Technical FAQ with Lennard Zinn: Tire pressure

Lennard talks to a reader skeptical that lower tire pressure could improve rolling resistance

Dear Lennard,
Zipp says that decreasing pressure reduces rolling resistance. Nothing is said about the surface being ridden. I cannot understand how decreasing tire pressure can do anything but increase rolling resistance. The less the tire flexes and deforms the less energy is lost. We can discuss how lowering tire pressure helps handling and comfort and how that translates to being faster over a given course, taking into account the surface being ridden. But how anyone can say there is less rolling resistance at a lower tire pressures baffles me and apparently track racers agree.

If lowering pressure actually reduces rolling resistance, as Zipp claims to have found, then why aren’t the track guys running 120psi?
Bryin

Dear Bryin,
The assumption, if not explicit, that I made is that Zipp’s research applies to riding on the road. On the track, it has to do with the smoothness of the surface they’re riding on.

Riding a mountain bike or motorcycle on rough terrain is faster with suspension, and tires, your first line of suspension, act similarly.

As with suspension, the argument is about sprung vs. unsprung weight. If you ride a tire pumped up to, say, 140psi, on a chip-sealed road, every single chip you hit will lift you and the bike, sucking up energy and slowing you down. If you instead ride over that chip seal at, say, 80psi, the tire tread will move up and down over the chips without the wheel being deflected (and hence the bike and rider as well). This will result in lower rolling resistance.

The rougher the surface, the lower the pressure for minimizing rolling resistance. Cyclocross racers run tubulars at 26-28psi for this reason. Believe me, you can feel the enormous additional rolling resistance if you instead ride them at 50psi on most ’cross courses. Obviously, the converse is also true; you can use higher pressures efficiently on smoother surfaces.
Lennard

Dear Lennard,
Thanks for the succinct assessment of virtues of tubulars in VN Tech. I never switched and may be alive because of that. Was sweeping down through a hairpin on Alpe d’ Huez in ’03 when my front tire blew. I rode it out.
Phil

Dear Phil,
Glad you’re still here to tell the story.
Lennard