High Speed vibration, can't find what's causing it.

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RockinRobin

RockinRobin

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In the last month or so I started to notice the race car vibrating when I get above about 90 mph on a pass. Does it only while the car is still accelerating. When I let off it goes away.
It crosses the finish line 107-108 mph
No vibration during burnout or until 3rd gear (727)
So far I have:
Checked u joints and trans mount, also trans output shaft doesn't seem to have any side to side play.
checked motor plate
New front tires (was vibrating before the new tires)
front end alignment ( Stock front suspension. I do need tubular upper control arms as alignment is at its limit, but all ball and other joints are fine)
I would think that warped front rotors would show up before 90mph so I haven't looked at those
Suggestions??
 
I don't see front end/ tires/ rotors generally, causing problems only under acceleration.

Have you tried winding the engine up stationary? Jacking up the rear, tying it down "safe" and winding it up with the rear tires off the ground?

Then try removing rear tires, and do same thing with it in gear but no tires?
 
Have you had anyone watch your pass closely to see if anything is visable from the outside, have you narrowed it down to what section of the car it's coming from? It could be any number of things, or just one thing.
 
Just a thought but is it possibly something in the differential like pinion walking on the thrust while loaded and when you back off it settles down. Sounds rotational speed related to me.
 
did you move the springs or mess with the perches at all? possible pinion angle issue?
 
Solid trans mount to go with the motor plate? Driveshaft runout? Any pinion bearing preload left?
 
The other thing is, I have no idea what a failed sprag might do, auto?
 
Sounds like you may have lost a wheel weight on one of your rear slicks. If you balance them. It’s possible one of them came out of balance whether you balanced them when you mounted them or not. I have had to re balance mine after some usage in the past. I use a harbor freight bubble balancer, sticky wheel weights, and I duct tape all of my weights to the rim.

Not saying this is the problem, but it’s a possibility.

RJ
 
71Demon528 said:
Sounds like you may have lost a wheel weight on one of your rear slicks. If you balance them. It’s possible one of them came out of balance whether you balanced them when you mounted them or not. I have had to re balance mine after some usage in the past. I use a harbor freight bubble balancer, sticky wheel weights, and I duct tape all of my weights to the rim.

Not saying this is the problem, but it’s a possibility.

RJ
Click to expand...

First thing I thought of. Definately easy enough to take them off and check and rebalance
 
71Demon528 said:
Sounds like you may have lost a wheel weight on one of your rear slicks. If you balance them. It’s possible one of them came out of balance whether you balanced them when you mounted them or not. I have had to re balance mine after some usage in the past. I use a harbor freight bubble balancer, sticky wheel weights, and I duct tape all of my weights to the rim.

Not saying this is the problem, but it’s a possibility.

RJ
Click to expand...


I graduated to Gorilla Tape on my weights. Give it a try sometime RJ.
 
I got a good buddy out of a pinch years ago at our Bracket finals at maple grove in Pa. He had a vibration and i suggested putting a hose clamp on the driveshaft just tight enough that he could forcefully turn it. Make a pass then tighten it down where it stopped. It worked great and he ran it like that all season then addressed it over the winter. Something very quick to try. We put it close to the rearend.
 
pittsburghracer said:
I got a good buddy out of a pinch years ago at our Bracket finals at maple grove in Pa. He had a vibration and i suggested putting a hose clamp on the driveshaft just tight enough that he could forcefully turn it. Make a pass then tighten it down where it stopped. It worked great and he ran it like that all season then addressed it over the winter. Something very quick to try. We put it close to the rearend.
Click to expand...
Pretty clever fix !!
 
pittsburghracer said:
I got a good buddy out of a pinch years ago at our Bracket finals at maple grove in Pa. He had a vibration and i suggested putting a hose clamp on the driveshaft just tight enough that he could forcefully turn it. Make a pass then tighten it down where it stopped. It worked great and he ran it like that all season then addressed it over the winter. Something very quick to try. We put it close to the rearend.
Click to expand...

Whatever it takes! I’ve heard of that before and saw it on a customers car.

The gorilla tape I’ll be sure to give a try John, thanks.
 
Don’t overthink the balance job on slicks. Many that have tried other fancier ways have circled back to good old bubble balance jobs.
 
pittsburghracer said:
I can’t take all the credit as I read about it in an old Hot Rod or Car Craft tech article. Back when I had an open mind and I actually bought those magazines. Lol
Click to expand...

That method is actually in the Mopar FSM's as well for determining if the driveshaft is imbalanced, possibly where the magazine guys got it from? Lol. Those old Chrysler engineers were clever.

One thought I had is the rear leaves could be starting to wear out and allowing too much axle windup (dynamic pinion angle) when on the throttle since OP mentioned it goes away when letting off the throttle.
 
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