Tuesday, January 3, 2017

M101A1 Axle Swap

The M101A1 is a great choice for a full size camping and bug-out trailer.  I've had several smaller trailers, around M416/Canadian M101 size, and find myself both running out of room and never taking them on anything more that fire-trails.

The 3/4 ton M101A1 has been in service with the military for decades, and you can be sure they are built as the best compromise between weight and ruggedness.  For example, I find the thickness of the metal making up the bed to be very thin and easily bent and dented.  But the reality is that it's strong enough to haul over 2,000 lbs and cosmetic blemishes won't end an overland adventure.  The frame and axle, on the other hand, are built very strong (and heavy).


Here's my M101 with no modifications.  I purchased it from GovLiquidation for $500, and it appeared to have just gone through a military rebuild.  The paint was fresh, tires were new, and axle hubs looked like they had just been serviced.  I towed it back from the Sierra Army Depot in Herlong, CA to the Bay Area, a five hour drive, with no issues.

My first modification is to convert the wheels from the military Budd type, to a standard lug pattern so I can use a "normal" spare tire if I ever get a flat.  A few people have tried converting the hubs with no luck, and consensus seems to be to simply swap in a new axle.  Technically, a 3,500lb axle should be sufficient and I could get hubs to match my Jeep Wranglers 5x5 lug pattern.  However, the military used something closer to a 6,000 lb axle and I have to believe there was a good reason. I'm sure a 3,500 lb axle would be fine if the trailer never left the asphalt, but I'd like to know I could run this down washboard roads with nasty pot-holes for days on end.  Here's a great video from the Adrenalin Campers folks for inspiration:


One wrinkle is that 6K axles can't accommodate the 5x5 hubs to match my Jeep.  It's possible I could find an adapter, but that would push the wheel centerline far outside the trailer hub face and put extra stress on the smaller outside wheel bearing, as well as introduce another possible failure mode with the aluminum adapter.  In the end, I decided to go with a 6x5.5 hub that matches many Toyotas and Chevys.  In a pinch, I'll just go make some new friends :)

For the first phase of my build, I'm using idler hubs and will lose the parking-brake feature.  I'll run this for a year, and if I can't live without the brakes, I'll add them back in.

Here's the axle build tag.  Hopefully this helps someone else with their measurements:


Note that the 6K axle has been derated to 4,879 lbs (lower right corner).  This happens when the spring perches are too far in-board from the hubs.  I'm assuming Dexter's computers calculated this new value.  For this trailer, 4,879 lbs is still far over the GVWR and I have the benefit of the bigger bearings.  But I'm left to wonder, are the 3,500 lb axles derated to under the GVWR... does anyone know?

For tires and wheels, I scored a set of three well worn 33x10.50 BFG tires mounted to 15" steel wheels with a 15mm offset, all for $60.  Thank you Craigslist!

The axle swap is very straight forward.


I've heard problems with spinning off the u-bolt nuts over the years of built up paint, so I wire wheeled them first.  The air-impact wrench made short work of removing the nuts, but only one of the bolts broke free from the lower casting enough to move.  The rest were frozen/rusted to the casting.  I used liquid wrench and a heavy hammer to try to free them up, but after several days of soaking I broke down and simply cut the u-bolts off.


Next wrinkle was to free up the Budd wheel lug nuts.  Turns out the nut size is 1 1/2 inches and I didn't have a socket.  Both Orchard Supply and Home Depot didn't stock sockets that large, but Lowes had one for $7.  One important note is that the lug nuts on the right hand side are left-hand threads, meaning you turn them *clockwise* to loosen them.  The ends of the lug nuts are labelled with a "L" or "R" to help identify this.  With the impact wrench and the proper socket, they spun right off.

Next up was to put the trailer on jack-stands, remove the wheels and tires, unbolt the parking levers and cable stays, and drop the axle.


Ten minutes later the axle was liberated, and a fresh new one bolted in its place.


I used a new u-bolt and plate, tightened the u-bolts in a cross-pattern to 60 lbs, and put on the new wheels/tires.


I'm very happy with how this turned out.  Here's the trailer cost so far:
  • $500 M101A1 trailer
  • $265 Custom Dexter axle (see axle tag above for details)
  • $42 UPS ground shipping for axle
  • $60 (3) 33x10.5r15 BFG tires and steel wheels
  • $15 3" x 7" x 1/2" u-bolt kit with plates and nuts
The axle swap only cost $382 and I now have a spare, a lug-pattern that's readily available, and hubs with bearings I can service with standard parts. 

Another benefit of an axle swap is to drop the trailer weight down.  These trailers weigh 1340 lbs with the canvas top, bows, original axle, tires, and wheels.   Here's a break-down of what I removed:
  • 112 lbs each for 9.00x16 military tire and Budd split ring wheel
  • 208 lbs for the axle with drum brakes
The new axle and tire weights:
  • 69.6 lbs for each 33x10.50 BFG tire (well worn) and steel 15" rims
  • 65.8 lbs for the axle WITHOUT drum brakes
Running the math, I saved 42.4 lbs on each side by swapping the tires and wheels, and another 142.2 lbs by going to the Dexter axle.  Total weight savings was 227 lbs.

What's next?  I'm going to cut off the lower u-bolt castings from the old axle and bolt it on the new axle.  This will let me use the stock shocks and bump stops.

I hope this helped answer a few of the questions I was left wondering after reading hours of posts on M101 axle swaps.  If you have any questions please post in the comments section below.