Questions about M720 and brakes

[Coug]

So I'm currently buying an M720 shelter dolly. I've been reading up, and can't find anywhere that anyone that has done an electric brake conversion with one of these.

I do not have an air brake capable vehicle to tow it with, so looking at what are my possible options.

it looks like there are only 2 possible ways to do a conversion.
way 1 is buying 2 separate electric over hydraulic actuators, and install one on each the front half and rear halves of the dolly.
While this is the simplest method, it's also pretty spendy (looks like parts would be over $1500 for 2 actuators) and also the problem that I do not know if a standard trailer brake controller is able to power 2 actuators at the same time. If not it adds a lot more parts and gets stupid. Up side is it is 100% reversible no problem.

Way 1.5 would be just put brakes on the front dolly set and have no brakes for the rear. Wouldn't be an issue with a larger tow vehicle, especially if I wasn't loading the shelter dolly up at max.



Way 2 is the one I'm considering to be better from a complexity standpoint is to get the spindles replaced with modern trailer spindles. Would require new hubs and drums, as well as backing plates as well. Not sure on cost as I have no idea what a shop would charge to replace 4 spindles, but I'd imagine in the end would probably cost around the same as the electric over hydraulic. I also can't use the current backing plate flange on there as it is 6 bolt, modern electric brakes use either 4 or 5 bolt backing plates (I could probably drill holes in the backing plate, but that requires a little more precision to center the holes properly than I currently have available to do)

Plus sides with this one are parts have been standardized for a long time, and I don't expect the industry to change them any time soon. after the conversion parts would be readily available at any trailer parts store.

Down sides, I can see several. One is the fact that is is non-reversible. I suppose I could take parts off of a second dolly to repair it, and there seem to be plenty of these out there. main downside for me is I would have to either change to a different bolt pattern for the wheels, or the actual axle capacity would be lower.
I'll explain that last one. The spindles for 3500 lb axles have matching hubs with the 5 on 5.5 bolt pattern that the dolly currently has. 2 axles like this = 7k lbs. This is about 1k lbs lower than the current rating. Other spindle would be for the 5k and 7k axles, but the hubs only come in 6 and 8 bolt lug patterns.

(and before anyone starts arguing that it's only a 3 ton dolly set and that 2 3500 would be fine, like everything else it's rated for 3 tons of load. The dolly itself weighs just over 2k lbs, with 3 tons of load that's an actual weight of just over 8k lbs, or 4 tons)

I'm sort of leaning towards the 3500 lb spindles because I don't see myself ever putting 4300 lbs worth of crap into an S280 shelter (empty weight around 1650, at 7k lbs total weight I'd still have over 3k lbs of available load). I'm primarily looking at it as a possible camping shelter, though using it as a portable tool room isn't out of the question, which could get heavy. I could also use the current wheels with the 3500 lb spindles.
On the other hand if I go with 5k spindles, I could bump up to 8 lug hubs. This gives me greater braking capacity (3500 lb use 10 inch drums, 5k uses 12 inch, it currently has 11 inch which hasn't be used for decades apparently other than military) With the 8 lug wheels I could, in theory, put HMMWV tires on it (in theory a lightly loaded M720 with S280 is still within the HMMWV towing capacity) which would increase ground clearance, make it so I have to carry fewer spare tires, but also make it so I have to have the shelter jacked up in order to hook up and detach the shelter. Also not sure if there would actually be enough clearance for the larger tires, but other sizes of 8 lug wheels/tires are readily available (edit: looking at pictures, there isn't a huge size difference, so I think they would fit no problem).

(not my dolly, was just getting it hooked up to a shelter behind the barn where the 5 ton the dolly belonged with wouldn't fit)

(to answer another possible question, the hubs it comes with cross reference to WWII era Bantam T3 trailer and also front axle on jeeps of that era. The spindle is straight, not a stepped down configuration that takes a larger diameter bearing on the inside and smaller diameter outer bearing. Both inner and outer bearings use the same part number)


I've also toyed with the idea that maybe a sleeve could be machined to fit over the existing spindle to match it to the 5k spindle profile. i theory it shouldn't change the weight capacity of the existing spindle, and still reversible. Would have to modify either the brake backing plate or mount for backing plate to match this.



Does anyone have any thoughts about this? I would greatly prefer constructive thoughts and reasons to go along with those thoughts, as opposed to just flat out telling me it's crazy or won't work.

 

[Zed254]

Is there a cheaper way to install air brakes on your tow vehicle? I'm thinking an electric/air system on your truck and then run the air hoses to M720 axles. Need air compressor and the electric actuation system. I have no experience with air brakes, but thinking 1 system should be less expensive to assemble.

