Bucking/Shuddering When Towing
 

I just got my truck back today, with a new block (see Back In The Saddle on 24V engine). I left my car at the shop and drove the truck home, hooked up the trailer to go get the car. I now notice a bucking/shuddering, kind of like riding on a washboard, at 30 mph, then at 40 mph, perfectly smooth road. It happens whether I am cruising, accellerating, or coasting. This did not happen before I had the engine out to get the block replaced. I'm pretty sure the new block is a heavier, since I had a thin-walled "53" block in there. The new block has thicker walls and should weigh more, of course.

The only thing I can think of that might cause this is a broken tranny mount. I suppose a driveshaft balance problem could cause it too, but then again, this only happens when towing.

I have Rancho RS9000X shocks on the truck and they were adjusted about 3/4 tight, out of 9 positions. I tightened them all the way in mid-tow, and it did not seem to make a difference. Having the car on the trailer did not seem to make much difference in the bucking.
All tires on the truck have 57.5 to 59 PSI. The trailer is a 20 ft. flatbed with a bumber hitch. The trailer tires, 2 axles worth, all have 50 PSI in them. Trailer is 2K empty, and the car weighs about 3K.

Anyone know what's going on here?

Thanks,
Chris

I would take it back to the shop that did your engine work asap....could be a loose mount or a drive shaft not timed or a bad u-joint....have the shop look it over and drive it with you to show them what the truck is doing

Agreed. Take it back, have the tech ride with you. He will know right away what it is. Could even be tire balance.

You think they will know? This is the Cummins dealer, not a Dodge dealer. I figured they'd just know the engines.

Chris

This sounds a little like a torque converter is this an auto? torque converter trying to lock-up ??

The TC lock-up is very noticable as it is an ATS Triple-lok with an Emjay tranny controller. The TC locks at 35 mph. The shaking starts at 30, clears up by the time the TC locks up, then starts again at 40. I also notice it starts gain at 65 mph. When I posted the other day I had not yet had it up to 65 while towing. When at 65, everything's locked up and we're in OD. But keep in mind that the shaking happens whether I am pulling or coasting, so it probably is not related to a slipping part in any of the drivetrain. I still think the only possibilties are a mount is loose or broken, or there is a driveshaft problem.

Chris

Still needing help here. I took it back to the Cummins dealer. They guy there said they drove it and dynoed it and the shuddering went away when they put it in neutral. That actually isn't so. I put it in neutral when I drove it home, trailer attached, and the shuddering did not go away, but I went one further, and cut the engine off when in neatral, coasting down through 40 mph and 30 mph. The vshudder is still there when coasting down through those speeds.

Now I have to figure it's a trailer problem, since the Cummins dealer says there is not problem with any mounts. I wonder if I have a square tire on the trailer. All of the tires look OK. If I had a broken belt in a tire I'd think I have a shudder/vibration from it all the time, once I got up to any significant speed.


I guess the next step is to hook the trailer up to my neighbor's truck and see if that truck has the shuddering at the same speeds.

Thanks,
Chris

I've also encountered some weird shaking, vibrations, bucking, and jerking pulling different types of trailers. Sometimes it's common to experience such elements. Most of the time, the elements quit after the trailer is loaded or shift the loaded weight on the trailer forward or backwards to make the haul more comfortable.

Does it shake and rattle not pulling the trailer?

Do you have trailer brakes?

Yes, I have trailer brakes. This is a 20' tandem axle unit with electric brakes. The shudder is slightly different when loaded, but doesn't go away.

I found today I have a bad tire, but I do not know when it went bad. Although all of the tires have plenty of tread, in fact they look new, they are dry-rotted some and one of the tires lost a large chunk of tread. There are belts showing on that tire now.

I will need to get that fixed before I continue diagnosing this problem, if the tire isn't the problem.

Tommy,
Truck rides smooth as silk when not towing. I pulled another fellow's trailer today, with NEW tires on it, as of Friday, and it did the same thing. We looked at the tire pressure on it, lowering from 60 to 50 psi, and found one of the brand new tires has a knot in it. So he's going back on Monday to get them to replace that tire.

Thanks,
Chris

I thought about another culprit that contributes to vibrations. If you pull a trailer that has been setting around over time the tires will develop flat spots and it takes a few miles to let the tire warm up enough to smooth.

Some trailers have "Trailer Only" tires on them, which are load d or e tires like camper tires, they have thicker sidewalls and ride rough during winter trips. They're also harder to keep balanced and it usually takes a lot longer for them to smooth out.

Some have old car tires, which are b range. they ride a lot softer and smoother