Size matters

[halfmedley]

Not sure if this has been referred to on this forum before but I found this website interesting:

http://www.slofly.com/ebike/

Clearly the installation is a little 'Heath Robinson' but the size of the motor is appealingly tiny compared to current hub motors, yet seems to have plenty of grunt if the youtube video (there's a link on the website) is anything to go by.

I'm thinking of building a pared down, nimble, 'commuter special', using the lightest weight regular bike I could afford, then adding the smallest/lightest electric motor for occasional help on hills. Maybe this motor could be utilised, though not the friction drive!

 

[flecc]

This sort of approach with small motors using low voltage and very high current can give short term impressive power to weight ratio, but it's very inefficient.

Range on any given battery would be poor, and such motors are almost invariable noisy, often excessively so. Generally speaking, this sort of approach was used in the early days of bike motors but has long been abandoned. The last to try were Sachs with the SRAM Sparc first generation hub using two high output low voltage small motors, but after much criticism it was withdrawn and redesigned with better motors.

The motors on the market now are about the best that can be achieved with current technology, and where miniaturised, they tend to give trouble as the Nano is now showing.
.

 

[halfmedley]

Hmm yes, the motor did seem to make rather a racket on the video!

My thoughts were along these lines: an ideal electric commuter bike for me would be as light as possible, following the Colin Chapman philosophy of performance through low weight and simplicity, a sort of 'Lotus 7' of electric bikes if you will.

However, whereas the '7' was built for sports, this bike would be built for commuting. The bike would be used mainly unassisted and the motor would only be used for occasional boost when necessary and so have a limited continuous range (5 miles? most city commutes are less than this). Would following this philosophy lead to major weight savings (smaller batteries etc)?

It seems to me that ebike marketeers are going down the wrong route in their emphasis on range/speed, particularly as the technology can provide neither without considerable (in cycling terms) mass. Discuss.

 

[flecc]

I agree, there is definitely a demand for that sort of bike, assisting only in the toughest conditions with normal cycling at other times.

The Brompton Nano tested in the current A to B magazine is effectively that, since the bike was ridden at around 16 mph for most of the test in fairly flat terrain, the motor unable to assist at just over 14 mph and above, kicking in just for hills that slowed the rider's speed.

Again it has a choice of batteries right down to a tiny one of about 72 watt hours I think from memory (24 volt 3 Ah), and the motor is as light as hub motors can get. Sadly it has reliability issues with it's unusual drive transfer system.

That's the only one around at the moment though, and it would be good to have a choice of various powers and capabilities to suit all cyclists.

I don't think the present direction of e-bikes is wrong though, just too narrow in it's view and remit. The manufacturers need to have more vision, something I've often said before. One size does not fit all.
.