Little Wheels on E-Bikes

[energyman]

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]

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.

[Andre Jute]

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]

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".

[martinf]

"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.

[energyman]

Thanks for in-puts.
Trying another capacity battery to see if it is the battery.

[mickeg]

I have been out of town, have not posted for a few weeks.

Motors could very well be operated at an optimal speed, if you take them out of that optimal speed range they might be less efficient.  That could show up in distance per battery charge.

In the early 1980s I was in school studying things for my Geological Engineering degree, and that included a few lectures and problem sets on electric drive mining trucks.  Needless to say, I have forgotten 98 percent of what I once knew, but I know that the electric drive did not need a transmission and that the motor speed was proportional to the wheel speed.  But I know there was a gear reduction and I would expect that the gear reduction was selected for the most optimal running.

Some of the electric bikes that I have seen in my community look very heavy and bulky, in some cases those tires look like they would have very high rolling resistance.  Maybe the small wheel bike has less efficient tires than the larger wheel bike?