 

[Coug]

Is there a cheaper way to install air brakes on your tow vehicle? I'm thinking an electric/air system on your truck and then run the air hoses to M720 axles. Need air compressor and the electric actuation system. I have no experience with air brakes, but thinking 1 system should be less expensive to assemble.

The main issue with this is I would be limited to a single tow vehicle.
I have my HMMWV, and a Ford F450 that I own that can tow this. I also have an F350 available as well, then a dozen other potential trucks to tow it with if those all fail for some reason.

I agree, just adding an air system to the tow vehicle might be the cheapest way, as it's really just a compressor, tank, lines, and multiple switches and sensors, but it does add a lot more complexity as well. Doing either an electric over hydraulic or full electric brakes to the trailer keeps things much simpler overall.
My F450 might not be bad to install an air compressor on, but my HMMWV would be.

Reading through all the threads on this site and others, for other trailers with these types of parts (either air over hydraulic or surge, or even no brakes at all) the solution is almost always get an axle with the electric brakes on it made and installed. Bantam T3, M416, M101/M116 series, M103/105 series, the solution for towing behind civilian rigs and needing brakes that work is always go straight electric.

 

[Coug]

looked up the parts I'd need for the spindle conversion. Looks like for cheap parts it would be about $1k before the welding. Using Dexter EZ-lube parts (which I like) would be a few hundred bit more. After I have the dolly in my posession I'll take off the relevant parts and talk to some axle places to see what it would take to convert them.

Another downside of switching to pure electric I didn't list before is losing the parking brake feature. There are electric brake backing plates that have this feature on them, but they cost a lot more than the standard ones, so likely if I do it it will only be for one half of the dolly for now. (in reality the electric portion of the brakes is pretty cheap, so adding them later I wouldn't be out much up front)

 

[M1078MAN]

Old USAF dolly set you have there..and wow, what a great looking HMMWV.

Interesting thought, probably doable with the Doe Rah and some me. hubs are bolt on, I am sure there is some hub out there that you could fab to make your new electric hubs compatable. What kind of shelter are you trying to move

 

[frank8003]

So state how heavy is the puller and how heavy is the pulleee.

 

[Coug]

Old USAF dolly set you have there..and wow, what a great looking HMMWV.

Interesting thought, probably doable with the Doe Rah and some me. hubs are bolt on, I am sure there is some hub out there that you could fab to make your new electric hubs compatible. What kind of shelter are you trying to move

I've looked and looked. There is nothing modern that matches the spindle design, and nothing that bolts to the backing plate mount.
The current ring for the backing plate has 6 holes in it. All modern backing plates are 4 bolt for 3500 lb and less, and 5 bolt for larger than that.
The current design is a separate hub and drum, so there isn't any surface for the electric brake magnet to slide against as far as I can tell, plus no idea if the spacing would be correct either.
Brake drum part number eventually crosses to early/mid 50s cars. 11 inch drums just don't exist these days.

I could probably talk to Dexter or another manufacturer and have something made up, but I hate making phone calls, and custom work would probably negate any savings on the spindle replacement.

Currently I don't have a shelter, only picking up the dolly because it's a price I can't refuse for a complete, working dolly (even if I just use the tail lights off of it I got my money's worth out of it. Literally)
most likely will be an S280 shelter, and if possible in the future I'll come up with some type of flatbed for it as well that is capable of holding farm equipment and small cars.

 

[Coug]

So state how hevey is the puller and how heavy is the pulleee.

How heavy either one of them is depends on what I'm doing. This is the main reason for making sure the M720 would have modern brakes all around if possible

If I use the HMMWV I'm rated to tow 4200 lbs. My truck currently weighs about 8k lbs with the BEOD on the back and basic gear I carry. It's an M1123, so I'm still well within it's weight rating (10,300 lbs truck, 4200 towed)
For load behind the HMMWV, the empty dolly weighs just under 2100 lbs. An empty S280 shelter weighs about 1600 lbs. That affords me 400 lbs of gear inside the trailer itself, which isn't much but it's enough for things like a bed, chairs, a table, and so on. Anything with weight can ride in the back of the HMMWV during transport.

I wouldn't expect to do any off roading with the trailer behind me like the military rates everything for, so in reality I could probably get away using the H1 tow specs which is 8k lbs. I have no plans to do this, but just wanted to point out that what the military rates things for is not the same as what the civilian world rates things for, as they are 2 different environments and 2 different missions.
If you add the two ratings on the M1123, 10,300 truck and 4,200 trailer, it's 14,500 combined, so weight shouldn't be an issue when it comes to pulling. I'm more worried about making sure it can stop.

