RedLS1GTO

As most of you know by my random questions I'm relatively new to the world of racing (on full sized tracks). I'm getting my list of things that I need/want to do to the C4 ready for the winter. The car is relatively stable at high speed under power, but during high speed braking, or even just off throttle, it gets VERY tail happy. I was wondering what you guys have seen done on C4s that could get me some rear downforce. Is the best option going to be a wing, or is a lip type of spoiler the better way to go? I'm not opposed to either at this point although from what I can tell, mounting a wing correctly isn't an easy job in the C4 and involves a lot of cutting on the rear body panels to attach it so that it is strong enough and not just some POS bolted to the tail that is going to fly off.

0AmericanCustomInd

If you find that you are interested in replacing your rear bumper with a custom fiberglass bumper with a lip, take a look at our custom (part number ACG050).
This bumper also uses C5 tail lights.


This car is also outfitted with a rear wrap.

Our products have not been wind-tunnel tested, so I cannot say how much additional downforce you might gain from a modification like this.

They go for 626.84, but I can do 595.50 for forum members!

Let me know if I can help in the parts department

Solofast

We noticed a nice improvement with a SCCA Street Prepared legal spoiler on our C4 even at brisk autocross speeds. I'd start with that and see if that is enough, since it is relatively inexpensive to do with some plastic sheet, sandwich it between the rear cap and add a couple of aluminum braces running forward to the area around the rear window.

As you noted wings offer more downforce for less drag, but a decent spoiler might do what you want for a lot less effort in terms of putting in bracing between the frame and body back there.

RedLS1GTO

Do you have a pic of this? I'm having a hard time picturing braces going forward as opposed to down and backward.

66IISS

Solofast,

Any pictures of this setup? What kind of gains did you notice on the auto-x course? We have a very tail-happy BSP car and I've toyed with the idea of a spoiler, but I haven't followed through with it yet. I still like being able to put a cover on the car as well.

red goat: I've had my C4 out on Road America doing 150 at the end of the front straight, and 140+ into turn 5 and 12, and I haven't noticed any tail-happy tendancies. I run into a little power-on oversteer if I'm not smooth on corner exit, and mid corner grip isn't as good as I'd like (I think I need more spring)

That being said, I feel like it's mostly the rake of the car that's helping me. I added about 1/8" to 1/4" rake before my last track day and I had virtually no stability issues even with 3/8" toe out in the front and 38 degree temps.

RedLS1GTO

Here is a video of VIR... if you look at the 00:24 mark you can see what I am talking about going in to turn 1. I know I wasn't perfectly straight because of the other car in front of me stopping short, but it got about that same amount sideways every time. Even a very small steering input under braking makes the back want to come around very quickly, and anything off camber will step the rear out every time. It doesn't really look like a lot, but you can certainly see that it gets twitchy.

...and no making fun of my driving. It was the first time I had ever seen VIR.

rfn026

I seriously doubt if you have an aero problem with your C4. If you want something to do remove your lisc. plate and vent some air out of the lisc. plate opening. GM wind tunnel tests found that the C4 traps a lot of air in the back. Removing the the plate and opening things up solved the problem.

I run my C4 at both Sebring and on the banking at Homestead and have never felt any need for a huge wing/spoiler on the back. It's more a matter of getting used to driving the car than a real problem.

My basic rule is don't touch the car for the first year. You need to run 6 to 10 events before you spend any money on the car.

Richard Newton
Intervention Motorsports

ScaryFast

C4's are a bit squirrely under hard braking, it's partly (mostly?) a function of rear suspension geometry.

Quite frankly, much of this could be attributed to the car not being pointed straight when you're braking hard. This seems to be the case for me when I'm really pushing it in a braking zone, like at the end of a long straight. If the car is not pointed straight the back tends to step out in the opposite direction than the wheel is pointed.

I'm with Richard. You're new to the car, new to the RR thing. I'd give it time before you start making changes. If you start the mods, you're already changing the variables. Then, when you improve you'll have no idea if you're becoming a more capable driver or if your spoiler really helped. Until you're running lap times consistantly within 0.5 sec or so of each other (barring traffic) making changes is like shooting in the dark.

I especially caution you to consider your long term plans before making changes. As we've discussed in other threads, if you're seriously considering going to the next level soon (time trials or racing) any mods you make now are going to push you farther and farther from a class where you can keep up with the Jones's. You know, the guys that have annual race budgets bigger than our net worth.

Aardwolf

Watching the vid my first thought was to high of torque rear brake pads making the rear wheels lock up. Instead of venting the rear, consider a rear diffuser.

