Author Topic: Little Wheels on E-Bikes  (Read 273 times)

energyman

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Little Wheels on E-Bikes
« on: June 19, 2019, 02:09:31 PM »
I'm trying to get my aged head round a statement I heard today.
"Twenty inch wheeled E-Bikes will not have the same range as, say twenty eight wheeled E-bikes, as the motor has to work harder."
Same battery capacity and same motor.
Comments welcomed.

martinf

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Re: Little Wheels on E-Bikes
« Reply #1 on: June 19, 2019, 10:34:34 PM »
Depends on the motor gearing.

If the motor is geared proportionately to the wheel size and the tyres are of similar quality there should be very little difference between motor performance with 20" and 28" wheels.

Without assistance, I found very little difference in performance between my lightweight 15-speed 700C derailleur bike and a 16" wheel Moulton set up similarly with drop bars, good quality lightweight tyres and 7-speed derailleur system, so long as I kept to reasonably good roads. No reason why this should change with electrical assistance.

On relatively smooth roads, all other things being equivalent, small wheels theoretically have "slightly" higher rolling resistance than large ones, caused by greater tyre carcass deformation of the contact patch at a given pressure. This is offset to a large extent by the reduced air drag of a smaller wheel.

Bigger wheels absorb less energy on rough surfaces, which is one of the reasons why off-road vehicules tend to have big wheels.
« Last Edit: June 20, 2019, 08:08:25 AM by martinf »

Andre Jute

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Re: Little Wheels on E-Bikes
« Reply #2 on: June 19, 2019, 10:45:11 PM »
I'm trying to get my aged head round a statement I heard today.
"Twenty inch wheeled E-Bikes will not have the same range as, say twenty eight wheeled E-bikes, as the motor has to work harder."
Same battery capacity and same motor.

Very unlikely to be a meaningful, perceptible difference. In the real world of practical cycling it's  the next best thing to "not true".

Bigger wheels are, pari passu, more comfortable to ride on over uneven roads.

energyman

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Re: Little Wheels on E-Bikes
« Reply #3 on: June 20, 2019, 08:42:26 AM »
Thanks folks, that's what I thought.  The reason for the shorter range must be something else yet to be identified.
(My daughter-in-law won't ride my Moulton because "you have to pedal faster on a little wheeled bike".
« Last Edit: June 20, 2019, 08:44:34 AM by energyman »

martinf

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Re: Little Wheels on E-Bikes
« Reply #4 on: June 20, 2019, 11:43:22 AM »
"you have to pedal faster on a little wheeled bike".

This is true if the gearing isn't adapted to the wheel size. To get the same gearing as with a large wheel bike you need a much larger chainring, or a much smaller rear sprocket (or combination of the two). Or a suitable hub gear setup, which is IMO better suited for small-wheel bikes than derailleurs.

I've done about 53,000 kms with small wheel bikes, mostly 16" on old Moultons then Bromptons, plus I had a 20" Moulton TSR for a while. Kitted out with the Moulton racks, the latter was an excellent on-road touring bike.

The main advantage of small wheels for me is compactness when using other transport.
Mainly trains, but sometimes cars, buses, I've even been on the Paris Metro with a bagged Brompton.

The main downside for me is poorer performance on rough tracks, which is why I sold my Moulton TSR and bought a Thorn Raven Tour.