If I tow it behind my F450 the empty truck weighs over 8k lbs. It's tow rating is 10k on the bumper tow (mine has a permanent pintle/ball combo rated for 16k lbs) and a tow rating for 5th wheels at 18k lbs, for a gross of 26k lbs.
Trailer in this case could weigh up to the maximum of 8100 lbs (2100 for the dolly, 3 tons for shelter and anything I want to put in it) and it wouldn't matter. I regularly tow trailers heavier than that as either bumper pull or gooseneck with my truck and it doesn't care. Different beast though using the dolly, as there is no weight being added to the rear axle, so once again, the importance of fully functional modern braking.



No matter what I did, the main limitation driving this setup would be speed. I'd pretty much be restricted to 55 or less (likely less) to keep things under control, because this particular type of trailer setup isn't exactly known for high speed stability.

 

[frank8003]

"No matter what I did, the main limitation driving this setup would be speed. I'd pretty much be restricted to 55 or less (likely less) to keep things under control, because this particular type of trailer setup isn't exactly known for high speed stability."

Always get where your goin. Speed of transit is not that important.
The world time about You goes really fast. Slow down and enjoy.

 

[M1078MAN]

Thats no joke, speed that is. What I meant buy fabricate..is have a backing plate made to match the M/XM720 hub square. Then you can bolt on your hubs of your choosing.. Tires, rims

They do great going one way, but you better know what your doing when you go in reverse.. Cant tell you how many bent towbars I have replaced

 

[Coug]

Thats no joke, speed that is. What I meant buy fabricate..is have a backing plate made to match the M/XM720 hub square. Then you can bolt on your hubs of your choosing.. Tires, rims

They do great going one way, but you better know what your doing when you go in reverse.. Cant tell you how many bent towbars I have replaced

I've backed a few of the 1022s when I was in the military in the motor pool, but only because nobody else could. (would love to have a 1022, but that's definitely above my tow ratings)
Also done A trains when I was getting my CDL back in the day. If I really get into a place I can't handle, the answer is disconnect the front and back dolly and reverse their positions, then how it out with the shelter on backwards. A whole heck of a lot easier than trying to make it reverse.

Can also use ratchet straps to secure the tongue and keep the steering from moving to one side or the other, at least for some backing. Also makes it easier/more stable to tow when you don't have a shelter, but so far the most stable solution I've heard of id secure the tongue/front dolly to a long beam going from end of the tongue to the rear dolly, and secure it in such a way that it lifts the front dolly off the ground. At that point you can tow it any speed you like.

 

[Mullaney]

How heavy either one of them is depends on what I'm doing. This is the main reason for making sure the M720 would have modern brakes all around if possible

If I use the HMMWV I'm rated to tow 4200 lbs. My truck currently weighs about 8k lbs with the BEOD on the back and basic gear I carry. It's an M1123, so I'm still well within it's weight rating (10,300 lbs truck, 4200 towed)
For load behind the HMMWV, the empty dolly weighs just under 2100 lbs. An empty S280 shelter weighs about 1600 lbs. That affords me 400 lbs of gear inside the trailer itself, which isn't much but it's enough for things like a bed, chairs, a table, and so on. Anything with weight can ride in the back of the HMMWV during transport.

I wouldn't expect to do any off roading with the trailer behind me like the military rates everything for, so in reality I could probably get away using the H1 tow specs which is 8k lbs. I have no plans to do this, but just wanted to point out that what the military rates things for is not the same as what the civilian world rates things for, as they are 2 different environments and 2 different missions.
If you add the two ratings on the M1123, 10,300 truck and 4,200 trailer, it's 14,500 combined, so weight shouldn't be an issue when it comes to pulling. I'm more worried about making sure it can stop.

If I tow it behind my F450 the empty truck weighs over 8k lbs. It's tow rating is 10k on the bumper tow (mine has a permanent pintle/ball combo rated for 16k lbs) and a tow rating for 5th wheels at 18k lbs, for a gross of 26k lbs.
Trailer in this case could weigh up to the maximum of 8100 lbs (2100 for the dolly, 3 tons for shelter and anything I want to put in it) and it wouldn't matter. I regularly tow trailers heavier than that as either bumper pull or gooseneck with my truck and it doesn't care. Different beast though using the dolly, as there is no weight being added to the rear axle, so once again, the importance of fully functional modern braking.



No matter what I did, the main limitation driving this setup would be speed. I'd pretty much be restricted to 55 or less (likely less) to keep things under control, because this particular type of trailer setup isn't exactly known for high speed stability.

