The featured custom build this month is from Miles, one of endless-sphere.com’s most helpful and respected members, who lives in S.E. London, in the UK. Rather than the high-powered builds that we normally showcase, Miles’ E-bike is a high-quality and very efficient city E-bike that is light, compact, and runs extra-quietly. The design process for this project started around 2006.
The drive configuration is a dual parallel right-side drive, with both the pedals and the motor independently driving the rear wheel. In order for the two drives to remain completely independent of each other, the motors’ rear pulley includes an integral freewheel, as does the pedals’ front chainring. A major influence on Miles’ decision to pursue this style of configuration was a custom E-bike that was built by Randy Draper of Maui (in Hawaii, USA). Randy was a pioneer in the practical development of this type of drive system.
The rear wheel incorporates a SRAM “Dual Drive” hub, which uses three internal gears, in conjunction with an external derailleur-activated sprocket set on a freehub. This provides the motor with three speeds to choose from, and the pedals have 12 speeds at their disposal.
The motor is a very quiet direct-drive unit, which powers a single-stage belt drive to the rear wheel, which also runs very quietly. The entire drive system is “barely audible in most surroundings”
The Moulton name might not be familiar to most North American bicyclists, but it is a respected name in the UK bicycling community, and they are known for their frame engineering.
The history of this unusual frame is interesting. Alex Moulton had a background in aircraft design working for BAC, and he made several bold and unconventional choices when he started this bicycle design in 1962. His family business was connected to the rubber industry, hence the desire to incorporate rubber suspension units, if that proved to be appropriate. He is most famous for his work developing the light and compact suspension of the original “BMC Mini” car.
Alex Moulton died at the age of 92 in December of 2012.
The distinctive feature of Moultons is their smaller wheels. The trend for adult bicycles since the “Safety” bicycles of the 1880’s had been towards larger diameter wheels coupled with fat tires, in order to deal with coarse road conditions. Smaller diameter wheels are stronger and lighter, and the smaller cross-sectioned tires have less rolling resistance.
The decision to eliminate fat tires directly influenced his next innovation. This model has full-suspension, which was quite rare on a bicycle in the 1960’s. The length of suspension stroke is not long, but it is adequate for civilized road conditions. One of the main design considerations for this type of full-suspension is that; the design of each part of it was chosen for it’s lightness. The front girder-style fork has its spring very cleverly hidden inside the frames head-tube.
Concerning lightness, this has been the comprehensive theme of these bicycles. This is apparent in the most eye-catching feature of the bike, the space-frame design of the bicycles main frame. It is very strong and rigid, and yet it remains surprisingly light. Moulton was also a leader in promoting a uni-gender frame shape that has a low top-bar for easy mounting.
This build is based on a Moulton TSR27
-Custom-built Planet Rider brushless permanent-magnet motor.
-SRAM Dual Drive, 3-speed Internally-Geared-Hub (with 4 derailleur sprockets retained).
-Gates Powergrip 8M GT3 belt transmission (16T to 100T, 6.25:1 reduction). Rear pulley is 263mm diameter.
-Sprague clutch in hub of rear pulley
-NiCad power tool batteries, 36V 4-Ah, in backpack (this was the battery he was using in 2008).
-Weight (without battery) 16.5 kg (36 pounds)
-Averaging 12-WH per mile
“…With the present 36 volt set-up, I’m getting a top speed of about 19-MPH so, with a more optimal 48V set-up, about 26-MPH – which is all I need…The belt drive is pretty quiet, as is the motor – noise is something I care about. You could approach someone from behind in a park and they wouldn’t hear it…I’m convinced that belt drives are the way to go…It’s designed for hill climbing, top speed is 22-MPH at 39V. It pulled up a long 20% gradient with ease…Over mixed terrain, for speeds under 18-MPH (with no pedaling at all), I was getting 15.7-WH per mile…”
“…I’d prefer it if I could fit sturdier tyres on mine, but using the Stelvios was the only way to create enough space for the motor…” (Schwalbe Stelvio 406mm / 20-inch)
“the nice thing about this motor is the form factor – the case is only 60mm deep…this motor is one of a small batch made by a company called Planet Rider, almost 5 years ago, but that was it…It never went into production, I’m afraid…”
The maximum speed rating for the motor is 4,000-RPM, weight is 5.5-lbs
Kv = 59-RPMs/V
Ke = 17 V/kRPM
Kt = 0.17 Nm/A
Resistance: 0.25 Ohms
Inductance: (approx.) 200 uH
12 Poles / 12 magnets
Air-gap radius: 40mm.
Motor-case diameter: 142mm
Motor-case width: (excl. shaft) 60mm
Shaft diameter: 5/8″ (15.9mm)
Nominal input voltage: 18V-48V (max. 55V)
Continuous output current: 20A
Peak output current: 35A
Output power (max.): 1,000W
Measured no-load current of motor: 0.8 A @ 38V
Miles’ choice of rim brakes instead of discs was in keeping with the original design theme of lightness for every component. This bike can easily be carried indoors, or up a stairway.
The custom electric drive:
“…The 100T rear pulley itself is from the crankset of the Strida 5.0 (you can buy the whole crankset as a spare part), I had to re-machine it a bit, though. The 16T 8M motor-pulley was a custom job, the smallest commercially available is 22T and the smallest recommended size is 18T, but it works fine…16T to 100T is only 6.25:1, though….It’s just a standard one-way clutch bearing (from Tsubaki [Japan] and is equivalent to a CSK35P), used as a freewheel substitute. I only made the housing/adapter for it…
…To get a greater reduction ratio, in a single leap, you have to move to 5M size belt, but this means a completely custom large pulley…5M pulleys are available down to 12T but 16T is probably the smallest of any use in a one-step drive…Probably a good compromise between torque transfer, reduction ratio and size would be to use 18T to 180T 5M (10:1)….”
“…..The cranks are freewheeling, using a White Industries ENO 22T freewheel on trials cranks. I had a chainring made especially for it, but others have made adapters to standard chainrings. The 22t ENO has handy circumferentially milled weight-saving slots that one can use for fixing…not that it’s a problem to drill the fixing holes, with a suitable drill…
…So, the one-way bearing isolates the motor drive so that you can pedal as a normal bike, without any extra drag, and the freewheeling cranks decouple the pedals when you use the motor without pedaling. For normal freewheeling, the original freewheel in the hub gear kicks in, of course…”
“…I’m using 36V 5-Ah high power density NiMH cells…I have to carry my bike up and down steps whenever I use it. Having the batteries in a backpack makes the bike itself an easy lift…”
Miles’ prototype motor design
Miles has been pleased by the performance of this drive system, but since the original motor is not in production, he has spent quite a bit of his spare time these last few years by studying motor design. He did this in order to establish what kind of a design could provide the same performance, but in a smaller package, and with the highest possible efficiency.
This project is nearing completion and here is the discussion thread for his 90mm (3.54-inch) diameter motor.
The eMoulton build-log: http://www.endless-sphere.com/forums/viewtopic.php?f=6&t=5313
Written by Ron/Spinningmagnets, October 2013