MTFCA Forum

My 26 coupe vibrates quite a bit when running down a steep hill in high gear. The crank and flywheel have been dynamically balanced and the car will run up to about 50 mph with little to no vibration. It's only an issue under engine braking. Any ideas?

Thank you,
Stephen

Too much exhaust back-pressure? Can't get out the manifold past enough? Just guessing. Almost sounds like a model T version of Jake brakes!! Someone else who knows more than me will no doubt chime in and help.

I would suggest that you may have driveline issues. Are you confident that the Universal Joint, drive shaft end float and crown wheel / Pinion are in good condition?
Alan in Western Australia

Maybe the outside of the brake drum has a little runout (not perfectly concentric with the centerlines of its bushings). It could be checked by putting a dial indicator on the drum between the band ends and cranking over in high gear with spark plugs removed and rear wheels off the ground.

Maybe a drive shaft or u-joint issue. There are some circumstances where slop in the thrust between thrust bearing and u-joint could present differently between power and braking. Off-centering of the u-joint could potentially cause vibration (if the yokes of the u-joint aren’t perfectly centered inside the ball.

It’s possible that an imbalance in the flywheel may show up when the engine is holding the car back, but not under power, but since your engine is balanced, is unlikely. How was it balanced?

How is the idle? A rough idle will be noticeable under compression braking. Then the engine might run rich and have some pops in the exhaust system and any looseness in the drive train will be more noticeable because when the engine fires it will go forward, and then on compression it will go backward and set up a vibration similar to a front wheel shimmy. It will bounce back and forth.
Norm

I'd check the U joint and related parts. If it feels more like an engine misssing under load, a weak valve spring is a possibility. A weak exhaust valve spring could allow the exhaust valve to open while decellerating with a closed throttle, which would reduce the "braking effect" of that particular cylinder

I would say hard spots (heat glazed) on the brake band causing a friction problem.
What is the band material??
I would first reline the band and try...N.

I took the problem to be under trailing throttle with the car still in high gear. That rules out any problem with transmission drums/bands. The driveline is a more likely source. It probably means taking out the rear axle, and starting with an inspection of the U joint.

Allan from down under.

Allan,
I would love to know where Stephen has his right foot placed when going down a steep hill in high gear, especially in a Model T Ford... ..N.

Any thought's ?...N.

I have a '30 Model A that used to vibrate noticeably when decelerating in high gear against the engine compression. I installed a high compression head on the car and the vibration stopped. I remember seeing a post on the Model A forum about the reason years ago but can't remember what it was.

browning wrote: ↑

Sat Mar 20, 2021 10:06 am

I have a '30 Model A that used to vibrate noticeably when decelerating in high gear against the engine compression. I installed a high compression head on the car and the vibration stopped. I remember seeing a post on the Model A forum about the reason years ago but can't remember what it was.

Now that is something that makes some sense... On the complete balanced and blueprinted T engines I do, I cc the cylinder head. Sometimes the combustion chambers are pretty close, but many times the combustion chambers get progressively larger front to back or vice-versa. His head could be the issue. Of course, it still could be some errors in balancing. I’ve seen pictures of T flywheels that were professionally balanced and have seen some that just look like a plain single plane balance, not dynamic. It’s still a great improvement over not balancing, but sometimes doesn’t do a great job eliminating gyroscopic forces that can cause vibration at some speeds or power settings.

Have you tried varying the spark advance/retard?

Just throwing ideas against the wall.

I seem to recall an old Italian joke about something hitting the wall !!! And yes, I am !