Also: Reducing tire pressure in the rain and tire pressure warning systems
Lennard Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.
I am an ole’ guy who has moved to Continental 28 clinchers (100psi) on my town bike. I like the ride and it’s a little better dealing with small potholes and road cracks.
Am I a trendsetter? Should I consider it for my road bike? Can you review the physics and thought process of rolling resistance with a wider tire at lower pressure?
Yes, you’re a trendsetter! And yes, you should consider it for your road bike, although if you climb a lot or sprint a lot, the rolling resistance gain would be offset somewhat by the heavier weight of the bigger tire.
While I have often pointed out that bigger tires roll faster at the same tire pressure than smaller tires that are otherwise identical, readers often don’t believe me. And those same readers also often don’t believe that dropping that bomber pressure they run in their tires will reduce their rolling friction. While I’m sure I can’t convince some people, I can explain in simple terms the reduction in rolling friction with increased tire width. You can equally apply this reasoning to increased tire diameter (i.e., why a 29er rolls faster than a 26er).
Tire rolling resistance on the road is caused by: (1) internal friction and hysteresis within the tire’s materials, and (2) on rough roads, small bumps lifting the bike and rider slightly on each little impact. Look at the diagrams by Continental and Specialized; they show that the wider tire will have a shorter contact patch and hence less tire deflection. By definition, if the tire pressure is the same, the area of the contact patch must be the same to support the same load. However, the shorter the contact patch, the lower the vertical depth of tire deflection and hence the less the internal friction and hysteresis within the tire’s materials.
If a wider tire is made of the same materials in the same thicknesses as a narrower one, it will roll faster, because the internal friction and hysteresis within the tire’s materials will be lower, and because the surface imperfections in the road will be absorbed into the tire more easily (since it has more deflection available), thus lifting the bike and rider slightly less on each little impact.
The go-to independent tire-testing lab for the bike industry has consistently found in its testing that fatter tires roll faster than narrower ones. I visited the facility during a recent trip to Finland and will be having some testing done for VeloNews there.
Here is what Wolf VormWalde, tire designer for Specialized, says: “’Wider is faster’ holds for clincher and tubular tires under the condition of the same air pressure. Test results (below are) from Wheel Energy in Finland.” See the table above (Image 3/3) to illustrate his point. He explains, “the last column shows work (Watts) to keep constant speed (40km/h) of a test drum against the resistance of a loaded (50kg) tire.”
The table from VormWalde shows that, in each case of two tires of the same materials and construction method (first two clinchers followed by two tubulars) the wider tire in each case has lower rolling friction than its narrower brethren.
In looking on the Internet I notice that the tire pressure application of piezoelectric tire pressure sensing and display has not been applied to bicycles. I’m wondering if you may know of an application.
The reason I ask is that my wife and I recently crashed our tandem because of a low-pressure rear tire in a corner. We had picked up tire wire from a debris area on a highway. When rounding a right corner in town at about 15mph the rear tire rolled from the rim, causing the back-end to slide out. We’d ridden 4000 miles with one other flat on our 28mm Gatorskin Continentals (great tires). I recognized that flat as we rode but had no indication of the one that caused us to crash before we went into the corner.
Unfortunately she suffered and continues to suffer with a broken femur and dislocated shoulder. I suffer from apprehension of putting her at risk on the tandem again.
While I don’t think there is anything out there, I hope you may know of something or are able to verify my findings.
That is a great idea, particularly on a tandem, where it is harder for the person doing the steering to sense what is happening with the rear tire. I know of no such thing in the bike industry. I hope that when it comes, it works better than the ones that have been on a lot of rental cars I’ve used that have forced me to tolerate tire pressure warnings on the dash despite the fact that that the tire pressure was fine.
So sorry to hear about your wife’s injuries.
Rainy race weekend here in the Midwest. Why is it standard practice to reduce tire pressure in the rain? Wouldn’t this decrease rolling resistance? Maybe when it starts to rain and gets slippery you want to go to max inflation for more rolling resistance?
You’re confusing traction (sliding friction) and rolling friction. In the rain, you want an increased contact patch. The greater surface area of contact will increase grip. Yes, it will also reduce rolling friction, but it will increase sliding friction.
Feedback on a previous column:
In reference to the dude in Holland wondering if the cold caused his (carbon) frame to crack, consider the Boeing 787 carbon components, which are subject to much more temperature extremes than -18C (0F).