Smaller wheels and hub motors

[amberwolf]

I’m guessing 18” rim in bike terms means 14” tire in motorcycle terms.

I don't recall a crossover in this size; hopefully I'm wrong on that.

There's a thread here somewhere that lists all the crossover sizes between motorcycle and bicycle, for tires and rims. Also another website out there that has a page on it that I ran across once a few years back (don't recall the name though).

 

[MylarStar]

Yeah I love tearing things apart and rebuilding. I’m pretty new to the e part of e bikes so I’ve been gathering a hoard of parts and motors to mess around with.

I’m kind of obsessed figuring out how to mod a motor to be like 15:1 reduction for super slow hill crawls that won’t force the motor down to a third of its revs it wants to be at with full current.

 

[amberwolf]

I’m kind of obsessed figuring out how to mod a motor to be like 15:1 reduction for super slow hill crawls that won’t force the motor down to a third of its revs it wants to be at with full current.

That would be: stick it in the frame and use a reduction on it's output.

In theory some DD hubmotors have enough space inside to install a reduction, but it would require completely redesigning and rebuilding the whole motor, because the part you need to reduce is the part the spokes are attached to. :/

Geared hubmotors are already built the right way,but except for some of the fatbike versions there's not enough space inside to put additional planetary reductions. At least, not at a width that would support the torque required without excessive wear or failure.

 

[MylarStar]

This is how far I’ve gotten with my dream bike. I’ve been crazy about this frame since the minute I saw it but I didn’t want to go through the whole process of replacing all the stock e bike parts again, I’m not a fan of the whole fat tire/ fat rim thing. I like dirt bike tires on BMX rims. So just bought the frame, motor, and controller/display direct.

 

[MylarStar]

That would be: stick it in the frame and use a reduction on it's output.

In theory some DD hubmotors have enough space inside to install a reduction, but it would require completely redesigning and rebuilding the whole motor, because the part you need to reduce is the part the spokes are attached to. :/

Geared hubmotors are already built the right way,but except for some of the fatbike versions there's not enough space inside to put additional planetary reductions. At least, not at a width that would support the torque required without excessive wear or failure.

Yeah I know it’s a tall order but since I have a 190mm frame now so there might be some way to join together two shells to make room or something.

There is one motor I almost bought that’s a 750w AKM and those are double reduced. It makes sense because at first the stats for the motor seemed weird. Like it’s 750w, has 90nm of torque but they had it on a 20 amp controller and it only peaks at 1000w. I was thinking the gearset from that one might be usable.

I saw this old TDMH motor or something like that that has an internal gear hub built into it but they are probably very hard to come by and highly sought after as a collectible. Have you ever seen one of those?

 

[amberwolf]

Yeah I know it’s a tall order but since I have a 190mm frame now so there might be some way to join together two shells to make room or something.

That's a possibility. Poeple have double-wided dd hubs (including rewinding the paired stators! so it's psosible to do the mechanical work. Fitting the gearing in would be easier.

There is one motor I almost bought that’s a 750w AKM and those are double reduced. It makes sense because at first the stats for the motor seemed weird. Like it’s 750w, has 90nm of torque but they had it on a 20 amp controller and it only peaks at 1000w. I was thinking the gearset from that one might be usable.

There's some posts referring to various double-geared-down units, at least one big thread for that that neptronix posted in if it helps to find it. had some test data, takeaparts, etc. bearing discusdsions, bevel vs straight, etc.

I saw this old TDMH motor or something like that that has an internal gear hub built into it but they are probably very hard to come by and highly sought after as a collectible. Have you ever seen one of those?

TDCM; the gear hub is not part of the motor power path, just the pedal path. IIRC grin tech has it listed in their discontinued items. I have a thread around here somewhere or a post that pondered some ways to use the igh for motor power, but don't think i came up with anything useful.

 

[bananu7]

15:1 gearing sounds cool, but realistically... if your geared 750W has 90Nm of wheel torque...

A 12x5 Mxus 3K Turbo has 1.4Nm/A. With powers touching 5kW, and a controller capable of driving high enough phase currents, it could probably out-pull virtually all smaller geared motors, while still remaining usable on the straights, would have much less maintenance and is readily available (and reasonably priced).

It's a bit outside of the specs I mostly spend time looking at, but e.g. Sabvoton ML7260 is spec'd for max 170A of phase currents. If my math is right here, such a setup should theoretically produce ~238Nm at the wheel.

If you can accept a 9.4kg motor in your rear hub, I'd give it a thought.

 

[MxusMick]

BMX bikes have very short wheel bases, maybe squirrelly at higher speed. Hardtails take a hit in big potholes, I imagine. Make sure the motor axle can fit your rear drop outs and it's wide enough as well. The weight of the motor, battery, and rider is gonna be max capacity IMO. I have seen BMX ebikes but don't think it's the best way if you want speed.

 

[Chalo]

TDCM; the gear hub is not part of the motor power path, just the pedal path.

SRAM Sparc was like that too, but incredibly unimpressive with its two internal 100W brushed motors that couldn't benefit from variable gearing. It was at the time more of a confirmation about e-bikes for pedal bike snobs than it was a worthwhile product.