Small wheels revisited

[pm124]

I'm writing a paper and procrastinating as usual. I came across this: http://www.biketechreview.com/archive/wheel_theory.htm.

The article suggests that rolling resistance is 5X more important than wheel aerodynamics in terms of wattage. It's nice in that it presents a number of different scenarios.

On rough surfaces, unsuspended small wheels with high pressure tires should have significantly worse rolling resistance than unsuspended big wheels with high pressure tires. However, Moulton suggests that small, suspended wheels have lower rolling resistance that big unsuspended wheels. Greenspeed suggests that small unsuspended wheels have significantly lower rolling resistance than big wheels on smooth surfaces.

Thus, the question surrounding whether smaller or bigger wheels are more efficient ultimately depends on suspension losses (which can be mitigated by proper riding technique) and road surface.

[jur]

Moulton's tests showed that tyre pressure and material is dominant and that wheel size is insignificant.

However, at least in one film, I saw that the test surface was a 1' drum which diminishes the effect of wheel size since the drum curves away from the tyre faster than the tyre curves away from the drum. I am not sure to what extent he validated his test results due to this effect.

Having said that, Moulton was not born yesterday either and I assume he considered this effect.

From a purely geometrical perspective, a small wheel rises faster on a square bump compared to a big wheel. But how often do you encounter such square bumps in a typical road ride? Perhaps none at all. Besides, the tyre softens the bump. So, with rider inertia and tyre pressure, the 2 cases may experience very similar trajectories over the square bump anyway.

Then you have to consider small road irregularities. These are typically of a shape and size where the tyre wholly or mostly absorbs such impacts.

So from what I can see, wheel size is indeed not dominant on rolling resistance.

Suspension, there I can't comment with the same degree of confidence.

[LittlePixel]

This article by Tony Hadland (Moulton Biographer) might also be useful:
http://www.hadland.me.uk/page15.html

[LWaB]

Greenspeed suggests that small unsuspended wheels have significantly lower rolling resistance than big wheels on smooth surfaces.


On smooth surfaces, a steel wheel/tyre has the lowest rolling resistance. I've ridden replica 'boneshakers' on roads and the rolling resistance isn't particularly good...

[pm124]

So, assuming that we are riding a suspended small wheeled bike, that the measured differences in rolling resistance on a steel drum reflect real world differences on suspended bikes (about -33% using Primo Comet tires when going from 451mm to 349mm), and that there is no headwind, a lone rider on a flat surface might expect a small wheeled suspended bike with 32 spokes to be about 3% more efficient overall based on rolling resistance and an additional 1% more efficient based on front wheel drag. But requisite suspension may also sap watts and these are huge assumptions.

Kind of funny that this has never been tested with a power meter on real roads.