Hub Motor / Direct Drive Axle Stress Safety Discussion

Hub motor and direct drives place additional stress on axles and trucks compared to gear drives or unpowered skateboards due to the torque the motor applies directly to the axle (a force not present in belt/gear drive systems). There have unfortunately been various instances and reports in the past of wheel separations (wheels falling off the board while riding) with some direct drive systems. Over time some manufacturers have attempted to varying degrees to address the issue by designing custom made trucks and axles with improved strength. Still, I’m not aware of any consensus or studies that have addressed the question: what is the minimum safety standard for axle/truck designs which are intended to support direct drive motors?

As more companies consider entering the direct drive market, we run the risk of repeating some of the safety incidents which have occurred in the past.

If you have ever experienced a wheel separation or axle damage on a hub motor or direct drive setup, please share your story.

If you have an opinion on what the minimum safety standard of strength for direct drive axles should be, please share your opinion.

Hopefully here we can once and for all address some of the known issues with direct drive truck & axle stress so that as more manufacturers begin producing direct drive systems in the future, we won’t ever have any repeat of the incredibly dangerous wheel separations which have occurred in the past.




I don’t see how the torque is applied directly to the axle considering that the motor sits on multiple bearings. That would be a horrible way to try and apply torque to an axle by using bearings. Also sounds like you are confusing hub motor between direct drive which should be specified otherwise confusion will exist.

What is your experience?


^8mm steel axle (later versions were upgraded to threaded 12mm)

Dexter, you know not what you have done.


We’ll isn’t that by design why that happened. lol Doesn’t seem like there was that many that actually were damaged though or was there?

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Fortunately it was a test version being ridden by the designer and to my understanding only the upgraded 12mm designs were shipped out to customers.

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Feel like the 8mm axle broke for different reasons.

Gonna upset Brian



peak pounds force on 8mm axle from torque (75kv, 120a motor): 855.1lbs

pounds force on axle from normal force (200lb vehicle, 4 wheels): 50lbs

assumptions: roughly 75kv 120a motor current, 8mm diameter axle

axle radius: 4mm

torque per amp: 0.127nm/a = 60/(2 * pi * 75kv)

total torque: 15.24nm = 120a * 0.127nm/a

total force at axle: 3817.5 newtons = 15.27nm / 0.004 meters axle radius

total force at axle (pounds force): 855lbs force = 3817.5n * 0.224 pound force per newton

^ the force applied to the axle from torque from the motor was a factor of 17 times greater than the normal force caused by someone standing on the board.

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You’re still talking about an 8mm axle that didn’t go into production?

years later folks still don’t agree on the root cause of the failure… some claim leverage

What’s the normal force from someone riding? Down curbs, over large tracks, doing ollie’s and power slides?

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that’s a bit more tricky to calculate because some of the energy will be absorbed by the tire, and it depends on the rider’s velocity, size of the bump & radius of the wheel, or drop distance if you’re getting air, and the weight of the rider.

I just don’t think saying

Is relivent then. We don’t just stand on skateboards.

17x more sounds like alot but the base it’s really reflective of what trucks normally go through.

keep in mind it was reported to me the 8mm hub motor axle failure occurred while the rider was accelerating from a low speed in a flat, smooth area

this is a true color image with increased saturation & contrast and I personally interpret the bluish, curved sections of the failure pattern as resulting from torque, not leverage.

(right side up)

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How do we know that the axle didn’t already have fatigue damage?

I’ve broken few axles. All bent first before snapping so I knew it was coming.

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Well the truck was only a few weeks old and I’ve never known the rider to do any jumps or other high impact skateboard tricks.

I broke one after 5 weeks. And it was strictly a A - B low power esk8. Bike paths and sidewalks only.
But I blame that on impurities in the case.

Still, whatever stress any 8mm truck would ordinarily go through from beginner or advanced riding, the motor puts an additional force above and beyond that which in this case was in the neighborhood of 855lbs, or ~17x the ~50lbs normal force per wheel.

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What about the bearing thing dexter pointed out?