Why so stiff springs guys?

[swong46]

Hey guys, I was at Fontana checking out other peoples cars and noticed that a lot of the team's wheels did not come down (springs did not decompress) when they lifted their car up. So there is no rebound on the inner wheel when cornering, whats the deal?
Are they just using the stiff springs to boost up the roll rates and what not?

[swong46]

Hey guys, I was at Fontana checking out other peoples cars and noticed that a lot of the team's wheels did not come down (springs did not decompress) when they lifted their car up. So there is no rebound on the inner wheel when cornering, whats the deal?
Are they just using the stiff springs to boost up the roll rates and what not?

[nowhere fast]

It is preload rather than stiffness that determines if the springs compress under the static weight of the car or not. Even a very stiff spring will compress by some amount unless it is preloaded.

Droop limiting also has the effect of increasing the roll stiffness since normally the suspension of the inside wheel pushes the chassis in the direction of roll while the outside pushes against the roll. But if the inside suspension tops out then it no longer pushes into the roll.

I would be surprised if teams were intentionally running so much preload on all wheels that they have no rebound travel.

[Test Driver]

Because Race Car!

[DMuusers]

What do you think are stiff springs then? How many N/mm? Besides, you also have to look at the IR when you consider spring stiffness. A 30 N/mm spring on an IR of 0.5 isn't the same as a 30 N/mm spring for an IR of 1.0.

[Garlic]

Most FSAE cars are probably droop limited when the driver is not in the car.

If you witnessed cars being lifted without drivers, likely the wheels had no rebound left because the car was ~60% lighter than design weight in the front and ~35% lighter than design weight overall.

[exFSAE]

There are a number of teams (mine did this, somewhat stupidly) that run high wheel rates in the hope of better yaw responsiveness - when in fact roll stiffness is at best a 2nd order contributor.

I would consider >= 100 lbf/in (17.5 N/mm) to be quite stiff for cars this light. But that's just my opinion.

[wagemd]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Test Driver:
Because Race Car! </div></BLOCKQUOTE>

+1

[swong46]

I forgot to mention that they told me they were running 300-500 lb/in springs per corner. I don't care what your IR is, that is one STIFF spring!

[Chris B]

lol if 17.5 N/mm ia considered stiff then we're screwed, 30-60 N/mm is the range of the stiffness of the springs we have.

I personally feel and always have that this is miles too stiff for this application, however due to monetary constraints we're kind of stuck with the current springs and shocks at the moment.

although there are other ways of affecting ride stiffness other than through the spring. i.e you can modify your motion ratios on your rocker, etc. this is the avenue we've had to go down for the new car and so far.

[exFSAE]

Even if you have 30 N/mm springs, it's still the wheel rate that counts. Can knock that rate down quite a bit. Otherwise indeed, 30 is a hell of a lot for a wheel rate. 60 is absurd.

It's not necessarily that you're "screwed," since the ride requirements on these cars are relatively non existent - though there are some detriments.

Bigger point is, throwing spring or bar stiffness at these cars isn't necessarily going to do anything past a point - particularly if your suspension has the compliance rate of a dishrag and/or the tires you've chosen are pretty numb.

[Chris B]

yeah that was the conclusion we came to with being to get a lower wheel rate for a given spring depending on how the rest of the suspension etc is designed.

[Some Guy]

Of the teams running "stiff springs" what percentage of those had wings. Of the percentage with wings what percent had them unsprung?

[swong46]

None of them had wings actually (that I talked to)

[Felippe Vieira - Fórmula FEI]

The real question isn't about a stiff spring, but about what is the frequency of sprung mass.
I read something around 3 Hz for non-aero race cars, like almost all fsae cars. Are you guys working around it?