Vibration

F

fastroadster

Senior Member
Seems when I am coming down a grade and back off of the gas I start getting a pretty good vibration is this common?? It does matter what gear I'm in.
 
I Feel the vibration in the steering wheel and really the whole car. Will check u-joint and pinion. any other ideas would be appreciated.
 
I concur with the U-joint being most likely culprit. 'Been there and the symptoms matched yours. If you don't feel the vibration while accelerating, but do feel vibration when you back off the throttle, the U-joint is probably bad. What happens is while the engine is accelerating, pressure (torque) is applied to the two U-joint halves so that they push against each other, taking up any slop. But when you back off the throttle, the U-joint halves resume their "neutral" relational positions and vibrate because they are loose, either because the rivets have gone bad or because the shanks are worn inside their bores. I'm not sure I explained that clearly, but I think you get the idea.
Loose pinion gear nuts can also cause noise and vibration. A car I restored for a friend a few years ago developed noise and a vibration that could be isolated to the front of the rear axle pumpkin. Upon disassembly, it became evident that the repo pinion gear lock washer's "ears" were too short and didn't hold when I had hammered them over the nuts. My bad. I thought they would hold, but clearly didn't. So, the front nut loosened, followed by the second nut = noise because the drive shaft and pinion gear were bouncing around against the ring gear teeth. Vibration was also noted, although the noise was the major complaint, more of a clunking sound that telegraphed up to the steering wheel. The noise and vibration were present both during acceleration and deceleration, unlike with the U-joint. Use that as your litmus test to determine whether the problem is the U-joint or loose pinion nuts. No matter which is at fault, that rear end is coming out!
Marshall
 
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I'd rev it in neutral to about the cruising engine speed then let up on the accelerator quickly. If you feel the vibration it's in the engine/flywheel/clutch...not the rest of the drivetrain.

Did this just start happening?
 
My 29 Special Coupe was vibrating and it was as Marshal stated the pinion nuts had come loose.When I disasemble the rear I found it had a 4.11 ring and pinion gear set in it. I put a 3.54 gear set with new bearings and nuts and locks and it ran great.
 
DON'T get excited. Model A's are notorious for NOISES/VIBRATIONS that come & go. Just make sure that important stuff is SECURE & drive on!!!
Bill W.
 
There is no noise just the vibration. Maybe I will live with it for a while and then later on pull the rearend (really not looking forward to it) Thanks Marshal
 
fastroadster said:
There is no noise just the vibration. Maybe I will live with it for a while and then later on pull the rearend (really not looking forward to it) Thanks Marshal
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If it is just a light vibration, you might try loosing the nut on the bottom of the front motor mount. It some times helps.
 
My first thought was the same as Steve's, that the pinion nuts are loose. I've seen this happen a couple times, and it would vibrate while engine braking, but not while accelerating. Anyway, you'll get to check the U-joint at the same time.
 
I too would like to know the answer to Dick's question, did this just start? I'm guessing you're new to Model As.

All motors vibrate more when decelerating, especially when backing down in lower gears/high rpms. The reason is that with the throttle closed, there's very little "cushion" of fuel/air mixture in the combustion chambers for the fast-revving pistons to slam into. Unlike at the bottom of a piston's travel, where it can decelerate and reverse direction gradually as the big old crank throw takes its sweet time getting around, at the top of its travel it has to reverse RIGHT THEN.

Incidentally, this what really hammers the bearings, especially in low-compression engines, and also partially related to what frequently causes backfiring under the same (too lean) conditions.

Or, maybe it's the U-joint, although I don't know why that would depend on what gear you're in. I suggest you get an experienced Model A guy to go for a ride with you.
 
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I too would like to know the answer to Dick's question, did this just start? I'm guessing you're new to Model As.

All motors vibrate more when decelerating, especially when backing down in lower gears/high rpms. The reason is that with the throttle closed, there's very little "cushion" of fuel/air mixture in the combustion chambers for the fast-revving pistons to slam into. Unlike at the bottom of a piston's travel, where it can decelerate and reverse direction gradually as the big old crank throw takes its sweet time getting around, at the top of its travel it has to reverse RIGHT THEN.

Incidentally, this what really hammers the bearings, and also what frequently causes backfiring under the same conditions.

Or, maybe it's the U-joint.

I wonder what caused this duplicate post and how I delete it?!
 
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steve s said:
Or, maybe it's the U-joint, although I don't know why that would depend on what gear you're in. I suggest you get an experienced Model A guy to go for a ride with you.
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someone with good ears, you just drive and concentrate on replicating the sound and driving safely while the passenger can just concentrate on the noise.
 
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