The Stovebolt Site Forums The Shop Area Engine Shop Vibration at 2500 rpm

My 1942 1/2 ton Chevrolet PU has the original front axle and spring assembly with knee action shocks.

At around 2500 rpm, the front end starts to bounce....I have had the tires rebalanced and would like to know if anyone has an idea as to what would cause this.... It lets up when I reduce speed....

Ted Talbert,

Sometimes folks use terms loosely. Your post indicates the front end/wheels bounce up and down. This is different than a shimmy felt in the steering wheel...right!

If this is actually a bounce, then it seems like it would be a out of balance issue with the rotating tire or tires.

When you had your tires rebalanced, were the spin balanced? Is it different now than it was before the balance?

If this is really a shimmy situation, I think jacking up the front of the truck and giving it a inspection would be the plan of the day!

A tire separation might cause havoc when on the rig, but maybe not on the wheel balancer.

My feeling is that bad shocks aside, the tires should run smooth by themselves. Keep us posted!

Stuart

The tires were spin balanced at the local Goodyear store...front and back..

I feel the vibration in the steering wheel for sure... was wondering how you check the knee action shocks....they seem real tight to me...

The king pins and front end was rebuilt by Yesterdays Chevy Truck Parts and I believe the front end is tight...

Ted Talbert,

I am not familiar with knee action shocks. My uneducated guess is that they should offer resistance in both directions similar to a modern shock.

If a steering component is loose or worn, play would be felt in the steering wheel and the truck might have a tendency to wander around.

It's my feeling that a shaking or vibration has to come from a out of balance rotating mass.

A vibration isn't the same as bouncing. The engine could produce a vibration and transmit it through the cab to the steering column.

If all front end components are relatively new, and are adjusted properly...this includes the steering box and wheel bearings, I would still suspect the tires.

Stuart

P.S. You must have a tach in your truck! How fast in MPH is 2500 RPM?

if it's a vibration and your front ends tight. i'd check your u-joints. i've had em go bad and it'll vibrate the whole truck.

It runs 60 mph at 2500 rpm

seems to me like that is usually the speed that you run when you pick up tire type problems but then I dont have a 42 model truck..does seems strange that it would start there,, what is the top speed you drive it? you might slip it out of gear at that speed for a moment and see if it stops.. if it does, then I would think it would be in your drive line..

Joker,

Does the drive line stop turning when you take it out of gear!


Stuart

I had been told one could test with this method,, also here is another guy that feels the same way. per his quote "That sounds like a good plan. Additionally, I would drive the car at the speed where it vibrates and put the transmission in neutral. If the vibration goes away then it's in the drivetrain. Try to determine if the vibration changes when you are on the throttle and when you're off. Rev the engine in neutral to see if you can duplicate it (while driving down the road)."

Switch one front tire with a rear tire.
Drive it and if there is no change do the same with the other front tire.

This should eliminate any question about properly balanced tires.


Straight front axles with any worn suspension joints are more sensitive to slightly unbalanced wheels or tires.
Check king pins, bearings, spindles, drag link, tie rod ends...


It can be hard to find a tire place with a good and calibrated balancing machine.
Many tire places think an 'old and slow truck' doesn't even need balancing.
And then you got to find an operator who takes the time to do it right.

When in neutral the driveline in unloaded, no torque on any of the joints, thats why it makes a difference. Have someone that knows a little something drive next to you and watch the tires for vibration at speed.

On the road again has it right in finding a possible tire issue quickly, move the fronts to the back one at a time and see if the problem goes away. A 60 mph vibration is most commonly a balance or radial force variation issue. If you come back to a possible balance issue, have your wheels balanced on a road force variation balancer. The balancer loads the wheel tire assy during balancing to simulate the load placed on it during operation. This will find a fault within the tire a standard balancer cannot. The balancer will give the force variation in pounds. I believe Hunter reccomends less than 36 pounds variation for a light truck and around 25 pounds for a car. I don't know how sensitive your truck is, on front wheel drive cars we shoot for less than 10 pounds to eliminate vibration complaints. A straight axle truck should be OK with much higher numbers.

John

Are the shocks knee action or lever shocks? I understand that leakage of the hydraulic fluid is the best way to tell if the lever shocks are any good.

I had a vibration at 50 mph coming from the rear. I found it on the balancing machine but I also jacked the rear end up a few inches and ran it in gear. You could see the tire was out of shape by the way it jumped up and down.

Some balancing machines use the wheel center hole to mount the wheel/tire on the machine. If your center hole is out of line or elongated you could get a faulty reading. Try to find a balancer that has the fingers which go thru the lug nut holes.

Ted,

What tires are you running?

I had VERY OLD bias ply tires on my 46 when I bought it. The truck had been parked in the same spot for so long, the tires would never return to round. The truck shook so bad in a curve that it would just about throw the truck off the road. I bought new tires and the problem was fixed. One shock link was off at the time, but with new tires, it did not matter...it rode 100 X better.

46 in Progress - did you buy bias or radial? I want radials on my 49 but the dealer says my narrow stock rims would make the combo unsafe. I am not against bias-ply if I can get some good feedback on them

Do truck tire shops still have dynamic balancers to balance the wheels in-situ? That was the only way I could get some of my cars and vans balanced in the 50s and 60s. This could be an unbalanced drum issue.

Ted,
I am confused. Does your truck shimmy and shake? Or does it bounce up and down? Does this happen on a straight and level road? Or a curvy bumpy raod? Also, did this ever happen before? Shimmy and shake on a level road USUALLY means an alignment issue or u-joints. Bounce up and down USUALLY means something is "out of round". Need more info!

I am running new Denman bias ply tires... the vehicle seems to have a bounce..up and down..not side to side at 2500 rpm or 60 mph. I think it might be the drum also but have not been able to find a shop that can balance the tire, drum and all on the car.. We used to have one here in town but the mechanic died a few years ago and none of the yound guys know how to do it.

take out the battery of the vibrator '' just kidding. let us know when you find out..

The vehicle just started doing this...happens on straight and level road as well as bumpy road.. bumps just make it worse. Still does it when you push in the clutch or put it in neutral at 60 mph.... making me think it is not the drivetrain. I cannot feel the vibration up the floor gear shift so I think its the front end somewhere... Kingpins have been replaced about a year ago..does not wear the tires unusually.

those bias ply tires are out of round - do as suggested and switch tires front to rear and I'll bet the problem changes .... but won't disappear completely

back in the day it wasn't unusual to find out of round tires and there was even a fairly common machine to 'lathe' the tires on the vehicle so they were true

Bill

Here is an old table about vibrations from an article by Bob Adler in Pickups 'n Panels in Print. The table was supposedly found in old shop manuals. I would assume the speeds would be for around a 4.11:1 rear end.