M1070 HET - Suitable for Toter-Home Conversion?

[john4002]

Interested in building a toter-home conversion on a M1070 HET, basically welding a small travel trailer to the rear frame, and use a flat bed trailer to haul a rock crawler offroad.

The platform has several advantages: decent offroad capability, automatic transmission, insane towing capacity, and the units being offered for auction have almost no use at an attractive price point.

Fuel economy is an issue, approximately 4-5mph, but the bigger concern is top speed for interstate travel.

Looking at the Oshkosh spec sheet for the M1070A0 HET, the solution which springs to mind is swapping out the Allison CLT-754 5-speed transmission for the Allison 4800SP 7-speed with overdrive from the A1 HET, while retaining the Oshkosh 55000 dual-speed transfer case. This combination would increase mechanical top speed to approximately 70mph:

Allison 4800SP final drive: 0.64
Oshkosh 55000 high range: 0.98
Rear Axle Ratio: 7.36
Tire overall radius: 25.9in
70.10mph at 2100rpm

Before I buy the truck, I thought I'd ask the experts for their input, and see if anyone has done an overdrive swap to their HET. Should I anticipate any issues with mating the M1070A1 transmission (Allison 4800SP) to the M1070A0 (Oshkosh 55000) transfer case?

 

[red]

For starters the 16x20"s are only rated for 55mph and yes that rating is at maximum pressure/max weight but still a concern considering the weight of the truck. And I'm one of the people that's alright with pushing the limits on tires but 70mph on those is a really bad idea.

The axle hubs are going to be getting very, very toasty spinning that fast and will come apart sooner rather than later.

Cost wise, much better options.

Fuel economy, cut it down to 1-2mpg realistically. 2 stroke Detroits and fuel efficient do not go together especially at those rpm's. Owners on here are doing great to get 4-5mpg puttin around town at lower rpm and speed. Chat with truck drivers who used to drive rigs with the 8v92 and it's the same thing.

Also welding a travel trailer to a truck frame will tear the camper to shreds because the truck frames flex by design. A spring loaded/pivoting mount setup is required.

If you're looking for a offroad capable expedition truck a m820 or m934 is a much better starting point. A realistic cruising speed of 65mph takes some work to get to safely/reliably but is the max without basically a completely different commercial truck. Doing all that to a m820/m934 will also cost less than just the purchase price of a HET, about half of the price actually.

 

[john4002]

For starters the 16x20"s are only rated for 55mph and yes that rating is at maximum pressure/max weight but still a concern considering the weight of the truck. And I'm one of the people that's alright with pushing the limits on tires but 70mph on those is a really bad idea.

Gearing for 70mph at 2100rpm means 60mph is 1800rpm, extra gears allow for increased performance and fuel economy. Not necessarily a desire to actually drive 70mph for any length of time, instead it's about reducing the profile of an M1070 HET as a 45mph roadblock on the interstate.

The axle hubs are going to be getting very, very toasty spinning that fast and will come apart sooner rather than later.

The hubs are engineered for a chassis payload of 45,000lbs, hauling a combined load of 240,000lbs, all generating heat; converted to a toterhome we're talking about a 10,000lbs travel trailer on the chassis, hauling 15,000lbs of steel flatbed and rock crawler. Removing the 5th wheel and some of the rear chassis would further mitigate this load.

I'm certain a hub replacement, and locating suitable parts, is quite the undertaking, but this risk seems marginal.

Fuel economy, cut it down to 1-2mpg realistically. 2 stroke Detroits and fuel efficient do not go together especially at those rpm's. Owners on here are doing great to get 4-5mpg puttin around town at lower rpm and speed. Chat with truck drivers who used to drive rigs with the 8v92 and it's the same thing.

This is the main concern, but the cost to repower the HET with a Cat diesel etc. doesn't make sense, as it's not a commercial truck, and use will be limited.

2-3mpg across 150,000 miles annually or more is real money, but an adventure toterhome which will see perhaps 3000 miles annually, it would take 10 years or more to break even on the cost of repowering the truck.

Doing all that to a m820/m934 will also cost less than just the purchase price of a HET, about half of the price actually.

Looking at pre-existing toterhome's on mostly commercial trucks, being able to take the toolboxes and home further into the trail, before offloading the toys, is appealing. 8x8 and 52" tires are the advantage of the HET, it's designed to carry the world on its back, at the cost of weight and fuel, which in this case serves as a disadvantage.

