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Anyone know the ground pressure for a FMTV?

DieselAddict

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Curious if anyone has measured the contact area of the tires or have seen any numbers for ground pressure on a FMTV or LMTV?

I've done some searching but haven't found anything useful yet.
 

Guruman

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I wonder how this is "officially" calculated? It's not like the contact patch is a constant. Do they measure aired down? on what surface? I mean concrete will give a smaller contact patch than soft dirt.

Seems like a bit of a moving target, hopefully someone has the answer.
 

DieselAddict

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For sure. I'm interested in hard packed gravel or pavement. If there aren't any published numbers I may see if I can make a contact patch print to use as a starting point to calculate it.
 

Ronmar

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The Goodyear military tire brochure list footprint area for the MVT’s at 95PSI(155 sq/in), 55PSI(174 sq/in) and 40PSI(220 sq/in). So a 20,000 pound LMTV would deliver 32.2, 28.7 and 22.7 pounds per sq/in to the ground at those respective tire pressures?
 

Green Mountain Boys

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While I am sure there is a dusty document somewhere that contains a nice chart of the data you are seeking, I am sorry to say I do not have it. But, if you do not require precise numbers it is easy to calculate some estimates.

Ground Pressure(PSI) = Weight of vehicle/Tire surface contact area in Square inches

Do not confuse Ground pressure with Tire pressure. Both are measure in the same units (PSI).

Weight of the vehicle is pretty easy to estimate based on the data plate and estimate your load weight.
Tire surface contact area can be a little more challenging to estimate. It will change based on the tire pressure and the surface conditions. On softer ground the tire will sink in a little and cause an increase in surface area in contact with the ground. This increase in surface area will decrease the ground pressure even though the weight of the vehicle remains the same.

If we assume the width of the tire in contact with the ground is 15 inches and the length of the contact area is 16 inches then 15in x 16in = 240in^2 for 1 tire.
Lets assume all the tires have equal contact area. Multiply the number of tires by 240. Lets say it is an LMTV with 4 tires. 4 tires x 240in^2 = 960in^2
Lets assume the vehicle has a small load on it and the total weight of the vehicle is 20,000lbs.

20,000lbs / 960in^2 = 20.8 pounds per square inch.

If the ground is a little soft and the tires sink into the ground slightly. Then you need to recalculate the tire surface contact area. Lets assume the tire width remains the same at 15 inches but the length increases from 16 inches to 18 inches and the loaded truck weight has stayed the same at 20,000lbs.

15in x 18in x 4 tires = 1080in^2
20,000lbs / 1080in^2 = 18.5 pounds per square inch

If you reduce the tire pressure using CTIS you will also increase the tire surface area and decrease ground pressure.

The numbers are not very useful unless you have something to compare.


ObjectGround pressure (kPa)(psi)
Hovercraft0.7 kPa0.1 psi
Human on snowshoes3.50.5
Rubber-tracked ATV5.1650.75
Wheeled ATV13.82
Diedrich D-50 – T2 drilling rig26.23.8
Human male558
M1 Abrams tank10315
1993 Toyota 4Runner / Hilux Surf17025
Adult horse (550 kg, 1250 lb)17025
Bagger 288 excavation machine17025
Passenger car20530
Adult elephant24035
Mountain Bicycle24540
Road racing bicycle62090
Stiletto heel3,250471


I can tell you from experience, I have driven my loaded LMTV across a muddy area without sinking in more than 2 inches and never spun a tire. If I had driven my 4x4 F550 dump truck that weighs 10,000 lbs empty across that same mud I would have been badly stuck.

I have also driven in deep snow up to 40" deep and not even come close to getting stuck. In snow, you have to watch the snow conditions. Traction is best when it it well below freezing. I like it best when it is below 20F, sub zero is even better.

I have driven on muddy roads and driveways and I leave less of a rut than a car or light pickup truck. I have used the LMTV to flatten out rutted driveways with the wide tires. Under the right conditions it can do a pretty good job.

In the spring in Vermont the roads begin to thaw from the top down. So you get a layer of mud on top of frozen ground. The frost can easily reach 6 feet deep and the mud layer forms on top. Under these conditions even the best gravel can turn to into complete mush. A road that was solid one day can quickly thaw and become impassable to almost any vehicle. Vermont towns will "post" the roads in spring conditions limiting the weight of vehicles on 2 axles to 15,000lbs in an efforts to protect the roads from damage. So when a road is posted I am not supposed to drive on it with my truck even though the LMTV has less ground pressure than a car or light truck. The law was not written with high floatation tires (low ground pressure) in mind. I spoke with my town road manager and showed him the truck and he said he would not mind if I drove on the roads in the spring with the LMTV as long as I used common sense. That won't help in another town but at least it is a start. Oops, I should say for those who have not been to Vermont, we have more miles of unpaved roads than paved roads. Both, paved and unpaved roads are "posted" in spring.
 

Awesomeness

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I don't know the ground pressure, but I can tell you that adjusting the CTIS mode drastically affects it. For example, in soft sand the tires will dig a lot (e.g. it feels like 2-3 rotations of the tire for what should be 1 rotation's worth of forward motion), and when you hit the Sand mode on the CTIS it's just driving normal again.

Acela has a nice CTIS guide with some pictures, and sources listed. Knowing that most of Acela's other documentation is borrowed from military documentation, it's probably out there (if it's not one of the listed sources). https://assets.website-files.com/59...Inflation Systems (CTIS) Explained 220227.pdf
 

Mullaney

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I don't know the ground pressure, but I can tell you that adjusting the CTIS mode drastically affects it. For example, in soft sand the tires will dig a lot (e.g. it feels like 2-3 rotations of the tire for what should be 1 rotation's worth of forward motion), and when you hit the Sand mode on the CTIS it's just driving normal again.

Acela has a nice CTIS guide with some pictures, and sources listed. Knowing that most of Acela's other documentation is borrowed from military documentation, it's probably out there (if it's not one of the listed sources). https://assets.website-files.com/5903c3c12540264c1656022d/621fc6c450a32e259722909f_ACELA - Central Tire Inflation Systems (CTIS) Explained 220227.pdf
.
Wow 3 pages of reading that completely explains what CTIS does and the very basics of how it works... Pretty amazing.

The pictures in the PDF show a lot about the tire footprint
Lifted the picture below from the ACELA document:

CTIS Pressure vs Tire Footprint.jpg

.
 

coachgeo

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.
Wow 3 pages of reading that completely explains what CTIS does and the very basics of how it works... Pretty amazing.

The pictures in the PDF show a lot about the tire footprint
Lifted the picture below from the ACELA document:

View attachment 860702

.
not that it is of any importance really... those pics are of a Unimog document....

 

Awesomeness

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not that it is of any importance really... those pics are of a Unimog document....

Good find. I knew the pictures weren't from an LMTV - tires aren't right, and there is a step in the picture - but I wasn't sure what it was from.

Here's a PDF copy.
 

Attachments

Mullaney

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not that it is of any importance really... those pics are of a Unimog document....

.
Agreed that it is different from what we have, but the idea is there. Especially for the newer guys and this will get them a running start...
 
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