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Vibration over 65 mph

Started by may2011, May 06, 2011, 10:26:40 AM

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I have a 1985 Firebird SE - Recently, the front struts, rear shocks, universal joints, U-Joints, engine mounts, clutch and throw-out bearing have all been replaced and most recently a new (not rebuilt) engine has been installed.  The drive shaft has been sent to the shop and rebalanced.  Wheels have been restored to their original condition and balanced and Firestone tires are new.

When I go over 65 mph I feel a shake in the seat of my pants which normally means some kind of problem in the rear of the vehicle.  I don't know what's left to replace back there.  Stabilizer bar appears tight and normal.

I realize the SE has very stiff suspension and one feels every defect in the road surface but at less than 65 the shake disappears.

Got any ideas?

Glenn Martin - Tempe, Arizona.  


Hello Glenn –

You have an unusual situation here, considering that you report that vibration starts at 65 mph.  Usually a defect will appear at all speeds, and/or get worse as speed increases – except when it involves tire unbalance, or the un-sprung rolling stock – i.e. tires, wheels, rotors/drums, axles.  Usually any other defect would show themselves at all speeds. 

There's no perceivable shaking at lower speeds at all?  If not, look to:


Since you've already balanced the rear tires, do you know what method was used to do so?  There are two major types, though most reputable shops will use high-speed.  The 'wrong' method  would be Bubble balancing, which only takes care of one 'plane' of a tire's weight issue.  They will place a wheel on essentially a giant level, and if one side is heavier than the other, a weight will be put on the opposite side to correct.  Very crude, and with today's wide hi-speed tires, not very effective.

However, it's improbable that such a method was used.  At the same time, there could be a defect that wasn't discovered when the tires were installed/balance.  An easy way to rule that out would be to mount another pair of rear wheel/tires on the car, and see if there is any difference. You can even swap front to rear (if same size), and see if the shake shifts from your 'seat' to the steering wheel.  If there is, you'd want to look for: a. possible bubble or other sidewall defect, b. a tire may have a shifted belt under the thread, which may or may not be visible. Or c.  There may be an undiscovered defect in one of the wheels.

One other question:  Does the car sit for long periods of time?  If so, one or more of the tires may have developed a flat spot from sitting.  This can sometimes be eliminated/helped by over-inflating the tires by 10-15 pounds and running them until they smooth out.


All drums and rotors are balanced at the factory, and most times will have weights welded/boned to them to make them true.  Sometimes (though rarely) these weights can get knocked off.  Doing so would cause the same condition as if you've thrown a lead weight off a wheel, and the same high speed vibration.

On drums, if a weight is missing, you can usually see where one was by the different color where the weight was compared to the normal rusty surface.  These weights are mounted to the face of the drum, where the wheel mounts.

On rotors, sometimes the weight will be in the grooves between the inner and outer disc, or on solid rotors, on the inside of the hub. One other brake possibility is either a chunk missing from the drum or rotor, or a piece of something stuck to them somewhere. Are the drums rotors true – any vibration when applying the brakes?

Are the pads shoes dragging, causing the drums or rotors to heat up at speed?  Can sometimes cause a shake – very unlikely, but still something to check.

Lastly, a defective axle or bearing – again doubtful as such a defect normally will cause shake at all or most speeds.

Let me know if any if the above helps or not.  If not, I'll look a little deeper.  Try swapping tires first though.  A lot of times, even if not 'seen' during balancing the simplest cause to treat.

Good luck – I know how annoying something like this can be.  Drives me nuts, too!

Joe Genera

National Firebird and Trans Am Club
phone/text: 773-769-7166

North America's oldest and largest club for all Firebirds and Trans Ams including GTAs and Firehawks.  Established in 1984 and offers color magazines (EAGLE), emagazines, technical advice, free member classifieds, member repair shop recommendations and much more!

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