I just put the car on scales after going to coilovers etc. over the winter. Any advice on where to start with the tweaking would be greatly appreciated. The numbers are as follows(with me in the seat) LF 848 RF 898 LR 853 RR 723 TOTAL = 3322 cross% 47.3/52.7

 

Can we assume that your ride height is roughly set, with ~ 1" to 2" rake down-in-front, and equal height side-to-side?

If that has already been done, and tire pressures are set to racing inflation psi, then

[1] Lower your RF corner a single turn on that threaded perch, and

[2] Raise the LF corner a single turn on its threaded perch.

[3] Recheck the scales.

You are hoping for ~ 875 pounds on each front. We would continue with the pattern of lowering the RF and raising the LF until you have the fronts (approximately) equal in poundage. That should result in your rears being much more equalized also, and a 51.3% wedge.

Ed LoPresti

 

You could also raise the RR instead of the LF.

 

that is a lot of cross weight.....but all of the above advice is good. Disconnect your sway bars, and make the adjustments until you are withing 20lbs of ZERO cross weight. Good chance in your case, this will throw your ride height off, and you'll need to correct, rinse repeat etc. Getting height, rake, and corner weights right the first time takes some time, but once you get it minor adjustments will be easy.

Make sure you have ballast (or a friend) in the seat for final adjustments also. It doesn't change corner weight a lot in a Vette, but you want it right when you get close!

 

There is some great advice in this thread. We agree with the posters above, they cover the main techniques we use when corner balancing our own cars. It sure is easier to make fine adjustments after it's set the first time, so it will get easier the next time you go through this process!

 

After several hours of tweaking im not much closer. i have the front end within 10lbs (FR=FL) but the RR is 100lbs lighter than the LR. I have even dropped that corner down 1/2" lower than the other and it was still light. Is ballast the only way ? I hate to add weight to a car im trying to lighten.

 

How much gas in the Car?

 

You cannot change Left/Right or Front/Rear weight without adding ballast or physically removing weight. Dropping the R/R is the last thing you want to do in your case.
Our cars are naturally LF heavy and RR light. I would raise the RR and not worry so much about the front side to side split. More rear weight = better off corner acceleration.
Post your specs.

 

If I may ask, where are you measuring this ride height/rake?

I set 1/4 inch rake at the tie down holes. I realize that rake gets bigger as you measure towards the ends of the car.

Thanks.

 

So it seems that re-locating the battery to the right rear recessed cubby hole in the trunk would greatly benefit corner weights AND front/rear ratio.

I plan on doing this sometime over the next few months myself.

I have a 2006 Base coupe. Keeping the stock size battery, about 45lbs would also make the most improvement in the rear....going to a compact/superlight battery wouldn't benefit as much for balance. Weight reduction yes, but not balance.

 

Thanks to the "pros" posting in this thread. So my take from this thread is that 50/50 crossweight is desired but not that important. We know that tweeking for crossweight can mess up the chassis rake/rideheight. We learned that LF/RF getting close to equal is important. So for set-up road racing the priority of the goals are?
1) set chassis rake/rideheight
2) The cross weight is initially what it is
3) try for equal weights LF and RF but don't go too far overboard and mess the crossweight? Any general percentages that are acceptable?

Do I understand this set-up plan correctly?

 

Well, let's go back to basics . . . . .

The point is, you are not going to get closer than around a 100 pound difference in the rear - not without physically moving weight around. For drag racing, we would be trying for more equalization on the rear corners, but not for road racing.

By the way, the farther you drop the ride height on a corner, the LIGHTER it, and its diagonal, become. One wants to keep the ride height as close to equal as possible, from left-to-right. And, in the end, an asymetrical car is going to be a series of small compromises.

Ed

 

We measure and record ride height between the bottom of the frame rails, just behind the front fender, and just in front of the rear fender; and the "perfectly flat horizon" on which the tires are sitting.

What reference points one uses is not nearly as important as [1] using fixed places on the entirely-sprung chassis, and [2] always using the same reference points, for consistancy. Naturally, ease of measurement enters into the process, also.

