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Greenspeed Magnum Big Wheel with 'stoke monkey' motor and Gates Belt drive

Aug 28, 2021
There's more to do, but this works well now.


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Very interesting setup - I think the motor arrangement is nearly the "ultimate" trike solution, as it allows retention of gears etc, and the GMAC even offers regen braking?

The Magnum looks well-suited, as I know it is strong, but also has room beneath the seat and a good mounting location. Had you considered belt-drive for the final stage? The open fork rear frame would seem to make that easier. I wonder if this idea is adaptable to any existing trike that have full suspension also? That way, the hub motor weight could be kept off the rear swing-frame element. As it is on a rigid frame trike, what are you seeing as the advantages over using the GMAC as a spoked wheel hub? Thanks
I haven't yet ridden enough to prove out all the possibilities, so some of this is speculative still. You can see the progression of electrics I have gone through in my signature - I would have stayed with the first if my body had cooperated, and the second, but events overtake me.

As well, some of my choice is due to circumstance of location and supply chains. Another major factor is the goal of transportation that will last the balance of my life - I am near retirement now, and I'm attempting to choose transport I can maintain and get parts for. It can also be my main power source - I'm investigating other approaches to do without refrigeration, heating, etc. I need electricity for a computer, a light or two, and perhaps a radio. I think this could also run some power tools.

With that, keeping in mind some of this is speculative:
  • The GMAC motor provides regen. Although it is marginal, I think the not-yet-released Grin All-Axle rear motor will also fit (they tell me the outer shell is the same - and I designed for that, although my fabricator may not have perfectly accomplished the location). The GMAC is geared, and I am much more interested in transport that will carry me and perhaps a trailer up hill as I age. Going fast is not my priority - function and reliability are, along with minimal dependence on supply chains and low carbon footprint.
  • The regen may mean I can charge the battery by pedaling on a stationary stand. This is not proven - there are other priorities first. But that means both power and exercise with a comfortable seat, even when the weather is poor. Grin tech support and I have discussed the possibility, so I'm open to finding a configuration that provides it.
  • The regen means I may never wear out brake pads in normal riding. There is enough pull in the brake levers to trigger the brake switches before any mechanical braking occurs, while still applying full mechanical brakes before the levers bottom on the bars. Grin says the regen is enough to permit a constant trickle that overcomes any motor drag while pedaling even when I don't want to add motor power. So far, this trike uses the least power of any of my conversions.
  • It's not a Magnum, it's a Magnum Big Wheel. It is the only Greenspeed model that can mount fat tires, and I have 65mm rims for all the wheels, although I haven't built them yet - there are details about motor mount and position to work out still. Fat tires provide suspension, and also are notably better off road. Again, I am not interested in speed (well, enough speed), I'm interested in quality of life. I have done experiments with the stock 2.125" tires, and they will go through all but 'sugar sand'. I think the fat tires will let me ride out onto the beach, and I like that. I also wouldn't mind living on a semi-rural property, and the tires may make the difference in what I can access.
  • With the motor mounted nominally as the rear wheel, the derailleur and chain are close to whatever brush, sand, mud, water I ride through. Now, they aren't. With the motor mounted nominally, I would require two motors to be able to switch between road and fat wheels. Now, I only require one motor to change wheels - the extra hub is about 1/8 the cost of the motor. I am using 'Dirt Jump' hubs and ignoring the single-speed freehub on the right. The flange spacing is wider than a hub for a full cluster so there is less 'dishing' required. I bought fat rims with the spoke holes nearly all at the midline, as opposed to many that have them staggered along the outside of the rim, so the triangulation will hold for those, too.
  • As well, with the motor moved, I can enclose the secondary drive between the motor and rear wheel. I may also enclose the primary drive from pedals to cluster and derailleur. The crankset is an Efneo 3 speed planetary drive - it no longer contributes to chain length change, and I may shift when stopped.
  • I have kept the stock 11-36 cluster, but moved to the short cage version of the Microshift deraileur so it clears the frame cross-piece as it goes through it's range. Using the Efneo limits the chain length change thus to 25 links.
  • I can vary the overall gearing by my choice of motor and wheel sprocket. I used Problem Solvers 16T and 22T sprockets, and the gearing works well. I do want it lower, and the Problem Solvers are not engineered for this - the mounting holes are too loose for the rotational speed and load, and the chain is not seating fully in the sprockets. I found VeloSolo for the 16T sprockets, and I'm investigating having the corresponding wheel sprockets made.
  • The Big Wheel rear fork does not taper, and mounts a 135mm OLD hub. The arms are oval, and the span between is about 130mm. A Gates belt is 12mm, and then play is required, and distance between belt and tire on the inside. If I forgo fat tires, I think a Gates belt would work. My fabricator is a Gates (and Rohloff) associate and not afraid to try such things, but my desire for fat tires and concerns about supply chains are leading me to go with a chain drive. Waxed and enclosed, I should get long life from a chain, and they are very widely available later. Another problem with the Gates belt is the sprocket on the motor - the motor output is the 44mm BCD ISO brake rotor mount. Immediately outboard from that is the torque arm - there is very little space. The motor housing tapers away towards the inside then, but there are no Gates sprockets which mount to an ISO brake rotor pattern, and fabricating something is quite uncertain. I'm not sure Gates wants to share their material specification to permit high quality welding and concentricity is very important at these rotary speeds and clearances. It's in the too-hard basket, alleviated by my desire for interchangeability with fat tires.

