rear hop

[drileyg]

Hello i have a 71 m35a2 and when i drive at high speeds. 50 - 55 at random times it will hop from the rear of truck hitting brakes dont seem to change it. slow down and then it will go away. it is very random and doesnt do it alll the time.my rig has over 50k miles on it so it could be anyting. any ideas would be appreciated. thanks guys

 

[WillWagner]

Maybe some tires out of balance? Mine does it sometimes especially with no load or trailer. Maybe try experimenting with air pressures.

 

[hole]

Does it happen when hitting a bump/lip in the pavement? Mine "hops" after going over a bump/lip while empty.

 

[DieselBob]

There is some suspension checks listed in TM 9-2320-361-20 pages 2-12 and 2-13 you may want to try if you haven't already done so.

 

[drileyg]

it doesnt do it over bumps. just completly randow. thansk guys

 

[Heavysteven]

I would crawl under the truck and make sure non of the drive line bolts are lose first.

On mine the transfer case to frame bolts were not tight and the drive shaft between the trans/transfer I could spin by hand.

 

[stumps]

The problem is simple, but the explanation is not quite so easy:

Assume you have two massively out of balance tires on the same side of your truck, but on different axles. Let's call them Tire A, and Tire B. If the heavy side of Tire A is up, and the heavy side of Tire B is down, the extra weight on Tire A will move in the opposite direction from the extra weight on Tire B, and the net result is no hopping. If the heavy side on Tire A and the heavy side on Tire B are both down, the weights will move in tandem, and you will get mega hopping.

Ok. If that's understood, we will move on...

Suppose the two tires are not quite exactly the same size. Suppose that when you are driving down the road Tire A turns 10,001 revolutions for every 10,000 revolutions Tire B makes. This will mean that ever so slowly, the heavy spots in the two tires will go from being opposite each other to being on the same side and everywhere in between. You will feel this as the truck moving smoothly down the road, and then after a while starting to hop madly, and then shifting back to moving smoothly.

Further compounding things is the speed that the balance point will change will change with turns, bumps, how hot the tires are, ...

And, to further compound things, assume you have 4 very out of balance tires A,B,C, and D, all on different axles, and different sides of the truck.

The answer to the problem is to make sure that all tires on your truck are mounted concentrically, and are in pretty good balance.

Also, make sure that your valve stems on each dual tire pair are on opposite sides of the axle. They are supposed to be balanced, but rarely are.

-Chuck

 

[RANDYDIRT]

I vote that you have a tire size issue. The different sizes between axles will cause drive line wrap up. This will build until there is no other way for the stored energy to alleviate it self, then it unwinds. This will cause slippage at the ground-tire contact patch which results in a shuddering. This shuddering may set up reactionary harmonics in the rear suspension causing uncontrollable wheel hop. Slowing down lets the tires gain enough traction to stop the hop and start storing energy back into the drive line wrap.... then it will happen again.
If you must mix tire sizes mix them on the same axle so the differential will handle the mismatch.

To see if this is the issue, you can pull the rear-most drive shaft. If the problem goes away it would be almost certain that you have a wrap-up issue.

Back about a 100 years ago my dad had a blazer that would wrap up like this. I remember having to go jack up one of the front wheels to relieve the stress just to unlock the hubs.

Dirt

 

[Srjeeper]

What air pressure are you running and are they all kept the same?

 

[papercu]

Yer got any armadillos in Southern Minnesota? If so, next time it happen stop and check between the tires. may have one stuck in thar.

 

[Ord22]

check your u-joints in the driveshafts. i ran across an issue like this. go underneath your truck and push up and down on each driveshaft too see if the driveshafts are tight in the u-joints. the vibration at high speeds will make it seem like there is a problem in your tires or transmission. you shouldn't have different tire sizes on your truck in the first place. if thats the case, you would've felt something a long time ago. insure your tires are not loose, also.

 

[drileyg]

all of the tires appear worn the same. and around the same size i believe i got the pressures at 80 psi. i cant honestly remember.

 

[hole]

At 80 psi those tires will start hopping if you run over a pebble , try 50 psi in all the rears, and 60 psi in the fronts.

 

[Srjeeper]

That's your problem.....80 is way to much for an empty truck on NDC's. You'll feel every crack, seam, slightest imperfection in the road, plus they'll have no grip should you encounter a wet serface. Your only running on the center bar of the tire, so it'll act like a hard slick.

Drop them down to 50 on the rears. You can go 55 or 60 in the fronts to help her steer a bit easier, but still be careful in the wet.

I've already gone 45 in the rears on wet days and 50 in the fronts just to get more tire on the road. Don't leave the rears there all the time cause it'll eat the tires up.

Let us know how it goes......Good luck

 

[rlwm211]

I run my deuce with about 4000 pounds in the rear, S280 shelter and all my stuff, and I run 35 in the rear and 45 in the front. I made the gradual reduction in pressure after running for 3000 miles and checking tire contact patterns. At this point, the truck rides great, handles great and does not pound.

80 psi is way way way way too much for an empty truck....way too much!!!!!!

Military pressure is 50 psi. Experience says less in the rear, perhaps 50 or 55 in the front.

My criteria is handling, for one, and then tread pattern and wear. My truck is somewhat rear heavy, and since I installed a set of lockout hubs from JATONKA and installed my spline caps in place of the front rear axle shafts, I enjoy driving the truck and it handles far better than before.

I expect to drive almost 3000 miles this year like last and I am very pleased with where I have worked my truck to. Experience helps and you learn your likes and dislikes.

Lower your air pressure to 50 psi to start and see if the hopping goes away. With as much pressure as you have in them, I sort of doubt drive line bind is the problem, especially since you have tiny little patches of rubber on each tire on the road....

Hope this helps

RL

 

[doghead]

Water in a tire will run smooth until you get up to speed. Have you had a tire with low air pressure for some time, then re-inflated it?