Unlike DD, the freewheel clutch of an idle gear motor prevents paying a penalty when only one motor of a 2WD setup is powered. The clutch, low weight, and good slow speed torque made gear motors seem the way to go. A YouTube video from Cycle9 showing a two gear motor Big Dummy chewing up hills (like mine) pretty much sealed the deal ( http://www.youtube.com/watch_popup?v=pqEGkmGWMzo
With two motors worth of torque, speed winds seemed a better choice than torque winds and so I went with BMC V2S gear motors from Ilia at Ebikes SF http://www.ebikessf.com/
. Using two identical motors with identical wheels/tires has some simplifying effects that are discussed in a later post. Ilia laced them onto Sun Mammoth rims (no longer in production – replaced with the slightly narrower Rhino Lites). A couple of Fat Franks add a little suspension.Front Torque Arm
The Yuba folks use a fender stay mount to secure the torque arm for their eZee on the eMundo, but I wasn’t too happy with the short arm length. Unfortunately, the Mundo fork is not round but bladed which makes using a conventional hose clamp (I don’t like these much even for hoses…) kind of unusable since the band cannot pull smoothly around the tube – it needs to be bent with sharp angles ahead of time. This makes dropout pre-loading adjustment pretty much impossible. Instead, I use the adjuster components sectioned out of two T-bolt clamps (for turbo induction hoses) bolted back-to-back to allow parallel adjustments both inside and outside of the bladed fork tube. These use very thick SS bands and are way overkill, but the fabrication is simple with a cutoff wheel. Use small 1.5"-2.0" T-bolt clamps (eBay "T-bolt clamp"
) so the adjusters are smallish and get clamps with flat bands instead of those with formed edges.
Since this was a first build, for a few bucks more I overdid it and doubled up with two Ampedbikes
torque arms. SS Nylocks keep everything together and allow adjustment to first rotate and preload the axle in the dropout, tighten the axle nuts, then bring up the torque arm tension to remove the slack so both dropouts and torque arm share the load.
The Ampedbikes torque arms are very cool and will align with almost anything. The trick is that if you need a position halfway between two teeth, just flip over the inner toothed piece and the alignment will shift 1/2 tooth spacing
When mounting the front motor, I ground out the supplied torque washers (Dremel) to exactly fit the lips on the dropouts instead of using C-washers. The result is a closed washer that holds the axle in the dropout unless the nut is unscrewed quite a bit to let the washer slide up and out over the lips. A thin SS washer on the inside of the dropouts keeps the narrow BMC axle shoulder from sinking into the dropout. To get a tight fit to the axle, I opened up 12mm SS washers a tad with a Dremel sander drum so the washers screwed easily down the threads of the 14mm axle.
BMC front motors use doubled phase wires as soon as they leave the hub. I shortened these a few inches later and installed Andersons which mate to a 10ga wire extension to run back to the controller.Rear Torque Arm
The rear motor situation on the Mundo is pretty much a nightmare: the standard axle is 14mm so hub motor axle flats don’t work and the dropouts are cluttered up with stays, making conventional torque arm attachment difficult. Unfortunately, at the time of this build, Kiwi had not yet designed his spiffy rear torque plates for the Mundo V4 so I went after this a couple of ways, including working with Yuba on a custom Mundo-specific adapter design. In the end, Yuba went with a different more generic design which is sort of a torque washer on steroids.
After doing an install with the new Yuba Mundo Torque Washers, I was not that happy with the axle relocation that the washers introduced (about 5/8" forward and 1/4" downward). They cut down on the clearance for balloon tires and do little to make the derailleur and rear disk brake happy. To fix this, I had a local auto machine shop remove one lobe from each washer so the motor shaft seats in the original axle position at the bottom of the dropout slot. This cost about $30 and took a only day with just a verbal description of the job.
With the shaft size addressed by the Mundo Washers, a standard torque arm was added using an AmpedBikes universal arm with an extension to clear the trailing portion of the rear dropout (the only change to the AmpedBikes arm was to open up the existing holes with a #8 bit for a close fit for 5mm bolts). The SS extension shown was fabricated from a couple of donor torque arms.
To get both the washers and the torque arm to share the load, the anchor point of the torque arm needs to be made adjustable. This allows the motor shaft to first be rotated to pre-load the washers in the slot, then the axle nuts are run down, then the torque arm can be adjusted to just snug up play. This adjustment needs to revisited over time as the components deform slightly. An easy off-the-shelf solution is to use a SS 1.25" saddle clamp (DX Engineering http://www.dxengineering.com/Parts.asp? ... SAD%2D125A
) refitted with Nylocks so the length of the u-bolt to saddle can be adjusted.
This 'adjustable' solution really only makes sense for gear motors because they freewheel and the clamp is always held under tension - there is no regen/braking to apply a reverse torque... However, as shown in an image above, the torque arm extension can be arranged to contact the chain stay, allowing the clamp to simply be run down tightly for direct drive motors.
As with the front, this conversion uses a BMC V2S motor which has a 17mm motor shaft. The configuration of the Mundo Washers leaves only two small areas of the shaft (top and bottom) to rest on the inside of the dropouts instead of the much larger areas over the flatted portions of the shaft as is usually the case (i.e. (17mm-14mm)/2 = 1.5mm shoulder width). Running down the axle nut with much torque risks sinking the shaft into the dropout. A pair of thin stainless washers inside the dropouts give a large contact area to fix this. I used the same reworked 12mm washer gimmick as on the front to get a snug fit and pick up as much axle shoulder as possible.EDIT: Please see Oops – Rear Dropout Washers for revised washer solution.Kiwi rear torque plates
are now available and provide a tidy fabrication-free solution, but this got my build moving along fairly easily using other available parts.
Both torque arms are overbuilt (particularly for no regen) but the small extra cost and effort has been worth it for peace of mind. After 850 miles they haven’t budged even with 1500W per motor (not recommended – but fun!).