is geared hub motor feasible in a 20-inch rim?

[marka-ee]

I'm building a Tadpole trike with 20-inch rims all the way around. I like to be able to go at least 50 kilometers per hour, so I'm wondering if it's feasible at all to use a geared hub motor to achieve this. I typically use direct drive hub motors, but I'm curious about the geared ones, as they can be lighter. My concern is that the RPMs of the wheel might be just too high for a geared setup. What do you guys think?

 

[amberwolf]

You'd have to see what ERPM you'd end up with, and whether that's too high for your controller. Same for what voltage is required to drive the motor you choose to that speed under your loads in your wheel size.

The motor itself doesn't have an RPM limitation, unless it's an inrunner with SPM or a brushed motor, in which case things could fly off the rotor/armature above it's physical limits.

I know the GMAC / Phaserunner6 will do 20mph in a "20"" wheel (probably closer to 21/22" actual with the tire size I use), but it is VERY loud to me. (I prefer the silence of DD hubs, but I certainly like the startup and low-end torque the GMAC has vs any DD for the same power level in this wheel and application). It might do the speed you want; you'd have to see what Grin's site says for the PR's ERPM/etc limits, as well as what voltage it will take to make the GMAC spin that speed under your loads, in your wheel size.

 

[offGridDownUnder]

My concern is that the RPMs of the wheel might be just too high for a geared setup.

I don't know, but I can provide some experience. You'll need to do calculations.

I have a Greenspeed Big Wheel Magnum (see signature link). The rear wheel is 26", and I'm using a Maxxis Hookworm tire.

The motor is currently a GMAC fast wind mounted as a Stoke-Monkey behind the seat.

The motor has a 26T Gates sprocket on the ISO brake mount bolts which connects to a 60T Gates sprocket on the rear wheel ISO brake mount bolts - reduction is about 2.31. This means the motor spins at that multiple of the rear wheel rpm. Internally, the GMAC has it's own 5:1 reduction.

This spins the motor into regen on downhills or when I apply brakes - I am using a Baserunner with brake switches.

The motor self-limits to about 27km/hr in this configuration on downhills. With added field weakening current of 10, 15, 20A, the top speed increases up to 37km/hr. The temperature has not exceeded 60C in mixed riding for several hours in warm weather (Perth, Australia summer).

I don't know the limits possible. Tomorrow I expect to swap the GMAC out and replace it with a Shengyi SX2 leaving all other aspects the same (apart from possibly changing some current limits in the Baserunner).

 

[neptronix]

20" wheels and smaller typically have added gear friction per MPH that reduces the efficiency during cruise. This is a big negative unless you desire massive torque.

A DD is a lot more suitable for this situation, especially one that is undersized.

Grin all axle motor at 87% efficiency ( great! ):

( check this link to edit the simulation ) Motor Simulator - Tools



GMAC 8T in the same condition, only 82% efficient




These results are on an upright bike.. a tadpole trike will do better..

 

[Marc S.]

I'm building a Tadpole trike with 20-inch rims all the way around. I like to be able to go at least 50 kilometers per hour, so I'm wondering if it's feasible at all to use a geared hub motor to achieve this. I typically use direct drive hub motors, but I'm curious about the geared ones, as they can be lighter. My concern is that the RPMs of the wheel might be just too high for a geared setup. What do you guys think?

Yes its feasible. The small "250W" geared hub motors don't last long at these speed/power levels (BTDT), but the bigger "500W" geared hub motors can take it (Bafang CST, Grin V2, etc). Especially, if you use some 40-45ml ATF for cooling and lubrication of the internal free wheel clutch. If you need the trike to climb hills, you might even want to update the planetary gears to steel gears.

Do you have any experience riding tadpole trikes? Any experience building tadpole trikes? (a proper steering geometry is key!)

How do you wanna use it? (just for fun, or as a daily commuter?)

I love my tadpole trikes, but they need some experience, good brakes* and good tires** if you do 50km/h or more, regularly.

*I'm particularly fond of Sturmey Archer XL-SD 90mm drum brakes (Ginkgo is THE best source for drum brakes)
**Conti Contact Urban are reasonably quick, offer reasonably good puncture protection and are top of the heap in wet grip

 

[Marc S.]

20" wheels and smaller typically have added gear friction per MPH that reduces the efficiency during cruise. This is a big negative unless you desire massive torque.

A DD is a lot more suitable for this situation, especially one that is undersized.

Grin all axle motor at 87% efficiency ( great! ):

( check this link to edit the simulation ) Motor Simulator - Tools


These results are on an upright bike.. a tadpole trike will do better..

If you use "Semi Recumbent", the Motor Simulator computes results much closer to measured real world data with a tadpole trike!

