What causes deceration vibration

[d.j. moordigian]

Well, this should start something. Take the flywheel OUT of the equation. Start
it with the hand crank and see if it shakes on deceleration.
Yes, I know, your not going anywhere, and NO, I'm not joking about this.

Dudley

[Milton]

I would start with the heaviest rotating part first. Pull the plugs, floorboard and flywheel/clutch inspection plate. Use a dial indicator and check the flywheel for travel, both rotation and fore and aft travel must be true. While at this task the rear main should be able to check the rear main with the dail indicator and a crow bar. I've never done this but it seems possible however it will call for a dose of caution with a helping of common sense. I recall quite awhile ago using a glass of water to compare the vibration by mounting the flywheel in different positions and got the lesser of the two. Nothing great in the high tech dept. but anything can help. Sorry that this reply took so long but I had to get some stuff and check a surge tank on the radiator.

Jeez, I just posted this and the first thing I see is Dave in MN on the same track.

[Richard Lorenz]

Some vibration during deceleration seems to be normal. I have a '29 Tudor that runs nearly vibration-free at any speed. The crankshaft, flywheel, and clutch plate were balanced as an assembly. Just after the rebuild I noticed the deceleration vibration. It has decreased considerably after the break-in period, but I might also be accustomed to it now.

[Wick]

Cracked Flywheel housing or Pilot shaft bushing. Also take the belt off the fan and gen.
Wick

[steve s]

Originally Posted by 85930tudor

...I dont know what causes the decel vib but would love to know. ...Dave

Dave,
Check out my post above. The only thing that's different on deceleration is that the combustion chamber is relatively empty so no "springy" gas mixture is helping to cushion and turn the piston around--it's all on the wristpin, which passes the greater impact all down the line. All the various possible mechanical imbalances--flywheel, etc.--are the same on acceleration or deceleration.

Steve

[Dave in MN]

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Originally Posted by Bob Johnson

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

Bob, Those measurements are pretty good...better look elsewhere! Have you removed the belt? Maybe the fan is out of balance. Good luck in your search. Please post what you find to be the cause.

[85930tudor]

I went out this morning and looked at my suspect engine. the flywheel housing is cracked.Oh well I dont have anything else to do.....Dave

[sturgis 39]

The ring gear and pinion is probably the problem. The pre-load or bad bearings are probably the problem. This problem is transmitted to the transmission and engine. When pulling everything is tight. When decelerating there is no torque on the ring gear and pinion. There are other causes but the ring gear and pinion are the common cause.

[HoarseWhisperer]

Recently, developed vibration and noise that drove me nuts trying to locate.

Finally discovered three sheared bolts that hold the rear engine mount to frame. Replaced and all is good.

[1928Pickuppain]

If my A didnt Vibrate spit oil and fart id wonder who stole my ford. Just ignore it and keep driving.

[Proartguy]

Originally Posted by Bob Johnson

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

Maybe this subject has been answered, but I could not find it. Is the deceleration vibration inherent in the Model A engine? There seem to be a lot of suggestions but few answers. I have a '31 which has the same vibration as described in this thread and wonder if I expect too much from 85 year old technology.

[Tom Wesenberg]

Originally Posted by Proartguy

Maybe this subject has been answered, but I could not find it. Is the deceleration vibration inherent in the Model A engine? There seem to be a lot of suggestions but few answers. I have a '31 which has the same vibration as described in this thread and wonder if I expect too much from 85 year old technology.

Probably so, as with the throttle closed you have little air to compress and cushion the piston reversal at the top of the compression stroke. Combine that with a crankshaft with no counterweights and maybe too much clearance in the babbit and you have a good setup for extra vibration. When everything is built to new specks the Model A is a very nice and fairly smooth runnig engine, but for the life of the babbit, I'd add counterweights, or buy the new counterweighted crankshaft.

[Pete]

...

[George Miller]

I think deceleration vibration is normal with a flat flimsily crank long stroke big bore model A engine. What will help is only tighten the front motor mount enough to get the cotter key in.

I think Milton has the right answer. When the engine goes into the vacuum mode, the crank flexes the opposite direction. That changes the balance of the engine. No matter how good they are balanced they always have some vibration at one point in there rpm range.

[JBohannon]

I am a newbie and no one has mentioned this but I recently replaced my steel fan for an aluminum and my engine runs much smoother. It would be easy to isolate the fan my removing the belt for a short test.

[Art Bjornestad]

Has anyone talked to an owner of a '32 Plymouth about this problem? What about a '32 Ford, do the newer motor mounts help the problem?

I have a B motor with a cast iron Crager, 22 lb wheel, Float a motors and a Volvo o/d. I have no vibration whatever. My built '31 shakes like a rat. Art

[Mike Gipson]

My Monies on the Flywheel. I have a B engine with V-8 Clutch and Pressure Plate. The same wobble / Rattle you describe on Decel/Moving or Stationary turned out to be the Flywheel runout. It was at 8/thousanths. Fixed that and the woble/rattle quit entirely. Good Luck.

[Kohnke Rebabbitting]

And when it's all said and done, you still have a Motor that is tied to the frame!

[ursus]

I second the comment from Flamingo31. I once made up a jig to test several rebuilt transmissions for noise, thinking this would be a good way to check for fit when run in each gear. This involved a small electric motor with a chuck that gripped the pilot bearing race on the input side or some thread stock screwed into the output shaft end. I was amazed at the difference in noise between a simulated acceleration versus a simmulated deceleration. Same gears and same mesh but what a difference in both sound and vibration.

[Growley bear]

Weak valve springs can cause a shudder on deceleration.

Originally Posted by Bob Johnson

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