Some more on the lack of any legal status for kit motors:
None of the e-bike laws make any mention of kits in the UK and Europe. All three laws, technical standards EN15194, the exemption from motor vehicle type approval 168/2013 and the usage on the road law EAPC 1983 as modified 2015, they all only refer to complete manufactured electric assist bikes, trikes and quad bikes.
So both individuals and suppliers operate in a legal vacuum in which all try to supply and use as closely conforming to the complete e-bike law as they can, trusting that is acceptable. That has always worked elsewhere throughout all of Europe and the UK without any mention of a possible prosecution, providing the three main points of the law are adhered to, i.e. 250 watts maximum assist, 15.5 mph maximum assist speed and power only when pedalling.
However, I can tell you the theoretical UK legal way of dealing with a kit, although no-one has ever done it with a kit yet:
1) Buy and fit the motor kit.
2) Make an appointment at an approved vehicle testing station, paying the £55 test fee.
3) Normally the purpose of this is to get an SVA (Single Vehicle Approval), entitling it for use as as type approved motor vehicle. However your aim will be to get the inspector to agree that it meets the pedelec requirements so is exempt from being a motor vehicle and is approved as a pedelec as well as having SVA.
There is only one testing station set up for this, the one at Southampton!
That's because an arrangement has been made there for pedelecs with an added otherwise illegal throttle to be tested for L1e-A approval to make the throttle legal in the UK only under a DfT approval. So it kills two birds with one stone, legalising both the fitted kit and the throttle, but on that one e-bike only.
But to my knowledge they've only ever done this once.
In summary, legislators don't like kits, too many variables, so avoid their mention like the plague. Mainland Europe killed the kit car industry long ago and our laws have damaged it here too, so we are perhaps lucky that the lawyers don't pay attention to our little pedelec kit niche.
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