Another "bounce" question

[Sparky]

There is a current thread about bouncing while driving down the road but I didn't want to hijack that one and my question is a little different.

When I am driving down the road at about 45 mph or so, even if it is a smooth road, the transmission sometimes will start to bounce up and down rapidly. I can see this by the movement of the shift and brake levers. The car itself is not bouncing, but it seems like the entire engine is rocking about the rear mounts. I checked the mount bolts and they are tight (I have stock mounts, not float-a-motors). Is this a normal thing? Is is possible that the front engine mount springs are too weak?

[Tom Wesenberg]

That would seem most likely to be a U-joint or front driveshaft bearing.

[Sparky]

I'm not following. Do you mean the bearing on the input shaft of the transmission? I know that one is OK because I have had the transmission out and checked it over. Besides, since the transmission is bolted solidly to the engine I don't see how a bad bearing can cause the whole unit to shake.

I think the U-joint bearings are Ok also, or are you saying something might be bent?

[Tom Wesenberg]

Near the gear that drives the speedometer is a roller bearing. If it's loose then the U-joint can be good but still flop around. Both the U-joint and the roller bearing behind it must be good

[John Kuhnast]

Wheel balance?

[Sparky]

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Originally Posted by John Kuhnast

Wheel balance?

Maybe, but the frequency of the shaking is lower than a wheel vibration.

I suspect that since my car is so light and doesn't have a body to stiffen it the frame is simply flexing.