Why does my car start to vibrate at 75mph?

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90 Black Formula
5.0 TPI 5spd lotsamods

 

tires need balanced, driveshaft needs balanced, flat spots on tires. thats all i can think of

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have a beat to hell 87 GTA 305ci 700r4
t-tops,recaro,honeycomb wheels,digital dash
paid $600 at a junkyard for it (i had to save it)

 

more than likely jsut the tires

 

What about the mounts? trans. mount, eng. mount... etc. Would that have any bearing on vibrating?

 

mounts dont vibrate they clunk, the drive shafts on our cars are notorious for wobbling at high speeds. flat spots on your tire should be vibrating at any speed, so your prob. is probably the drive shaft or your tires need be re- balanced

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1991 RS LO3
mods: 14" chrome air cleaner, straight-piped after cat exhaust with one chrome dual tip on the left side.
future mods: headers, removing smog equipment, and as many cheap mods as i can find
aol s/n: budman8503

 

Here is how to test for the out of balance.

Jack up the rear end and put on stands, chock the front wheels. Put in gear and run the engine upto 75 mph. If no vibration, then it is in your front end.

If there is a vibration, take off your tires and put on the lug nuts to hold on the drum/rotor.

Now run the engine up to 75 MPH. If the vibration is gone, then it's the tires. If the vibration is there, then it is the drivelive.

Have a friend check for out of round while the tire is at 75 MPH.

When shutting down, first ALWAYS kill the ignition, apply the brakes, then put the car into park.

 

I don't know about you but there is no way in hell I would do that with an balanced shaft..like mine. I am not gonna have my car on jacks with the wheels going 75 MPH..Doesn't that scare you??

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• Red '88 IROC 5 Speed 305 TPI w/3:08
• Dual Friction Centerforce Clutch
• Accel 300+ Racing Ignition
• Accel 300+ Pro Sleeve Plug Wires
• Accel U-Groove Plugs
• Flowmaster Force II 3 Chamber Cat Back Exhaust
• Hypertech Chip
• Hypertech Airfoil
• Removed, Then Re-installed (out of fear) MAF Screens
• Gutted Air Box
• Best E/T 14.686, 60' 2.205, 1/8th mile 9.461, 1/4 MPH 93.91
• Best 60' 2.17

 

Heck no!!!!!

Now, if there is a bad vibration, I'd be a little leary, but I'd still do it.

I've buzzed the speedo upto 120 MPH for a 20 seconds. Kinda fun...

Tyr it. The brakes can stop those wheels in a couple of tenths of seconds (0.2 seconds).

Do this on a level surface, chock the front wheels (front and back), and with the jack stands with u-tops firmly placed. The car isn't going anywhere, and if it does teeter, the driver has plenty of time to react.

DON'T do this in a small garage, give yourself some lee way. I though that was kind of obvious.

 

i had a vibration in my iroc that strated around 65-70 and got real bad by 85. i tried all the normal stuff. i decided to replace the tires instead of rebalancing them, they were maybe 2/3 wore, but still had soem life left in the. anyway i got 2 new t/a radials for the back and when they broke the tires down i got about 2 quarts of water and aboutthe same in mud. fixed my vibration problem. i doubt if that's your problem, but it was mine.

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ICON Motorsports

1st & 3rd

 

I had a defective cord in one of my front tires, and mine would start to vibrate at about the same speed. You can see a dimple in the sidewall of the tire that was defective on mine. When I put snowtires on the back, and rotated the back street tires to the front, no more vibration at 75 mph!

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89RS w/350 TPI; 69RS/SS w/450 HP 350/Muncie 4-Speed "Too weird to live, too rare to die."