By popular demand (well, two people, but for this site that’s practically beating the doors down), I’ll pen some notes about fitting the TSDZ2 electric assist motor to my Kona Ute, where it’s been happily spinning away for the last 2000km since we last spoke.
(For all my posts about this excellent e-assist retro-fittable motor, try this tag.)
Given that I’ve already gone through installing motor in interminable detail, we’ll just talk about the main difference: dealing with the fact that the motor expects you to secure it with a bolt between the chainstays:
and the Kona Ute (and Yuba Mundo, for that matter) looks like this behind the bottom bracket:
Happily, the solution for this is dead easy. If we think about the forces that chainstay bolt is resisting, we end up with a nasty diagram like this:
That’s looking at the bike from the non-drive-side, the front of the bike is to the left, and the cranks and pedals have been omitted (you philistines). The motor is driving the chainring in the direction of the blue arrows, pulling the chain to the left at the top (anticlockwise). To do that, the motor has to exert an opposite force on the bike frame (red arrows, clockwise), which is resisted by the bike frame at the chainstays (black arrow, bike frame pushing up equal to the force of the motor-securing-bolt pulling down).
Slight aside – it’s not quite that elegant, because the motor isn’t free to rotate within the bottom bracket shell. Instead it’s clamped in place by big nuts around the pedal axle that press against the bottom bracket shell:
(Ignore the big red arrow – I’m just being lazy in reusing a previous picture. We’re looking down on the motor from above now, front of the bike is to the bottom of the photo.)
Anyway, the punchline of this whole escapade is that we’re going to rotate the motor under the bottom of the bike until it pushes up against the downtube, and then tighten those big nuts (stop smirking at the back) to secure it in that position:
Front of the bike to the left, the seat tube is obscured behind the pedal crank. There’s a folded up piece of inner tube between the motor and the underside of the downtube to ensure the force is spread over as wide an area as possible.
Now, rather than the motor pulling down against the top of the chainstays, it’s pushing up against the underside of the downtube. The nuts securing the motor around the bottom bracket shell ensure that the motor doesn’t swing down under its own weight (and have been doing so without any bother for over a year).
Given how much I dislike clamping things around the chainstays anyway (Surly went through a phase of saying you’d void your frame warranty if you used a chainstay-clamp-style kickstand), I’d probably recommend using this method on all frames, even if you weren’t forced into doing so through the frame design. It also gives you a touch more ground clearance on the motor casing (maybe an inch or so).
The only other snag you might encounter in fitting the motor to a longtail is the length of the cable for the sensor that picks up the rear wheel magnet to determine the bike speed. You can either buy an extended cable, or build the sensor mount out a little so that it can pick up the wheel magnet nearer the rim. Once again, folded inner tube and cable ties are our friend.
In terms of reliability, I’m now just shy of 4000km on the system, and it hasn’t missed a beat despite being on turbo almost the entire time. It really is very good!