Cross Weight problems

[Yunlong Xu]

This year we have encountered some servere cross weight problem again in Germany with one diagonal weighing about 60%, while we still have a 50-50 front to rear and left to right.

So my question is, what can be the cause of this cross weight? my biggest suspect is a static chassis twist due to tube length or welding problems. Maybe suspension compatibility as well?

Say we have FL and RR as the lighter diagonal, how can we correct for it? I would assume increasing the pushrod length on FL and RR, should that work?

cheers

[thewoundedsoldier]

Get it close with pushrod length, then dial it in with the shock pre-load. Best to find corner scales and tweak all four corners together until you get it right. You don't want to change two parameters blindly and then hope it got close. Claude has an interesting anecdote about changing chassis settings.

[Yunlong Xu]

Originally posted by thewoundedsoldier:
Get it close with pushrod length, then dial it in with the shock pre-load. Best to find corner scales and tweak all four corners together until you get it right. You don't want to change two parameters blindly and then hope it got close. Claude has an interesting anecdote about changing chassis settings.

Do you mean using pre-load to fine tune the weight distribution after making most of changes by changing pushrod length?

Increase the pre-loads of the lighter diagonal should have the same effect as making the pushrods longer I suppose?

[Silente]

Yes the effect of preload is similar to that you can obtain working on push rod length.

But working on spring platform can have the undesired effect of changing also the working point of your suspension. And if motion ratio is not constant (or more or less constant, for example if you designed your suspension to be progressive) you will have a different wheel rate on each wheel.

I would work on push rod length.

Anyway such a difference on diagonal weights could ask for a too big difference in length on push rods. I would check also something else.

[Yunlong Xu]

Originally posted by Silente:
Yes the effect of preload is similar to that you can obtain working on push rod length.

But working on spring platform can have the undesired effect of changing also the working point of your suspension. And if motion ratio is not constant (or more or less constant, for example if you designed your suspension to be progressive) you will have a different wheel rate on each wheel.

I would work on push rod length.

Anyway such a difference on diagonal weights could ask for a too big difference in length on push rods. I would check also something else.

What else can it be? I was thinking of static chassis twist. Any other possibilities?

[exFSAE]

Your chassis and your suspension aren't fabricated to perfection. That's life.

Also, your springs aren't all identical rate even if they're stamped as such. Nor are your tires.

I wouldn't use preload at all. Just change pushrod length to tune it out. Then I'd also scale the car empty and with your heaviest driver just to make sure you don't have any crossweight change with load.

[Tim.Wright]

Working on pushrod or platform will change the operating point of the suspension but this is required since the installation sounds twisted. Or not twisted, but fabricated with a twist.

I would prefer the push rod adjustment since it changes less

Tim

[Yunlong Xu]

Originally posted by exFSAE:
Your chassis and your suspension aren't fabricated to perfection. That's life.

Also, your springs aren't all identical rate even if they're stamped as such. Nor are your tires.

I wouldn't use preload at all. Just change pushrod length to tune it out. Then I'd also scale the car empty and with your heaviest driver just to make sure you don't have any crossweight change with load.

Agreed. And to solve cross weight problem, we should be increasing the length on both FL and RR pushrods at the same tight right? Cos otherwise it will just simply transfer load from FL to RR or vice versa(I think...)

[exFSAE]

You have to work on a diagonal, yes.

Just get the wrenches out and start playin with it.

[Silente]

I agree with exFsae.

i would check with care that pushrods are the same length left and right before to start to move anything.

Maybe this can sound offensive to you because you already know how to set the car up before to use it (and if so i apologise), but for what i have seen in formula student one of the thing the guys very often don't do at all is to correctly prepare the car before any practice or race event.

Obviously you have to place the car on a perfect plane to be sure the wheigts you read on your scales are correct.

Maybe your problem is only a problem of setup. Maybe it's beacause your parts are not perfectly built, and this is possible as well.

[Yunlong Xu]

Originally posted by exFSAE:
You have to work on a diagonal, yes.

Just get the wrenches out and start playin with it.

That's what I thought.

Does that mean if we get unequal spring, it will in fact just shift weights between the two corners on the same diagonal? The only way that an softer/harder spring can cause a crossweight is by getting the car in a static roll (I think)

[jpusb]

I don't know why you haven't put the car on scales and start playing with it, that is the fastest way to learn how the adjustments affect the corner weight.

Now, as everyone has already told you, FSAE cars aren't built perfectly, so is very likely that you will have to use different length push rods, how different depends on you chassis fabrication quality. I dunno about the cool CF monocoque cars out there, but steel space frames are never good enough. I remember our 08 car had VERY different pushrod lengths, because rocker pivot placements was very bad controlled (no one knows what happened there hahaha). That, of course, is not ideal, but that was our reality and the car ended up being a really good, fast, and predictable car.

What you want to know is the base push rod length of each corner of your car, to guarantee that the shock will use its travel as you want it to. That will place every shock in the sag position you want for ride height and will more less give you about the same spring loads in every corner (if your motion ratios don't change wildly with rocker pivot position and angle) and from there you fine tune corner weights with the push rod length, so fabricate your push rods to permit length adjustments in both directions.

I just told you where you need to get to, how to do it is your homework.

[exFSAE]

Originally posted by Yunlong Xu:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by exFSAE:
You have to work on a diagonal, yes.

Just get the wrenches out and start playin with it.

That's what I thought.

Does that mean if we get unequal spring, it will in fact just shift weights between the two corners on the same diagonal? The only way that an softer/harder spring can cause a crossweight is by getting the car in a static roll (I think) </div></BLOCKQUOTE>

Not really following you here.

But, let me give you an example. Pretend your car has all equal wheel rates and the CG is in the dead center of the car. Your corner weights will all be equal.

Now, let's say you increase the wheel rates on the RF & LR (stiffer springs... different as-built motion ratios... tire build variation or air pressure differences... different compliances...). You will now have more corner weight on the RF & LR -> adds crossweight -> screws your handling.

You can shorten two of the pushrods to even out the corner weights and get the crossweight back down to 50%. But!! as soon as you change the load of the car (driver change for example), the crossweight will come back.

This is why I recommend scaling the car with a really light (or no) driver, and a heavy driver.