Mid-drive PAS tandem MTB mod

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Jun 27, 2018
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I'd like to add a motor to my tandem.
At present I only want pedal assist, ideally this would be controlled by a torque sensor.
We still want to ride with other non e-tandems sometimes, so it can't be too noisy.
It would be nice if it looked stealthy too, but that's not so important.

My initial plan is to keep both sets of cranks (which are square taper) and replace the front BB to a torque sensing one, add an additional chainring to the rear non-drive side and allowing the geared motor to drive that. The issue here is the motor would have to operate the 'wrong' way round so the freewheel would have to be mounted backwards. Is this hard to do? I guess it depends on the motor. What would people recommend? I'm considering a cyclone, not sure what size yet.

My second plan is to mount a chainring to the torque-sensing bb and connect the motor to that in the conventional way, however I'm not sure about whether its possible to set up the control system to work properly, as pedaling harder would result in the chain tension decreasing.

My third plan is go from a 3x to a 2x by adjusting the limit screws on the front derailer and use the motor to drive the unused ring. It'd be a shame to reduce the gear range though.

Any advice appreciated, especially if you've built a system like this.
 
Mid Drives have noise because of the gears, and the mounting of the motor will be noticeable.

You want stealth then this is how you do it.

Buy a rear direct drive hub motor of your choosing. Be sure its black, use black spokes and a black rim.
Buy a Sinewave controller because that is what you want for ultra silence. Paint it black!
Buy PAS components.
Buy a rear pannier rack, make it black.
Buy rear panniers, yes black.
Rear panniers hide the motor, everything is camo'd.
Install controller on the top of the rack; camo
Put batteries inside the pannier bags; hidden

As the teenagers say Chive On!

But you want lots of gears in your power transmission to climb hills or lots of stop and go traffic. That is the main reasons why one would go mid drive. Well then, sure a mid drive would work. BBSHD or Cyclone or the 8Fun are all choices. I had the 4kw Cyclone, and people could hear me coming from behind them when casually cruising and trying to be silent. Perhaps the others one are more silent. If I remember correctly, I dont think there is much of a noise difference, maybe the BBSHD is slightly less noise. But do the same thing, just camo it to the bike frame color, black is good. Chive On!
 
killerisation said:
At present I only want pedal assist, ideally this would be controlled by a torque sensor.
We still want to ride with other non e-tandems sometimes, so it can't be too noisy.
It would be nice if it looked stealthy too, but that's not so important.

FWIW, a DD hubmotor with a true sinewave controller (rather than the cheap typical trapezoidal types) will be basically silent in operation. I didn't actually believe they could be, but I'm testing one now designed by Incememed, and it doesn't have any driving noise, unlike the very loud trapezoidal generic controllers I've used up to now. I had a hard time being certain the motor was actually still working during cruising at first, unless I hit the throttle hard and then could feel the acceleration. :lol:

Middrives may be quieter if they use sinewave controllers; if they use trap controllers they'll have noisy motors *and* noisy gears. I dont' know which ones already use sinewave, if any--but there are some threads about using a Phaserunner (sinewave) in place of the original BBSxx controllers.

Stealth depends on your bike's layout and other stuff you already have on it (or can put on it). Attach some pics of your current setup and we may have suggestions on where to put some things.

Torque sensors are easy; they come built into some of the middrives (TSDZ2?, maybe others), or you can get a few different BB torque sensors; Sempu, THUN, and TDCM come to mind, might be more now.

The options below may lose you a chainring in front. But depending on the way your stoker drive and captain-to-stoker chain is setup, you could add a shifter and extra rings on the right side back there.


If you use a non-torque-sensing BBmounted middrive like a BBSxx (I think they just do cadence?) then yo could put that in the stoker BB, and put your PAS/Torque sensor stuff on the captain's BB. Then wire that up to a Cycle Analyst 3.x that you can configure however you want to translate the sensor info into throttle info to run to the BBSxx. You'd disconnect the BBSxx internal sensor stuff (there's at least one thread for this, about BBS on throttle only), so it doesn't get retriggered by the stoker BB turning, and it only responds to the throttle signal from the CA. You don't need a trhottle on the handlebars; it can all go by pedals. If you like you can have a button (or dial) for presets or whatever, to tell the CA to give more or less assist depending on situation.

A TSDZ2 would have to go on the captain's BB, as the sensors are built into it; might be able to do the same as the BBSxx but since it already has the torque sensor, no reason to.

Most likely either of those options would be cleaner and easier to deal with than a Cyclone setup with leftside freewheels and whatnot, especially since many people seem to have to modify or make new mountings for Cyclone stuff, upgrade freewheel parts, etc., just to get it to work properly.
 
The thing about a BBSHD is it would have to go on the front BB, theres just too much going on at the rear for it to fit. This would mean the front BB cannot have a torque sensor (unless I got one that measured chain tension). This is the reason, along with value, I was leaning towards cyclone.

EDIT: BBSHD would also increase the q-factor significantly which isn't ideal.
 
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