LMTV vs other military trucks - offroad

[Aernan]

I have long been an admirer of the Unimog products but alas I can't afford one and have purchased a M1078 LMTV. I believe for my use (off road camping) it should be perfect.

I have a lot of new questions about why the US military chose the LMTV and it's design features. Also how those compare with other solutions in the market. Lastly if there is a compromise how much is the compromise.

Areas of interest.

1. Ground clearance and tire size
2. Portal Axles
3. high low range transfer case and gears
4. Automatic vs manual transmission
5. Engine Torque and HP output
6. Overall vehicle weight
7. Locking/Limited slip differentials
8. Overall fuel consumption
9. Use of computerized systems
10. Intended Terrain/Environment of operation
11. Leaf vs coil springs
12. Independent vs solid axle
13. Roll Center and max side slope

From my reading it looks like the LMTV started it's life as a Steyr 12M18 but is considerably different. I think the Unimog U1300 or U1500 would be an appropriate comparison. Are there other vehicles I may have missed like a Tatra, MAN, Kamaz?

 

[Aernan]

I'll take a stab at what drove the choice of suspension on the LMTV. If anyone knows better please correct me because this is just all speculation.

The US military wanted a whole family of trucks based on the same base design so there would be some parts interchange and it would be easier to train people and keep them running.

I believe the main role of the M1078 is to haul troops and light cargo as well as tow loads. The intended terrain is on paved roads with some hauling over dirt road and unmaintained dirt roads. Fording or rivers and driving over obstructions and trenches also required.

This is the point at which I can't figure things out. The truck is very heavy compared with the 12M18 it came from at 6.3 tone vs 11 tone. Also the fuel economy 12 mpg vs 8 mpg. I could be totally wrong with both of these figures. I believe the LMTV to weight 16,500 empty and based on the fact that the 12M18 has a 6.6 liter engine I'm not clear on why the MPG would be so much higher.

If the intended role is to crank out long miles then I would guess that fuel economy would be something to optimize for. It may be possible that the extra weight is due to the much higher power engine and the automatic transmission. I have heard automatic transmissions weigh more than manuals. Lastly the top speed of the LMTV at 59 mph seems low if distance is big factor. But I have no experience driving any large diesel truck so perhaps these are normal top speed numbers.

If the above is true the most tried and true and simple suspension would be leaf springs with straight axles. This would make the truck somewhat capable off road but I would expect there to be at least a rear locking differential. Next the transmission is automatic. It's heavy but it's very easy to drive. Unless you manually select a gear it will pick a fuel efficient ratio and make driving up hills simple. My Range Rover has a high/low select allowing me to divide my gearbox in half. The LMTV only has an ultra low gear. This removes the two gears in the transfer case. It does simplify the truck but removes some of it's off road capability.

Has anyone done any crawling or technical off road in an LMTV? Is the 1st gear low enough?

 

[DiverDarrell]

The LMTV also uses a lot of commercial off the shelf parts. Something that not all MV's do. This helped to lessen the purchase cost along with the cost of maintenance. You will find lockers are available, and some have swapped in higher ratio gear sets for better highway use. Personally I would have liked to seen a true selectable 4x4 with a high low range for the auto. But the awd, works and the system chosen was the lowest bidder to meet specs. My truck has 90% of an a/c system installed. A rear locker, and high speed gears. What it really needs is an exhaust brake (newer trucks have). It's really easy to drive, quite sporty on the take off lol. Even with high speed gears the engin will still spin all 4 when hooked up to something. I love to drive my truck

 

[Aernan]

DriveDarrell can you tell me more about exhaust brakes. I've never owned anything large enough to have one. I did see a build with one retrofitted to the LMTV.

So much of what you say makes a whole lot of sense. Commercial off the shelf parts (check). I am actively investigating both lockers and high speed gears. I'm on the list for the California upgrade Certainly the truck needs an AC system and more insulation to make the cab more bearable. I'm looking forward to the auto trans. I know purist enjoy the full manual but I'm green with big trucks so it should be easier to pilot at the beginning.

It sounds like the military put out a bid with min requirements and they picked the lowed bidder. I've heard this is common so it sound plausible. Does my assertion that truck was mostly designed for reliability and road/dirt hauling as it's main use. That is opposed to a Mog with it's amazing offroad and low range hauling capacity which is excellent for very steep hills and mostly offroad (tractor like) duty.

Sounds like you have had some time to play in the dirt. How well does the LMTV perform on steep climbs and side hills? Does it outperform 4x4 trucks in the mud? I'm trying to gage how extreme I will be able to go.

 

[DiverDarrell]

Most new diesel trucks have a exhaust brake built into the variable vane turbo. Basically it blocks exhaust travel out the exhaust pipe. No matter how you look at it, it's a 17k pound truck. Serious off road is not it's thing. With a high cg side hills are a no go. Mud? 225hp diesel that can't fling the mud free from the tires. Mud=stuck. The truck absolutely needs a locker. It's one thing to get a 7k pound jeep un-stuck, but 17k is another beast. I'd take mine up some crappy logging roads, forest roads and what not. But the off camber suffer is a no-go. I've got a rubicon, or my husqvarna 701 for when the going gets tough.

