What causes deceration vibration

Bob Johnson

Bob Johnson

Well-known member
As I posted previously. After my engine rebuild I have an increase in the vibration when the engine decelerates. This is not an issue with the drive train (u-joint), as it happens even when the car is not moving when I rev the engine and then let off the gas. I have felt this vibration in all Model As that I have rode in. The only difference in the various cars is the intensity of the vibration. Some have alot and some have very little.

My question is --- What causes this vibration? My thoughts are the following. Since this only occurs during deceleration it is likely not due to the flywheel, clutch or crankshaft because if these where out of balance I would guess that the vibration would occur all the time and not just when decelerating. I also guess that the valve train would not cause this vibration. That leaves the pistons and rods. It seems reasonable to me that the vibration would be coming from the pistons and/or rods because the pressures on them are different depending on if the car is accelerating or decelerating. The above is just my guesses. Does anyone have any other thoughts on this or actually know what is causing the vibration.

Bob
 
I'll take a stab at it, even tough I don't like doing so.. I'd rather know an answer before typing it.. That said, my thoughts are running toward balance issues.. The rear journals tend to be overdampened by the heavy flywheel and the fronts underdamped for the same reason.. I would think the quality and extent of the balancing could cause such an issue..
 
Bob Johnson said:
My question is --- What causes this vibration? My thoughts are the following. Since this only occurs during deceleration it is likely not due to the flywheel, clutch or crankshaft because if these where out of balance I would guess that the vibration would occur all the time and not just when decelerating. I also guess that the valve train would not cause this vibration. That leaves the pistons and rods. It seems reasonable to me that the vibration would be coming from the pistons and/or rods because the pressures on them are different depending on if the car is accelerating or decelerating. The above is just my guesses. Does anyone have any other thoughts on this or actually know what is causing the vibration.

Bob
Click to expand...

Do not dismiss the crank/flywheel so readily. When under acceleration the combustion forces act, along with gravity, to cradle the crank in the main caps. With deceleration another force comes into play, vacuum. Now the crank is no longer being pushed down, in fact, the power stroke has become a vacuum stroke that wants to lift the crank against gravity. Couple that with a best case clearance of .001 to .0015 and a 62 lb flywheel and I think that provides a method for vibration. The limited number of A motors I have disassembled always shown main wear from front, least, to rear, most. To my way of thinking this supports the premise about flywheel whipping.

Disclaimer: I am not a "master mechanic" as others here profess to be, nor do I possess the "magical touch" as others claim to, neither do I recall much of the Physics training I was exposed to. I am just mechanically inclined/gifted and damn lucky at those times when it is necessary. :D
 
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What about clearances in the transmission gears?? With a load (acceleration) they would be minimal. but when decel they could be all over the place setting up different harmonics??
Paul in CT
 
I believe that Patrick is right. It seem that engines with very light pistons do not vibrate nearly as much as one with heavier pistons. I'm sure that pressure on the piston, vs a vacuum has a lot to do with it.
 
Milton,
Hey..'disclaimer'? Does that mean that you don't stand behind your work!
Whatshisname said (as to physics)..for every action there's an opposite and equal reaction...meaning, I think that you're answer is spot on and you are more physically inclined than you let on,eh!!
Hmm, I pick you for my team.
 
If you have engine splash pans installed, I would check to see if any of the four tabs that attach it to the oil pan have broken. This can be a source of vibration on spool down.

Tom Endy
 
It's not just Model A Fords that vibrate on decell like this. I have had modern (sbc) V-8s that do it as well. It must have something to do with the high vaccum in the combustion chambers.
 
Normally, the gas/air charge mixtures acts as a cushion on the compression and, to a lesser extent, exhaust strokes. This softens the impact the piston makes on the rod, crank, and everything they touch when it has to change directions by 180-degrees nearly instantaneously. On deceleration, the throttle is closed and there is minimal cushioning charge, together with high rpms, and that's what makes our mirrors shake. For this same reason, high compression heads are actually less stressful on rod bearings UNTIL a cross-over point is reached at higher rpms. It's the lower wrist pin bearing surface that takes the hit when the piston tries to keep going up but the crank has to yank it back down; if the problem were compression/combustion force, the upper bearing would take the beating. It's not such an issue at the bottom end because of the rather spread out circular path the crank/rod bearings take in the process of turning around. However, there are other balance/vibration issues that derive from what's going on down there.

Or something like that.

Steve
 
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Had my spare engine rebuilt,new rods,pistons,crank and cam reground flywheel and clutch balanced together,rebuilt the trans and have the same vibration that Bob Johnson has.I had the same vibration with the old engine in the car.The only thing that has not been looked at or rebuilt is the rear end. ???
 
