The MGA With An Attitude

At 05:45 PM 10/29/04 -0600, Cal Wine wrote:
>".... I have a vibration between 52-59 MPH. I put new tires on high speed balance, ...."

"Vibration" is a very broad word. First question is, what is the frequency of the vibration? Next, where do you notice the vibration? Steering wheel, seat, shift lever? Is it a high frequency teeth rattling vibration, or a lower frequency bone shaker?

If it was a wheel or tire problem, I would expect it to shake between 60-65 MPH, and smooth out to a little less shaking above 70 MPH. That shake would be at wheel speed, about 12 to 14 cycles per second at 60 MPH (depending on tire size). For a front wheel it would be very noticeable in the steering wheel. For a rear wheel you would notice it more in the seat of your pants. Please read about Wobbly Wheels

General looseness of anything it the front suspension or steering gear can aggravate or amplify the smallest front end shimmy into something much more noticeable. Please read about Front End Shimmy

>".... new driveshaft, a complete service and tune up and nothing seems to affect the vibration. .... any suggestions?"

In 4th gear the propshaft runs at engine speed, around 3500 rpm at 60 mph. Vibration there could commence at any speed above 30 MPH if it's bad. Otherwise expect the vibration starting between 45-50 MPH and getting worse with speed. Frequency is nearly four times faster than wheel speed, almost 60 cycles per second at 60 mph (like electrical hum in your house). If the front u-joint is bad you will feel the vibration in the shift lever. If the rear u-joint is bad you may feel it in the seat of your pants. Bad u-joints vibrate worst with forward load, and might almost cease to vibrate the moment you take your foot off the throttle to coast. A bent propshaft will be out of balance and may vibrate at similar speeds, but the vibration will not change with throttle setting. Please read about Bad U-joints

A bad tail bearing it the gearbox can cause shake similar to bad front u-joint, but will not change with throttle setting. Please read about Replacing the Tail Bushing

Bad input bearing in the rear axle might theoretically cause a propshaft shake similar to a bad rear u-joint, but not changing with throttle setting (but I've never known that bearing to go bad). Check the bolts in the propshaft flanges to be sure they are all tight, and that the flanges are properly seated with no gap at the mating surfaces.

The double flanged propshaft has a sliding spline joint just aft of the front u-joint. If greasing is neglected for a long time, these splines can wear badly, causing wobble that makes vibration.

Also with the double flanged propshaft, the front yoke must be assembled with proper orientation to the rear yoke. The lateral pivot axis of the front and rear articulated flanges must be in the same plane. If the sliding spline is assembled incorrectly, putting the front yoke out of phase from the rear yoke, you can get a vibration similar to a bent propshaft. This might start as low as 30 MPH, gets worse with speed, and does not vary with throttle setting. Read this:
The picture shows the correct orientation for the end flanges.

Worn and loose wheel bearings can cause wheel wobble type shakes. In the front it might feel like an out of balance wheel. In the back it might feel more through the seat of your pants, but maybe hardly noticeable. Bad wheel bearings also commonly make noises, giving off a cyclical rumbling noise that comes and goes at a frequency about 1/3 of wheel speed (orbital speed of the rolling elements). This might vary around 2-3 cycles per second at city speed, more like 5 cycles per second at highway speed. Please read about Front Wheel Bearings

If you haven't figured it our after all this, gimme another hoot.

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