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Hi members eres something that I would like to know. .. I have a viking gravity 20 inch ebike wheel motor. If I exchanged/refitted this motor into say a 29inch wheel with all the same parts/kit controller etc, would this then make/alter the speed/cutoff mph? Thus improving top speed and limiter on the 29inch sized bike.

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    Yes, but the speed readout on the display will underread and the thing will probably struggle on gradients, more than you'd expect.
    – Noise
    Nov 19, 2020 at 20:26
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    Note that you'd probably have a spoke count issue.
    – Daniel R Hicks
    Nov 19, 2020 at 21:11
  • @DanielRHicks the wheels are 32 spoke so spoke count least of his worries
    – Noise
    Nov 20, 2020 at 10:13

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Depends how the limiter works. If it knows about RPM of the motor/wheel then the increase in diameter will effectively raise the top speed.

Example - if the motor's limiter is at 18 mph on the 20" wheel, then that's 63 inches circumference.
18 mph is 19008 inches/minute.
That works out at 301 RPM on the motor.

So reverse that, 301 RPM on a 29" wheel (which is 91 inch circumference)
is 27,391 inches/minute is 25.9 mph.

However there's always a tradeoff. In this, the cost is in torque/acceleration. Your bike would take much longer to accelerate, and would "stall out" sooner as the grade increases.

Additionally, some locals have legal restrictions about the top speed a pedal-assist motor can provide. 25 mph may be faster than your local laws permit. While you're very unlikely to be pinged for the bike, if there was an accident you may be legally "at-fault" even if its not your fault in a road-rules sense.

More practically, your wiring may not be long enough for a larger frame.

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    The motor, the control electronics and the battery will work for most of the time in overload conditions and heat-up significantly which is detrimental to their life expectancy.
    – Carel
    Nov 20, 2020 at 8:14
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    @Carel: But the same happens on longer ascents or against a strong headwind where the motor is working at maximum power.
    – Michael
    Nov 20, 2020 at 10:09
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    @Michael: True, but here it will also happen on the flat and even more so on ascents.
    – Carel
    Nov 20, 2020 at 12:32
  • Great information about this post..And it's just a thing that' I've wondered about, Thanks members/Group& Criggie for a Gr8 calculated Answer...
    – Alan Walsh
    Nov 20, 2020 at 14:45

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