All four cylinder engines have an inherent vibration, but acceleration does seems to dampen it. A F.A.S.T. friend (cute, huh?) noted that some cars just seem to run better than others, and he attributes the difference to slight variations in clearances, wear, assembly, etc in about everything from front pulley to clutch that just add together to produce a less than satisfactory result, even though any one particular part may not be much off.
It is the painstaking and untold time and attention to detail that eliminates those little imperfections that allows Jim Brierely's engine to go 160 mph. Sure, he had speed equipment, but it would have all been for naught had not every little piece and fitting and clearance not been at least triple micked and tested. That said, I would guess that JB would have felt some engine vibration had he decelerated at the end of his run rather than, I assume, shutting down.
I rebuilt my engine with a counterbalanced crank, light flywheel, balanced, etc and was kinda surprised to find the decel vibration did not change much. But it was smoother, if that makes sense. It is more like an even hum than an erratic rattle.
My point is that isolating a single cause is difficult, absent some gross error. If it is not too bad, and only you can decide that, in the immortal words of another Club member, have fun and drive it 'till it breaks.
 
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I had the same issue (vibration on deceleration) after installing a rebuilt engine plus flywheel/pressure plate that I had balanced by a guy whose competence I questioned after seeing his chaotic pigsty I mean shop and how he worked despite high recommendations. The funniest thing I ever saw was watching him try to figure out why a mill he was using to work on a Chevy head had travel issues when it was bumping into a radio that he had duct-taped to the wall. After a few months, I pulled it and sent the flywheel+crank+pressure plate to a different shop for dynamic balancing but they said they did nothing since it was all good. I checked main and rod bearing clearances (no changes to rods, one shim pull on rear main).

I subsequently installed a Mitchell overdrive and NOS u-joint and then got no or very little decel vibration while in high overdrive. But still decel vibration in low overdrive.

This weekend I rebuilt the 7 toofuss steering and the vibration seemed to be much less but I still have some nagging noise from a repro emergency brake lever, homemade floor boards, and worn brake components.

I am starting to conclude that the decel vibration clearly originates in the engine, but its manifestations are due to problems peripheral to the engine... in other words, there is disharmony due to harmony.
 
As Bob stated he has vibration when decelerating when the car is not moving. I too have this problem to the point of almost a shudder. I'm hoping the cure is not an internal engine problem as opposed to a flywheel, clutch, pp, unbalance. My body is off the frame and when time allows I'll disconnect the above items and see if there is any difference. Upon reading some of the previous suggestions it was mentioned that the front pulley could be a source of vibration. Mine has a broken off 'web' which I recently noticed so that's something else to check into.
Bob. Let us know if you find out what's causing your problem and I'll do the same if anything I try works. It's been a nagging concern.
Rusty
 
Thanks for all the information. I would like to point out that I had said that the vibration occurs when the CAR IS NOT MOVING. Thus it is not due to the transmission, u-joint or rear end. It appears that there are many theories on the source of the vibration.

New question:

Is there a way to isolate the source of the vibration WITHOUT removing the engine from the car? Note that all parts were balanced before the engine was assembled. Pistons and rods statically, crank, flywheel and clutch dynamically.


If I had the money and time it would be interesting to take an engine that was running very smooth and experiment with balancing. Adding weights, one at a time, to various parts, flywheel, clutch, crank, pistons, rods ... and see what vibrations show up. Then one could find out what various amounts of imbalances produce vibrations.

Bob
 
I'll place my bet on the flywheel and pressure plate

Bob,
Do you have any records as to the run out and wobble of the flywheel when it was installed? If you took careful dial indicator readings while assembling, they will confirm a problem. If you pull the engine, take these readings before removing the flywheel.
A crankshaft ground slightly off center will cause an independently balanced flywheel and pressure plate to create a vibration you are describing. A flywheel wobble will also create a like vibration even though you do the best job of balancing the assembly. Needless to say, this condition is very hard on the rear main bearing.
I switched out a flywheel for a gent early this summer and we sent it back to the machine shop for evaluation. I know we are going to find it was the cause of the problem. A replacement I had on the shelf calmed the shakes!
Good luck!
 
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There's several factors that influence the vibration in the engine. Balancing certainly is the first item always mentioned, and does play a big role. The dimensional qualities of everything (moving or not) within the engine plays an equal role. This is often referred to as blueprinting, and it's expensive which is why most don't do this. Having four rods within a gram of each other is great, but won't eliminate the vibration if the center line of the wrist pin and bearing holes vary by .003-.005". Not much, but consider the distance the big end travels across it's rotation and it adds up. Or how the difference in center line affects the piston height, witch affects the volume of the cylinders. That would produce a vibration that no amount of weight could help.
 
I have seen cracked and weak rear motor mounts intensify the vib. but when fixed still have a slight one as was the case with my AA .Had a horrigle knock and vib in my tudor but the flywheel had been installed at some time in the past without the dowel pin retainer and the pins had come out. I dont know what causes the decel vib but would love to know. may be then we can all keep our nut`s tight!...Dave
 
I just checked the flywheel for wobble. I took out the starter and measured with a dial indicator.

For the front to back wobble I took the measurements on the front face of the flywheel inside of the ring gear (not on the ring gear). I measured every 45 degrees. Before each measurement I made sure that the flywheel/crank was pushed rearward to eliminate the thrust play. The results were 0, 0, 1, 2, 1, 0, 0, 0.5 all in 1/1000th of an inch.

I made a tool to measure the side to side wobble through the starter hole. I measured just in back of the ring gear. I again measured every 45 degrees. The results were 0, 0, 0, 1, 0, 1, 2, 2 all in 1/1000th inch.


Bob
 
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