Vibration travelling at speed

[LizardsA]

Good evening folks,
Seems like it's my weekly cry for help! I took my 1930 Sport Coupe down the freeway on Sunday, and any speed over 40 to 45 mph created a bone-jarring vibration which felt like it didn't come from the wheels. What could cause this?
Kind regards
Mike

[KGBnut]

I know you said it didn't feel like it came from the wheels, but that sure sounds like wheel balance to me... or possibly a bent wheel.

Rule of thumb, if you feel it in the steering wheel, it is a front wheel. If not, it is a rear wheel.

[newshirt]

While traveling at high speed, try pushing in the clutch. Do you still feel the vibration? If so, it may be the wheels. If not, it may be the engine, u-joint or other possibilities.

[columbiA]

A safe cruising speed for a bone stock A is around 45 mph.Due to the lack of counterweights on the crank shaft,they will vibrate around that speed.Prudent drivers would limit their speed to 45.If all internal parts are properly balanced,the engine will be very smooth,but without C- weights it is hard on the center main brg.To many drivers these days, push their cars beyond beyond there design limits.If you want to drive faster on todays roads,then install an overdrive & a 5.5 cyl head

[Mike V. Florida]

Originally Posted by columbiA

A safe cruising speed for a bone stock A is around 45 mph.Due to the lack of counterweights on the crank shaft,they will vibrate around that speed.Prudent drivers would limit their speed to 45.If all internal parts are properly balanced,the engine will be very smooth,but without C- weights it is hard on the center main brg.To many drivers these days, push their cars beyond beyond there design limits.If you want to drive faster on todays roads,then install an overdrive & a 5.5 cyl head

What facts do you use to come to this conclusion?

[Tom Wesenberg]

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I agree with columbiA. I'd want a higher speed rear end or overdrive, a counterweighted crank, and a 5.5 head if I wanted to drive 55. Without counterweights the center main takes quite a beating due to the heavy off center weight on a thin 3 bearing crank.

[Mark Earl]

I know there are 2 schools of thought on this but here goes: I remember years ago (maybe 25 years) my grandfather's 30 Roadster would not go over 40 mph without vibrating itself to pieces. He installed a Float A Motor kit and that solved the problem of the vibration. The car is now mine and it will run along at 50 mph no problem. Are my bearings still taking a beating and the FAM kit is just masking it?

[H. L. Chauvin]

Just my humble opinion:

With a stock Model A engine, original head & cam, original transmission & differential, & everything else original including a dead flat windshield with the visor scoop above; & the two (2) normal Model A open, front fender air scoops, I would be most hesitant to cross a tall, Mississippi River Bridge, in a mild 40 mph rain storm cross wind at one (1) mph over 50 mph unless I had five (5) extra heavyset Wal-Mart shoppers crammed in the Model A with me.

Not sure if all (5) could fit -- but, just a compassionate "Safety" suggestion for a new Model A driver trying to drive his/her newly acquired "stock" Model A at high speed for the first time.

[Tom Wesenberg]

Originally Posted by Mark Earl

I know there are 2 schools of thought on this but here goes: I remember years ago (maybe 25 years) my grandfather's 30 Roadster would not go over 40 mph without vibrating itself to pieces. He installed a Float A Motor kit and that solved the problem of the vibration. The car is now mine and it will run along at 50 mph no problem. Are my bearings still taking a beating and the FAM kit is just masking it?

Yes, that's the reason all crankshafts today have counterweights. Think of a wheel that spins free on the front spindle. It has equal weight all the way around and doesn't vibrate if it's in perfect condition. Now add a 5 pound weight on the outside of the wheel, and add another 5 pound weight on the inside of the wheel exactly 180* apart. The wheel will still be in good static balance, but spin it up to speed and you will have a lot of force trying to make the wheel go in one direction, and then the other direction. This is very poor dynamic balance. Connect two of these wheels back to back and you have 10 pounds of weight in the center wanting to bend the wheels one way and 5 pounds on each end wanting the bend the wheels the other way. It's the same thing with the Model A crankshaft without counterweights.

[LizardsA]

Thanks for the advice everyone, To get it straight, I'm not looking to push the old girl beyond 45, it's just that I noticed the vibrations which worried me a little. Saturday I am getting some different wheels, so I will check her out after that.