My situation:

  • A Bakfiets with a bad controller (hills too much for it I think)
  • Supplier has offered to send me a new controller
  • Supplier has also offered to supply a whole new separate battery - controller - motor system to mount on the front wheel (to prevent a recurrence of controller failure). This is a unit he has unused and it is designed for a rear 26 inch wheel.

This sounds great, but I am worried that his second controller won't properly drive my 20 inch front wheel.

My question - will this system work? Do controllers need to be configured to drive different wheel sizes? I feel the answer is yes, otherwise, how can a controller know the speed of the bike - it only has two ways of knowing this - the pedelec or the motor.

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    There's an upper speed limit for assistance (Europe-wide). The motor rotation rate is probably known or assumed by the controller and used to enforce this. The motor isn't just a simple 2 wire connection. So yes, I would think it would need to be reconfigured. – Chris H Jan 4 '18 at 18:27
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    Also can the (hub?) motor in question actually be built into a 20" wheel, and who would pay for it to be done and deal with failures of the rather short spokes? Hub motors for 20" wheels do exist so it's possible, but I'd be wary – Chris H Jan 4 '18 at 20:37
  • Oh, I neglected to mention that I intended to buy a custom front hub motor. It still doesn't solve my speed issue though - thanks! – Caspar Harmer Jan 5 '18 at 1:37

You may be able to reprogram the wheel size controller from the lcd control panel (or usb reprogramming cable). But this assumes a certain level of intelligence in the controller and yours might not have this capacity. You’d have to check with the vendor.

Even if it were reprogrammable you have the following challenges:

  • Motors have optimum torque and speed ranges. Your motor may be wound for slower rotations of a 26” wheel and won’t give you enough speed that a 20” needs

  • controllers may obey the ebike assist speed limit by limiting max RPMs. If your controller isn’t wheel size reprogrammable you’ll find yourself limited to a much reduced speed and even if you can, the physical/electronic max rpm limit may still limit your top speed (my Bafang geared hub motor/controller doesn’t seem to go above a certain rpm irregardless of the wheel size; this may be either a motor or controller limitation)

  • you’ll need to build the wheel and you might find some of the spoke angles are going to be very tight

  • a rear ehub will not fit normally in the front forks because of the hubspacing and freewheel clearance.

  • I forgot to mention that I intended to buy a custom front hub motor. That still leaves the speed issue. In some ways, it might not be a problem, because when I'm going slowly up a hill, it'll be working, which is when it's needed. I could view it as my special low speed boost motor. – Caspar Harmer Jan 5 '18 at 1:39
  • You’ll get a lot of drag from a direct drive hub motor that’s not energized. A geared hub won’t have as much drag but is noisier. – RoboKaren Jan 5 '18 at 2:01

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