M939A2 / M923A2 bucking/bouncing at 25 to 35 mph

[Tinstar]

Well 90 miles sure would have done the trick if that was it!

 

[Alexsha]

Well I finally was able to pick up some 12t jack stands and 20t bottle jack and check the front axle. The 6x6 is plumbed correctly and is engaging/disengaging correctly, so I'm not running 6x6 all the time. I'm thinking at this point it's either a tire out of balance/round or the ctis not keeping even pressure. Ctis was next on my list of things to check anyway since I get the 5 light flashing error. Going to try and adjust the governor up to 130psi from 120psi and see if that kicks the ctis back into line.

 

[98G]

Slow speed and short trip will not smooth out the flat spots.
A nice trip that allows the tires to heat up and higher speeds

Lol!!! I drove that truck 800miles in 2 days at highway speeds while pulling my 1 ton Dodge behind it on towbars. This included through the mountains.

I'm guessing everyone's tolerance and/or detection of buck and shake is different. It felt the same as my m925 does on super singles. They all try their level best to beat you to death as far as i can tell. Alex ' s truck has 14r20 Michelin xzls with about 2500-3000 miles on them. No idea on the date code.

Or it may be doing something different now. Would be interesting to have two of these beasts side by side to make a comparison over the same route at the same speed. If a pronounced difference was noted we could start swapping parts until we isolated the cause. This is why I feel obligated to have 2 of these things on hand at all times.

I watch with interest to see what Alex comes up with as a cause and a cure. He struck me as pretty sharp. I hope to learn something.

 

[Alexsha]

I dunno about pretty sharp, but I love to tinker. I need to photo and write down everything I do so I can compare to what's on this forum.

Back to the bucking... It might be totally normal, but this is the first 5-ton I've owned and I don't have any others nearby I can compare to right now. I did hear about a couple trucks in the area that might be 5-tons, but the people around here think my truck is a deuce and a half so the info might be wrong. Lol

 

[wheelspinner]

I'm on my third 939 truck. Each has had a speed that they hated and bucked, all different. The current truck HATES 48-53 but then drives right out. One other was an A0 with 11-20 tires and one was a 931A2. Its not shake, its just like a really big harmonic. I just avoid that speed whenever I can.

 

[Jason O]

This is the tire buffing / shaving process I mentioned a few posts back. It removes material to eliminate high spots until the tire is in round. I had already tried balancing beads, air pressure adjustments etc. 95% of the shake is gone, and my truck was really bad when the out of round tires would sync up and toss the truck around. I put over 1000 on the truck, hoping they would smooth out before doing this. Going to have my other 5T done next.

 

[brasco]

I was driving my M923A2 this weekend and noticed a bounce at around 25-30 mph as I was drving in a 30 MPH zone. I think this is normal in just about every larger truck I have driven or was a passenger in. (I still recall my school bus years ago bouncing at certain speeds and it was really pronounced.) Personally, I think this has something to do with the resonant frequency of the tire-vehicle combination. I really don't think much about the bounce anymore – I just speed up or slow down depending on the situation.

 

[1 Patriot-of-many]

Could also be bent driveshaft, wheel bearing tightness or something as simple as Ujoints not lubricated. I have a wicked vibration between 55-65, once I hit top speed it disappears. TM's have some troubleshooting on the issue, I haven't gotten around to dealing with mine yet.