Adding an electric motor to a gravity bike (no pedals, just brakes)

[Grantski]

Hello!

I am about to embark on making a gravity bike (without pedals, just brakes) to ride at freeride downhill longboarding events (in Europe, where I live); I have attached an example photo.

I want to incorporate an electric motor into the machine, for the times when we head for the practice hills near where we live in Budapest, Hungary, so as to take myself ~ and ideally also to tow my 2 sons on their longboards ~ back up the hill.

I realise now that I have set myself a significant challenge, and therefore I am turning to people who understand electric bikes, drive systems, batteries and electronics to try to shorten the learning curve and get some helpful advice upon which to base the design.

I would need advice on the optimal components to buy, brands, sources, etc., and specifications for the components so that they can then be incorporated into the design.

So far I have received the following advice (from Ying at Momentum, a manufacturer of electric bikes in the UK), that I should use:

- a 250-350 watt Bafang (or similar, high quality) electric geared hub motor, weighing 4-7 lbs (2-3 kg), [will this cause friction in descent mode? I would like to minimise friction]

- Lithium Ion batteries [size, spec, capacity - I need help with these],

- 20” wheels [I would like to keep the bike as low as possible to aid with cornering, and to improve speed up hill]

- high quality controller and throttle [I have no beginning spec for these and would need some help, as well; for the throttle I would like to source the thumb operated type]

- charger that works in Europe - 230-250 volts

Alla nd any comments welcome, and with thanks in anticipation...

Grantski

[ykick]

Hi and welcome to the forum!

I’ve got possibly similar experience with pedal-less platforms and hub motors.

Here’s a goofy looking one -


And something with smaller wheels -


All my projects were from years ago (checkout that NiMh battery pack, haha) but if I were to build something similar (and possibly what you’re doing?), I’d probably go with this little double geared hub motor kit - https://lunacycle.com/double-gear-double-torque-motor/

The lower mechanical gearing option would serve you well towing uphill, me thinks?

[Velodrome]

Dang ykick! I've wondered bout putting a hub drive on a kick bike. Hows it go??? To the OP if this is a down hill race the hub is gonna cause drag on the way down. I could VERY easily be wrong but a strong mid drive that you could de-chain quickly at the top and remove battery if the bike has a max weight limit for the race, might be best. A BBSHD or 3000W Cyclone WITH the right gearing may well get you and tow kids up the mountain. But with that gearing for tow, you wont be going very fast. PS: That looks like a dnagerous butt load of FUN! :)

[ykick]

I love powered adult kickbikes or footbikes. Even using cheap steel frames they're relatively lightweight and of course no chains to make a mess indoors. Just simple fun for the most part.

They're also kinda "grey area" in regard to sidewalks and pedestrian malls/walkways. Since your feet so readily touch the ground you're kinda treated like a pedestrian in most situations. Particularly helpful in extremely crowded areas.

I'd love to put a Luna double gear/torque hub motor into a 20" wheel kick bike someday? That would be a great match since DD motors don't handle steep grades very well. The double gear motor would shave off even more weight too.

[Grantski]

Dear Ykick, thanks for the link, that does looks interesting!
Dear Velodrome, the hub drive seems optimal, as I will have no chain. There is no max weight, and these are freeride events, not races, per se, and the added weight helps with stability / cornering, I'm told; I'd like it to go fast! So I like your thinking!