Questions on Suspension Geometry

[Zach3794]

Okay, so I've been reading up on spring rates, motion ratios, and how to get the wheel rate out of all that. I understand how for example running a 6k front, 7k rear setup in our car will actually make the front stiffer.

What I can't seem to find, and need advice on, is how to factor in weight distribution and roll bar stiffness.

Can anyone enlighten me on the proportion or some kind of equation for this?

The way I see it right now, if wheel rates and weight distribution were the only things to consider, it makes sense to run a slightly higher *wheel* rate in the front than the rear since the twins are a bit front heavy (53/47?)

But by how much is my question.

And after that, how do I factor in a motion ratio or whatever the proper term is for sway bars?

And then of course there are alignment settings with camber and whatnot.

My end goal is as close to completely neutral handling characteristics as possible.

[fatoni]

theres a lot more to it than that. i dont really have an answer for you but i would imagine things like camber curves and such. i think thats why you see so many options that work. i think what you want to be searching for is the front roll couple. thats a pretty good indicator of how a car will drive. i had something more to say but i forgot. im sorry

[Zach3794]

It's okay, I frequently forget things I've had on my mind all day. :P I have a good set of wheel rates for the springs in mind already, so with that I guess I can always just fine tune with sway bars like the vast majority of others out there. I'll have to do more research on camber curves.

On a separate note, I'm pretty sure I shouldn't have to worry about adjusting roll center unless I drop the car more than 1.5", correct?

[Turdinator]

From what i understand there is a lot of black art to suspension setup. Google is your friend. I have read some interesting stuff on spring frequency and balancing front and rear rates. I've also read that sway bars are the final thing to play with as they are more for fine tuning the balance. But best bet is having a chat with people with some hands on experience @robispec and @Racecomp Engineering spring to mind.

[robispec]

Thanks

[EarlQHan]

Originally Posted by Zach3794

Okay, so I've been reading up on spring rates, motion ratios, and how to get the wheel rate out of all that. I understand how for example running a 6k front, 7k rear setup in our car will actually make the front stiffer.

What I can't seem to find, and need advice on, is how to factor in weight distribution and roll bar stiffness.

Can anyone enlighten me on the proportion or some kind of equation for this?

The way I see it right now, if wheel rates and weight distribution were the only things to consider, it makes sense to run a slightly higher *wheel* rate in the front than the rear since the twins are a bit front heavy (53/47?)

But by how much is my question.

And after that, how do I factor in a motion ratio or whatever the proper term is for sway bars?

And then of course there are alignment settings with camber and whatnot.

My end goal is as close to completely neutral handling characteristics as possible.

I'm not sure on how to calculate the MR of the anti-roll bar because it isn't independent for each corner but what is the MR going to tell you? If you want to calculate the roll rate required from the ARB, it is

KthetaA = additional roll rate needed, lb-ft/rad
KthetaDES = desired roll rate, lb-ft/rad
Kw = wheel center rate, lb/in
t = track
Kt = tire rate, lb/in

KthetA = [ KthetaDES * 12 * Kt * t^2 / 2 ] / [ 12 * Kt * t^2/2 - KthetaDES ] - ( 12 * Kw * t^2/2 )

So you need to know what roll rate you want, and the tire rate, which is different for each tire. This is the most important thing to keep in mind. To calculate the wheel center rate,

Kr = ride rate, lb/in
Kt = tire vertical rate, lb/in

Kw = ( Kr * Kt ) / ( Kt - Kr )

The problem with finding a neutral handling car in absolute terms is that it is fleeting because the car is transient, rather than steady state.

[Zach3794]

Originally Posted by EarlQHan

I'm not sure on how to calculate the MR of the anti-roll bar because it isn't independent for each corner but what is the MR going to tell you? If you want to calculate the roll rate required from the ARB, it is

KthetaA = additional roll rate needed, lb-ft/rad
KthetaDES = desired roll rate, lb-ft/rad
Kw = wheel center rate, lb/in
t = track
Kt = tire rate, lb/in

KthetA = [ KthetaDES * 12 * Kt * t^2 / 2 ] / [ 12 * Kt * t^2/2 - KthetaDES ] - ( 12 * Kw * t^2/2 )

So you need to know what roll rate you want, and the tire rate, which is different for each tire. This is the most important thing to keep in mind. To calculate the wheel center rate,

Kr = ride rate, lb/in
Kt = tire vertical rate, lb/in

Kw = ( Kr * Kt ) / ( Kt - Kr )

The problem with finding a neutral handling car in absolute terms is that it is fleeting because the car is transient, rather than steady state.

I was hoping that with the motion ratios of the roll bars I could calculate the wheel rates of both stock springs/bars and how they change according to different coilover/roll bar setups. I was hoping that this could get me much closer to the neutral with very slight oversteer for corner entry that I desire.

This stuff is complicated but I'm determined to have a better understanding!

In the meantime, moar driving for now.

[Carlitoz3]

I'm so lost, feels like I'm in high school all over again lol.

[mla163]

Not necessarily gospel, but here is a decent primer

http://farnorthracing.com/index.html

There is a lot to suspension dynamics. You can come up with an ideal spring rate and it doesn't work for crap in practice. The important thing is testing. As much as it is a "black art", auto engineers don't start off with a blank slate.

Theory. Test. Theory. Test. Theory. Test. Theory. Test. Test. Test.

[mav1178]

You do realize there's one thing that isn't a constant, right?

The driver.

You can spend all day playing with suspension setups, but unless you are consistently the same driver around a given course, your suspension setup won't mean jack.

Plus, do yourself a favor and invest in a properly set up alignment that provides a more "neutral" feeling first, then play with parts.

You can only test if the person testing (you) are as consistent as the results the car can deliver.

-alex

[Racecomp Engineering]

Originally Posted by mla163

Not necessarily gospel, but here is a decent primer

http://farnorthracing.com/index.html

There is a lot to suspension dynamics. You can come up with an ideal spring rate and it doesn't work for crap in practice. The important thing is testing. As much as it is a "black art", auto engineers don't start off with a blank slate.

Theory. Test. Theory. Test. Theory. Test. Theory. Test. Test. Test.

It's really interesting...I see different approaches applied at different levels/areas of motorsports. The heavy focus on suspension frequencies explained by DG in the link above has gotten a lot more common at the serious track day and auto-x level and seems to be the direction the OP's mind is heading. Others continue on past that and get into more on geometry, roll centers (or if they're baller status they talk about Force Application Points), and the actual weight transfer dynamics. It can get very physics/math heavy if you get really into it (suspension frequencies are nothing in comparison). Others still do the "make it really stiff and you don't have to worry about geometry" thing and with a good driver you can still be fast on smooth pavement.

- Andy

[robispec]

Or be really old school and look at tire graining patterns..(i swear I feel like an Oracle at Delphi when I do this)..

[EAGLE5]

Don't forget front and rear track, bushings, joint /bearing types, tire width, height, stiffness, and compound, aero, driver weight, driver style, driver ability level, driver consistency, and driving surface. As I hear it, US tracks are not as smooth as European tracks, leading to higher ride heights here and differ aero. What makes Pobst fastest might not make me fastest. Schumacher supposedly drove his car with an unstable setup. Instability increases response but can lead to trouble for idiots like me. Etc.

[Zach3794]

Driver input definitely has a lot to do with it, and I have no problem admitting that I could probably stand to improve my technique more than most track guys on here. :-)

Robi, your post gave me a vision of an old blind bearded man reading a tire like braille. :P