Ideal ride height for autox/ road racing

[mitsugsx95]

What would you say the ideal ride height is for a 2g gsx setup for autox/road racing. Now I know that autox and road racing are two totally different beasts in themselves, but there is some compromise in the height setup between both. Also, I am just trying to get a general height to start with, as I want to setup the coilovers with corners weights and adjust them accordingly (if i can ever find a stupid place that has corner weight scales ). Would I have to first find out how much the suspension compresses before I can make any adjustments. (I do have a general understading of suspensions, but i am still learning myself)

Secondly, where exactly would you want to measure it from? I was thinking from the center of the hub/ axle nut. Thats how i have it setup now, but its only done by eye and a little measuring (again, no corner weights).

I have D2 coilovers with 9/7k spring rates (504/392lbs/in)
(yeah, i know the 50/50 compression/rebound rates suck )

Thanks.

 

[wret]

You can try my cantilever scale. My ebay mail order business wasn't making me rich so I gave away the plans.

 

[DG-FNR]

This is one of those things that sounds harder than it is.

1) The proper ride height (if we assume a car with minimal aero - no tunnels) is as low as you can go without bottoming the car out, either on the ground or on some portion of the suspension.

2) When the car is lowered, in most cases it means that the suspension travel is reduced. When the suspension travel is reduced, the spring rate must increase in order to keep the car from bottoming out. So suspension travel and spring rate are linked.

3) But spring rate also affects other aspects of the car's handling, so you cannot just keep cranking in more spring to get the car lower. There is an "ideal" (more correctly, a range of ideal) spring rates.

4) Once the spring rate has been set, you now know (via either calculation or through trial and error) how much suspension travel you need. You then set the ride height to give you this much travel, and there you are.

5) Next you will want to do a bump steer check, to see if your static ride height has moved you onto a suboptimal portion of the steer-in-bump curve - and if it has, you correct it.

6) At this point, you're 90% of the way there. You have the proper springs in the car, you have the ride height set to give you the amount of travel you need to use those springs, and you have the bump steer neutralized. You can now set the corner weights and be pretty much where you need to be.

7) If you want to go all the way, you can examine camber compensation curves and the location and movement of the roll centres - but on a 2G DSM with reasonable springs (900/400 for a pure race car on race tires, 600/300 for a double-duty car on street tires) the amount of travel you'll need to keep the car from bottoming will limit the amount of lowering you can get away with, and that in turn limits how much the camber and roll centre locations will move - in other words, for you to lower the car enough such that camber compensation or roll centre movement becomes an issue, you would have run out of suspension travel a long time ago anyway.

DG

 

[mitsugsx95]

Thanks. This is they kind of info i need ("Tune to Win" doesnt exactly have street car setups discussed in the book ).

A couple quick questions on that note.

4) I believe I have it at a pretty good ride height at the moment (as low as i can go, without getting the "pogo" from bottoming out the suspension), but how can I tell if I have the corrrect (or at least close to) travel by just driving it?

5) As for the bump-steer check, how exactly would I test for that? Im guessing all will have to be adjusted by driving and doing some testing/tuning around some turns, but does it matter what speed it is, or just enough to shift the weight of the car during the turn (as in can i just go to an empty parking lot and test for this).

Thanks

 

[DG-FNR]

Tune to win does discuss this; you just need to be able to filter it through a street-car lens.

My next race car will be a Formula car though. Anybody want to trade a Ralt RT-4 for a Talon?

In any case:

Do the zip-tie test. Put a zip tie on the shock rod, and then exercise the car throug it's dynamic range of motion - ie, do a few laps of the track you'll be running at, or a few autocross runs, or a bunch of street driving. Once finished, look at the zip ties (which are acting as travel indicators)

If they are into the bumpstops; too low. If there is a huge gap, possibly too high(the travel limit may lie elsewhere, like tire->fender, or on the 2G front, upper control arm->fender)

On a street car, I like to see a little fudge factor (so a little higher than strictly necessary) A race car can flirt with the limits a little higher (but be aware that the penalty for going too far is usually sudden terminal understeer)

Bump steer is the change in toe as the wheel moves up and down. Prepare to Win page 145.

DG

 

[mitsugsx95]

"Tune to win does discuss this; you just need to be able to filter it through a street-car lens."

I guess ill have to go back and read it a bit more with my "street goggles" on.


"Do the zip-tie test. Put a zip tie on the shock rod, and then exercise the car throug it's dynamic range of motion - ie, do a few laps of the track you'll be running at, or a few autocross runs, or a bunch of street driving. Once finished, look at the zip ties (which are acting as travel indicators)"

Now, how would this work if the springs were too stiff, as in they dont compress enough for the shock to even hit the bump stop or come close? Does that mean that the spring rate is too high, or am I just thinking to much into it?


"If they are into the bumpstops; too low. If there is a huge gap, possibly too high(the travel limit may lie elsewhere, like tire->fender, or on the 2G front, upper control arm->fender)"

What would be a good way to check if the upper cont. arm is hitting the fender, if there is one?


"Bump steer is the change in toe as the wheel moves up and down. Prepare to Win page 145."

Sorry, still havent picked up prepare to win. I kinda went in the wrong order when i read the books.

Thanks