spring rates consultation

[david.pr]

Hello guys,

get ready theres A LOT of details here!

I want to have your opinion on the calculated spring rates that have I got( based on Milliken chap 16). I got 250 lb/in spring rate needed at the front with 130 kg front weight and IR of 0.76 (for 2 inch wheel travel).
And 200 lb/in needed at the rear with 170 kg rear weight and IR of 0.7 (with same wheel travel as for the front)

TLLT is about 49:51 (F:R)

Roll stiffness dist is about 56:44 (F:R)

Roll gradient decided to be 1.13 deg/g

We are also using ARB both at rear and front.

Sum it up, the relevant rates are :

K ride F= 122 lb/in
K ride R= 105 lb/in
K roll F= 300 lb ft /deg
K roll R= 236 lb ft /deg

The contribution of the ARB to the total roll stiffness are:

K arb roll stiffness F= 47 lb ft/deg
K arb roll stiffness R= 36 lb ft /deg

So am I in the right direction with the spring rate that I got?

[Will M]

Read these and tell us what you think.

Article 1
http://www.optimumg.com/docs/S...mpers_Tech_Tip_1.pdf (http://www.optimumg.com/docs/Springs%26Dampers_Tech_Tip_1.pdf)

Article list
http://www.optimumg.com/technical/technical-papers/

-William

[Luniz]

I refuse to calculate anything in imperial units ;-)

[BillCobb]

A roll gradient calculation usually includes the tire stiffnesses because that's how you normally measure it with instrumention. Then you separate out the tires from the chassis when allocating the roll geometric responses (steer and camber for example). Even so, I suggest your roll gradient target is too low. The structural requirements to contain such a stiffness means (to me) something is going to break from fatigue. Besides, the weight of the bars and mounts to hold onto this value is substantial. Try for 2.5 deg/g (including tire effects). The values for full size racing vehicles are alot higher than most of you would guess. Driver comfort and motion perception factors play into this as well. Yes the operating tire pressure now gets into the picture.

This would be different if you had an active suspension. The only thing that would change, though is the SIGN of roll per g. You still want some geometry to play into the game.

[Dettle]

I'm sorry.. like Lutz said... send us your data in SI units and we might check something. http://fsae.com/groupee_common/emoticons/icon_wink.gif

What about the chassis roll moment for a given lateral acceleration and track widths to check the roll stiffness?

The chassis roll gradient of 1.13deg/g sounds perfectly reasonable to me. I guess most teams run gradients between 0.6 and 1.0 deg/g which is pretty stiff!

@BillCobb:
My experience is that, with the low roll moments we see in FSAE, a chassis roll gradient as high as 2deg/g seems not beneficial (tires will add about 0.2 to 0.3 deg/g). With only suspension springs contributing to anti-roll (no ARB's), you would probably end up with natural frequencies in heave of about 2 hertz. If you want ARB's working, reduce that to 1.5 hertz or something like that. This migth be good for road holding etc., but will require total wheel travels way beyond 100mm.
Additionally, with the high resulting pitch and roll angles, you will most definetely become an issue with camber.

But I totally share your point! Many teams run to stiff suspensions in heave & roll.

Regards
Hannes

[exFSAE]

43% nose weight and a slightly rearward biased TLLTD?

Get ready for Tokyo Drift.

[Warpspeed]

Be careful guys.
I America metric units equate with terrorism.

[M. Nader]

@BillCobb

Out of curiosity what for example would be the roll gradient for a sports car: corvette ZR1 for example and what would that be for an offroader like a Landrover or Cherokee or even a Vitara