* Are wide tyres faster than narrow tyres?
* What is the optimum tyre pressure?
There is of course a load of data on the Bicycle Rolling Resistance website, but their test method measures rolling resistance of the tyre in isolation and so is really a measure of "casing losses" only. The additional effect of the road surface roughness upon the bike and rider as a whole, sometimes called "suspension losses", is not measured by the BRR method.
Ever mindfull of Lord Kelvin's "When you can measure what you are speaking about, and express it in numbers, you know something about it..." I have attempted my own "real world" measurements of rolling resistance. After a year of on/off evolutionary development I finally have a working test method.
First, a few notes on the scale of the problem. If you start freewheeling on a level road at an initial speed of 20km/h, the next 100m will take 23.885 seconds (rider + bike = 80kg, Crr = 0.005, Cd.A = 0.400). If the rolling resistance of the tyres is increased by 10% to 0.0055, the time will increase by just 0.390 seconds. It will not take much of a headwind to produce the same decrease in time. Clearly accurate measurements and controlled conditions will be neccessary for meaningful results.
I started by constructing a custom data logger using a microcontroller development board from Texas Instruments. I added a Hall effect sensor and wrote the software for this so that it measures the time for each revolution of the front wheel with a resolution of 30us.
The test method is then to roll down an incline whilst recording the time for each revolution of the front wheel using the data logger.
To find the rolling resistance, a set of equations is constructed which describe the energy of the closed system over time, based on the ideas proposed by Robert Chung. The equations have a number of inputs all of which must be measured, calculated or estimated. The output of the equations is the rolling resistance Crr.
This method is not without its drawbacks, the principle one being the requirement for zero wind. The incline is also important: to measure Crr it has to be just steep enough to keep rolling but not too quickly: about 1% gives good results. A steeper incline is used in the same way to determine a value for aerodynamic drag CdA. The height of the incline was measured with a barometric altimeter: not accurate enough really, but all I have available.
The first results were obtained using Continental Grand Sport Race tyres (28mm front/32mm rear) on a smooth road surface.
First a roll down a steep incline was done to get an unimpressive value for Cd.A of 0.520m^2. To double check this I took a photo of myself on the bike head-on with a metre rule for scale and counted the pixels with a photo editing program: my frontal area came out at 0.593m^2. As the coefficient of drag Cd is typically around 0.8 to 0.9 for a cyclist, my measured Cd.A should be in the range 0.415 to 0.534. I used a value of 0.500 for subsequent calculations.
To measure rolling resistance, two runs were done at four different pressures down a gentle incline on a smooth road surface. The results are shown in the following graph, with the min/max indicated by the error bars. The absolute values cannot be guaranteed, but the shape is indisputable. As can be seen from the graph below, rolling resistance decreases as the pressure increases from 60psi to 80psi but then markedly increases at 90psi. The 80psi runs were done on a different day when the wind was not quite zero. Harder is not always faster!
Gratifyingly, this result replicates the findings first published by Tom Anhalt in 2009, see https://www.slowtwitch.com/Tech/What_s_in_a_tube__1034.html
As a data quality check, the calculated elevation profile for each run was plotted on the following graph. They all line up pretty well.
So, what next? I'd like to test some wider and narrower tyres. However I have a very limited collection of odd tyres to choose from and I
cannot afford to buy a shed-load of tyres purely for testing. Voyager Hypers are popular with some on this forum and I have one in size
37-622. Is anyone interested enough to lend me another?
[Edit: updated graphics]
