Vibration on acceleration when towing

[Johne0512]

Hi. I have a 2020 Ram 3500 Limited w Cummins (non H/O) w six speed transmission. it has 21K miles and I purchased it with 13K miles on it to tow my 14Klb 5th wheel w 2900lb pin weight. I've run the numbers and I'm within tolerances for weight and payload. When I'm towing, I get a rather strong vibration on acceleration starting right as it's rolling and lasting to about 20 or so mph. It does NOT do it when I'm not towing anything. I get the vibration regardless of the grade I'm on but it's worse the harder I press the accelerator. But it's worse depending on the torque needed to start pulling (so uphill is worse than downhill). I would be very grateful for any input anyone might have. Dealer can't replicate it because I can't pull my RV to them to show them. Appreciate the help!

 

[Riccochet]

Pinion angle would be my guess.

 

[Lettikka]

Get a go-pro and set it up looking at your driveshaft. Reproduce the issue and check the video. If you cant see anything there aim it at the rear axle and see what you get. It is certainly in the driveline and would think that someone who knew these trucks could find something wrong without having to reproduce the issue. Your description of it is pretty clear. My jeep does the same thing but only during take off and a little through first gear. I know it is my pinion angle. Anything done to your machine suspension wise that could change some of your geometry? If not I would look for something broken or loose. The video would help id the issue and could be taken to the dealer for them to review.

 

[Johne0512]

That's for that feedback. When I bought the truck, it had spacers in the front that were about 2.5" to 3" that I had the dealer remove when I bought it. But that was it with the suspension. I'll have to get a go pro. Much appreciated.

 

[Riccochet]

You might also want to look at your rear spring perches for angled shims. Previous owner may have installed shims to adjust the rear pinion angle when they did the level. Vibration during acceleration is almost always a high pinion angle, where vibration during deceleration is a sign of a low pinion angle. Could also be a worn pinion bearing, center bearing or u-joint.

I'd take to a driveline shop and have them check everything.

 

[Lettikka]

You might also want to look at your rear spring perches for angled shims. Previous owner may have installed shims to adjust the rear pinion angle when they did the level. Vibration during acceleration is almost always a high pinion angle, where vibration during deceleration is a sign of a low pinion angle. Could also be a worn pinion bearing, center bearing or u-joint.

I'd take to a driveline shop and have them check everything.

Ric may be on to something.... When I bought my 2007 GMC from a dealer it had been traded in with 5000 miles on it. The dealer had pulled the wheels, tires and lifted suspension off the truck and returned it to "stock". I found remnants of the aftermarket suspension in the mix over the years. Nothing harmful but it was still there. I wonder if yours had a lift that was pulled off and the shims at the spring perches were left in there?? This would change your pinion angle and could cause what you are describing. They will look like triangle wedges of metal between the axle and the spring.

 

[Johne0512]

Interesting thoughts, Guys. I'm assuming you mean the leaf springs (as there are no coil springs)? Thanks!

 

[Riccochet]

Interesting thoughts, Guys. I'm assuming you mean the leaf springs (as there are no coil springs)? Thanks!

Yes. The perch is where the spring sits on the axle. Some call it a seat, I call it a perch, potato tomato. Some kits use shims there to raise the pinion angle of the axle. There may have been a lift on the truck at some point and those shims may not have been removed.

 

[Johne0512]

Great. Thank you! Will check it out.

 

[JayLeonard]

When you change the angle of any universal joint you change the dynamics. The 2 joints need to properly phased ( angle of one to the other).
I worked as a design engineer for the company that makes the steering intermediate shafts for these trucks ( along with many others) and phasing is critical. We had special software to get it correct.
Driveshafts are even more critical because of the speed they rotate.

 

[Johne0512]

Had it checked and no remaining lift hardware. Only other likely culprit (other than u-joints which look good) is squat. Going to add airbags to adjust the squat when the 5th wheel is attached and hopefully that will keep everything more in alignment. Wish me luck!