RedLS1GTO

Thanks for the input as always guys... I know that the problem is suspension geometry induced rather than aero. However, I am hoping that aero might be a solution, or at least a partial one, since I am not anywhere near the point of fixing suspension geometry. I am not looking for a gigantic wing like the big guys are using. I'm looking for just a little help in keeping the tail planted. I have spent a lot of time in various vehicles and have at least a decent "feel" of what is going on with them in general although I might be relatively new in this car. I have compared this one to my 02Z and the difference under breaking is unbelievable, even at a relatively low speed (~100mph). The Z is planted and goes where I want it to where as the 96 feels like it is on top of the track and going where IT wants to. I have had multiple instructors who have a lot of experience in cars over the course of the year and they have all said the same thing. Even farther than that, I have let a couple of people (with a lot more experience than me) drive it and again, the one "complaint" is the rear of the car. I have no wind tunnel data obviously, but by the eyeball test, the big Greenwood wing out front, the hood vents, and the angle of the cowl hood are going to give at least some downforce at speed. Is it possible that is upsetting the balance a little compared to the stock nose on other C4s? Looking at pictures the C4 in your Avatar has some sort of spoiler doesn't it rfn?

I actually do run with the plate out because I have read that before. I'm really considering taking the rear part of the Greenwood stuff off because at least by the eyeball test again it looks like it would trap a lot more. As for the first year, I agree, and that is what I have done. The first year is one event from being over and the only thing I changed this year was the brakes, and even that was a safety issue in my head rather than a performance one. I ran this season whenever possible and I have done quite a few events in the car at a few different tracks and with a bit of seat time under my belt I know what I would like the car to do (within reason) compared to what it actually does.

Scary- I'm still looking at where to go with it in the future and at this point I'm leaning towards ST-2 where none of this would matter, or perhaps another year of HPDE in the car as it is and then going through it top to bottom and building it specifically for a certain class.

NOW were talking... on both counts. I will try some different pads and see if that might help. Although they aren't locking up completely, they might be skating a little bit and causing the problem.

I don't think it would be too hard to fab up a pretty decent diffuser out of aluminum with the equipment I have access to and a few expert friends. My winter project might have just gotten more interesting.

BrianCunningham

option 1

option 2

66IISS

Aero in the front with none in the rear could deffinitly be part of the problem. Some of the experienced guys should chime in here, but I remember there being a statement that for X number of inches of front splitter, you should have X number of inches of rear spoiler. Aero is a system, so there should probably be something in the back if there is in the front...

Solofast

No pics, but imagine a piece of flat stock about .125 thick that went from the groove around the hatch, in two places, one near each corner, to near the top of the spoiler at about a 45 degree angle...

shakedown067

Dude if you pull that off I gotta have a how-to. I'd love a diffuser for my C4.

ScaryFast

Ya know, i started thinking that as well. Looking at the pic in your avatar and based on your comments, you have some front downforce and nothing in the rear. This is definately imbalanced.

You're absolutely right, if you're going ST2 aero is fine. But I do question your comment that you'd rather tackle aerodynamics over suspension...

Aero is a black art, and very few grassroots guys get it right. Suspension is pretty much a known commodity with book upon book written about it. It would be a heck of a lot easier to read and understand suspension effects than aerodynamics, and even more easy to quantify on track.

Take a look at the "Congrats to Ingle and Bob Mayer" thread. On about page 3 the discussion of aero comes into play. It seems that the wings on Bob and Robert F's cars slow them down more on the straights than speed them up through the corners, and these are $100k+ pro built race cars. It's as easy to do more damage with aero as it is to help you out.

I'm not saying don't do it, just saying make sure you know what you're getting in to, and make sure you can take it off if necessary

RedLS1GTO

I've already started to put that expensive engineering degree to work. The biggest problem I see is that to do it best from an aero point of view, I would have to go under the exhaust. I have to look at it more and see how much heat would build up in the area it created.

It is definitely unbalanced. The more and more I look at it, the more I see how far off it really is. I was looking at a lot of the Greenwood stuff for the original C4 race cars. I'm going out on a limb to say that theirs was actually balanced pretty well. I have the exact same thing in the front and nothing in the back... Actually, I have just a little more in the front with the closed off cowl on my hood. Theirs also has a smooth bottom in the rear that from the looks doesn't catch any air, although I'm hesitant to call it a diffuser since it doesn't look like it would add actual downforce to the car. I do have the bottom piece at home in the garage, but it doesn't fit with the exhaust setup that is under there. As for choosing aero over suspension, I think that the suspension is very well balanced right now. The car hooks up good, and corners incredibly well. The only problem is the light tail. No adjustment that I can think of would help that aspect without seriously hurting in other areas.



EDIT: I know I could get the Greenwood wing... but **** that thing is ugly!

Mine for reference in the front:

Kubs

What are your rear suspension measurements? Some aggressive chamber and little toe will make it loose. I run -1* chamber and about 1/8" total toe. I have a very neutral car (slight understeer but I still have rubber bushings in the front for the time being).

astock165

I just ran across this used wing F/S.

Tom03Z06

Sent you a PM, non-sponsor ..

66IISS

Tom03Z06:

I'm very interested in that, could you send it to me also? Love the exhaust exits on that car... That's cool. This makes me want to do a whole lot more to my BSP car than is already done.....