 

[neptronix]

If you use "Semi Recumbent", the Motor Simulator computes results much closer to measured real world data with a tadpole trike!

Yep.

 

[marka-ee]

Yeah, kind of what I expected that the best would be a direct drive hub motor, which I'll have to somehow get a hold of. It's difficult because I'm in the EU. In the meanwhile, I have an 8fun (bafang) geared hub motor from a folding bike which I can use temporarily for playing around. I know it's not going to go very fast though. The bike is an Optima Rhino, which is rare. They only made about 36 of them with full suspension, like what I have. It has hydraulic disc brakes and I'm switching the front tires to 1.75 inch wide Continentals with flat protection. I didn't want to get a non-suspended bike as the bike paths around me are in lousy condition. I'm looking into the aluminum-spoked 20-inch scooter type hub motor wheel combos as that might be an option if I can figure out the drivetrain and the brakes. If anybody has tips on where to order something within the EU, so I don't have to pay exorbitant taxes and shipping, that would be fantastic.

 

[docw009]

To go 50km/hour on a true 20" wheel, you need a motor that can turn 500 rpm, I was looking at the specs for the Bafang CST. and it says that is the top limit. It might be a little tricky to spoke into a 20" rim and keep a 1X crossing pattern. Sure, I think a geared motor is feasible.

I would prefer geared over direct drive. Seems like a DD motor with enough torque would fill up most of the inside of a 20" wheel. Also don't care for the added drag when pedalling in low power or coasting, but that's me.

 

[Chalo]

It might be a little tricky to spoke into a 20" rim and keep a 1X crossing pattern.

By the time you can't get spokes to insert straight into the rim with cross-1 lacing, radial is always better.

I was skeptical about that at first, but my oldest, highest mileage hub motor wheel has radial lacing to a 20" rim, and it not only hasn't ever broken a spoke, but it hasn't ever gone the least bit out of true or lost any tension in many years and tens of thousands of miles. This despite having been on a ~500 lbs GVW cargo bike the whole time.

I would prefer geared over direct drive. Seems like a DD motor with enough torque would fill up most of the inside of a 20" wheel.

I have helped three coworkers set up Crust Clydesdale forks with Jump bike Bafang G020 hubs laced to 20" rims. Those bikes all work great and see lots of mileage. The entire electrical system fits tidily under the load platform. I've even considered swapping out my old Crystalyte X5305 front hub on my front loading cargo bike for a lighter, freer-running geared hub, but so far I've been content to leave well enough alone.

 

[marka-ee]

I've been playing with the motor simulator and trying different things with a 9C motor and have some basic concerns because on a 20-inch wheel it seems that the speed I would get would not be good. When I plug in the specs for my other 2 wheel recumbent with a 26 inch wheel using a 9C motor on one of the variations I do get the 50 kilometer per hour maximum speed which is a confidence builder on the simulation. However for these motors available on Amazon I don't know how many 'turns' they use. They don't specify of course and I'm a little bit worried about ordering one and getting something that would be quite limited in speed on this small wheel. I'm still considering one of the 1800 watt Kunray/Vevor motor kits running a timing belt and with this approach I could have some control over the gear ratio to optimize the speed but it would be a lot more work. They sell these motors at a rating of 1800 watts and 3000 watts, but the motor actually looks identical. I'm wondering if the internals are different. Also, the cooling holes look more suitable for a salt shaker than cooling holes. Could they have made the holes any smaller? Sometimes I'm really confused about these red Chinese engineers!

 

[Chalo]

Also, the cooling holes look more suitable for a salt shaker than cooling holes. Could they have made the holes any smaller? Sometimes I'm really confused about these red Chinese engineers!

If the rotor has a pumping effect, then small holes probably move enough cooling air without unduly inviting in pieces of magnetic trash to cause damage to the motor.

Chinese "copy of a copy of a clone" engineering is still somewhat more trustworthy than "that don't look right" engineering from somebody who hasn't done any testing or made any measurements.

 

[marka-ee]

For an interim solution on this 20" tadpole trike, I installed my old Bafang 8-fun geared hub motor which was stamped with a 250 watt sticker. running it at 10s or sometimes called 36 volt battery pack I managed to get a flat straight line speed of 34 kilometers per hour which is more than I was expecting from this old motor. The steering is a little bit more sensitive than I was expecting. I wouldn't call it twitchy, but it's certainly not as confidence-inspiring as a normal bike or my two-wheel recumbent. Perhaps this is caused by the unsophisticated single arm suspension, I'm wondering? I'll be replacing the 1.25 wide tires with 1.75 in the next few days. I hope that helps it a little bit.

