nose dive and generally to make sure that the front end is always more stable than the rear, so the driver can modulate with the gas pedal. i thought for sure that i was running way more spring than most teams. i guess that's not true.

i have not yet run anti-roll bars. i don't think our drivers are quite up to that level yet, myself included. straight springs are more linear and predictable than anti-roll bars, but i know that most top teams, probably with top drivers, all use them.

yofa

[Bob Wright]

how does 100lb/inch all round sound?

Bob Wright
Monash University
Australia
http://www-personal.monash.edu.au/~fsae

[ben]

24N/mm?

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

[MercerFSAE C. Burch]

100lb/in. sounds about right. I was thinking around 150-250lb/in., but as I have limited, read "NO", experience with these things, it was just a guess. It has since been hinted to me that my guess is off by about a factor of 2., which puts 100lb/in. right in the ballpark!

You don't bother to change your rates front-to-rear, or for different events? I guess you've got a lot of other variables to work with on the Monash car!

-Chris

[Frank]

i should just comment to fix a mistake, or possible misconception before,

when i said the pushrod is at an included angle of 30 deg.... i meant included angle to the transverse vector, and this only (sort of) works if the rocker is near in plane with the vertical-transverse plane

the math to solve the motion ratio is, of coarse, more complictaed, and includes tyre deformation, and in our experience is easiest to solve and re-iterate with a CAD package

Frank