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.
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.
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.
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.
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.
