5th wheel placement (moving rearward, effect on axle loading - Steel Soldiers::Military Vehicles Supersite

Rattlehead · 07-14-2008, 16:26

Ok, so I have been thinking of the issues with tire clearance to the landing gear when towing some trailers with military tractors, and decided to figure out some math on the effects of moving the 5th wheel back.

Specifically, I am considering this on a M932 w/w, though the weights and dimensions should be nearly identical for the M818 version. I am running 14's, which causes even more clearance problems than the 11x20 NDT's.

If my math is correct, you can figure out the change in front axle loading by taking the distance of the kingpin from the centerline of the tandem, divide that by the wheelbase (167" in the case of M818's and M931/932), then multiply your result by the expected weight to be applied to your kingpin. If your kingpin placement is in front of the tandem centerline, then the resulting number is ADDED to the front axle weight. If its behind the centerline, SUBTRACT this weight. For a simple example:

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Frt axle weight unloaded = 10,000 lbs
wheelbase = 167"
stock kingpin placement = 4.5" in front of tandems

So, a 5,000 lb load on your kingpin would result in:
4.5/167 = 0.026946.......
0.026946 x 5,000 = approx 135 lbs, added to front axle loading.

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If the kingpin placement was 10" BEHIND the tandems, then the front axle weight would be REDUCED to:

10/167 * 5,000 = 299 lbs REDUCTION.

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Does this math look correct? If so, the question is, how much weight can you safely pull off the front axle? Attached is a pic from the M939 transport guidance manual that lists axle weights for winch (932) and non-winch (931) tractors, both empty and with the max semitrailer load of 15,000 lbs on the kingpin and 35k lbs total trailer weight. If I can assume that, with a winch-equipped truck, as long as I don't reduce the front axe loading more than the allowed weight of the non-winch truck with full trailer load, I should be ok. You might be able to go more, but that would be the equivalent of simply towing the same load with a non-winch truck. In this case, the EMPTY front axle weight of the winch truck is 10,870 lbs, and the LOADED frt axle weight of the non-winch truck is 10,195, so I could effectively reduce my front axle loading by this difference, 675 lbs, safely.

Remember that this is considering a max capacity trailer, so lighter trailers should provide more of a safety margin.

Also, see the attached spreadsheet, which shows some calculations on various kingpin loads at different placements, IF my math example above is correct? I first show the stock placement. Then 11" behind the tandem centerline, as that is the max distance I can move it back and still attach to the angle iron mounting that is riveted to the side of the frame. I also threw in a 24" just for an extreme, to see the effect. The red shaded portions of this chart are where I would cross the line and bring my front axle loading to BELOW that of a fully loaded non-winch truck.

Does this look correct or am I way off base?

Rattlehead · 07-14-2008, 16:28

Here is a drawing of the M818. The 900 series manual has the wheelbase wrong, so I didn't use it.

98hd · 07-14-2008, 17:27

I'm going to look over your stuff closer when I get off work tonight, but one thing to consider:

Stock 11x20 is what 44" in diameter? So for me a 16x20 is about 52" That is a 9" difference. 1/2 that is 4.5" (coincidentally what you say it is ahead of the axle centerline by), so you would only need to move it back to the tandem centerline to be at the same spot as a tractor on 11x20's. Correct?

Rattlehead · 07-14-2008, 21:00

Quote:

Originally Posted by 98hd

Stock 11x20 is what 44" in diameter? So for me a 16x20 is about 52" That is a 9" difference. 1/2 that is 4.5" (coincidentally what you say it is ahead of the axle centerline by), so you would only need to move it back to the tandem centerline to be at the same spot as a tractor on 11x20's. Correct?

I think so. But also consider what trailers that you may want to pull. Some won't even work with 11x20's.

98hd · 07-14-2008, 22:06

Quote:

Originally Posted by Rattlehead

Quote:

Originally Posted by 98hd

Stock 11x20 is what 44" in diameter? So for me a 16x20 is about 52" That is a 9" difference. 1/2 that is 4.5" (coincidentally what you say it is ahead of the axle centerline by), so you would only need to move it back to the tandem centerline to be at the same spot as a tractor on 11x20's. Correct?

