michaelkrelina

Of those who experimented, what height, thickness and angle do you recommend for autocross. Is it worth the effort?

c5_4_me

Autocross? Spoiler? Focus on improving the loose nut in the driver's seat!

Solofast

We tested on our BSP C4 and it does help. Even though you aren't going all that fast a big spoiler (ours was to the limit of SP rules) made a difference in lap times and "feel". It wasn't that we could say that on the skidpad we got .02 g better, it was just that in fast transitions the car was just "planted" and it was more confidence inspiring.

Think about it this way. The faster you go the more a rear drive car is prone to oversteer. The reason is that you have to keep increasing the amount of thrust from the tires at higher speed and that takes away from their lateral capability. A big spoiler can negate that and over about 30 or 40 mph (speeds you are often at in autocross) it will help plant the back end, and you definitely can feel it.

A big spoiler lets you set the car up a bit looser at low speeds, and the car stays planted as you go faster.

I've heard the comment from folks using huge wings and downforce in autocross that you don't so much notice that it's there, you just seem to go faster with it. That was our experience too.

Without the spoiler we had to be very careful in fast transitions, with it we could just horse the car through those and it just went were it was pointed, the front end went and the back end followed, no matter how aggressive you were.

Lastly, a comment about "smoothness".. Everybody will watch you from the outside and if you are hooked up and flying, will comment on how smooth your driving looked. They aren't inside the car and aren't seeing that in some cases you are absolutely reefing on the wheel to get the car where you want to go. If the car is hooked, and isn't loose, you will be fast and look smooth. If it's loose you won't be fast or look smooth, so that's why keeping the back end planted is so important.

Our spoiler was as big as the SCCA SP rules allowed, it was simply sandwiched between the body and the rear cap of a C4, so that defined the angle. Pretty simple.

michaelkrelina

Thanks for the very helpful response. The thickest lexan sheet I can get is 0.5". Is that enough from the structural point of view? What thickness did you use?
Thanks again.

michaelkrelina

Thanks for your imaginative and non-judgmental input.

alextz

I think it helps.....you onely need 3/16" lexan. save weight.

ltborg

I helped a friend use 1/8" Lexan to build a spoiler for his ESP Camaro. I thought it might be too thin, but the flexibility helped it conform to the curve on the back end of the car. With four spoiler supports, it's worked great for him.

Solofast

I think Larry used 1/8 Lexan, but he made a couple of supports from aluminum and went forward from the spoiler to the rear hatch and secured the aluminum under the weather stripping an got them to sneak around and come out in the gap between the hatch and the body. The other thing was that while it looks pretty straight, there was some bend to the joint between the body and the bumper, and that gave the spoiler some bend when viewed from above on edge and that made it a lot stiffer.

SouthernSon

PM sent

SouthernSon

Should anyone else be interested I have a great spoiler for autocross that is up for sale....cheap.

alextz

3/16 bends pretty well too

c4cruiser

Do you have any pics of the car showing the spoiler and how it attached? I might like to try one for my '87 coupe.

Solofast

Sorry, no I don't. It was pretty simple looking but Larry mentioned it was a PITA to do. There are a ton of bolts that hold the rear cap on, and he took it all the way off, cut the Lexan, drilled all the holes in it and then put the cap back on.

The brackets were put under the rubber for the hatch so that if you took it off you wouldn't leave any holes in the body that would show.

The brackets were made long and were installed under the rubber first, and then bent to go to near the top of the spoiler. IIRC the brackets were 1/4 inch thick aluminum, but they might have been thinner. The trick was making them with a step where they came out from under the hatch. That let them clear the glass by going almost straight up in the area where they had to come out of, and then they bent back towards the spoiler after they cleared the body..

dgoetz

I found that after installing spoiler that a higher AX speeds the car was squatting and had some push. Installed a #100 higher rear spring, from #700 to #800. Car works better at every speed now.

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michaelkrelina

What thickness lexan is that?
It seems strong with the widely spaced attachment points.
What speeds have you had it up to?
MK

dgoetz

1/4" no more than 80mph. Installed 3yrs ago no problems. would remove and install maximum 3" for higher speeds as in hill climbs and track use.

VGLNTE1

Why would you need a spoiler or wing going 30mph?

1991Z07

The fairly obvious answer is to get more grip...low speeds is where that is most important.

Solofast

There is one thing I have learned in racing. If there is anything you can do to make your car faster, you have to do it. Even if it is small, you have to do it. We used to use the long drives to Kansas every year to try to brainstorm and think of ANYTHING that we could do to make the car faster.

The reason is that even though it is a small advantage, if you can find 10 things that are all worth a tenth of a second, at the end of an autocross run you have found a whole second. And a second is a week.

I lost an SCCA National championship by .06 seconds, over the two days of running. This sport is so competitive, and since it's all measured by the clock, any time gained counts, you can't afford to give up anything if you want to win.

Now if you want to drive around and have fun, that's fine, but at the top levels of the sport you need to do anything, and everything you can to gain a few thousandths of a second.

If you don't, someone else will, and you will get beat.

ltborg

It's also about getting a better total balance. Cars tend to get looser at higher speeds than lower speeds (excluding the effects of aero). In a non-aero class, you need to compromise on the car balance (a little tight low speed, but a little loose high speed). With a spoiler, you can set it up more towards oversteer low speed and still have the locked down feeling people have mentioned high speed.