 

[amberwolf]

Is there any slop in any of the joints? Meaning, while holding the bars still (or tied in place securely) can you make any part of the steering system move, at all, from any other part of the steering system other than the part that's secured? Even a tiny bit of slop can make for steering issues.

If it's really just too sensitive, changing the ratio of handlebar movement to steering movement can help, or sometimes adding a damper.

 

[marka-ee]

Is there any slop in any of the joints?

Not really. When I lifted it off the ground and moved the wheels around, I was surprised how tight the joints were actually. I'm thinking a damper would be a good idea, so I might try that soon. Also, I'll change the tires and see what that does. I measured the amount of toe-in and I think it was excessive close to a quarter of an inch. So I'm going to try to adjust it to be closer to 1/8. Would the amount of toe-in affect the jitteriness of its tracking at high speeds?

 

[Daytriker]

With Tadpole trikes, your geometry & steering is essential for decent control at speed. ANY error in Ackerman, tracking, frame flex, or front to rear weight balance is going to effect your handling, braking & control. The faster you go, the more these deficiencies will show themselves. Riding a Tadpole trike at a constant 30 mph is not something that is enjoyable unless you have a sophisticated suspension system. Take a look at HP Velotechik Scorpion, Azub Ti-Fly, Steintrikes Wild One & those suspension systems will give you an idea of how much engineering goes into a good suspension & steering system. For a home built unit I would go with your 500 or 750 watt geared motor (Mid Drive or Hub) as they both have Pros & Cons. You might not hit your constant 30 mph but will probably find your trike is a handful at a little slower speed anyway.

 

[neptronix]

Yeah, kind of what I expected that the best would be a direct drive hub motor, which I'll have to somehow get a hold of. It's difficult because I'm in the EU. In the meanwhile, I have an 8fun (bafang) geared hub motor from a folding bike which I can use temporarily for playing around. I know it's not going to go very fast though. The bike is an Optima Rhino, which is rare. They only made about 36 of them with full suspension, like what I have.

ebikes.ca's Shengyi SX2 handles being in a 20" better than any geared motor i've seen. It's also small and supposedly pretty quiet. Probably the best non-DD choice unless you have mega hills to conquer ( it doesn't have the thermal headroom to take acceleration for long periods of time )

 

[Hummina Shadeeba]

Is there an internally geared hub motor that has a strong enough planetary gear that it won’t break when doing the torque limit of the motor?
I assume it would need to be a steel gear and preferably helical to reduce noise.

Are there speed limits to mechanical gearing? Why?

 

[neptronix]

No, the mechanical limit of the gears in every geared hub motor i've seen is lower than the absolute maximum torque.
The small case and inefficient heat dispersion is another thing that keeps you from maxing out a given geared motor.

The MAC has very, very strong gears... but not the efficiency in tihs size of wheel ( way too many poles )

 

[Papa]

I assume it would need to be a steel gear and preferably helical to reduce noise.

Are there speed limits to mechanical gearing? Why?

I strongly discourage steel planetary gears in gear hubs. Why? Because composite planets acts as a mechanical 'fuse'... and the cheapest to replace if/when it fails.

There is always limits to gearing - and likewise... limits to physical size, weight and cost when attempting to make them stronger.

 

[neptronix]

Also noise.

 

[Hummina Shadeeba]

I strongly discourage steel planetary gears in gear hubs. Why? Because composite planets acts as a mechanical 'fuse'... and the cheapest to replace if/when it fails.

There is always limits to gearing - and likewise... limits to physical size, weight and cost when attempting to make them stronger.

Mid drives have gears that are able to survive the max motor torque and I see no reason a hub motor’s gearing shouldn’t be able to do the same. A hub has more torque potential..why isn’t there a motor with a burly steel helical planetary gear?

 

[neptronix]

1) 3x the added friction = less efficiency
2) Achilles heel of a geared hub motor is it's much more limited ability to shed heat. You just made that worse with more friction. The gears survive the torque, but the motor overheats faster now, and your Nylon gears melt anyway.

 

[neptronix]

Mid drives have something geared hubs don't:
stator/rotor contact with the case, which can now act as a heatsink.

In a geared motor, your only real path is air to the motor shell ( not great ), or the axle.

This is why we can make insane torque but not handle the heat that comes with it.

 

[Hummina Shadeeba]

Mid drives have something geared hubs don't:
stator/rotor contact with the case, which can now act as a heatsink.

In a geared motor, your only real path is air to the motor shell ( not great ), or the axle.

This is why we can make insane torque but not handle the heat that comes with it.

Im assuming a hub with gearing, despite having less room for actual motor, generally runs cooler given the same load than without a gear.

There’s no hub motor that comes with strong steel quiet internal gears or these replacement gears?

Maybe with the planetary gear heat transfer is further obstructed?