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

When driving my 49 at any speed I get this strange motion coming from the rear end. Difficult to describe except friends following me say the back end is jumping up and down a bit and I get that same bobbing feeling through the seat. It is not a rough motion in fact very slight but you can feel it and it makes a drive quite tiring.
I have 4 new tires, all balanced and the wheels are fairly true. The truck had lever shocks on the front which I changed to tubular. The rear lever shocks are still in place but are dead. No resistance at all.
Could the lack of rear shocks be the cause? Could it be in the driveline?
Thanks.

I could see it being the lack of shock absorbtion in the rear. My brother's 66' needs new shocks and it feels pretty bad in the truck, I've never followed him, so I can't describe if it is like what your buddies have for your truck.

Morning Bill, what your describing is exactly like the problem I’ve had with my 1950, 3604, which is completely stock. At around 45mph I get a vibration at the rear, I can see the tailgate hopping up and down in the mirror and feel the hopping all through the truck. I don’t have any buddies any more, their all dead or can’t drive anymore so I don’t know if it can be seen from the rear but I can see it.
I first thought it was the 15” split rims, which were in almost new condition since the truck had spent most of its life in the fire barn. I broke them down and discarded the old rubber, blasted them and PPG’d them with DBU, mounted brand new Power King bias plys, tubes and flaps. I found a dealer that had the newest dynamic load or Road Force balancer and the tires/wheels runout checked out OK. I indicated them on the truck to see if the spindles or axles were bent, OK there. All that didn’t help. Found a real nice set of 16.5” tubeless rims did all the same as above to them as far as prepping the rims and mounted new Coopers up on them, and had the Road Force balancing done, that didn’t stop it.
I put new double action gas shocks from JC all around, that didn’t help any.
I put a thousand lbs of bagged sand over the rear to see if it would change the mode, that didn’t change a thing.
I had the complete driveline out, stripped to bare metal and inspected for damage or wear, all was like new there. I rebuilt all of that with brand new u-joints all three of them, replaced all bearings, bushings and center support rubber. Dial indicated the open drive shaft for runout, OK there. I put it up on jack stands and ran it up to the speed that the rear end hopping starts at, that be right around 45mph, and it was fine on the stands, no back end throbbing or related vibration other than the normal vibration one would expect from a machine running at 45mph. So that tells me that it I’m back to something to do with the tires/wheels when they are on the ground.
Every bit of the front end steering and suspension is completely rebuilt and aligned, there is no slop in the steering and there is no oscillation in the steering wheel at any speed.
My final conclusion is that it is a resonance node that sets up a vibration at around 45mph, unless someone comes up with a better idea. I’m really looking forward to getting the cab and box off the frame and see if there is a problem somewhere in the frame that I can’t see with everything mounted up.
Good luck and please do keep us posted if you get it figured out.
Denny Graham
Sandwich, IL

I had this problem on a nice Mazda I once had. Strangely enough I had just put brand new tires on it as well. The tires were balanced perfectly but it turns out that they were not round. when I jacked the car up and spun the wheel by hand, there was almost a 1/4 inch variation. Apparently this was right from the factory. You might want to check that. I was given a set of replacements.

Thanks for the input. I also had this same motion before I changed the tires too. After thinking about it I guess the best description is like an oscillation. Its not harsh, just a wave like motion.
I will swap front wheels to rear and check that out.

Bill I get the same things SOMETIMES too I put it down to the road condition... Does it happen all the time or just on certain roads in Victoria. On certain areas of West Saanich i get that bumpy feeling..Peter

Peter
Good question. It is there even on the smoothest roads and at any speed. Very mild mind you. I do feel it on West saanich Road as you mention.
Bill

Denny,
do you have any contacts in the garage business? Where we live, we have to get our newer vehicles emission tested yearly and they run them on a roller affair, like a dyno. If you could get a run on a set of them, maybe you could see the hop.

