touchy steering on M818 with 16.00x20 's

[OPCOM]

i have a drveability issue and I hope the experts with experience can help.

I have my tires issues pretty much worked out, have fixed the spare, etc etc.

The truck is an M818 with an M109 box, 16.00x20's, and the box is 3" above the frame via oak planks. The other mods are a lift gate and a low-mounted generator set where the driver side fuel tank was removed.

Instead of 100PSI, I am now runing 70PSI on six 16.00x20 tires on Goodyear HEMMT rims. I lowered the prssure from 100 because the entire tread was not on the ground as evidenced by tracks on concrete. It seems to be now. The air pressure change did not make any real difference in this.

Is the steering touchy on the M818, or is this the tire size, or do I need some alignment?

The way it behaves is that a turn of the wheel in one direction results in a certain amount of direction change, and a turn in the wheel in the other direction seems to result in a greater change in that direction.

The reference point for "straight or center" the above statement is when going down the highway, straight, at 45-50MPH. Speed does not matter but the faster I go the more pronounced any steering issues are going to be.

I am sometimes in fear of losing it because the steering seems over-eager and the truck wants to over-steer and unevenly at that. Not like the deuce was. It happens at all speeds, and of course at 35 is an annoyance, and at 50 it is scary.

I have swapped tires around on the truck and there has been no change. I cannot find anything wrong but maybe I don't know where to look.

There has been no sidewall heating. What is the correct pressure for Goodyear 16.00x20 on a 25,000 lb 6x6 truck at 50MPH?

Is the steering variable ratio?

could the tuck be going straight when the steering box is not "centered", because of a wrongly done toe alignment?

A lot of questions - I hope the combined experience on this board can answer them. Before I am driven crazy. It is to the point I am curtailing driving for safety reasons.

 

[cranetruck]

How is your caster? What happens when you let go of the steering wheel when driving on a level straight road?

 

[doghead]

I have seen several loose steering gear boxes, that have made the 809 series trucks, wander or drift. I would check that. Have a second person turn the wheel back and forth about a 1/4 turn, while you look at the steering gear box under the fender.

As far as I know, the ratio is not variable.

A simple tape measure will get you a decent toe in measurement. I would check that also. Specs must be in a TM. I haven't looked for them recently.

Also, have you inspected/serviced the front wheel bearings recently?

 

[hklvette]

bad toe alignment on one side will cause the steering wheel to no be centered while going straight and require effort to keep going straight. What I do is get the steering wheel pointed straight, the look at the tires to see where they're pointing. If its a bad problem, its normally obvious, especially with large tires.

 

[gringeltaube]

Hmmmm...... tall, wide radials, a tall & heavy box plus liftgate, all that resulting in a relatively high CG ......... all playing against stability/drivability/fun to drive and safety of course.....!

Yes, probably not sufficient caster and also - did you check the toe-in?

Wide radials have better grip and traction in ALL directions, so correct toe-in is more important here than for the stock bias ply IMO. See http://www.steelsoldiers.com/deuce-.../16353-deuce-front-end-alignment-radials.html
(Personally I prefer 0 - 1/16" measured on the tire tread, for my 14.5R20 contis)

As for correct caster the 5ton specs call for +3º, IIRC. I would confirm that first and eventually add some shims to go to +5º or even higher (provisorily, for a test ride only) If it helped then the spring seat wedges could be accordingly machined, later.

Even with the box not perfectly centered the truck should be able to run straight and stable at any speed. I have no explanation, though, for the described left - right turn difference in steering action if that also happens at a very low speed. Box centered or not shouldn't make a difference here; as far as I know these boxes are constant ratio.

G.

 

[Tow4]

Have somebody follow you and watch for the body leaning while you make slight turns (lane changes). I have had tall radials do that on small 4X4s. Watch for the rear axles shifting too.

As for the quicker turn in one direction. Is the truck heavier on one side? If it turns quicker to the right, it could be heavy on the left causing the body to lean more in a right turn.

 

[sandcobra164]

I'd also point towards slightly loose suspension components. I drive MTVR's all the time and they run 16R20 Radials and weigh more than that setup with no drift or erratic steering at all.

 

[nk14zp]

IF the toe in is correct and nothing is wore out my guess is caster.

 

[OPCOM]

Thanks for all the things to check. I am not sure about caster, I thought it was built in and have not heard of shims being used, but I will investigate. The front end seems tight, nothing seems loose that I have been able to find. Is caster the setting that makes the difference between understeer and oversteer?

The weight on the truck- The generator set on one side is heavy, maybe 400 lbs? but no more than a 50 gallon tank of of diesel and 4 batteries, and it is slung low, well below the frame.

 

[USMC6062]

The slight difference in left and right turning probably has to do with the steering ram. The drag link on these trucks is more of a mechanical backup than actually steering the tires. If you notice when the truck is not running, there is quite a bit of movement left and right at the steering wheel and no movement at the tires. This is operating a valve in the steering box that sends pressure to the steering ram. By design a single ended ram has more power extending than it does retracting, it also will extend slower and retract faster at a set pressure and flow. So, the way a 5 ton is made it would turn faster to the right , slower to the left, but have more turning force to the left and less to the right. Make sense. haha