Vibration

[ling102]

Just changed the u joint on my 1946 Super Deluxe. It has a 39 Ford transmission.
I have new bias ply tires that I personally balanced on a spin balancer.
The balance is absolutely perfect.
The wheels are true.
The tires are true.
Now that I have this issue corrected, I have a noticeable vibration that starts at 35mph and ends at 45mph. I ran the car all the way up to 65mph with no other vibration.
The car also vibrates on deceleration...whether the clutch is in or not.
The vibration comes on at 45pmph and leaves below 35mph.
Possible pilot bushing? Clutch? Pressure Plate? Bearing in the transmission? Torque tube bearing?

Any help/ideas would be great!
Thanks Guys.

[drolston]

The speed dependency indicates some sort of harmonic related to wheel/tire rotation. Have you checked for a tire out-of-round? It could be a poorly constructed tire that distorts as tire rpm goes up.

The suspension dynamics also get into how the harmonic vibration behaves. Try switching the tires from front to rear and see if the behavior changes. If that does not improve things, switch tires left and right to change the direction of tire rotation. If it is a tire construction flaw, that might help.

[Flathead Fever]

Just sitting in the garage is the engine smooth at the same rpm that you feel the vibration while driving. That would help you decide if it's an out of balance engine or something else. It's a little risky but you can put the car on "good" jack stands and run it at that speed and see if you can find it. I only use those heavy-duty truck stands for stuff like that. Do not get under the car while doing that. We did that all the time up in the air on the hoists at work or put them on the smog machines' dyno. A vibration can be tuff thing to find when it feels like it's coming from everywhere. Do you feel it in the seat of your pants or in the steering wheel. Motor mounts can cause a vibration if they are letting the engine or trans just barely touch metal to metal somewhere. Broke baffles in the muffler, something slightly loose in the suspension or steering, wheel bearing adjustment... Sometimes rearends will make a noise at just certain rpms and so will transmission gears. It still might be the tires, could be the tread design or an internal flaw in the tire. Spinning them on the balancer is not the same as having a load on them going down the road. Try an air pressure change and see what happens. We had so many wheels and tires mounted at work it was easy to just try another set or swap them with another vehicle. That's one of the only good reasons I can think of for being a fleet mechanic. I always had identical vehicles to swap parts with. Never tell the drivers you took the bad tires form bobs' van and put them on theirs for an experiment. Never let them know you were there!

[swedishsteel]

Is it entirely speed related, i.e, does it do it 2nd gear at that speed and high gear at that speed? How about sitting still with the engine revved to that rpm? I just figured out a vibration in a '67 Mustang that I built--felt vibration in steering wheel and seat at cruising speed, 57-63 mph. Driveshaft rebalanced, new slip yoke, new tail shaft bushing, new harmonic balancer. Realized had the same vibration sitting still revving the engine to the rpm corresponding with the highway speed, worst at 2200 rpm. No vibration felt at all on the engine if revving it using the carb linkage under the hood. Disconnected tailpipe from mufflers and body, built a 1/4 resonator, studied frequencies and how they interact, nothing worked. Turned torque convertor 180 degrees. Was down to only two things I hadn't checked: tranny mount and motor mounts. Put a jack under the rear of the tranny and took out the home made tranny mount: no vibration. The tranny mount was too solid and transferred the harmonics of the engine/tranny into the unibody. I made a "rag joint" style mount of a piece of truck tire and it took away 95% of the vibration. Moral of the story, vibrations can come from strange places

[FlatheadTed]

I think you may have answered your own question ,start there,Stick on weight through the trans inspection window , Ted

[Ken/Alabama]

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So if you run it up to say 60 mph and push it in neutral it will vibrate as the speed slows ? If that’s the case I’d be looking at the u joint or torque tube bearings.

[fordor41]

We had a '41 ford w/ F/H and 3 speed. always had a slight vibration. Replaced the engine(302), trans (C-4), Ford 9" in rear, new rims w/ radials F & R, disc brakes in front and completely rebuilt stock front suspension and still had the same vibration for 25 yrs! Ours was sort of a "rolling" vibration. would come and go about every 4-5 sec.

[ford38v8]

So you punched a hole in your '46 floorboard to accommodate the '39 shifter? Maybe the hole is too small and the floorboard is touching the shift tower?

[ling102]

To respond to some of the questions, the vibration starts at 35mph and goes away at 45mph, whether or not the clutch in, and it happens on acceleration and deceleration.
As I am not familiar with the possible issues, I was hoping for a definitive answer so I am not tearing apart more than need be, and I am not making purchases that are not necessary.
Again, I truly appreciate the help.
Thanks Guys.

[Brian]

Probably not your issue, but i once had an ot car that exhibited the exact same trait of having a vibration between 35-45 MPH only. Turned out to be loose front wheel bearings! Tightened them up, vibration disappeared!

[cadillac512]

Originally Posted by ling102

To respond to some of the questions, the vibration starts at 35mph and goes away at 45mph, whether or not the clutch in, and it happens on acceleration and deceleration.
As I am not familiar with the possible issues, I was hoping for a definitive answer so I am not tearing apart more than need be, and I am not making purchases that are not necessary.
Again, I truly appreciate the help.
Thanks Guys.

When you changed the u-joint were the bolts holding the clamshell the correct bolts? Those need to fit quite closely in the holes since they are responsible for centering the torque tube with the u-joint and transmission output shaft. I put regular ol' 3/8 coarse thread bolts in mine and had to replace them due to vibration issues...learned that lesson that I should have known,lol.
Hard to diagnose this kind of deal from a distance....



Terry