An existing commercial truck toterhome for $50k with 300,000 miles or more, and it'll drive like a semi, or a 4000 mile M1070 HET for $15k; I think they're the same money by the time everything's done.

 

[red]

A functional M1070 HET for 15k nowadays or are you talking about initial auction price? Usually they go for 2-3 times that at auction or private sale.

Bear in mind I work on heavy diesel equipment/trucks. And you're far from the first person to want to make any of these trucks faster. For which, state your actual goal when it comes to speed. Saying that your aiming for 70mph on 53"s gets you input towards that goal. Coming back and saying it's 60mph instead changes my opinion on the tires but still not on the hubs. Also with speed you need to talk to people who drive the M1070 already. Apparently the rear steer is a hazard at higher speeds.

Correct these trucks will haul alot of weight but high rpm takes far more of a toll on mechanical things. Working as a heavy diesel/hydraulic mechanic the easiest example I can think of is with a wheel bearing. Holds up loaded trucks at highway speeds with no issue for years but spin it fast for just a little bit in your hand and it will come apart violently. High rpm is far more destructive than weight.


You're discussing this with someone who is working on a similar expedition rig plan, trying to sell me on the advantages of more drive wheels is already known haha. It's a cool idea just the wrong platform.

 

[Shark Bait]

The mighty HET. I loved driving mine (just sold it after a few years). Mileage wise, here in the hills of WV is more like 1-2 mpg. The rear steer if aligned correctly has never been an issue, at first mine wasn't aligned very well. It is a big truck! Two lane roads are not your friend. Put foliage on the roadside and your mirrors will get beat up. Without weight on the rear (15k lbs might help) you'll be engaging the front axle just going up gravel hills. It is a fun truck to drive though, above 47 mph. Maybe 50 or so, the over speed light will blink on the dash. Downhill I've seen 51mph. You'll use a couple tubes of grease to lube that thing, and a bit of time to do it. There are a lot of grease points on it. Tire assemblies are heavy, around 700-750- if I remember correctly. Don't get a flat. Lol. It is a smooth ride compared to other MV's I've had, I'd say smoother than my LMTV even. But, the HET is over 40k lbs empty so it should. If you do build one please keep a pictorial diary of it on here, we'd love to see it grow!

 

[Nomadic]

Awesome idea. It would be so fun to go off-road in a HET toterhome and spend time out in the country. I spend most of my time as a nomad in the Nevada backcountry and have been eying these millitary trucks and researching the practicality of owning one and camping out of it while doing my work. For me, if the max speed minus ~5 MPH is too slow then I need to find another vehicle. These vehicles were designed for a target max speed. So many engineering decisions revolved around that target number.

Also think about the speed of travel climbing grades. If you go from Vegas West on NV-160 to head out of town these trucks will be driving very, very slow over the Spring Mountain grade. I'm guessing maybe 30-40 MPH with a tail wind.

On the flip-side, leveraging the design of the vehicle can work around some of the slow speed problems. Like taking the mining and power line roads across the mountain passes instead of the freeway. Having a streetable canam escape pod to make a beer. grocery or parts run from camp into town can solve some of the on-road impracticality issues too. Driving at off hours of the day like avoiding leaving town on a Friday. Travel at night.

If the value of the HET is negated by the slow max speed, another thing to consider is to find middle ground such as converting a used power line truck. They are big and bad, 4WD, cost less than a big rig and they will do the speed limit on flat ground.

If you are going to stick to Nevada, I think you should go for it. Nevada is the place to accommodate that kind of vehicle with most of the land being public and accessible by a HET type vehicle. Diesel is sold everywhere. You could bring 50 gallon drums of fuel and a pump with you to extend the range.

 

[Goomer214]

If your going to go to all this trouble your talking huge money. Find a crane chassis that has all wheel steer. The. No later the size you can go a lot of places. Fat hydraulic pumps
Make
For good winches

 

[ketsueki82]

For a time I was considering heavier off road vehicles for the expedition build I'm wanting to do but I trimmed it down for a couple reasons that might not be a concern to others.

First reason I found out there are weight limits in some park/game lands, this is due to conservation issues for some areas I guess.

Second reason is where I will be camping the most in the beginning has quite a few bridges with a low weight limit. The boys in blue don't care that the bridge is so short it only holds one axle at a time and the axle is technically under the weight rating of the bridge they go by the GVWR.

Just some food for thought before you pull the trigger on such a beast, it might not apply to your situation at all though.

Sent from my SM-A215U using Tapatalk

 

[ROARKY]

I’m working on a short camper, m818. Have considered doing a m920 as well.