If you’re one of the 50,000 spectators expected to turn out for the Amgen Tour of California’s Sacramento stage on Sunday, you’ll notice that pro road racing amounts to very lean athletes riding bikes with very skinny tires.
But those tires are not as skinny as they used to be.
It turns out that fatter is faster.
While that may seem counterintuitive, a tire that’s slightly wider than the industry standard of 23 millimeters experiences less rolling resistance. Tests have proved it and pros, who make their living by being efficient and going fast, have inspired a trend in recent years that’s just now trickling down to amateur cycling enthusiasts.
The new standard on many pro teams is 25 millimeters (or almost an inch). That’s more than a 30 percent increase from the 19 millimeter tires regularly used by riders not too long ago.
For decades, conventional wisdom was that pizza-cutter-thin tires pumped to rock-hard pressures were faster. Cyclists assumed they were more efficient, more aerodynamic and more responsive on their lightweight bikes, even though they were uncomfortable to ride and punctured easily.
But science had something to say about those assumptions. Several years ago, Bicycle Quarterly editor Jan Heine wrote about tests he conducted that proved wider tires with supple casings were significantly faster than skinny ones, especially on imperfect surfaces like roadways. It also became clear that wider tires handled better in corners.
Heine found that 25 millimeters was the best width for peak speed, though even wider tires could be more comfortable and less prone to punctures without giving up speed. (At some point, though, extra weight and wind resistance become an issue.)
Leading tire manufacturer Schwalbe says the less rolling resistance – and more speed – found in wider tires can be explained by measuring deflection, or the small amount that a tire flattens against the road when you ride your bike. That’s called the contact area, or the contact patch.
A 25 millimeter tire has a shorter and wider contact patch than a 23 or 21 millimeter tire. Narrower tires have a slimmer and longer contact patch. The longer contact area means a thinner tire loses more of its roundness and ultimately rolls slower. “In the wide tire,” Schwalbe notes in a summary published on its website, “the radial length of the flattened area is shorter, making the tire ‘rounder’ and so it rolls better.”
“I started out on 19 millimeter tires (in the 1980s). That was conventional wisdom,” Heine said in a phone interview from his Seattle office. “By 2011, all the pro teams were ordering 25 millimeter tires. The tires make the biggest difference and the pros know this.”
John Marsh, the publisher of Road Bike Rider, an online publication read by cycling enthusiasts worldwide, says the move to wider tires “is something I think most serious road riders have seen happening over the past three years or so. That’s when I made the switch from 23s to 25s. I really didn’t know anybody 10 years ago who was riding 25s, and if you went to buy tires, 25s were almost unheard of.”
Vincent Gee, a veteran bike mechanic dating back to his days with the U.S. Postal cycling team, says his current team, Optum Pro Cycling Presented by Kelly Benefit Strategies, ordered only 25 millimeter tires for the 2015 season. Optum is one of 18 teams competing in the 2015 Amgen Tour of California.
“There are a lot of small factors that add up to one big factor,” Gee said. “I would sum it up as rolling resistance. The fatter tire actually rolls faster. There is deflection with every pebble you hit on the road and there might be 2 million pebbles on the ride. You can’t feel it, but you can measure it.”
But Gee says most casual road cyclists have yet to catch on, and it’s still difficult to find a variety of 25 millimeter tires in bike shops.
“On the bike trail, I would say 80 percent of the cyclists still uses 23s,” he said.
Steve Rex, a highly regarded custom bike frame builder based in Sacramento, says wider tires started catching on about a decade ago. But he noted that many factory-made road bikes don’t have enough clearance on the frame and fork to accommodate anything wider than 25mm tires.
“I currently use 28s on my road racing bike because of the comfort,” Rex said. “I’ve used wide tires off and on for a long, long time. I never thought of it as a speed issue. The only drawback is a little bit extra weight.”
Part of the popularity with wider tires is making dirt roads accessible to road bikes, once confined to paved roads because their skinny tires handled poorly.
“That’s a super-hot segment,” he said. “People are really embracing that. It’s a great way to ride a bike.”
Amgen Tour of California
What: The Sacramento stage kicks off the eight-day race.
When: 10:50 a.m. Sunday (men); 11:15 a.m. (women)
Where: The start/finish line is at 11th and L streets.