 

[Aernan]

Would an exhaust brake be worth retro fitting to the truck? It looks like for turbos there are dual turbo (small/large), VNT and twin scroll turbos available. I think you can turn up the base HP but increasing the torque the transmission is torque limited. I'm not clear on what is needed maybe just a larger torque converter.

What HP/torque do you think it would take to flink mud from the tires to make it good in mud? I know for certain the Mog does not have high HP numbers so I would assume it's in the same boat. Both the Steyr and Mog are lighter so that could help but they are both also heavy compared with passenger trucks.

As far as lockers. I totally agree. The absence of one shocks me. I'm just going to guess the folks in the military must spend a bunch of time getting these things unstuck in mud/sand. Maybe this is a lowest bidder problem.

 

[DiverDarrell]

I will be adding one to my truck. If you turn up the power on a 3116, you have to have a plan for the egt's as the engine builds heat quickly. My goal will be the 290hp that the 1088's have. The military has a course for un sticking these things, but it always involves lots of people and lots of recovery gear. The lmtv is an un-armored supply truck, not usually needed to go off road in today's world. Look at the conflicts were currently in, urban, and such. Long gone are the days of crossing the steps overland. It's a great truck, but has limitations of weight, width, height, poor cg. And after completing a 3800 mile trip on my enduro, I think it could have done all but a few trails of that trip with the 1078. The limiting factor being height, and washed out roads. On a side note, if you rock crawl it, I'd like to buy your door handles if they survive the roll over best way to find out is to take it off road, and see what it can do, just have a recovery plan if you get stuck. Dirt every day did a show on it.

 

[Aernan]

I do plan on adding a pyrometer and doing water methanol injection to control the EGT and possibly improve fuel economy. I have no idea why the military didn't at least put in the gage. Looking at the CAT spec you can certainly turn up the power output of the power plant a whole lot.

Given the fact the truck is so easy to get stuck because of it's weight. Is the Mog any better at the mud? I know for certain the ultra low gears make it easier to climb very steep grades. So in that way it excels. It also has portal axles claiming some 20+ inches of clearance against the 12 for the LMTV. I could imagine some lines involve a lot of cab lean that might be avoided if you drive over the obstacle due to ground clearance. On the flip side I just watched some videos of Mogs and it looks like the lowest component is the sway bar. Not sure how undrivable the vehicle becomes if you smash that.

For center of gravity. I plan on adding 100 gallons of fuel and some 100-150 gallons of water at the frame level. I intend on locating it as low as possible. If it can be near in line with the axles it should help the side slope instead of hinder it. Chances are I won't be able to get it that low. Like all vehicles total weight is not the dominant force in side slope it's weight above the axles. Having very heavy axles and a light cab you can drive on very sloped hills.

One of the big advantages of the LMTV and Steyr platform is the cab forward design. It shortens the wheelbase which improves offroading a great deal allowing for tighter turns and easier U turns. My Rover has an excellent small turning radius. I'm hoping to find the LMTV to have the same properties. The reverse is a longer wheelbase makes for more stability at speed (irrelevant in this application). It also causes higher frame flex due to the length of unsupported member. Given the mog engine sticks out in front of the vehicle I think it's just wasted space.

 

[Aernan]

I wonder if the fuel economy of the other vehicles is higher because the tires are run at a higher PSI and the contact patch is smaller. I heard the LMTV runs 75 PSI but you can get the CTIS reprogrammed to run 100. I know commercial trucks run over 100 PSI like 110-115. The other factor might be more working gears meaning lower RPM at speed. With the high speed gear conversion I wonder if the MPG could be pushed beyond 10.

 

[DiverDarrell]

Military doesn't put an EGT guage in because at their fueling setting it never gets to a point of needing to monitor it. I wouldn't go much past 290hp tuning. They do make 350hp injectors for marine applications. But they have a much more efficient cooling system with a raw water system. The military did put 350hp engines in the Stryker with improved cooling. But those engines were overhauled at a schedule only the govt can afford. With power added you reduce life of the engine. As far as MPG. It's still a 6.6 liter engine pushing 17k. Turning up the power means more fuel. The rotational mass of our heavy wheels doesn't help. Awd all the time adding more friction. The wheel gear reduction steals more power. You would be better off putting in an 8.3 for better mpg. That way you wouldn't have to be running at max capacity of the engine all the time.

 

[DiverDarrell]

On the road I run 100 psi in my tires. Helps to keep them cool. I've only heard of someone attempting to reprogram. I've also seen a full manual system that pretty cool. I do have a problem of blowing a hub seal every time I use it. Pretty sure due to a nick in the hub where the seal presses in. RTV'ed the last seal in to attemp a fix, but have yet to try it. As for mud, the mvt's and XZL's load up quick and don't clear well. And then they dig in. Ground a jeep can run over may be too soft for the weight of an lmtv, then surprise your resting on the axles. Sand, air down, way down, easy throttle input and away you go hopefully.

