The Stovebolt Site Forums The Shop Area General Truck Talk strange motion when driving

I think I'm gonna get my self out of the way and turn the focus back to Bill's truck. It was his thread and I think some of the replies meant for him are getting mixed up with mine. So one more time, sorry for butting in on you ya Bill and I hope you or one of the guys comes up with a solution.
Denny G
Sandwich, IL

I don't think he minds Denny. Although your trucks have different drive lines, troubleshooting the vibration is very similar.

Has anyone considered the harmonic balancer setting up a vibration at speed?

Denny I don't mind. There are enough good suggestions coming in that maybe we can solve both of our problems - hopefully.
Bill

Could it be a brake drum out of round or out of balance? I think you guys have covered all the bases with the exception of the drum itself. Seems like a long shot, but if the drum was out of round or out of balance, then it would stand to resaon that the smaller diameter of the drum would produce vibrtions or the oscillation described only at a higher speed. Too bad nobody balances wheels like they did in the old days,(On the vehicle)then you would know for sure if the vibration was wheel/tire or driveline. Too be truthful, I don't know how one would balance a drum without factory equipment. Maybe change out the drums when you do the next brake job. Good luck w/ this one, Joe

Good tip Joe. Still working on it.

Well I gotta jump back in for a minute and share the results that I found with my '50, 3604. I'd done all of this on the first set of rims and tires but never recorded and numbers so this time I charted all of my measurements.
Put her up on jack stands this weekend and ran it up to the magic 45mph mark and did notice some of that throbbing happening although not as bad as when it’s on the road. I next dropped the rear (open) drive shaft down and ran it up to 45 again. NO vibration. I reconnected the drive shaft and put an indicator on it and came up with about .010” to .015” RO.
Next I indicated the machined surface of the hubs just inside of the axle flange, both sides had less than .010” RO.
The rims were the next step, outside of the bead seat gave me .025” on one side and .038” on the other.
The really important RO measurement was the radial RO of the tire. I ran a 1” masking tape around the center tread for a track and got an indication for total RO of .035” on one side and .040” on the other. I orientated the rotation of the wheel to the hub to cancel out the high spots and since the high spots were not exactly opposite each other I only reduced the condition by a couple of thousands. Rotating the tire on the rim might give me a few thousands less RO but one must consider that the Roundness on a steel truck rim and production rubber tire probably isn’t going to be good enough to give you better results so it gets to be kind of a fruitless effort.
The last measurement was for the lateral RO of the tire, the results being .035” and .050”.

Now if I were going to run this truck at 150mph I would no doubt try to get these numbers closer or running at 85 or 90 on the freeway these number would be about at the top of the RO tolerance and one might feel some vibration. But at 45 or 50 mph this is well within tolerance.
So this leaves me with one last rotating part to address, the drive shaft, and I’m going to take it over and have it balanced this week.

When this doesn’t solve the problem then there is only one answer left. There does exist some Radial Run Out on every rotating system. This is going to set up a vibration at some point at the structures natural resonant frequency unless there is some method of dampening it. This is normally absorbed by the automobiles suspension. When I look at the original 7 leaf heavy rear springs on this 3/4 ton truck there isn’t much give at all and with them being 58 years in service there probably isn’t much sliding going on be twix the leaves. So I think what might be happening is that any run out of the tires is being transferred almost directly to the frame and at around 45 mph this frequency begins to resonate with the frames natural frequency of vibration and the two add together and create this amplified throbbing that I’m feeling.
There is one last thing to try, and that is to remove the springs, separate them, clean and inspect them real good and against the factory recommendations, lube or add those Teflon strips. This might help absorb some of the vibration from the slight yet inevitable tire run out and keep it from transferring to the frame.
Denny Graham
Sandwich, IL

Thanks for the update, Denny. Keep at it - You 'da man! I admire your troubleshooting. Mine rattles and shakes so much, I would be afraid if it rolled smoothly. (Kind of how parents are scared by silence - It's usually an indicator of trouble brewing.)

Denny I think I'll just sit back and let you do the grunt work solving this problem. When you have it finished let me know. In the meantime I'll be thinking of you working away as I chug my beer!!
Actually going back to the teflon strips between the leaves, I have done this. I bought a sheet of teflon and cut it accordingly. Some of my springs actually have notches in them from the spring above or below rubbing. However the ride is smooth.

