M915 odd, heavy thumping on rough roads. WTHeck?

[JH1]

My M915 has a very distinct, heavy thumping on mildly rough roads. It sounds as if there is a 50 pound weight on a few inch long lever arm that is banging around down there in front of the driver. It sounds like something is loose and hitting the frame. Nothing seems to be loose. Runs fine, steers fine, and the thumping is not felt in the steering wheel. Anything typical for this symptom? Thanks.
Jim

 

[wheelspinner]

Is it rythmic like its on the tire, or random, only with the bumps?

 

[Digger09]

Could be a bad cab mount.

 

[Ferroequinologist]

I know in my M915, even though everything is tight and checked out, rides terrible on rough roads without a trailer, and makes all kinds of nasty sounding noises.

I don't recall a noise quite like yours. I might check and see if the shocks are worn out, and the axle is hitting the stops.

 

[waayfast]

Motor mounts??

 

[jdknech]

could it be the hood?

 

[JH1]

Gents,
All good suggestions. The sound is not rhythmic at all. It happens even on paved roads. I checked some of the cab mounts and they were tight, but I'll check the rest of them. I think it is not the motor mounts because even slight bumps launches whatever it is. I watched the hood and it is not moving during the thumping. The axle is pretty far from the stops, so I don't think it is that. I checked the rubber inserts at the ends of the shocks and they seem fine. It really sounds like something very heavy is dangling loose and smacking against the frame.

I'm surprised nobody suggested the noise was from the loose nut behind the wheel bouncing around inside the cab.
Jim

 

[jonesal]

Trunion bearing?

 

[rchalmers3]

Noises like that bother me as well. May I suggest you raise the front end off the ground and give the front steering and suspension a good inspection.

I use a long lever bar to pry against rubber mounted items like motor and cab mounts. I have found that some rubber mounts and bushings look OK under compression but fail to hold in extension, which can lead to the type of noise you are describing.

If the motor is too heavy to lift with the bar, you can set the parking brake real hard and slip the clutch in first and reverse to load the drive train. You will need a spotter to watch the motor and trans mounts under load and determine their condition.

Another item I use for rattles is to thump different parts of the truck with a rubber hammer, but this works better for identifying lighter rattles and clangs.

Rick

 

[JH1]

I'll try that. The M915 doesn't have a standard clutch to pop, though. It is a semiautomatic 16 speed. Thanks. Will report back.

 

[rangereter]

It sounds like your steer axle rear main leaf eye (driver or pass. side) is hitting the lower frame flange.
Regards, Bob

 

[rmgill]

Step out on the running board after setting the hand throttle and hold the steering wheel while you lean around to try to locate the sound...










No, don't really do that.

 

[JH1]

Thanks. I'll look under there again, specifically at the back end of the leaf spring. There should be a grommet of some sort isolating the leaf spring from the support bolt, shouldn't there? If it's shot or missing, that might do it.

It sounds like your steer axle rear main leaf eye (driver or pass. side) is hitting the lower frame flange.
Regards, Bob

 

[rangereter]

JH1, Absolutely... most of the time you would not consider this failure on an ex-military truck because of the inherent lack of miles or wear. On a commercial civilian class 8 truck with a half a million plus miles on it I would be looking for the contact of normally isolated suspension components to chassis every time I hear or feel what you are describing.
Regards, Bob

 

[JH1]

I got my back dirty today and found something. There is a large triangular bracket that holds the air dryer to the frame, and the bolts holding it to the frame were a little loose. I gave it a couple of swift kicks and it sounds like maybe that's the problem. I'll report back. Thanks for all the suggestions, guys.
Jim

 

[Ferroequinologist]

Glad you (maybe) found it. I wouldn't have thought of that being loose. It pays sometimes to just crawl around kicking stuff! lol.

Hope that's it an it will be another 'happy Steel Soldiers ending'