The young man that I bought my - M989A1 Heavy Expanded Mobile Ammunition Trailer (HEMAT) from rigged up a small air compressor and auxiliary tank in the back of his Dodge 1 Ton truck. Hooked the "out" on the compressor tank to the emergency line on the trailer. He used it that way regularly. He hauled the trailer down from Ohio that way... Slowly opening the valve applied the trailer brakes on the back axle.

That is definitely NOT the ultimate solution, but it would allow you to move the trailer around on your patch of dirt and have braking power... I personally don't think you would want to be on the highway that way. Especially if you were in an accident!

 

[Coug]

The young man that I bought my - M989A1 Heavy Expanded Mobile Ammunition Trailer (HEMAT) from rigged up a small air compressor and auxiliary tank in the back of his Dodge 1 Ton truck. Hooked the "out" on the compressor tank to the emergency line on the trailer. He used it that way regularly. He hauled the trailer down from Ohio that way... Slowly opening the valve applied the trailer brakes on the back axle.

That is definitely NOT the ultimate solution, but it would allow you to move the trailer around on your patch of dirt and have braking power... I personally don't think you would want to be on the highway that way. Especially if you were in an accident!

Yes, I definitely agree, it's physically doable, but definitely not ideal. I'd much rather do it the right way and get full usability from it, rather than do it "just enough" to make me the first one at the scene of the accident.

 

[Mullaney]

Yes, I definitely agree, it's physically doable, but definitely not ideal. I'd much rather do it the right way and get full usability from it, rather than do it "just enough" to make me the first one at the scene of the accident.

I particularly like your analysis of why "hacking" a solution isn't a good idea. Being the first one at the scene of the accident will definitely cause you to have a really bad day. Especially when the lawyers get involved... And that assumes nobody got killed...

 

[fasttruck]

Remember, the USDOT standard, which has been adopted by reference in many states to keep the highway trust $ coming in, requires some type of operational brake, hydraulic, air, electric or what have you, for any towed device RATED for over 3000# gvw, not what it weighs empty. Post 12 has an idea for multiple tow vehicles; make a skid mounted compressor with a reservoir, trailer brake valve, lines to trailer emergency and service glad hands and an electrically operated applicator valve working off the trailer electrical plug on the truck. Power it with a 12 volt electric motor or a pony motor if nothing else. Otherwise spend the big bucks to change the brakes on the trailer or get a deuce with airbrakes. This can be like making lemonade with oranges.

 

[Coug]

Remember, the USDOT standard, which has been adopted by reference in many states to keep the highway trust $ coming in, requires some type of operational brake, hydraulic, air, electric or what have you, for any towed device RATED for over 3000# gvw, not what it weighs empty. Post 12 has an idea for multiple tow vehicles; make a skid mounted compressor with a reservoir, trailer brake valve, lines to trailer emergency and service glad hands and an electrically operated applicator valve working off the trailer electrical plug on the truck. Power it with a 12 volt electric motor or a pony motor if nothing else. Otherwise spend the big bucks to change the brakes on the trailer or get a deuce with airbrakes. This can be like making lemonade with oranges.

Umm, yes, that is the entire point of this thread that I am indeed going to make sure that I have brakes that function fully and to do this it would involve changing out the brakes, which will very likely cost the big bucks. I'm just seeing if I am overlooking anything major somehow with all the possible options out there. The industry seems to have gotten pretty standardized for everything, so the idea would be to modernize everything to current standards (option 2 at least, option one would still leave me with 50 year old brakes and difficult to source parts for them if they ever fail)

What is listed in post 12 is about as sketchy as it comes, I would NEVER recommend someone make up a system that requires independent operation from the vehicle brakes. Add to that the fact that if you made one that tied into the vehicle brake pedal, you'd still have to tune it for every vehicle you move it to.
I also have no idea how large of a compressor would be required for this, but I have never seen a truck that used an electric air compressor to provide the power for braking, and don't even know if that is legal.

As for the 3k lbs, that is commercial code. For non commercial it varies by state. In Texas for example, it's 4500 lbs gross weight (not gross weight rating) before brakes are required, and between 4500 and 15k lbs they aren't required as long as you don't do over 30 mph.

 

[Coug]

So to add some pictures and some more info/thoughts since looking at it closer.

HMMWV wheels won't work on it, too much backspacing, and I'd rather not make the spindles stick out any further than they already are.

The wiring input into the dolly is actually a commercial 7 pin round plug (not 7 pin RV) with the other end 12 pin NATO. Intervehicular cable between the two dollies is 12 pin NATO. This means adding a circuit for electric trailer brakes will be extremely easy.

