M923A2 Shaking at 55-60

[OktobrVega]

Hi my 923 shakes badly at 55ish to 60ish. Doesn't shake at all before and really after these speeds. It does sometimes go away all together. But for the most part, its a pretty regular shimmy during those speeds. Changed rear tires and still doing it exactly as before.

What can I check?

Should I get a new set of tires?

Is this a balance issue since its certain speeds. Since I figured low spots would shake throughout 0-65.

 

[M35A2-AZ]

I would think it is the tires, Balance, CTIS assy., ETC.
What type of tires do you have?

 

[OktobrVega]

Goodyear singles

 

[atomoros]

My 1990 M923a2 does the same thing at 50mph, not before and not after. So far, I just stay out of that "zone" and all is fine.

 

[M35A2-AZ]

You may try moving them to the back and trying some from the back, but the A2's are bad about jumping around.
You could look at getting them balanced, if you can find someone to do it.
Not sure I would use the balancing beads in them.

 

[OktobrVega]

Thanks for the help and suggestions guys, what would I do without SS.

 

[KaiserM109]

If ya can't fix it, use it: put a couple of potatos in the glove box along with an open pocket knife. After 10 miles or so, ya got sliced potatos. If ya want diced potato, use a two bladed knife.

It sounds like a sideways shimmy, not a badly balenced tire. Out of balance tires don't usually have a speed range, they bounce all the time, the faster, the worse.

General rule about a shimmy, you need 2 things wrong to get a bad shimmy, something loose to let the tires move and bad alignment. Check all your joints and look for slop. While stopped, turn the wheel back and forth and look for movement. Brand new, the tire should move a little with a little movement of the wheel.

I have no idea how to align a big truck, but I'll bet there's something in the TMs about that. Good Luck.

Search the TMs for "toe in". On civi 4x4s that is the most common cause of problems.

 

[1 Patriot-of-many]

Mine also shakes badly from 50-65, if i get at top speed it stops shaking.(think it reaches harmony with the world going fast?) It's so bad I had to wrap a strap around the spare tire holder nut to keep it from turning loose! TM's say Universal Joints lubricated? Then Propeller shaft flange nuts torqued? Propeller shafts all in good condition? Center bearing assembly in good condition? Then notify your supervisor. I think that's Doghead?
-23-1 page 0044
Let us know if you figure it out. I haven't gotten around to following the troubleshooting train with this truck yet. With 5 others and facebook you know.

 

[davo727]

Apparently it is a standard feature requested by the Army, both mine do it. Im thinking both front wheels are several pounds out of balance.

When I get some time im going to rig up some kind of a wheel spindle on a hydraulic lift stand to try and do a static balance on a wheel tire assembly. Taking a 5 ton spindle and a hub and take the wheel seals out of it and put some real light grease in the bearings to try and cut down on the drag when spinning by hand to do a static balance..... worth a try at least.

 

[KaiserM109]

A different kind of harmonic haunted me while ferrying my M923 from Ft. Benning, GA to Ft. Worth, TX. I'd get going about 50 (which I later learned was only 45) and I got this horrendious noise that sounded like something in the power train was ripping itself apart. I pulled over and crawled under the truck twice.

Then I was filling up in Mississippi and I happened to lean against the spare tire rack. IT MADE THE SAME SOUND! The nut was loose, but rusted in place, so I wrapped a nylon strap around it and went on, relatively peacefully.

 

[OktobrVega]

Hahah, The mystery sounds.

Also, my truck makes a slight clunk when turning right or left. Like something is shifting. You know when you start into a slow turn. Its a light clunck/knock center of truck it seems. Same sound, either turn.

 

[Jeepsinker]

You have a bad tire somewhere, or loose/ cracked wheel studs somewhere. When you got the truck had it been sitting on any flats for a long time? That ruins the sidewall and will make them do this. I found two tires on my wrecker that were bad like that, replaced them, and no longer have a shimmy or vibe. Also note that I removed my front driveshaft because the slip joint was worn out and vibrating, then later found out that I had some cracked wheel studs the hard way.
Lesson: Remove front driveshaft first, then replace tires with known good tires two at a time, and check wheel studs while you have the wheels off.

 

[gringeltaube]

................. General rule about a shimmy, you need 2 things wrong to get a bad shimmy, something loose to let the tires move and bad alignment. .....

Agreed for a BAD shimmy... but you could have all nice and tight and still experience a slight (but still very annoying)shimmy-kind of vibration which is normally caused by front tire imbalance.

.................Taking a 5 ton spindle and a hub and take the wheel seals out of it and put some real light grease in the bearings to try and cut down on the drag when spinning by hand to do a static balance..... worth a try at least.

Yes, it works! And even better if you replaced the tapered bearings and adapt ball bearings instead. And don't use grease, just gear oil. See this post...

Hahah, The mystery sounds.
Also, my truck makes a slight clunk when turning right or left. Like something is shifting. You know when you start into a slow turn. Its a light clunck/knock center of truck it seems. Same sound, either turn.

That noise is probably coming from the rear spring ends shifting inside their pockets (on the axles).


G.

 

[OktobrVega]

You have a bad tire somewhere, or loose/ cracked wheel studs somewhere. When you got the truck had it been sitting on any flats for a long time? That ruins the sidewall and will make them do this. I found two tires on my wrecker that were bad like that, replaced them, and no longer have a shimmy or vibe. Also note that I removed my front driveshaft because the slip joint was worn out and vibrating, then later found out that I had some cracked wheel studs the hard way.
Lesson: Remove front driveshaft first, then replace tires with known good tires two at a time, and check wheel studs while you have the wheels off.

So remove front driveshaft, Can I run the truck with this disconnected due to the fact this is hwy use only?

 

[davo727]

Perfect, Thanks for the confirmation and the link.

Yes, it works! And even better if you replaced the tapered bearings and adapt ball bearings instead. And don't use grease, just gear oil. See this post...

 

[corvette9]

My wrecker shakes around 50 MPH. My cargo dose not. The cargo has counter weights on the front wheels, The wrecker dose not. I think thats where the problem is. Do you guys with this problem have the counter weights on the front wheels? If not I think we need them. As for the clunk you here when doing a slow turn. It is the rear axle assy moving side to side in the spring pockets. There is no panard bar to support side to side movment. If you look at your very rear spring pockets you will see where the noise is coming from.

I must call some of the parts dealers and see if anyone has the counter weights for the front tires and see if that fixs the problem.

 

[OktobrVega]

I checked mine and it does have the counter weights. But i bet that would play a part if i didnt. The shake seems to be whole truck. center based. Maybe its drive shaft itself? Its shakes to all **** thats for sure not a fast shake but a slow bam bam bam. doesnt speed up or down just staedy at those death speeds.

 

[Bolkbich]

I've owned / driven a bunch of M939's. Only 2 trucks shook , both had goodyears, 1 had 14's the other had 16's. The truck with the 16.00's drove smooth as glass till I converted it from duals to singles . They seem to be flimsy compared to the Michelins.

 

[OktobrVega]

I've owned / driven a bunch of M939's. Only 2 trucks shook , both had goodyears, 1 had 14's the other had 16's. The truck with the 16.00's drove smooth as glass till I converted it from duals to singles . They seem to be flimsy compared to the Michelins.

What if its just that. Maybe Goodyears are more prone to this over time. Mine are all Goodyear 14.00.

 

[Jeepsinker]

You only need counterweights if you have ctis.