What's the appropriate "rake" for a C5? I seem to remember that the front should be 4mm lower than the rear. If so where should it be measured from? The lift points? The frame behind the front wheel well and in front of the rear wheel well?

 

If you measure ride height from the ground to a) the rocker panel or b)the fender lip, your measurement may not be the same at the same suspension ride height as someone running a different tire size.

When we measure ride height at our shop we are concerned with suspension travel and bottoming out so we always measure "ride height" from the spindle centerline to the fender lip. In that way we take tire size out of the equation.

To answer your question, we usually run the rears 1/2" to 3/4" higher than the front, unless the customer requests something else. We found that the cars handle well in this configuration and when the ladies overpack for a weekend adventure, you aren't going down the road bottoming out the rear suspension.

I personally like running 14 1/2" front and 15 1/4" rear. With stock shocks you will bottom out once in a while on rough roads, running this low. If you go to a Bilstein sport shock or one of our shocks, they allow the suspension to compress about one inch more so the bottoming out problem goes away.

I hope this answers your question.

Mark

 

1. Put your car on a absolutely flat floor
2. Disconnect swaybars
3. Measure from the frame to the ground behind the front wheel and in front of the rear wheel. The diff should be about 1/2 inch.
4. Adjust and repeat #3. You might want to have the suspension to settle between each adjustment.
5. Connect the swaybars. adjust the endlinks to make sure the swaybar are unloaded.

You might want to have someone with the same weight as you sitting in the drivers seat while doing the above :)

:cheers:

 

I measured from the spindles and find the rake on target, but question whether I'm in a safe zone at 14" front and 14 1/2" rear. I am running Bilstien Sports. Found quickly (actually an autox instructor pointed it out to me) that the stock shocks would bottom under severe loads.

 

Sknight,
I think that you are still running too low. I need to measure a fully compressed Bilstein fronts. If memory serves me correct, their fully compressed length is 13" (from mounting pin to mounting bushing mounting surface). If that is the case, then the "bottoming out measurement is about 13 1/4". If you are running 14" then you only have 3/4" of compression before you bottom out. Not too good.

Even with the shortest shock that can be made the "a" arm bottoms out against the frame at 12" ride height.

I'll measure the Bilsteins tomorrow.

 

Thanks! Look forward to what you find out!

 

Okay Sknight,
I finally had time to measure the Bilstein Sport Front shock.

Fully compressed, from the bottom of the bar pin to the top of the bottom mounting bushing is 13 1/2".

That means that your front suspension should bottom out at somewhere around 13 1/2". So if you are running at 14 " you only can compress your suspension about 1/2 inch before bottoming out. That should make for a pretty lousey ride as you will always be bouncing off of the bump stops. Based on that I wouldn't run any lower than 15" in the front and 15 1/2 would offer a better ride. :thumbs:


[Modified by C5stein, 6:10 PM 2/12/2003]

 

Wow! That would take me back to the stock ride height, since it is lowered 1 1/2" from the original ride height.

I'm kind of struggling with this analysis. If in fact the Bilsteins have an additional travel of 1" over the stock shocks, and I'm now only 1/2" from bottoming, that would suggest that I could not have lowered the car this far, which I did, when it had the factory shocks. The front suspension was not (and is not) noticably harsh, which I would think would occur with a suspension travel limit of <= 1", although we found it an issue in the rear with the factory shocks during autox runs. The change to the Bilstein Sports corrected that problem. I suppose if I really wanted to determine suspension travel I could ask an alignment shop to hook up a come-along to the front end and pull down until the suspension bottoms out.

Sounds like you're recommending a suspension travel from "rest" to bottoming out in the 1 1/2"-2" range.

Does the outer fender lip, from where I obtained my 14" measurement, lie in a horizontal plane with the upper mounting point of the shock? If the lip is actually lower, our calculation would be off by the difference, buying me some additional travel to the stops. :crazy:

 

Sorry for the confusion.
1)Most of the cars comming in here with "stock Ride Height" are at around 16" from Spindle CL to fender lip.
2) I love suspension travel and will usually give up a bit of looks to keep it.
The top shock mount is a bit lower than the fender lip, and the shock sits at an angle, so it is more coincidence that the two measurements are very close to each other.

keep in mind that I could only compress the shock down to the bottoming bumper & the bumper in the shock probably has 1/2 to 3/4 squeeze available.

That being said:
If you are not bottoming out the shock, then run as low as you want because you can't hurt anything. If you are bottoming out the shock alot, then crank it up 1/4" at a time until you stop hitting the bottoming stop.

Here is what we do to set up for different tracks. We run the car as low as we can until we start to bottom out, then we go up 1/4" at a time until we only hit the bump stops maybe once per lap. Then we run it! :yesnod: :thumbs:


[Modified by C5stein, 12:45 PM 2/13/2003]

 

Excellent information! You obviously know your chit! Do your small adjustments affect your alignment (at least your camber) enough that you need to adjust that once you've settled on an optimal ride height for a given track?

 

In the middle of the ride height (let's say 15") lowering the car 1" changes the Camber & Toe. The Camber grows by about -1/4 degree, so if you were at -0.50 it will go to -0.75. Raising it 1/4" will hardly be measurable without putting it on an alignment rack. Watch out for too much negative camber. -0.75 and more is fine for racing , but I find that it is a bit too aggressive for street & Highway driving if you are concerned with tire wear (especially if someone is driving on runflats). As far as toe, the wheels will toe in about 1/8 degree when you lower it an inch, so raising it 1/4" isn't a big deal either.

So the simple answer is that you won't wipe out your tires while you figure out your perfect ride height. However once you get the height that you want, I would suggest getting a 4 wheel alignment. When you are ready for that e-mail me at Mark@motorsportimage.com and I will give you my recommended suspension settings for alignment.