How it sits on a 12 ft flatbed trailer. Tires come exactly to the edges sidewalls do, tread is a couple inches in), and my truck is 8'6" wide.




picture of the hydraulic pump plate just in case anyone needs it


Tire load rating, absolute max load on the tires alone is over 11k lbs.


Data plate on the chassis giving inflation pressures for the airbags at different loads. I find it amusing that the manual states max gross of 8k, but the info on the trailer goes up to 10.5k.


This last picture with the data plate makes it pretty obvious that I shouldn't try to cheap out and use the spindles for a 3500 lb trailer (would max me out at 7k lbs) and instead need to use 5200 lb axle spindles.

So current hubs have a 5x5.5 bolt pattern, only available these days on 3500 lb axles, so I will have to bump up to a different bolt pattern, either 6 or 8 lug. The current wheels also seem to have the bolt face centered, so finding wheels that match that might be a little more difficult.

Now doing some more searching, the cost of converting the spindles is even higher due to requiring new wheels and tires (though can probably find some used ones of reasonable price) I'm still up at least in the range of the electric over hydraulic, so I've been looking into them a bit more. I'll do my thoughts on that as the next post.

 

[M1078MAN]

Those are new style 85up (in my mind 750x16 tires on 1 piece rims, with gobs of offset, I seem to remember there being an 84 and older 700x16 NDT tire and wheel that fit the bolt pattern with less offset.

At least I seem to remember swapping between new and old style rims

Yup, that was ofter disputed the 10k thing

 

[Coug]

So here are my current thoughts for going with an electric over hydraulic (EOH) brake actuator (or rather using two of them)

I found that someone does actually make a brake controller that works with 24V electrical systems.

Then discovered that the EOH actuator only requires 5 amps from the brake controller, it takes the rest of the needed power from the 12V accessory system. Only thing to note with this is depending on weather, each actuator requires 25-40 amp feed in order to function (40 amp is below 0 F so for my area I'll only require 25 amps)

Here is a list of parts I am considering for this conversion.

Brake controller:

Tow-Pro Elite Electric Brake Controller

The award-winning Tow-Pro Elite is suitable for both 12 volt and 24 volt vehicles and is the only brake controller on the market that offers two types of braking options in one dash mounted unit.

redarcelectronics.com

The award-winning Tow-Pro Elite is suitable for both 12 volt and 24 volt vehicles and is the only brake controller on the market that offers two types of braking options in one dash mounted unit.

Electric over Hydraulic brake actuator:

HYDRASTAR®

Hydrastar Electric Over Hydraulic & Trailer Disc Brakes

www.hydrastarusa.com

Then just have to do the break away battery and switch.


Now, the issue I'm coming into here is with 2 actuators I really need to have a 50 amp fused 12V input for them to share. I suppose I could run 24V accessory power through one of the wires in the intervehicular cable and then use a converter to bring it down to 12V at double the amps. The issue here though is there is no marking on my cable, so I don't know the wire gauge inside. I doubt though it's the 10 gauge I'd need, it's much more likely to be 14 gauge (which is the marking I can find on the pigtail for my M116A2 trailer) which means 15 amps power at 24V would only give me 30 amps of 12.

What I am thinking about this issue is installing a larger battery on the trailer. The minimum required is a 5 amp for the breakaway system, times two for two actuators, so 10 amp minimum. I will do this, but also considering adding an additional battery to act as the power source for the brake actuators. This also fixes the problem of being able to use the trailer with a 12V vehicle (once I change out the light bulbs at least) as the vast majority of the driving I do doesn't involve a lot of braking so plenty of time for the battery to recharge between activations. Larger is better, so I was thinking something at least 20 amp hours capacity, and maybe higher. Hooked in parallel with the actuators, it will act as buffer for heavier power draw and recharge when not being used.

If I just install a cheap 24-12V converter in the tow vehicle then it only charges at 15 amps, but on the other hand with a simple 7-12pin adapter I'll be able to use the trailer with any 12V tow vehicle with no further modifications. If I run the 24 to the trailer then I'd have to use a different intervehicular cable in order to use it and keep the battery charged.

Anyone see any major flaws with this idea, or possible additions/upgrades to it?

 

[Coug]

If you can't tell, I'm using this thread to write down all the various thoughts going through my head on the subject. I still don't know what I'm going to do or even if I will end up doing anything, but found this to be an interesting problem to work through and find possible solutions for. Hopefully it does help someone else in the future, if for nothing else other than to see they aren't the first one to think about it and gives them a starting place or questions or answers they hadn't considered.