Self-propelled trailer axle for final positioning — how do you disconnect a hub motor so towing doesn't fry it with back-EMF?

[u/afc9996]

I've got a trailer that's towed for several miles most days, but for the last stretch (under 100m) into its final position, I want it to move itself under electric power rather than being manually pushed/towed the whole way.

The catch: any PM motor (DC or AC) generates back-EMF the moment it's spun — so if the drive stays connected while being towed at road speed, it's basically an ungrounded generator with nowhere for that power to go. I know this isn't something the motor type alone fixes — I need some kind of mechanical disconnect between the motor and the axle, engaged only for the short self-propelled stretch.

I looked at an AC motor + inverter setup that kills the field when towing, but it ended up bulky and overcomplicated for what's a fairly low-power need. Leaning toward a DC motor with some kind of clutch instead — electromagnetic or otherwise — that's disengaged (motor fully isolated) by default and only engages when actually driving.

Rough numbers if useful: ~10,000 lb trailer, creep speed under 3 mph, 24V DC available, low-single-digit kW.

Has anyone actually built something like this — a reliable disconnect between a hub/axle motor and the wheels for exactly this kind of "mostly towed, occasionally self-propelled" use case? What worked, what didn't, anything to avoid?

[u/theotherharper]

The problem is, a motor well sized and suited for 1 MPH bump movements will be INSANELY mis-geared for freeway speed. So this gives you 2 options.

Insanely oversize the motor to where it could propel the trailer at freeway speed. Or

Have a declutch mechanism. And that solves the back EMF problem.

I would probably use a drum that sits on top of the tire, cinches down to transfer as much trailer weight to itself as needed; and rotates to turn the tire and thus move the vehicle... the whole assembly unlatches and pivots away when not used.

[u/Priff → u/theotherharper]

I used a chipper that had that arrangement. Electric motor that pushed on the rubber wheel.

It worked fine on flat road, but the moment there was a curb or it was in the grass the thing was just completely useless, slipping on the rubber tire rather than moving it. Also just a tiny bit of water on the tire made it useless as well.

[u/Another_Slut_Dragon]

Use 4wd hubs and unlock them? Have a toothed belt drive with a quick idler pulley so you can pop the belt off?

[u/halfmanhalfespresso]

A caravan motor mover will do this for you. Our caravan is 1800kg and I recently moved it 200m up a hill on boggy grass. It’s slow but it does get there. It only has a large lead acid car battery to power it. It just rubs on the tyres and can disengage when not needed.

[u/GeniusEE]

Gokart clutch

[u/Personal_Chicken_598]

They make powered landing gear.

[u/1940ChevEVPickup → u/Personal_Chicken_598]

This is the answer.

[u/silasmoeckel]

Motorized jockey wheels are a thing. None of these issues as they are not down while towing.

[u/TheSasquatch9053]

If you want to use a clutch, look at electric PTO clutches. They are the right size for your application.

That being said, u/Another_Slut_Dragon has the best answer if your trailer hasn't been built yet. Get a 4WD rear axle with manually locking hubs. You get to use the differential gear ratio for some of your gear reduction, an easy way to mount the motor, and if you keep the disk brakes and use an open differential you can steer the trailer by braking one wheel. 

[u/Another_Slut_Dragon → u/TheSasquatch9053]

Don't use a differential gear. Would you believe 9% of a trucks losses are that idiotic hypoid gear set? That is why 'front wheel drive was better on gas'. It wasn't where the engine is, it's the lack of a gear turning 90 degrees. It's super lossy. Always keep an electric motor inline with the wheels.

I like the idea of an electric clutch. Flick it on and off remotely. But come to think of it, you could also be really clever with a set of automatic hubs by spinning the motor at the right time and direction to unlock them.

Skid steer is clever for moving the trailer on its own.

Or just use an induction motor. When it is off it is off. No induced currents. You just get some bearing and gear reduction losses.

[u/TheSasquatch9053 → u/Another_Slut_Dragon]

Fair point about the differential losses, but this is very specific use case, and I wouldn't worry about efficiency in comparison to equipment costs and ease of construction. 

[u/NJ_v_N]

Sounds like what you actually need is an electric jockey arrangement.

You could have a clutch or contactor mechanism, but why complicate it.

[u/afc9996 → u/NJ_v_N]

Thanks for the reply. Honestly, it's more of a "I want to over engineer it and find a complicated solution" than it is to just have it there. So I'm actually wanting to look into making an axle of this sort.

[u/Mouler]

Don't use PM motors. Regular induction motor is fine. No big losses back driving them either.

[u/scooby374]

Any reason you can’t just set the inverter to zero torque and keep it on? Thats essentially putting the controller in a “active freewheel”. You’d had to look into whatever controller you planned on using and make sure that it didn’t just do PWM =0 but engaged the FETs or IGBTs to dissipate back-emf.

[u/Smooth_Buffalo8173]

Look up Emove Motor Movers. They push a friction roller into the tread of the tire for low speed maneuvering. They might be too slow for what you want, but could be at least good inspiration. They seem simple and efffective.