I think so. But also consider what trailers that you may want to pull. Some won't even work with 11x20's.

The one thing I question is the calculation itself of the effect on the front axle. Is that a standard formula for calculating that figure? I did some searching and didn't see anything about calculating the loading.

I did see this: http://www.classadrivers.com/phpBB2/...c.php?p=391945

m16ty · 07-14-2008, 22:30

I wasn't very good a math in school but I have loaded my share of tractor/trailers with sliding fith-wheels. I don't think I'd want to go back past the center of the tandems or your front end could get awfully light with a heavy trailer load.

What I'd do if I was you is get a sliding fith-wheel off a civilain truck. Then you could move it where ever you needed it for weight or clearance issues. I've got a few laying around here if you're interested.

jimmcld · 07-14-2008, 22:52

I would think you would also have to consider the angle of the king pin to the centerline of the tandem. If you get behind the centerline, you are putting a lot more leverage on the rear of the frame. This would tend to take a lot more weight off the front axle when torque is applied. The extreme would be that the front end could come off the ground when in a high torque situation such as starting from a stop.

Just my two.

Rattlehead · 07-16-2008, 13:14

My previous calculations only dealt with comparing a winch-equipped tractor to a non-winch. I thought about this more and decided to look at the effect of pintle hitch loading on front axle weights. Below are the dimensions for the 900 series 5 ton's, which I used because they gave the distance from the tandem bogie to the pintle where the 800's did not. But the truck weight is similar, and the wheelbase dimensions are identical.

The 5 tons allow for a max pintle load of 1,500 lbs, no matter what chassis. Seeing that the distance between the tandem centerline and the pintle varies considerably on each model, I would have to think that they came up with this 1,500 figure because it is 10% of what they came up with for an acceptable towed load, 15,000. 10% is your typical tongue load goal for this style of a hitch.

For my calculations, I subtracted 2" from the distance between the tandem centerline and the hitch, as they are measuring to the end of it and not exactly where the load is applied. 2" is close enough. Here is what I came up with for the front axle weight reduction when a 1,500 lb tongue weight is applied:

5 ton dump and/or tractor = 51.2 / 167 = 0.3065 * 1,500 = 460 lbs reduction
5 ton cargo = 79.7 / 179 = 0.445 * 1,500 = 667 lbs reduction
5 ton extended wheelbase cargo = 119.7 / 215 = 0. 5567 * 1,500 = 835 lbs reduction

The effect on the front axle of the cargo truck is about the same (667 lbs) as was in my 5th wheel example in the first post, when comparing a winch truck to a non-winch truck. The extended wheelbase truck is affected even more. To me this seems to indicate that you could safely move the 5th wheel behind the centerline, depending on loading and how far it is moved. But the industry consensus just simply says "don't do it". How far back do the sliding 5th wheels typically go?

For reference, the front axle weight loadings are as follows

wo/w w/w
cargo
empty 9700 10750
loaded 9855 10905
tractor
empty 9820 10870
loaded 10195 11245
ext wheelbase
empty 10200 11230
loaded 10205 11235

EO2NMCB · 07-16-2008, 14:07

Here's a pic of my 5th wheel situp, I do what they call power only in trucking. Just last week pulled a single axel trl that I had to slide all the way back on. The whole slider has been moved back a bout 13 or 14" from the factory. I can just about put it on the rear axel grossing 80k with no problems, will only change the front axel by about 1k. Hope this helps some.

m16ty · 07-16-2008, 22:54

EO2NMCB, That air ride truck is going to be different than a 5-ton. All the weight of the back of the truck on a five ton is centered between the tandems because the weight of both axles rides on the trunon between the axles. On a air ride each axle is attached to the frame independent to each other so moving the fith wheel back won't change the front axle weight as much. On a 5 ton each back axle will always have the same amount of weight on each axle. On a air ride or Rayco the further you side the fith wheel back the more you will load the back axle.