M923A1 rocking back and forth

[Arny]

Hello all,
since I have my M923A1 I have noticed the truck rocking back and forth a lot while driving. It doesn't seem to be at sepecific speeds but it occures often (only?) when I'm using the motor brake.
I haven't noticed this on other 6x6 all wheel drive trucks with single wheels.
Does anyone have a similar problem?
What could be the rootcause for this?
Tire pressure does not seem to have a big effect.

Thank you in advance

Cheers Arny

 

[sandcobra164]

Arny,
Try to be more descriptive but I think I understand what you are saying. If I let me M923A1 sit for more than about 2 or 3 weeks, one of my tires will feel as though they have slightly flat spotted. I feel like it is my passenger side inner axle tire that is the culprit. It isn't constant or speed dependent so it may be a combination of the passenger side rears get lined up just right with each other to cause the bounce. After about 3 miles, it goes away and if I drive the truck on a weekly basis, it does not happen at all. Rears are aired up to 70 psi on my truck as per the TM which should prevent the occurrence but I know others who don't have issues leaving the rears on 35 or 40 psi.

Joseph

 

[Gunzy]

What do you mean by, "Motor brake?" The NHC does not have engine brakes(Jake Brake) on it unless someone has modified it. Sandcobra is right though about flat spotting from sitting.

 

[simp5782]

If you just got the truck or you bought it from the auctions then more than likely the tires are just spotted and prob left flat for a long time on a lot. Ruining them.

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[Arny]

Thanks for the answers!
Sorry it's harder for me to exactly describe the Situation since i'm not a native Speaker.
I think you also call that phenomen "bonanza effect".
If the tires have flat spotted the Problem should appear no matter if the vehicle is accelerating or decelerating, right? Shouldn't that disappear after some miles?
With "Motor brake" I just meant if you are in coasting, downshifting on a slope. That's where it built up a lot last time. I didn't realize it on the flat before.

 

[Gunzy]

If you have the resources I would start replacing tires.

 

[simp5782]

Are you located in Stuttgart, AR or Germany?

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[jbayer]

I would say Baden-Württemberg(BW), a state, so to speak, in Germany.

 

[Arny]

In Stuttgart, Germany "BW" is the "state" so to say.

When I was driving a longer distance to a vintage truck show this weekend the rocking got better but still did not disapear. It still occures mostly when braking or going coasting on a slope..

 

[simp5782]

What brand of tires are on it? I have an A1 with AT2s and I have to run various pressures to make it not seem like there is one out of round. I run 90 in the front and 35 in the rear the other day and it didn't do too bad.

 

[Arny]

I have the GoodYear AT-2As on it. I tried with 75 psi at the front axle and 55 on the rear axles. But the shaking still occures strongly around 32 - 36 mph and while braking.<br>Isn't 90 at the front a bit much?

 

[simp5782]

Well playing with the pressures may make it ride smoother but it may also get you to the location of finding which one is separated inside. 90 isn't that big of a deal. I run 90 to 100 in the fronts on the trucks I tow. If It is in the seat that is shaking its in the rear if you have it in the steering wheel then it is for sure in the front. I have a set of those on a truck and yes it shakes like its got one out of round or separated. I have yet to get the motivation in this heat to go out and swap a tire off and on each hub to see which one it is.

 

[Ford Mechanic]

The combat rims on the M939 trucks are only rated at a max of 85psi. It's stamped between the outer set of nuts.

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