Fred

There is a possibility it could be the wheels, but not in the way one might think. Sometimes the lug holes won't be concentric with the wheel. That can cause the problem, especially if you've changed the wheels to a non-oe wheel. There's 2 different types of centering on wheels, lug centric and hub centric. many older wheels are lug centric, meaning that the wheel is cetered to the hub by the wheel studs. That style is not very accurate. Most, if not all, newer vehicle use a hub centric, or hub piloted if you will, style wheel, using a center hole andhub pilot that are machined to center and fitment. I know that if you change wheels on cars designed for hub centric wheels to lug centric or do not use a hub centering ring, they tend to have the very problem decribed.

Well, I can certainly tell you that no shocks will cause a vehicle to bounce even on smooth roads. My grandma had a car with no shocks at all and if you hit one rock it felt like a pot hole. She jumped the railroad tracks one day.... Didn't feel good. Who knows, maybe replacing those two shocks could fix the problem.

I'm thinking it's the shocks too.

Bill
These are OE wheels and I never checked to see if the lug holes were elongated or enlarged. When they balance the wheel would a problem like you describe not show up? I can't recall how they balanced them, with the wheel centre hole on the balancing mandrel or if they actually used a 'finger-like' attachment. I know that whichever way they did it the set up balanced nicely.
Bill

they Balance using thr centre hub hole...

Sounds like an out of round tire.

The out of round tire was thought of. I had an out of round tire previously and it was very noticeable so this is not a new problem although better than it was. When these were put on the tires checked fine but I wasn't there and the fellow who did the work is one who would tell me if there was a problem with the tire. He is very fussy. In fact he is so fussy and meticulous that I have handed over to him almost all of the repairs on the truck, even things I used to do myself.

Worn spring shackles, loose shackle mount rivets or a cracked spring center bolt will also cause the symptom you describe. Other than all the previous great sleuthing I'd just adapt and make milkshakes while travelling down the road. ;o)

Sorry I neglected to mention it but the shackles/bolts are all new. It is not a rough ride, just an oscillating motion that after a while could put you to sleep.

This morning I pulled both lever/arm shocks. One has a lot of resistance when pushing down on the arm and the other has some resistance. Somehow I am thinking of ruling out the shocks.

Maybe that could be the reason that it is slight? Or part? If one had a LOT of resistance and the other one didn't that could be the reason that it is just barely happening. As mentioned before it could also just be the resonance or vibration in the frame.

I have read this thread with some interest and have a few questions. Are they nylon ply tires? When the tires were balanced, were they checked for roundness? You can balance a square block, but it won't ride good. If they are nylon cord bias ply tires or even nylon cord radials, I'm leaning toward a tire out of round or a bad (or flat)cord. Its odd that it only happens at a certain speed which would lead me to suspect a tire, wheel, bent driveshaft, bent axle housing, axle bearing, or bent axle. I think if it was bad shocks, it would do it any sustained speed after hitting even the smallest bump in the road. Joe

The tires are new Firestone Deluxe Champion made from polyester I think but definitely not nylon. Being bias-ply I was expecting the cold ride bump but they don't even do that. They are smooth.
This problem occurs at all speeds and again I want to stress that it is not a severe ride, merely a soft oscillation. I am thinking as you, maybe bent axle. It is a torque tube and the bushings at the front end are new.

Here's a thought. Could it be that the wheel just isn't tight. I know that I drove a jeep with loose lugnuts and the wheel ran true except at certain speeds. I'm sure that you've checked the lugs but is there any play in the wheel at all when it's on the jack stands?

Bob I have checked that as well. Everything seems tight. There is a company in Vancouver which apparently makes old wheels better than new. I may consider going to them just to rule another thing out.
What Denny Graham said earlier in this post is starting to make me wonder. Maybe there is some strange oscillating node which will never be detected. But with help I'll keep on it.

Well Bill, I certainly hope your find the cause, and if your do please post it for the rest of us, especially me! I'm at my wits end, I've tried and checked every thing and I'm flat out of ideas and that taint like me. I just don't know what sort of method one would use to isolate the source.
Denny Graham
Sandwich, IL

The best way I can decribe it is that the wheels should be mounted to the balancer the same way that they are centered to the vehicle. A lug centric would use a plate with aligning studs that mount to the lug holes, a hub centric would use a cone through the center hole. If it's not mounted to the balancer correctly, that can cause the balance to be inaccurate.

