What causes deceration vibration

[Bob Johnson]

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

[Patrick L.]

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..

[Milton]

Originally Posted by Bob Johnson

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

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.

[1931 flamingo]

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

[Jim Brierley]

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.

[hardtimes]

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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.

[Tom Endy]

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

[John S]

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.

[steve s]

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

[Dennis/CT]

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. ???

[PC/SR]

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.

[Jerry in Shasta]

My money is on a bad U-joint or a loose U-joint

Just my w.a.g at the problem.

JB

[Rich in Tucson]

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.

[Milton]

My vibration turned into a very slight buzz when I got rid of 35 pounds of excess flywheel!

[Rusty Homestead Fl]

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

[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

[Dave in MN]

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!

[azmodela]

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.

[85930tudor]

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

[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

[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

[Bob Johnson]

Mike my post #20 08-29-2011 04:41 PM says:

"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."

I think that means that flywheel run out is not the problem. The flywheel crank and clutch were dynamically balanced.

Mr. Bear;
The valve springs were replaced with new ones at the recent rebuild. I check the spring for proper tension before installing them.

usurs;
The vibration occurs with the card not moving, clutch in or out which means it is not the transmission.

I have driven many Model As and I know that vibration is normal for a 4 cylinder engine. However this one has too much vibration as compared to others.

Thank everyone for the suggestions. Still looking for a solution.
Bob

[Chris in CT]

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[/QUOTE]

Yes, you can do a Fast Fourier Analysis of the vibration in your engine, an expensive, time consuming operation if there ever was one. Two channel, with sine and cosine functions covered would be best, with magnetic trigger that measures the exact position of the crankshaft with the reading of each harmonic peak in the rotating system. If you had the time and money, and could get the assistance of someone who knows how to set up the system, you could in fact determine exactly where the vibration is coming from. Here is the problem: you may find out where it is coming from, but there is no guarantee you could eliminate it. To balance the sine function in an I-4, you could very well unbalance the cosine function at the same time. Its one of physics' little jokes on us all.
Enjoy your Model A!

[amishman]

Deceleration vibration really bothered me a lot, and a friend says it's a Model A, what do you expect? My first A was a 31 Roadster with a new rebuild from AER in Skokie IL, it had a 5.0 head, and balanced crank. I had my 1930 Townsedan rebuild this last winter with AER also, this one with 5.5 head, Burlington balanced crank, larger intake and exhaust valves, and Stipe .340 cam. The deceleration vibration was about the same. But let me tell you this, because I don't understand all of the physics talked about above, Last weekend I installed the Weber downdraft carb. and First Street electronic ignition and the vibration is gone. Most might not believe this, but it is true I think I'd read something like that in a post before. This weekend weekend we put in the Mitchell overdrive transmission. My wife can't believe it's the same car, she thinks driving from Michigan to Tacoma won't be so bad after all. Hope to meet some of you Fordbarners from the West Coast next summer.

[steve s]

Amishman,
Check out my explanation on post #9. If the Weber setup lets in more fuel during deceleration, that would explain it.

Steve

[Pete]

Posts numbered 5,9,25,32 and 44 pretty well cover it.

[Duffy1]

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So is type of vibration detrimental to the engine?

[Pete]

Originally Posted by Duffy1

So is type of vibration detrimental to the engine?

Definitely not.

[markbartlett]

Question how long was the car without engine just sitting in the weather my bet also is the u joint

[Mr Ed]

I have a 29 Model A with 600 miles on a rebuilt engine, it also has a substantial engine viberation on decelaration exactly as you described under the same conditions you described. Some years ago I had a stock 30 Model A that vibe rated only slightly under decel. I also have owned 1954 6 cyl Plymouth that vibe rated on decel. My Dad had given it to me because he thought it had a broken crankshaft. Long story short , the flywheel was lose on the crankshaft . Tighten flywheel and viberation was completely gone. Have you been able to lesson or almost elimate your viberation? Do you know if anyone has ever put a harmonic balancer on a Model A engine?
Eddie Condrey

[JDupuis]

Hey Mr Ed, Welcome to the barn.
Do a search for the "harmonic balancer" I believe this has been discussed in the recent past. I think the guys in New Zealand and Australia may have a source for them.
Jeff

[roy green coupe]

While observing my running engine on my 30 coupe, I noticed the belt seemed to be going through a harmonic vibration at certain rpms.I first thought the crankshafts pulley was the problem as the belt seemed to run in and out some.I did notice the belt was rather loose and had been reluctant to tighten it not wanting to put excessive load on the water pump.I was trying to leave well enough alone but I overcame that and decided to order and install the brace for the generator and tightened the belt.I was surprised when the belt now seemed to run smooth and true on the crankshaft pulley .I was also able to notice the engine seemed smoother in high gear at speed.I have read where others have taken the belt completely off for a short test.I think think the high placement of the fan and water pump magnify their vibrations.I have wondered if slowing down the fan speed with different size pulleys would accomplish some of the benefits people talk about with overdrives.