 

[Aernan]

I do like the idea of adding a manual override to the CTIS in case of computer fault. I had a first generation rover with air ride. The system failed endlessly and everyone swaps it back to coil springs. I found and fitted a manual system which made it possible to air up and down the air springs but was never able to get the system reliable. I suspect the CTIS could be a similar system. Amazing when it works but endless pain when it randomly breaks. It does look like the manual and automatic systems can co-exist which is the best of both worlds.

As far as mud. I heard waggon wheels are very tall and very narrow so they slice through the mud to get down to hard pack ground and avoid generating massive drag. So that means if you have enough clearance and narrow enough tires total power is not the limit. When it comes to soft terrain like mud, snow and sand. I think if there is a hard base deeper and you can squish/dig your way down because of your weight you can go places others can't. On the flip side if it's endless depth (sand) then you just end up digging in and getting stuck to the frame.

I know for getting traction you want the largest contact patch you can get and airing down increases that contact patch. That's why tracked vehicles can go over terrible bogs. I have seen tracked vehicles go places jeeps could not. I guess there is some ratio of contact area to total weight that I don't understand that makes it possible for such heavy vehicles to go places lighter vehicles can't. Any ideas there?

Please tell me more about this air leak problem. Can you post any diagrams of this hub seal?

Of all the terrains to get stuck in I fear sand the most. There is seldom a place to hook a winch to. I have read about burying a tire. I have also heard it's a massive amount of labor. I know there are land anchors but I think they only work in dirt.

 

[coachgeo]

I do like the idea of adding a manual override to the CTIS in case of computer fault. .....

A member in here has documented in a thread how he converted his CTIS to manual only. Might can do this as override I guess. Just little extra plumbing?

 

[Aernan]

With any luck someone will make a kit with usable instructions.

 

[DiverDarrell]

Sounds like you have a project I'd buy one

 

[tennmogger]

FMTV vs Unimog, depends on conditions. As long as the mud is not deep, the slope is not great and the surface has no topography, then the FMTV will do fine. Otherwise the Unimog wins every time. Kinda like comparing apples and oranges.

If conditions favor the FMTV, it's a heck of a pulling machine. Lots of weight, ability to lower tire pressure, and with Goodyear MVTs (great tires IMHO), the FMTV does well.

For those bad conditions where opposite tires are high (or low), having no locking diffs results in 'no-go'. The only work-around is lowering tire pressure to allow tire contact, and that is very limited. I have thought about using the kneeling cylinders to help force tire contact but have not pursued that because I keep my LMTV out of the bad stuff.

 

[Aernan]

Deep mud:
I think deep enough both vehicles will get mired. The extra ground clearance gives the Mog a huge leg up in this race.

Steep Slope:
the max slope looks to be a function of how low gearing do you have. I know the mog has a whole basement of low range working gears. Absent of having massive low range gears a very high torque motor can drive higher gears enough to pull up steep hills. The U1300L is listed as producing 363 Nm which is 267 ft lbs of torque. That's not a terribly high number. The turbo version (U1700L) has 520 Nm 363 ft lbs. Still not an impressive number compared with the S&S at 700-900 ft lbs. Which is now low compared with the cat C7 1166 Nm 859 ft lbs of torque.
the other limit to steep slope is traction. Given the LMTV with a larger contact patch I would assume it actually would out climb a Mog.

Suspension and Articulation:
At this point the Mog has coil springs which have a lower "stiction" and react quicker to bumps. This would be excellent if it has all independent suspension but it instead has solid axles which are slow to respond due to mass and they further impact this by adding portal axles. Any idea which vehicle has heavier axles? When going slow enough the portals really help with ground clearance. In all other applications the extra mass is a big problem.

This is just all speculation on my part. I have not seen any truck trials involving both the 12M18 and any mog or S&S and anything else.

Your last point about the locking diff. I totally agree. I have no idea why the military did not spec a solid locking diff in the rear and possibly the front. I plan on adding one in the rear for certain. The front locking diff may have been left off to avoid drivers snapping the axles when making tight turns on hard surfaces. Or maybe it was cost.

 

[tennmogger]

I believe you have researched and already have your own answers!

 

[DiverDarrell]

For 99.9% of what the military uses the LMTV for it does not need a locker. It's a non combat supply truck for moving people and supplies in the rear. If it gets stuck they call for a wrecker. And when operating forward they have tow vehicles in convoy and the truck pre rigged to be towed. The LMTV is not a front line vehicle where off road ability is a must. It's only awd to allow for continued operations in less than ideal road conditions think Subaru. The mog is probably better at a lot more things, but parts are harder to get, and I got my lmtv for 6k, it's hard to find a wrecked 404 for that. And it's made in America

 

[snowtrac nome]

mine is currently stuck lockers I don't believe would have helped broke a bunch of 4 inch tow straps last night trying to move it with a Volvo 180 gave up going back with a hydraulic excavator on tracks so I can lift it and reposition it. I don't think the oe winch would have been much help but I'm now thinking 5 ton wrecker so I have access to the killer drag winch.