Big vs little tire resistance test
August 27, 2007
It didn’t turn out the way I expected. “More research needed”, say researchers.
I switched from 700c x 28-32 tires (100-120psi), to 26 x 2.3 tires (60 psi) because I was tired of denting rims and breaking spokes on Our Town’s lovely streets. Unexpectedly, the time needed for my commute to work seemed to go down, and the little odometer reported a higher average speed (yes, I corrected for wheel size, and cross-checked the mileage against Google/Bikely). Didn’t feel any harder, either. I had always been told, and believed, that the high pressure tires had lower rolling resistance.
So, I had some time to kill, and I still had my old wheels, so I decided to do a rolling test. We live on a cul-de-sac that T’s into a street part-way down a gentle hill. I decided to just start the bike rolling, and see how far it went. I also made a video of my first two trials (all told, I did seven runs) both for timing and methodology checking (I did not do the first two runs exactly alike). The two tires were Bontrager Race Lite (700c x 28, 120psi) and Schwalbe Big Apple (26 x 2.3, 60 psi). I only changed the front wheel, to make it easy.
The executive, surprising, summary is that the fat tires with lower pressure had a longer roll-out. They were also slightly faster in the one section that I measured (you can check this yourself on the video). Three of the four Schwalbe runs made it past a fire hydrant near the end, and none of the Bontrager runs made it to the fire hydrant, and none of them made it as far as the shortest Schwalbe run. I did an eyeball timing on the video that was consistent with this; for a fixed distance, the Schwalbe tire got me there slightly faster.
0:11-0:50 (schwalbe, first-stripe to past last drain) (39 seconds)
5:34-6:14 (bontrager, first-stripe to past last drain) (40 seconds)
It is perhaps possible that the greater rotational inertia of the Big Apples reduced my maximum speed (and hence, wind losses) and then carried me further on the flats. However I am plenty heavy (220 lbs) and the bike itself is not light, and I also carried the off wheel with me to keep the total weight the same. Measuring maximum speed is tricky because the wheels are not quite the same size and my odometer would need to be recalibrated.
I timed my last (Schwalbe) roll to get a rough idea of times and speeds. That run took 57 seconds, covered 0.13 mile, average speed of 8.2 mph, maximum speed of 12.5 mph.
Here’s the movie, if you want to review it yourself.
6:50, 19.7MB, Streaming QuickTime. I did the Schwalbe and Bontrager runs that you see here, then I did two more Bontrager runs, and three more Schwalbe runs, and the results were consistent with these two.
And yes, the methodology is not super-fancy, but I think it is an adequate model for everyday use of the tires. We ride on roads, not in labs.