The jump from motor-in-rear-hub to stoke monkey was a big one, cost twice what my fabricator estimated, and I'm very pleased that it has worked out as well as it did - instead of wondering whether to put more $$ in it, I'm riding a very workable solution while noting some more items that need change. It could have been messy, but it's confirming as it is.
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That Magnum can fold for transport I think? Do the modifications prevent that? I wonder if anyone has done something like this arrange the cassette on the output or second stage side, so that the ratios are available to the motor? Seems like the function would be basically the same as a crank mid-drive? Not sure a cassette and derailleur could function well on the drive rather than driven side.

You mentioned your fabricator works with Rohloffs etc - I think an IGH hub would offer advantages - but I understand you want easy and inexpensive way to swap for wider tires etc.

My trike is road-only, driven by BBSHD at the boom, and shifted through a Shimano IGH at the rear. So far it's a good road combo. Perching the motor weight out front and pulling all the power through a long chain is not the best, but it works. Have to resist the urge to use all the power and just fly around go-kart style, rather than dial it back to a real pedal assist machine. No regen or motor braking, unfortunately.
That Magnum can fold for transport I think?
The trike can fold. I have not tried yet. The motor mount may fall ahead of the cross-members, but I suspect instead I will need to remove the motor to fold.

I agree an IGH would offer advantages, and it could work with this set up. I did decide that the possibility of enclosing the cluster and derailleur was good enough to get me what I wanted, although it was a gamble. That's down the road, but seems feasible now I have the concrete implementation working.

I'm in Perth, Australia. All the states and territories here have adopted European style electric bicycle rules - power cuts out over 25km/h, no more than 250w, throttle only to start moving from stopped and not past 6km/h (in some states), max 10km/h on footpaths. I can configure this in the Grin CA, (and just plain ride sensibly) so I'm legal on the road here. Off-road I can use the full power. The consequence is that I have another reason not to aim for speed - if I behave politely on the roads and paths, no one will bother me. But I have too much invested, and gain too much from this, to risk having it taken away. Off-road I can use the power, and on the road I can still go 50km/h+ if I can pedal that fast, or on a long downhill (which happens).

Here, a throttle-controlled 'bicycle' is classed as an electric motorcycle even if it has working pedals. Using it on the road requires passing all the laws that apply and getting insurance. I want to stay in the actual bicycle space, as an electric bicycle doesn't require that.

As time goes on, I expect the electric bicycle laws here to change, but I still don't see a good reason for me to go faster than a healthy person on a non-electric bicycle can go. I do look forward to relaxing the power limits to facilitate cargo loads, though. I'm still working on the gearing for this - I can use a few more ratios below what it can do now - 18.8 gear inches.
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How is this different from a normal rear geared hub?
If you look at the pictures, you can see that the motor is not in the rear wheel - it's behind the seat. There is more detail about the perceived and expected advantages in my reply above.

This motor is geared internally, but doesn't have any range of ratios internally. It does mount a standard freehub cluster on it's input side which permits me to change my input gearing to match my cadence, and the motor adds it's power to that and transmits it all through the chain on the left side to the rear wheel.

The second rider on a tandem bicycle has no steering input - they can only add power. It's common to refer to that person as the 'stoker' - they only stoke the engine. On an electric bicycle, a 'mid drive' or 'hub' motor designates the concept by where it is mounted. For an assist motor that helps pedaling, but isn't obviously one of those other types, the term 'stoke monkey' was adopted - it's a stoker, but not a person.

This uses the motor as a stoke monkey.
I built an e-trike based on HP Velotechnik Gekko for my son and now I'm thinking about building one for myself. I'm thinking full suspension and the same 2 speed hub motor by Xiongda. My son is using this set up for six years. It's a low power system. It could be OK for Australia.


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Very cool setup - the picture obscures the chain routing from boom front through any idlers and to the motor mounted cassette - that's a unique part of this setup. Can you do regen braking? I am assuming the right side chain is freewheeled and the belt drives the motor continuously? I am thinking of a setup where a trike or recumbent idler under the seat is replaced by a motor similar to yours (maybe smaller), driven by the chainring first stage, but able to drive the second stage by passing additional power through to the rear second stage. Shifting still handled at the rear hub, as the motor can drive though that. Yours would seem to offer regen, unlike what I am thinking, which is technically not unlike a conventional crank- or "mid" drive, only moved aft to the idler position.
Can you do regen braking? I am assuming the right side chain is freewheeled and the belt drives the motor continuously?
The pictures at the beginning of the post show the right side. The only change from then is that the idler is now a silvery wheel (a sloppy temporary build to test the idea and to be replaced by a more robust implementation soon). Otherwise the chain on the right comes from the front chainring (an Efneo 3 speed planetary hub) through the stock tubes and the one extra silvery idler (to bend the chainline up to the motor/cassette). The motor freehub provides the freewheeling on the chain so that the motor does not drive the pedals.

The left side motor output is a sprocket bolted to the motor's ISO brake rotor mounting threads to drive the belt, which in turn drives the same arrangement on a rear 'dirt jump' hub. When coasting, the wheel drives the motor in the same way providing regen braking.

I am going to investigate if I can set the regen low enough to charge by pedalling the trike with the rear wheel on an exercise stand. I already use it on the stand as a stationary exercise bike when it's raining.

I read you will use a rear hub with shifting capability - that means the input chain or belt will be on the right. With the approach I am using of chain input to motor on the right and output on the left, you will require another axle (pedal axle, motor axle, transfer axle, hub axle) between the motor and the hub to transfer the motor output back to the right side - unless you have thought of another approach?
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