Bill now that I recall my balancing was done with a plate and aligning studs as you describe.

It seems like it might be either a bent axle or a bent drive line, or something is binding up in the rotating assembly, maybe a bad bearing race or a bearing race that has worn out of true. If it was the tires you should be able to make the oscillation change by moving the backs to the front.

Here's another thought- how are your motor mounts? The front of the torque tube is attached to the back of the transmission, and any slop in the front or rear motor mounts would telegraph right back to the back axle. The angle of the torque tube has enough lever action against the pivot points on the axle to be able to move it vertically slightly. You might try driving with your floor cover off and see if you see the back of the transmission moving around with the oscillations you feel.

Jeez I really feel like I'm steppin’ on Bill's toes here or stealing his thread, but we are experiencing the exact same phenomenon so that’s the only reason I’m bringing up my symptoms also. In my case, last year when I started going through the mechanical work on the driveline I actually started at the front. New front motor mount, two new rear motor mounts, all new u-joints, bushings, center support bearing and rubber. Originally bought the new Power King bias plies, and balanced them at three different tire shops. I swapped them front to back; I rotated the wheel on the hub one lug at a time to see if that made a difference. In desperation I caved in and put a new set of wheels and Cooper tubeless radials on to no avail. All new front wheel bearings, properly adjusted according to specs. New bearings in the rear full floating hubs properly adjusted according to specs. I inspected the rear axles while they were out and they were like new, and everything went back together just as it should with no binding or forcing. And once again, new gas shocks all around. I can’t think of anything more to I can replace other than the truck itself.

Trying to describe a feeling is like trying to describe a color; ya just gotta see it to fully appreciate it. This oscillation, vibration, hopping, tailgate bobbing, throbbing or whatever label one chooses for it is still there. I’ve been pulling what little hair I have left out over this one for a year now. It really makes for an annoying ride as it comes in right around the cruising speed, that being limited to about 45mph in my case.

The thing that shoots my theory of the frame flexing down is that Bill describes the exact same problem with his 1/2 ton as I’m having with my 3/4 ton, two very different frames. And apparently no one else in the world has this sort of a problem with his or her Advance Design truck.
So Bill I really can understand your frustration.
Denny Graham
Sandwich, IL

Have you guys tried hit "highway speed" with the axle in the air?
It will oscillate just like on the ground if there's a problem in the driveline.

As Denny says;

"Trying to describe a feeling is like trying to describe a color; ya just gotta see it to fully appreciate it. This oscillation, vibration, hopping, tailgate bobbing, throbbing or whatever label one chooses for it is still there. I’ve been pulling what little hair I have left out over this one for a year now. It really makes for an annoying ride as it comes in right around the cruising speed, that being limited to about 45mph in my case."
Its so true.

I have good motor mounts front, rear and transmission. Good front bearings, not seen the rear ones. On a smooth road the truck runs smooth but still has that same wave-like motion, albeit like always very gentle.

And Denny you are not hijacking this thread at all. Somewhere there has to be an answer and then we'll all be happy.

In my case Dave, page one, second reply ; "I put it up on jack stands and ran it up to the speed that the rear end hopping starts at, that be right around 45mph, and it was fine on the stands, no back end throbbing or related vibration other than the normal vibration one would expect from a machine running at 45mph."
So again, in my case, it's not the drive line that's setting up the vibration.

Yeah Bill, when I'm running down the road at 45mph, when I yell at my wife the "vibrato" makes it sound like I'm singing to her!

I've been giving some thought to getting someone to follow me with a camera and film it. I can then take it home and study it and see if I can isolate where it's coming from.

Also it is cyclic pulsing a couple of times a second. This is, typical of something being out of balance, but I'll be danged if I can see where.

