Lord.Baal

Hey,
i drive a c4 and was wondering if you can do an event like autocross with the targa top off. i would assume you wouldnt want it in the back because it would rattle like mad.
i mostly was wondering how much it effects how rigid the car is, i figure a t-top would be fine but wasnt sure if you could do a race with the targa off.

if anyone has raced with it off or if you can give me reasons why or why not that would be great.
i think it would be a lot of fun to race with the top off but wouldnt want to do it if it was going to really negatively impact your ability to drive the course.

thanks!

AU N EGL

Not Likely

Taking off the top will turn your car into a convertible, with those tech rules in place. ie full roll cage.

Best to just leave the top on.

Solofast

Geeze, this must be C4 week on the board...

OK, straight scoop. John Carriere and somebody else took a then new C4 down to Black Lake at Milford back in 1988 and set up an autocross course to find out if they should run with the top in or out. This was done purely in the interest of science and the advancement of technology, or at least that is what they had to tell their bosses to get permission for the test..

They ran the same car back to back with instrumentation, their findings were, like the old Blue Bonnet ad,,, (insert phony French accent here) "In cooking, in baking, no difference!"

Since the lateral g was the same. They came away thinking that it was faster with the top out because if they couldn't see an difference in lateral acceleration, the car would be faster with the lower weight... John and Tom always ran their C4's with the lids off. The only caveat about the testing was that this was not done with sticky tires, since they couldn't get sticky tires onto the proving ground, they tested on the OEM Gatorbacks.

Roger Johnson (as big a weightaphobe as you will ever find as he was always looking for an edge) always ran with the top off mostly because he couldn't fit without hitting the roof with his helmet on. Rog hated wet events because the inside of his car always got soaked.

OTOH...

When I first started with C4's I did the same based on John's testing. I found that with the top out the car felt like you were sitting on the rear axle and the whole front of the car was twisting. I could see and feel the cowl (including the steering wheel) moving around and I didn't like that. Since I'm not as tall as Roger I didn't have any issue with helmet hitting the roof and I also noticed that after you ran with sticky tires with the roof out a couple of times all the plastic in the car squeaked and it rattled more. Since I was using my cars as daily drivers too, I didn't want to listen to that so I kept the lids on after the first car and I NEVER took them out.

Bottom line is there doesn't seem to be an advantage either way. My feeling is that with sticky tires taking the top off hurts chassis stiffness but reduces weight, so there isn't an advantage in taking it out and the increased chassis flex is distracting and feels like crap so I didn't do it.

Long post to say "suit yourself", but there are pro's and con's so it isn't a cut and dried thing.

Bill Dearborn

I know how Roger ran his car and he was probably the top C4 autocrosser for several years so his experience can't be discounted. However, I always advise people to leave the top in. Even GM recommended leaving the top in. The body twists like mad with the top out and I can't believe it helps handling to have it twist that much. Just parking a C4 with the top removed and one front wheel on a slight rise can keep you from reinstalling the top due to the twisting between front and rear. Some people who have head room issues use a cross bar that ties the front and back together while leaving room for the head to stick above the car.

Bill

rfn026

I did some stories about Roger Johnson back in the day. It always fascinated me that with the roof off GM could measure the twist. Yet when the car was tested at Milford there was no difference.

It sort of reminds me of the old old adage "Just shut up and drive."

Richard Newton

KBoltz

I bought a Targa Truss and ran HPDE's with it as well. No one ever said a thing to me, since it is a helluva lot more sturdy than a thin piece of fiberglass.

93Rubie

I almost ALWAYS run with the targa on. My helmet barely clears the top. That being said, I've noticed some "stress cracks" on my top over the past two years. My guess is my auto-x, 20 year old top, and the fact its a C4=some twist.

As others have noted Roger Johnson was very fast in his C4, that cannot be discounted.

Solofast

Well, when you think about it, it's not that difficult to make sense of. Obviously you want as stiff a chassis as you can to allow you to tune and drive the car, but John was measuring grip (and that was with street tires too) and that wasn't different. So long as the tires aren't losing camber at one end or the other due to the flex, there's a good chance the grip will stay the same.

Chassis flex can also alter the balance, but if you tune the car to have the right balance with the more flexible chassis, then it will likely work OK. Roger was at or near the top of the heap in B stock and SS from 1982 in B until he stopped running the yellow car in about 2000.. That's close to 18 years running near the top of the sport.