The Stovebolt Site Forums The Parking Lot Big Bolts Still shakes like paint mixer between 30-40 then smooth as glass 40+

I dont get it.. why does the truck shake and hop 30-40? And then smooth? Fron t tires balanced and shaved for perfect roundness, even installed shocks on front which helped hard bouncing a lot. But why the shake? Tires lowered to 70#. Could it be something else? Bent or out of balance drive shAfts? They are new but long story short, the axle came loose before it was welded - on first mechanics test run 6 mos ago and broke the drive shaft bolts but the said it all appeared fine and bolted back up. I imagine if the drivestaft or joints were damaged it would be ar any speed. I could take air pressure down more but why only at certain speed range?

Let me add , new kingpins and aligned.. all radials, all four rear tires new and balanced also. Put in neutral still does it at that speed range.

38 1.5 ton Are your drive shaft yokes in phase ?? If it was that don't think it would clear up like that. How about toe-in measure it for no more than 1/4 in on the front. Have a friend who understands what you are looking for follow you maybe he will see it. Nothing out of round ?? They say fan blade can really shake,never actually caught one,saw where it flew thru the hood long ago. Carrier bearing secured and in good shape,drive shaft in balance,pinion bearings,in good shape ?? Harmonnic balancer rubber in good shape and round ?? Are pressure plate bolts shoulder bolts that center the pressure plate ?? We used to line each others wheels with balance weights half way around don't think it's that !! Let us know when you find it !!! How about tires and wheels that are not turning in a true plane,like lug wheels can do if not tightened evenly,dirt or rust in the way. See if rear tranny main bearing is in good shape,what about a thick and thin brake drum,saw a mud dobber nest on a drum somewhere truck setting for a long time. Sometimes big rocks get stuck between the duals. saw a pair of wheel weight pliers left in a 10.00-20. We used to spread big tires and hold open with a big line wrench to work in there . how about too many balance beads in a tire ??

I had a similar problem with a mid 70s pick up
You could carefully drive it up to 35 mph but hit a mild rough spot in road or just look at it wrong and suddenly a lot of wobble and oscillation
Going faster 44-55 mph it would calm down and stop
But a critical speed range was terrible

Turns out it was a combination of a soft wall bad tire and the shock absorber not sufficiently damped and the outside of the rubber tire would get out of round with the rim center and wobble at the resonance of the suspension and shock

I put new shocks on and better tires and never had a nicer ride afterwards
-s

Cracked/Broken frame.

Follow your gut, have the driveshaft/s checked and balanced.

Thanks for the ideas. It did this before the shocks were put on. There is no physical evidence of a broken frame unless its somewhere at the molecular level. Its been welded in a few areas with new bracing - if anything - making it stronger. It is aligned well. I am going to have to defer this one to the shop to figure out - yes perhaps starting with the driveshafts..

I'm voting for driveshaft issues. Vibrations are crazy things, with harmonic frequencies, etc. I still remember a lab in college with a coil spring clamped on a shaker table. It was smooth as silk until it hit the harmonic frequency of the spring, when it went wild.

The weird thing if I may add, its a bouncing/shaking of the front end; not like before after hitting bad bumps but its like there is a gorilla shaking the front of the truck at that speed bouncing it. Just weird; then like glass, like a cadillac - real nice ride, predictable - surprisingly for this type of truck at 40+.. <-- this is after adding the shocks and rounding the tires.. yes I will report back because I WILL fix it - God willing...

Sounds like death wobble. Check the alignment again, and the looseness of tie-rod and drag link joints. If you have the old spring ball socket tie-rod ends, those would be suspect. Can you stop it by making a turn?

When you had the tires shaved were they HOT from a 20 mile drive or cold? If they were cold, they might not really be round. If you shave a cold tire to get rid of the "flat spot" the next time the tire warns up that extra rubber that was pushed in becomes a new high spot as it flex's back out to normal. The process for shaving tires is to drive the truck to get the tires hot and immediately jack the truck up to take the weight off the tires. After it's in the air you can take your time before shaving as you won't be creating a flat spot.

Find a shop that has an old fashion tire balancer the spins the tire on the truck. This will balance the entire rotating assembly and also confirm the tire is really round.

Mike B

I'll have them check it all over again. Yes all that stuff was apparently tighten and checked..

just to share one of my experiences... I had a 79 C70 twin screw that was similar. Around 30'ish it would begin shaking. The faster I went, the worse it got. Not a vibration... but SHAKING! The hood was bouncing around and you could not keep anything laying on the dash or it would come off. After 40-45'ish... it would smooth out and be smooth as glass at any speed above 45-----
Really thought it was tires, but after new tires installed - same issue.
The truck was un-usable because of this shaking.
Found the problem when I went to a wheel upgrade on the rears.
This truck had 20 inch daytons installed and when I changed over to 22.5's - it all stopped.
I could see a "slight" movement or "wobble" in a pair of the old wheels on the back, but never thought it would cause that.
I am thinking I was incorrect on my thoughts.

Good luck.