And, as you point out, if one is using reference points "inside" the wheelbase, we would be looking for a lower rake figure (say, 1"), than if one were using reference points outboard of the wheelbase (where we might use 2"). If one can find a reliable surface on the frame, taking an angle measurement works fine also. We just prefer measuring to the horizon, because it ensures equal side-to-side ride heights with the same single step.

Hope this helps.
Ed

 

Thanks for all the input ! As the car sits it is way off on cross weight (52&48 %) However the front end is ballanced within 10lbs and the ride height is even front and back with about a 1/4" of rake between the front and rear jacking points. I will be running primarily autoX events with 4 track days planned this summer. In autoX the car is loose so i have tried taking all the rake out of the car and running sways and c/o's both on full soft hoping to get a little more grip out back. So.. wondering if having a 100lb difference between the RR & LR has something to due to the lack of grip ? The rears are 17x11 ccw's with 315 hoosiers. Thanks

 

tire pressures front and rear?

is the front end of the car setup soft too?

 

What Ed is recommending (equaling front corner weights) is fine for road course work where you are predominately turing right. It makes for faster lap times because you are getting rid of push in right hand corners, that comes from overloading the left front tire and that is a big issue in a race where you could cook the left front and it could really slow you down near the end.

That's all well and good, but remember you are also going to end up with a lot of push in left handers and it depends on the track you are running if that is a good payoff. If the fastest straight opens up from a left hand corner you could hurt yourself really bad. The very high weight difference on the rears with that kind of setup would bother me.

I'm not against tweaking corner weights to get rid of some of the inherent right hand front corner bias that these cars have had, and I did it a lot in my C4 to help get rid of the nasty push that those cars had. But I'm not sure I would go all the way to equal weights up front unless the track was really punishing the left front tire.

For a track day car and if I'm not going for ultimate lap times I might use some cross weight to get the fronts a bit closer, but I find that trying to equalize cross weights in the front results in a pretty substantial difference in the way the car handles between lefts and rights and I don't prefer that.

Another thing about asymetric setups is that you can use them to your advantage. I've run higher negative camber on the left side of the car to compensate for the higher weights on that side and it helped a bit. I've run 1/8 of a degree more negative camber on the left side to good effect.

I agree that if your car is overloading the left front tire and overheating it, that you could jack some weight and help that situation, but as with anything else you can overdo it and sometimes make things a mess.

 

No offense but, my God, that is a LOT of rake. Can you post a pic of a car you set up with that much rake?

 

Sure, Bob,

Specifically, this is a late C4, substantially lowered, with precisely 1" of rake down-in-front, measured as indicated above. We have no good side photos of the car as yet, but these might give you an idea:

http://forums.corvetteforum.com/pict...ictureid=83172

http://forums.corvetteforum.com/pict...ictureid=83173

We have just started testing, and, like all suspension settings, the rake is certainly subject to change as development progresses. It was our understanding that the C4 chassis favors exaggerated rake, and thus far the "feel" indicates we are not far off.

Dante is in favor of lowering the static rear ride height some, but I think that is as much for aesthetics, as for center of gravity considerations.

Ed

 

That's a pretty car, I had a 96 LT4 similar, best rear clip design I think GM ever did on Corvette. I guess I need to expand my ideas on cg/rake/balance (which are concepts I admit, I am just starting to get comfortable with).

When you build a car with a lot of rake, does that affect the spring choices, say softer rear spring, or are springs a separate issue?

 

Thank you, Bob, for the generous compliment. We, too, love the body treatment of the late C4s.

At the risk of getting our original poster's thread a little further off the subject of corner weights, your observations are absolutely "spot on"! Slightly higher ride heights certainly ALLOW for softer springs and/or wider tires. (They also CONTRIBUTE to significant roll!) That was not the primary reason, however, for starting with higher ride height in the rear.

It was our understanding, from those who race these successfully, that [1] these chassis tend toward initial understeer, and [2] they enjoy significant rake. Well, down-in-front rake can help cure turn-in understeer. As with any race project, one needs to take an educated guess, and then adjust from there.

Even with a 675 lbs/in rear spring, I did find excessive roll, at the rear, but only in the slower turns, which we were attempting to dial out with damper settings. Also, I was working the front slicks much harder than the rears, but that might just be a driving style adjustment as we learn the car.

And, in the end, you may be absolutely correct -- we might simply have too much rake!

Ed