Car Bounces after 60 ft. What is causing this??

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RockinRobin

RockinRobin

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This car started off this summer running solid 10.2 @ 130 mph with a little instability early in the run. The car has 2.5" taller tires now, and hooks just fine, but after the 60 ft. it starts to bounce so much I have to let off the gas. After the bouncing stops I can get back into it and finish the run just fine. This is a foot brake car BTW. Someone suggested the chassis is unloading as the front end settles down. I really don't have a clue. We've tried Caltrac and rear shock adjustments and torsion bar adjustments with basically no difference. Pinion angle? Lower the rear end?
 
Also check the front shocks. You have to remember that your shocks have the same mileage on them as your trailer.:) I am also with others on here, video would be nice. Keep us posted.
 
This is called proposing. I am having a brain fart but Google search yellowbullet proposing for info

Shackle angle or bind comes to mind
 
Traction Dynamics said:
You have to remember that your shocks have the same mileage on them as your trailer.
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My first thought was the "rears" going away.

sbmmustang said:
What size is the tire? But that seems to too low!
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Needing under 10lbs these days, is usually covering for another issue somewhere? Unless you're running a dragster. :)
 
Alignment? Car sits 1.25" taller and may have also moved the CG further back making the car ride even higher than that while under load. I a may be talking out of my *** but that is where I would start to take a look.
 
Whoops. We are taltking about rear tires. Unless we are talking about wide tires like 14" slick I think you need more air. Check the contact patch and tire specs. I would think 15 psi or more.
 
Traction Dynamics said:
Also check the front shocks. You have to remember that your shocks have the same mileage on them as your trailer.:) I am also with others on here, video would be nice. Keep us posted.
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Brand new Calvert shocks all around. Working on getting a video posted.....
 
MidTexCuda said:
Alignment? Car sits 1.25" taller and may have also moved the CG further back making the car ride even higher than that while under load. I a may be talking out of my *** but that is where I would start to take a look.
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Had it aligned after the taller tires were put on.
 
MidTexCuda said:
Whoops. We are taltking about rear tires. Unless we are talking about wide tires like 14" slick I think you need more air. Check the contact patch and tire specs. I would think 15 psi or more.
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Yes, we are talking about a 14" wide slick.
 
SpareParts has the right idea, it's called porpoising and is a matter of spring rates. Front and rear spring rates are allowing the weight to rock back and forth. A friends Volare does the same thing but, very mildly. Keep tightening the rear shocks or maybe try a lighter set of T-bars, until it quits. For as quick as the car is you don't seem to get much fronted rise, what bars are in the front?
 
I agree with tightening your rear shocks. Can not understand why you are not getting more front travel. With the front end not coming up you are not getting proper weight transfer causing the rear end to want to unload. JMO
 
sst3193 said:
SpareParts has the right idea, it's called porpoising and is a matter of spring rates. Front and rear spring rates are allowing the weight to rock back and forth. A friends Volare does the same thing but, very mildly. Keep tightening the rear shocks or maybe try a lighter set of T-bars, until it quits. For as quick as the car is you don't seem to get much fronted rise, what bars are in the front?
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I measured them at .8", so they are /6 bars. Also I raised the front end when I went to the taller tires in back. Maybe I should lower it back down and allow the front to come up more at Launch. I think I also need to try footbraking at a lower rpm to see if I get more transfer. I've had people suggest to go all the way down to 6.5lbs on the tire pressure.
 
IMO you have some other problems going on. Look around the front end for a bind. the car dumps to one side almost instantly, bumper doesn't come close to staying level.
 
it looks like the rear tires are loading and unloading. The front looks like it is coming up and transferring the initial hit but because the rear suspension is not letting the weight stay on the tires the unit is skipping on the low tire pressure. I bet if you upped the tire pressure the issue would decrease. I think the shock in the rear are causing you the issue.
 
I had a 63 Fury drag car that was green,got the nickname Frog because it would hop like that. Went to taller front tires and loosened the T-bars(Big block with slant 6 bars) solved my problem. Maybe? just a thought..
 
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