Small wheel hill climbing advantage

[andyh2]

I've seen several references to hub motors in smaller wheels being better at hill climbing, when compared to bigger wheels, assuming the motors are geared relative to their respective wheel sizes. I can understand that if the motor RPM is the same then the small wheel will climb better as it is lower geared then and this would be at the expense of top speed. But I don't really understand why like for like the smaller wheel is better.

I'm sure I found a fairly technical explanation here somewhere but I can't find it through searching.

Is it the same as if I put a book in my hand then if I fully extend my arm it is harder to hold or raise the book than if my arm is only half extended? My arm being a spoke in this analogy. Or the way it is easier to spin shorter cranks than longer cranks?

Following on from this would it be easier for a hub motor to maintain reasonable hill climbing ability when using a higher RPM motor, than its designed rim size, if that were done with smaller rims. eg comparing the hill climbing ability of a 210RPM motor with a 700 rim instead of a 175RPM motor against a 260RPM motor driving in a 20" (406) rim instead of a 16" (349) rim.

I'm not particularly interested in the increase in assisted speed, but I am interested in the potential to use an 80mm motor in a 20" wheel for a Dahon conversion, as that would reduce the fork spreading required compared to a 100mm hub. And, remembering last winter, marathon winter tyres are available in 406 size, but not 349.

Has anyone tried an 80mm tongxin in a 20" rim?

 

[flecc]

It's as you say Andy, the improved leverage of the shorter rim radius/spoke length.

Undergearing a motor does improve the climb, and a good example is the Nano-Brompton that A to B tested, hill climb ability beyond what one might expect with the Tongxin motor. The reasons are the big mechanical advantage of it's 16" wheel and the highest rev version of that motor only powering the small wheel to 13/14 mph, shifting the point of optimum torque and power coincidence to a lower speed where less power is needed for climbing a given hill.
.

 

[jerrysimon]

Andy,

I have thought about putting a 80mm Tongxin in my 20" Moulton wheel but enjoy riding it too much unassisted at the moment.

As far as Marathon Plus go, to my knowledge they are still available for 349 rims ?

http://www.brilliantbikes.co.uk/tyres/207-schwalbe-marathon-plus-16-x-1-point-35.html

I have been trying Kojaks recently and must say they seem to hold up well. Maybe mid winter I might be singing a different tune though.

Regards

Jerry

 

[andyh2]

Thanks Flecc and Jerry.

Average temp of 0.5C over 3 months last winter meant hardly any ice free days and very little riding, so I'm thinking of the spiked marathon winters, rather than the standard marathons.

I guess running a 260 RPM motor in a 20" wheel would be the 'standard' combination, whereas running it in a Brompton is under gearing it some.

I guess the choice is down to better hill climbing, and probably less stress on the motor, with a Brompton vs being able to use year round including when there's ice on roads.

One of Frank (Mekaniker?)s threads said the factory gate price of the tongxin is Eu20. I guess over here we'd happily pay double that manufacturing price for a more robust product with the same characteristics.