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Discussion Starter #1
I am having an isssue with one of my race cars on the starting line. The car is a 69 mustang full chassis car that weights 2600LBS (51.6% front, 48.3% rear). The car has a powerglide with a 1.80 first gear and 4.10 gears in the rear. The engine has 900hp and 800 ftlb. it has 15x33 tires, a 5500 stall and i leave the line at 5000 rpm. The rear suspension has 14" QA1 double adjustable shocks with 100Lb springs. I have adjusted the shocks tight on rebound and loose on compression almost all the settings that are available. The problem that i am having is the car is bouncing off the starting line. I had the IC @ 42" out and 5.5" up and i adjusted it to 48" out and 2.5" up. The car still bounces I've adjusted the tire pressure from 8psi to 5psi.

I have a video online to add a visual aid. The pass in the video has the IC 48" out 2.5" up, 5psi in the rear tires, leaving at 5000rpm. I have the shocks adjusted 12 from full stiff on the compression (30 clicks per adjustment). and 2 from full stiff on rebound.

I could move the IC out 58" and up 2". That is the only other IC location that i can use. Thanks for any help. George Shrader

http://media.putfile.com/Starting-Line-Problem
www.putfile.com/gas6999

The first link is the video, the second link has addtional videos of the 69 and some of the 1990 Boss

This is a great forum and i hope someone can give me some advise.
George
 

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It looks like it's bouncing on both the slick sidewall & the rear suspension, probably the sidewalls a lot more. In my opinion big slick sidewalls shouldn't show a ton of wrinkles like your car does as it stages. And 5 lbs of air pressure in the slicks sounds way too low to me, especially for Hoosiers. The sidewalls look to be repeating a wind-up/un-wind bounce cycle, so the tire never really gets "up on the tire" under load during the first 60-100 ft. I'm surprised the car doesn't exhibit some amount of tire-shake at this air pressure. You might keep adding air pressure until the slicks almost flat-out spin (zing) then back the air back down a little, how much air it really needs will kinda depend on the rubber compound (soft, medium, or hard) & the sidewall stiffness you have. You can also sometimes tell if you have the right amount of air pressure by how full/wide the burnout tracks look.

Part of the bounce might also be from the wheelie bars, I/C length/height, & both the front and rear the shock valving, but I would adjust the easy stuff first (air pressure, wheelie bars, & shock valving) before breaking out the wrenches & sockets and adjusting the harder stuff.

Your rear shock "rebound" (extension) valving setting sounds really too tight? I could see where you needed it that tight if you wanted to slow/calm the suspension's initial "hit" because you were using either a decent amount of nitrous, or a short I/C setting (or both). And a soft bump setting could also be adding to the bounce cycle. True, a softer "bump" adjustment is sometimes used for controlling how well the car stays hooked down course on a rough track. And also for calming down the effect the wheelie bars might have on unloading the slicks during a wheel stand, but your bump setting might be too soft . Are you sure your QA1 double adjustables (with both knobs on the bottom of the shock) have 30 clicks of usable adjustment each knob? I thought that they were only around 20-24 clicks each, but I could be wrong.


As a starting point, if your slicks are a medium to hard compound & fresh, you might try around 6.5 to 7.5 lbs of air in the slicks (shoot for little/no wrinkles while staging). If your I/C was around the 45 -50" long range and around the 3.0 - 6.0" high range you might try the rebound set around the 60-80% tight range, & the bump set around the 40-70% tight range. And make sure the wheelie bars are high enough that they won't slap the ground or mask a problem, but still act as a safety device just in case.

Good luck, nice looking car. And as I will say again, the '69-'70 Mustang was the best body style Mustang ever made......EVER! :twisted:
 

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I WOULD NOT RUN TIRE PRESSURE ANY LOWER THEN 7.5 LB IN ANY DOOR CAR WITH 4-LINK.I THANK YOU SHOULD MOVE THE IC OUT TO AROUND 53-56 IN,AND SOME WERE AROUND 3-6'' HIGH
YOU SHOULD TRY TO GET IT AROUND HALF THE WHEELBASE OF THE CAR
I DID ONE OF THESE CAR ABOUT A YEAR AGO AND THE WHEEL BASE ON IT WAS 114.THE FRONT WAS STRECHED 4''
YOU SHOULD NOT HAVE THE REBOUND THAT TIGHT ON A CHASSIS CAR WITH 4-LINK.MAKE THE CHASSIS WORK AND USE THE SHOCKS TO
FINE TUNE IT.
WERE ARE YOU FROM?
ANDY SMITH
PROTREE RACE CARS
 

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Discussion Starter #4
bounce

andy i live in fl, i race at bradonton track!! thanks to everybody for the input it really helps, im going to make a four link adj, and loosen shocks and start with more air pressure, and start from there!! thanks guys george.
 

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Its hard to see from your video but I see two things happening. Or more specifically not happening.

I see very little or no movement of the rearend in the car. Initial hit should drop out and then as the tires hook and the front comes up the body should plant down over the rear tires.

Looks to me like the tires are doing all the work.

If you try to push the back of the car down does it move?

I would be looking for something binding your rear suspension or something like that.

I would also look at softer springs in the front to allow the front to come up faster.


dkp
 

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Looks like Dave,Andy and Boss have it covered, and i cant view the video (dial up), but looked at the first frame and would like to ask if you have those 15-33s mounted on 15" rims? We found out on any hard leaving car you need to run as wide of rim as the width of the tire. We even run a wider rim on the 10" tire stuff. 12"on 10.5-29.5s. The wider rim seems make the sidewall work effectively and you can add air pressure to help stable the tire. Its just my observation...........Ron
 

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IC = instant center.

The point on a four link car where the four link bars would intersect if they where extended forward until they crossed.

dkp
 

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What is the rear shock measurement, heim to heim at ride height, compared to fully extended measurement? How much of the shock stem can you see at ride height?
 

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I had tire guy tell me that I was shocking the tires to hard and it caused the bounce. He said I was egging the tire and needed to work with the sidewall or use a shorter tire, or wider rim. So I went with a shorter tire and it really helped. He said the tire gets out around and I didn't have enough HP to over come the egging of the tire. 620 hp 3800lbs 4200 stall
 
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