Thanks for acknowledging my skills. I've been aligning and setting up race cars for the past 15 years. :thumbs:

 

C5stein,

I'll bet you have an opinion on this too.

I'm about to purchase a set of CCW's and Kumho's (I want to drive to the event on the race tires) for running autox (ASP). I'm struggling with size selection. It has been recommended to run 17x11's (front) and 17x12's (rear) running 315/17 and 335/17, respectively. I have some concerns on fender well/lip clearence. I don't want to burn up a lot of plastic! :eek:

Other suggested alternatives for the front are 275/17's or 285/18's. Since I've got your ear I'd thought I'd ask. :D

 

Well you know what people say about opinions :D

So I will start with IMHO-
I would never consider driving my "race tires" on the street as they usually don't do well with road hazards and I wouldn't want to heat cycle them any more times than I can get away with.
I like the Kumho tire but I don't really like their tire size selection for C5s.
I didn't know that they offered a 315 17 and you didn't say what the aspect ratio was so I really can't comment on that (if it is a 35 I would consider it strongly as the diameter should be around 25.7" on the rear with the 275/40/17 on the front).
I have seen guys running the 275/40/17 that has an overall diameter of 25.5". Combine that with the 335/35/17 on the rear with diameter of 25.9 so it shouldn't screw with your active handling, traction control and ABS. I like the diameter matchup but the car should push like a snow plow with that much rear tire. I know that I was faster in autocross with a bit of oversteer. You could probably tune that package with a larger rear anti sway bar. The fronts will fit in fenders fine but the rears will stick out, which I think looks tacky.

The option of running 275/40/17s front and their 285/30/18 is probably better in the fitting in the wheel wells, but the diameters suck and will screw with the abs, TCS and AHS unless you reprogram and this setup may cause a bit of an oversteer condition. I think the C5 likes about a 20 to 30 cm width difference between the front and rear and without reprograming, running a taller tire in the front than rear makes the ABS intrude on aggressive brakeing a bit too soon.

Even though I wanted the Kumhos I ended up with the Hoosiers running a 245/35/18 front and a 275/30/18 rear because I liked the sizes better. The good news is that the hoosiers are faster, the bad news is that they cost more and don't last as long.


[Modified by C5stein, 11:21 AM 2/16/2003]

 

Sknight,
Just as a sidebar:
On friday I had a beautiful Red Z06 in the shop. It was really low and looked really cool. I measured the front at 13 3/4" and 14" rear
The owner wanted me to raise the car, set corner weights and do a 4 wheel alignment. He loved the low look, but said if he nailed the gas, the rear bottomed out hard and it was constantly hitting the bottoming stops going down the highway. He is running the stock shocks. We raised the car to
14 5/8" in the front and 15 1/8 in the rear and then set everything up. When the guy was driving home, he was so happy with the greatly inproved ride and handling that he called me to thank me. He said that he had forgotten how enjoyable this car was to drive.

So to ammend one of my prior posts, I think you could be okay at 14 3/8 in the front and 14 7/8 in the rear with the bilsteins. You will probably still bottom out once in a while but shouldnt hurt anything. :cheers:

 

Great Info, Thanks for sharing :thumbs:

 

Took your advise. Look forward to seeing the difference.

Now, I hadn't started drinking before I pulled out the wrenchs, but....

I found that 1/2 of a turn on the front bolts raised the front ride height between 1/4"-3/8". Note before I continue, I did drive it around to re-settle the suspension before re-checking the measurements.

As I went to get that last tweek, I turned the bolts another 1/4 turn. Re-settled the suspension, and no change in the front ride height. But the rear dropped 1/8 inch. I repeated with another 1/4 turn, blah, blah, blah, and again, no change in the front ride height, but the rear dropped another 1/8". I vaguely remember this happening a year ago when I originally lowered the car.

What kind of funky dynamics are going on here? Is it affecting weight distribution, or.... I can't even come up with another possibility. Unless of course I was :cheers:

Also, I don't think you directly commented on the 315/35/17 (as a front tire, in combination with the 335/35/17 in the rear. To further complicate things, the 315's only come in the VictoRacers, and the 335's only come in the Ecsta V700's.

 

This sounds like maybe you made this into a 2 hour 24 pack job :cheers: :rofl: :rofl: :lolg:

I can explain some of what is going on,
In the front, when you jack the car up and make your first adjustment, sometimes the snubber on the end of the screw jack binds up and gives you a false reading of a bit high.Your subsequent adjustments probably took up the bind. As far as the rear going down, I have no explaination but have seen the rears settle over time even when we thought we took all of the bind out. So your front adjustments didn't have any relationship to the rear settling down.

I would check with the Kuhmo tech support group before I mixed the Victoracer with the ECSTA. Even though the sizes complement each other I think that the ECSTA's are grippier. If that is the case then I wouldn't do it as you will end up with terminal understeer. And that would suck.

What ride height did you end up with?

 

I'll give them a call.

I ended up with 14 5/16" front, 14 3/4" rear, when I decided to quit when the wrong end was moving. :banghead:

I jack with it again next weekend. Drive to work didn't seem too different, although I would expect that the difference will be more noticable at the track. (My mind was thinking...gee, this seems smoother.... but it's easy to read things in because you know you've made some changes).

 

Sknight,
I did not read all the posts here but are you figuring in the weight of fuel in the tank when you check the rear ride height? A change in the amount of fuel will affect the rear height, which can even affect slightly the height of the front.
The difference in the rear height of a full tank compared to an empty one is about 3/16". I measured it.
So if your fuel level is changing then you will chase your tail forever trying to set ride heights.
Good luck with it.
Dave