Denny,

The throbbing you mention sounds like you are observing a beat frequency, where you are getting a periodic aiding and cancelling of two or more separate vibrations. The run-out numbers don't seem to be excessive, but if they stacked up on one side, it would seem possible to get that condition. What happens if you accelerate slightly or decelerate when the throbbing occurs? Have you tried moving the tires/wheels front to rear? Have you measured drive shaft angles and checked the center support cushion and bearing? (I haven't read the entire thread, so I'm assuming a two-piece drive shaft from your description.)

I also note that these are all questions, and no answers to your situation.

Harvester

Eliminate one thing at a time starting at the easiest.

Find a known set of chevy rims and tire and put them on to eliminate or secure those two possibilities.

You also mentioned that your lever shocks worked with different resistance. (Check the oil levels.) That could also be a problem, but with no weight in the back and probably not noticable at all speeds, it seems a unlikely to me.

The drive shaft seems a likely suspect next. Balanced and good bearings?

If you haven't pulled the rear end and axles (did I read that you hadn't looked at the rear axle bearings yet?), I'd clear my concience of the rear end/bearings being the possibility. What about the axle side play?

Builder rear axle bearings are on the list as they have not been looked at. The driveshaft is suspect but being in a torque tube could it still be a problem? The front bushing in the tube was changed.

No, acceleration or deceleration have now affect on it. it comes in somewhere around 42-43mph and peaks at about 45-46 then fades away after that. The amlitude is cyclic when you hold it at a stead 45mph with the vibrations at about 10 or 20 per second and the amplitued varies every second or two. This is what gives you the throbbing feeling.
Short answer, all new, I won't go into detail gain, it was all answered earlier in the thread.

You know it is really hard to compaire my ride to other stock 3/4 ton AD's because it's been 50 years since I was in another Chevy AD. I don't know anyone that has one around here that I can get to take me for a spin down the hiway. I would think that this would be a pretty common occurance with the unmodified trucks.

Denny Graham
Sandwich, IL

Denny, there may be someone close by willing to help you. Go to the home page, click on the stovebolt office, and then the stovebolt locater. Then look for someone close and someone might be willing to help you, if this is a problem for you.

Run out on the wheels and tires at 50 thousandths does not seem excessive to me and I think that if that was the cause of the vibration, it would not go away but would increase at higher speeds(revolutions). When you pulled the rear prop shaft and the vibration went away, I believe you have narrowed the cause of the vibration to the prop shaft and rear end. I had a GM Cutlass that had a weight hanging off the front end of the pinion housing that was there to remove vibration(but that was a coil spring car) I am now of the opinion you have a pinion angle problem or a bad U joint. This would cause a vibration at certain speeds, but not at higher or lower speeds. Have you put an angle gauge on the pinion and the carrier bearing? Maybe the prop shaft is out of balance; but again, I think if it was, the vibration would get worse at higher speeds. I can't believe the springs would have anything to do with it because even if the springs were froze solid, the shackles would absorb a lot of the vibration of something that was out of balance in the driveline. I gotta be truthful with ya, I've drove a many of these trucks 1/2 ton to 3 ton and I have never felt anything in the seat of my pants the way you guys are talking. Joe

Everything is stock in the driveline Joe, pinion angle hasn't changed in 58 years. Springs and hangers are intact, as are the pad and u-bolts.
I did drop the driveshaft off at lunchtime to have it checked for balance. I hope they don't butcher up a perfect driveshaft on me. It has no rust, no wear, no digs, no dents is not bent. It has been stripped bare and repainted and has new u-joints installed in a bench press not with a hammer and socket. And one more time, the U-joints are Brand New on mine unless you were addressing your comments to Bill!
Denny G

Joe my truck has the torque tube so the pinion angle is fixed. The only change I did to the drive line was to put in a new bushing and seal in the tube which meant unbolting my u-joint. However this problem was there before I did that work.
Now here is another neat thought which arose after you mentioned seat of pants. I do know that my seat cushion is quite springy as I bounce a good bit on a bump. But when someone is driving behind you and mentions the truck is going up and down it is more than the seat cushion.