Bounce bounce?

[Marines42]

I have a bobbed 5 ton on 14.00 R20 tires. It bounces like crazy at low or high speed. Is this just the nature of all of them or a symptom of something I can remedy? Any suggestions are appreciated. I would mainly like to know what causes it.

 

[kastein]

Does it seem to come from the front or the back? What kind of road generally causes it, if it doesn't happen all the time? Does the bouncing get faster as you speed up, then go away?

I would bet on out of round tires, bad shocks, a bent rim, etc. The jointed cement highways in Pennsylvania also drove my M54A2 nuts at about 52mph, the rear axles would literally turn into a see-saw. Each axle was bouncing 3-4" off the ground. I got some pretty mean hopping/bouncing from the front till I swapped my badly scalloped unbalanced heavily worn tires off it and put newer ones on (truck came with em.)

 

[cranetruck]

Off hand, I'd say it has to do with "unsprung" weight, not easily damped...

 

[nk14zp]

Try changing your tire pressure.

 

[gimpyrobb]

Yup, what psi you running?

 

[Jimma]

Make sure all rear tires have equal tire pressure. That may help. Also when those trucks are empty or have no real weight on the rear they do tend to be bouncy.

 

[broman78]

You have shorted the wheel base on a truck that is designed to carry heavy loads off road. I not sure there is much you could do besides taking out some springs or adjusting air pressure.

 

[Marines42]

I picked it up in Florida and drove it back to GA hardball all the way. It did seem to increase as I sped up and then tapered off as I cruised. I asked the previous owner about it before setting off and he had set all the tires at 50 psi to try to remedy the problem. The tires are also probably out of round. I have a new set that I will pick up when I get home and hopefully that will fix a large part of the problem.

 

[gimpyrobb]

I'd drop to 35 and see what happens. I have my 395s pretty low too.

 

[PowerstrokeNC]

Yep, i have noticed this as well on mine.

 

[Marines42]

Isn't 35psi a bit low?

 

[cranetruck]

Not as big as the 14.00's, but I run my single 1100-20 NDCC tires at 70 psi on the highway, including the cross country trips and never had a problem.
Low tire pressure and soft radial sidewalls may not go well together either...

 

[mtruitt1971]

Isn't 35psi a bit low?

new to the
MV scene, but i recall an interview with bob chandler (bigfoot monster truck fame) saying he ran 10 psi in his truck. when asked if that was too low he said that that is a big tire and a lot of air


i wouldn't go to 10psi but i think 35 would be ok

 

[jimk]

My 813 w/1400 x 20 has it bad at 30mph +/-5mph. Looks like it is 1/4" radial run-out on 2 rears. Adding air (90psi) made it worse. It is a resonance. Either the sidewall or the spring is storing the energy, adding it to the next rev... I am going to try having a local re-capper buff them.

 

[gimpyrobb]

Isn't 35psi a bit low?

Not for me. It only takes 30psi to seat the beads. If ya don't like 30, try 40psi. Just letting you know what others have done. It will take some experimenting to get it how you like.

 

[jimk]

Update to post # 14. I had a tire re-capper* south of Rochester buff the three Goodyear 1400x20 radials. They were all off axially by 1/4 –3/8”. The whole process took 5 min per tire. Charge was $20/ea which he guessed at, said it was twice the going rate for the race car tires guys come in for. I thought it a bargain. They looked goofy spinning on the machine. Really clear how they can shake a 22K truck. Tires w/o rim are mounted on an automatic spindle. The machine spins them about 200 rpm while a grinder was moved across face. Guy removed a bit less than 1/8'” per pass. A vac cleaned up the waste. Looked like a lot like turning round stock on a lathe. Tire were more egg or pear shaped than non concentric. Not bulged. There was a local company (Commercial Tire - Syracuse) that does smaller tires but they did not have the $3000 adapter to do these. I may have them balanced since there is now less tread on one side.

BTW, the right one was almost new (99%) before. The others might have been 65-75% before. I asked him to stop grinding before 100% of surface was buffed, to try and minimize tread loss. That is why one on right looks untouched in spots. The other side got the buffing.



* Main Tire Exchange, Dansville, NY ph (585) 335-6018, (see Harold in cap shop)

 

[cranetruck]

Interesting.

 

[paulfarber]

Make sure your tires are all within 1/4 inch diameter.
All the same tire pressure (part of PMCS).
MV owners run ratty tires and wonder why the ride sucks.

Some mechanical things to check:

Front tie rods, king pin bearings and toe in.
Trunnion bearing and dog bones for wear.

These tires don't need balanced as they don't rotate fast enough to need it. Plus you would have to stick on a few lbs of lead to even make a difference.

Must tires on these M35s are shot. Bad wear patterns. First thing to do is replace all the tires with good rubber. If you don't want to drop $1000 on new rubber then don't buy a truck with 10 wheels.

 

[paulfarber]

Update to post # 14. I had a tire re-capper* south of Rochester buff the three Goodyear 1400x20 radials. They were all off axially by 1/4 –3/8”.

If you put these back on your truck you will have scuffing issues as the diameter of these tires are significantly less than the others.

For a 4 tire vehicle its not a big deal, for dual tire axles its a HUGE deal. Your tires will be fighting each other on every RPM until they all wear down.

You could minimize it by measuring the tire diameter and matching them as close as you can for each axle... but you will still scuff/have issues.

 

[3dAngus]

WOW. The tire buffing is a really interesting concept. I wonder how bad that woudl mess up some tires that were out of round from flat spots. You drove yours a long ways and took care of those issues, but I don't think I would want to be to quick to try it.

I had the same bouncy problem on my new M35A3. Rounding the tires out with 50psi took care of most of the problem for sure. Today, after spraying soapy water on the tires checking for leaks, and finding none, I took her out for a drive with my M1061 hooked up for the first time. The extra weight really helped by loading it down, though I could not feel the heavy trailer back there, the truck sure did, somehow. Top speed even increased to 55 mph from 52mph on the recovery effort. What a difference loading it makes. I'm Happy!

Glad you got it resolved, and thanks for sharing your experience and new found knowledge.