front roll center location & spring motion ratio
 

Does anyone have the locations of the suspension pickup points, as well as the spring location? Even better would be a spreadsheet already completed (although I'll make one if it doesn't exist), or perhaps a suspension analyzer file? Time to get real data instead of "it feels better"...

I have looked both here and frrax for this information without finding anything solid. The measurements involve a bunch of imaginary lines that move as a car is lowered or rased. I almost think that one would need 3D modeling to get it right. I will look for a good sketch of what is happening. Dean has posted one a couple of times.

I'm not worried about modelling it - its mostly trig, and the formulas can be found in Milliken. I was hoping to avoid spending a day under the car getting dimensions if someone has already done the work! :) The bigger part of this project is to calc out what front/rear roll center locations I like, as well as how the difference front/rear (angle of the line going through both instant centers) changes how the car feels. Once we have the motion ratio and roll stiffness, it's easy enough to drop the roll center, tweak stiffness on the swaybar to have the same roll stiffness, and note the change in feel.

here is one thread that has a sketch but there is a better one out there.
https://www.thirdgen.org/forums/susp...nt+roll+center

Deans pictures shows the instant centers for each side. The roll center, is the point where the imaginary lines drawn between the IC and the center of the contact patch, converge.

Just an added FYI. for other people reading.

Andris, if you end up plotting out the instant center migration and diffrent ride heights, could you pass that along to me?

For tweaking wheel rate with swaybars there is a spread sheet that can be found on frrax, with a search, that takes into acount arm length, diameter and most other possible variables that you can change. You will put your measurements and calcs into a spread sheet and post it right:thumbsup: My car is up with the suspension partially loaded (jack stand under the A arms) So if you tell me what you need I can measure it as close as a measuring tape will allow.

I have the rollcenters marked hypothtically with the green and purple x's.
https://www.thirdgen.org/forums/atta...g?d=1095738771

One note to take into consideration is the roll centers will change based ALSO on what camber you are running, not just ride height.
Dean

Thanks - I've seen that picture before, but it's difficult to get accurate numbers from a picture. I was looking for actual measurements, so I could make a change, and see what happens - i.e. run 1 degree more camber, and see how much the IC migrates. Drop the car an inch, and again, see where it goes. With a certain roll stiffness, corner at 1g, and see what the body lean is, and how that changes the IC on both sides... Basically - model the front suspension, where the inputs can be tweaked to do "what if I..." situations. I've been on a kick to try a setup w/ similar roll stiffness f/r, and see how that feels, and see if ultimate grip goes up, rather than making balance changes from an imaginary starting point which may work, but may be giving up a bit of traction in favor of balance (i.e. whatever is in the car now).

any progress with this???

You're looking at a minimum of one large and one huge spreadsheet (or one truly immense one). One that works up the camber curves, and a second one that does the lateral load transfer distribution. Somewhere in all of that you'll need to insert some attempt at estimating tire grip vs camber, tire grip vs vertical load, and probably the tires' vertical spring rate as well (you can get nearly a whole degree of camber change just from the difference in right side vs left side tire compression after the load has transferred).

I'm still not all that happy with the current state of a similar effort after having spent (off and on for, ummm, never mind how long).

To top it all off, the actual axis about which the chassis rotates in roll is not necessarily the roll axis defined by the geometric roll centers, which will throw your computed cambers off. I have a couple of write-ups posted by one Mark Ortiz (can't find them online anymore, though), and I hear the same thing from an OE-level suspension engineer.

I'm still working at it, though only sporadically. That last piece looks like it's going to cause quite a bit of spreadsheet 'heartburn' and other frustrations.


Norm

Norm,
I agree. The mid-level commercial programs are nice, but don't do anything more other than nice graphics compared to a spreadsheet. I'd love to see your spreadsheet, if you don't mind posting or emailing it.

The camber curve should be no problem, since that is straightforward trig. Getting the IC depends on what is good enough for you, and an estimate of the tire spring rate should be easy to get by jacking the car on the rocker until the tire is airborne, and then lowering it and measuring the distance from the rim lip to the floor every fraction of an inch. A curve should be visible with enough data points, and a quadratic equation used to simulate it (or a simple spring rate...). I would think errors in measuring the pivot (ball joint, strut mount, a-arm mount) locations would be more significant than tire spring rate, but I haven't done the math yet.

Tire grip vs vertical load will be very difficult to get, as most manufacturers don't even have that data, and if they do, they won't share it. It'll depend on slip angle, camber, temperatures, and pavement type as well. An accurate enough estimate can be gotten based on cornering g's from a data box, since that's all that is needed to drive the equations for lateral load transfer. The deeper you look, the more complicated it gets, and for now, I'll be happy with a quick and dirty spreadsheet that has a few assumptions and estimates that may not be fully accurate, but will be good enough.

I've seen good stuff from Mark Ortiz in Racecar Engineering magazine, and it's one of the magazines I actually keep. Thanks for the idea to go back and check that...