Whilst, the UK Government's
Webpage does state:_"
Its electric motor:
- "must have a maximum power output of 250 watts"
That appears not to be the industries or converter's interpretation of the law on which it is based, the "norm" interpretation is the "maximum continuous power output is 250 Watts".
In itself a massive cop out in that really the maker can put whatever figure they want on the "continuous" rating labelling their kit can stand, and basically under any conditions. The outcome is many "legal" commercial bikes have motors that peak above 250 Watts, and I hazard a guess near every conversion kit.
EDIT: I have found the legislation relating to "continuous"
HERE . chapter 1, Article 1 (h). Quoting, with my emphasis: -
(h) cycles with pedal assistance which are equipped with an auxiliary electric motor having a maximum continuous rated power of 0,25 kW, of which the output is progressively reduced and finally cut off as the vehicle reache
s a speed of 25 km/h, or sooner, if the cyclist stops pedalling,
Now out of the EU we can do whatever we want to bring into UK law, however those penning the reference quoted by the Prof, wrote that in a sloppy fashion by missing out the critical word "continuous", that is in the legislation.
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I have "looked", though went proprietary, at doing a DIY conversion, particular issues I found needing attention are:-
With rear hub geared drives the shaft is a larger diameter, and the hub width is greater than "conventional". So the chain stays need splaying, and the dropouts need adequacy to be enlarged, not something aluminium frames readily accept. Plus then you can encounter drive line off set between the crank ring and rear sprocket.
With crank drives, again drive line off set can be challenging, here as the whole power goes through the chain, the issues of living with an out of line drive are increased.
As asides: -
Most DIY conversions will have a simple cadence based, as opposed to torque sensing or complex blending of the two, motor control system. The finesse of the control system being an area the major commercial brands invest heavily in to achieve the "character" of their products.
For hilly area use the crank drive motors have an advantage in not only the rider, but importantly the motor can benefit from a selective gearing system, hub motors typically whilst geared only have a unique ratio than needs to suit riding on the flat or climbing hills.