Denny G

Denny, Seeing that your at the end of your rope with this and all the stuff you've done and all the other thoughts on this.
The only shop I've ever fully trusted with frame,alignment and balance ( and this goes back to the early 60's when Carl ran it and now his son Curt has it ) is Hartman's Beeline in Elgin on W.Chicago St. 847-695-7755 He has or did have the only on vehicle tire/wheel balancer in town. He's one of those guys that don't let anything slide by. It's at least worth the call to see if he has any suggestions or has come across the sort of thing. If he hasn't ask him to check with his dad who worked with vehicles of this age.
In all the years and countless stuff I took there I was never unhappy with what they did for me. And don't be pullin any more hairs out ! lol
Dan

I vote for shocks or tires. Put the rear on jack stands and let it idle in 1st and hold something flat against each tire and check it for round. If not that replace the rear shocks.

Dad would always use a dial indicator that would magnetize to the body/frame to check if wheels were true/out of round. Did you have the truck on stands, and jack the center of the axle up to it's normal "driving" position? I like the idea of having a spotter with you(on the road or on the garage) or have them in the truck, and you be the spotter. Your spotter could tell if one side is hopping worse, and if the axle is tracking properly. A dry cup in one u-joint could also cause the vibration, or a mis-aligned drive shaft/axle. Good luck, guys.

I have been following this discussion out of curiosity about this problem. I haven't seen the flywheel mentioned, and you guys seem to have tried most other things. Is it possible that the flywheel is not true to the crankshaft or the surface is machined at an angle other than 90 degrees to the crank? Is the drive shaft yoke on the front of the rear end true? I don't mean to but in but these things might bear a look. I'll keep watching to see what you find.

best wishes, Les

Hey Dan, thanks for the tip, I get up to Elgin often, I worked at Fermilab for 27 years and my boss was from Elgin. I’ll stop in the shop next time I’m up there and see what they have to say. Thanks again.

Big mine has new gas shocks 2000 miles ago.

Don’t know if Bill has checked his but mine was supported by the axle and I had checked the tire and rim runout last year with a dial indicator like David suggested. And once again my u-joints were new 2000 miles ago, as was all the work on the driveline and the truck gets a grease job every 1000 miles.

Naw Les, you would be having clutch troubles if the FW was loose and besides that would show up on the jack stands with any other driveline problem.

Denny Graham
Sandwich, IL

Good points Les, thank you. About 4000 miles ago I put in a new clutch. I had the flywheel refaced at an automotive machine shop. New pilot bushing too. If I am correct the flywheel can only go on one way due to the bolt pattern.

Three years ago I put in an Okie Assembly in the torque tube. I bolted up the u-joint as it was, not thinking about having it 90 degrees to the other as there is no other. The bushing I pulled from the torque tube was badly worn and almost oval shape.

This problem was there before I changed tires and I have checked the trueness but didn't use a dial indicator. It was a yardstick mounted on a stand. It did show me that one of my previous was really out of round.

Anyway I am still thinking wheels or axle, even though the wheels seem true. I will have someone drive it while I follow or have someone take a video while I drive it.

As to the ride, it is for the most part probably one of the smoothest riding AD trucks around. People who ride in it cannot get over it. Other ones I have ridden in are not as nice BUT there is that mysterious little oscillation that has been the subject of this thread.

Bill

Denny, I didn't mean loose, I meant resurfaced so that the mounting surface and the clutch surface were not dead true parallel to each other. I was thinking that might cause something like a really slow "chatter" if it was only a few thousandths. Just a thought.

Les

The same would be true of "slop" in the transmission, right? I've even seen engine vibrations give a "road humm" before.

Denny - do you use a hand pump grease gun? If so - stop in J-Lube next time you are out in the truck, and get a "lube only." A lot of times their pneumatic grease guns may be able to push the grease into places where hand pumps can't. Be sure they catch all the fittings.(Isn't it around 20?) It's odd that the oscillation disappears when the load is off of the truck, then comes back on the road.

Driveshaft phasing? If you pulled the shaft to swap all 3 u-joints, did it go back together in the same splines? I had a driveshaft built for a project once were the u-joint was off by about 1 degree. That caused a pulsating vibration as described, however at about 55mph. It was also bad enough that the shifter would vibrate to the same frequency that I was feeling with my backside. After going down the same paths, I took the only thing left to check to another driveshaft shop. They repaired the shaft, balanced it to a higher spec and my cyclical vibration went away.

Just a thought

Scott