Car bending on launch

G

GO FAST

Member
Joined
Mar 3, 2003
Maybe only trans brake cars do this ? I dont know. I recently had the T-brake installed , watching video of the car in Reynolds on the first really hard launch (tires about a foot off :D ) I noticed a significant vertical crease at least 10" in the rear quarter panel behind the rear tire. I'ts actually intresting to watch in slo-mo. As soon as the rear tires hit the crease appears , the front tires come off the crease gets bigger (looks like a big dent) and as they touch the ground the crease goes away. The car launches perfectly even no twist . Any comments, suggestions that might help would be appreciated as I dont want to bend / brake the body or frame.
 
What's happening here is that as the rear of the chassis is attempting to twist counter-clockwise, the torsion energy is attacking the weakest point of the rear portion of the body: the area you described, behind the rear wheel.

The problem isn't going to go away; In fact, it's going to tear your car up something aweful. If the front of the car isn't twisting violently, I'm guession that you've upgraded the rear chassis significantly. (Can't tell from your signature) Do you have a roll cage? If not, it's time.
 
The car has a six-point roll cage , rear suspension is southside lower control arms and bennett uppers with airbags. Could polyurathane body mounts help , or would that make things worse .
 
You said it's a "vertical crease", and it's behind the rear tire...be a little more descriptive about EXACTLY how the crease runs, if it's a straight line, the shape, etc. It'll help pinpoint the problem.

How do the rear bars of the cage come down to the frame? Through the package tray? Where and how to they attach to the frame?
 
The crease is a straight line up & down and forms about eight to ten inches behind the center of the rear tire. The rear bars exit through the factory speaker holes in the rear deck area and are welded to the upper floor of the trunk with steel plates.
 
Originally posted by GO FAST
...The rear bars exit through the factory speaker holes in the rear deck area and are welded to the upper floor of the trunk with steel plates.
Click to expand...
The bars aren't welded to the rear frame rails, or to the frame crossmember over the differential...Just to plates on the floor of the trunk? If so, that explains a lot.

Since I can't see the car...where in the trunk are these plates...ahead of the rear end, I'm guessing?
 
The plates are in the upper trunk area , i'd say just before the rear end. If I can i'll attach the video, but I doubt you'd be able to see this crease on the computer its hard to notice on a regular TV unless you're really paying close attention to the back half of the car.
 
From the sounds of the description of your cage, it is not NHRA legal for an OEM frame car. It sounds like that the rear end is flexing the rear of the frame enough to wrinkle the quarter panel. Since your cage is only welded to plates in the trunk floor the cage is not stiffening the frame, which is allowing the body to flex.
Here is a link to a NHRA tech page.
http://www.nhra.com/contacts/tech_faq.html

Note that in both diagrams it states that all cars with an OEM frame must have the cage welded to the frame. I think I'd have a talk with my cage builder.:eek:
 
Oh thanks a lot for the N.H.R.A tech on my roll cage :rolleyes: and a link to the rules , that really helps me a great deal .
 
Have noticed this on my car as well as a buddy's. I actually have a wave in my quarter now, almost a good sized dent. I would have thought someone leaned on it or hit it at the track, but it matches my buddies car that did the same thing. Passenger side quarter right behind the wheel, about the 10 or 11 o'clock position of the wheel well. We are both running 10.5 tires, 10 point cages tied to the frame. Rear bars going through the package tray and tying right above the axle. Didn't happen until after we installed the wolfe sway bar setup. Sixty foots are in the 1.35-1.45 range. 3600 lb Buicks running 9.0 in the quarter. I am about to repaint my car, so we will see if it happens again.

Greg Kring
9.05 @ 150
 
Greg, if you did'nt do any suspension changes it will happen again, whatch the car on video it's impressive to see. As of now I dont have permanent damage but thats what im trying to avoid, I was considering welding a solid bar to tie in the frame rails like the wolfe bar does, but if your car still does it with the wolfe setup so much for that idea. Anyone with a solution im all ears.
 
GO FAST Oh thanks a lot for the N.H.R.A tech on my roll cage and a link to the rules , that really helps me a great deal .

Man why be such an ass? The guy was just trying to point out that if the cage was'nt welded to the frame that it might cause the flex your talking about. Geez
Click to expand...
 
GNTooFast 4U, I believe GoFast was sincere with that statement. If not, 6APPEAL can deal with it as he feels.
 
If he was simply trying to make a point about the cage being welded to the frame or not , just say so, not come off right away and state "It is not legal for an oem car". I could build a much better , stronger cage than a "legal" one which may be illegal by NHRA rules.
Anyhow enough of that , this is gradually getting off the topic.
 
Not too worried about. Just repeating the NHRA rule.
GOFAST, NHRA makes rules to prevent deaths from accidents at the track. We had a guy locally think the same as you, he could build a stronger cage that was not NHRA legal. I will say the cage looked very impressive, kinda Pro-Mod style. But, he was kicked out of every NHRA/IHRA sanctioned track in the southeast since his cage could not pass Tech. He could only street race or backwoods track race. Well, the guy is 6 feet under now. 3 years ago he rolled his MCSS on top of the guard rail at about 150 on a little track outside Jackson. The body seperated from the frame since his cage was not welded to the frame. He was sliced into 3 pieces as the body came apart.
Don't take this as a flame. It's your car and your life. Live it as you may and I hope you don't meet a similar fate.
 
I was'nt even gonna reply... but, let me see if I got this right. If I have an NHRA legal cage I wont die in my car at the track:rolleyes: got it. Don't talk to me about death and dying, that is what I deal with day in and day out with my job.
 
Does the car have tubs? Wheely bars? Sounds like the frame is flexing between the rear body mounts and the next-to-rear body mounts. Two suggestions:
Box/stiffen the rear section of the frame. If frame rails were notched for bigger tires/tubs, that could be the flexy spot.
Run braces from the roll cage back to the frame, as far back in the trunk as you can without getting into the sheet metal.
 
Originally posted by Ormand
Does the car have tubs? Wheely bars? Sounds like the frame is flexing between the rear body mounts and the next-to-rear body mounts. Two suggestions:
Box/stiffen the rear section of the frame. If frame rails were notched for bigger tires/tubs, that could be the flexy spot.
Run braces from the roll cage back to the frame, as far back in the trunk as you can without getting into the sheet metal.
Click to expand...
Yes, what he said. or if you're running a cell and have room, try an x-brace under the trunk area attaching the frame rails.
 
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