Tire Pressure – Stop Guessing And Read The Science

Talking tires with Josh Poertner – new discoveries about the relationship between pressure and performance indicate new directions for research and development

Tires are a peculiar commodity in bike racing. There’s tremendous competition between manufacturers who spend extraordinary amounts on research and development to make the best tires for any given condition. And yet if you take a survey of competitors at your local Olympic Distance triathlon or criterium on any given Sunday, you’re likely to find very few people showed up with a dedicated set of “race tires.” Plenty of racers bring race wheels. A select few will even have a set for hilly events and another for aerodynamic performance on gentler slopes. The majority of these wheel sets cost $1500 or more. But a $240 set of tires saved just for racing? Not on your life. Tires are just tires, right?

That’s how they’re quite often viewed, much to the detriment of racing performance. But even for the discriminating athlete injecting Noble gases into tubeless rubbers, there remain considerations on the table that aren’t taken advantage of. One man who’s become extremely interested in these considerations of late is Joshua Poertner. The man who reinvented the bicycle wheel (and helped to publish three papers through the American Institute of Aeronautics and Astronautics in the process) has since turned his attention to inventing better pumps to inflate the tires going on those wheels. Just like rim width and dimples on those Zipp wheels he helped create, minute details like tire pressure and the gradation of road surface have become something of an obsession for him.

“For me, it began when we were testing the Zipp 303 wheel for Paris-Roubaix. We thought we had a great product that first year, and ended up having failures in the race due to tire pressure variance, and large riders choosing 24mm tires over the recommended 27mm…it was a rough day. We took a lot of heat from people saying we had done it as a publicity stunt and cost riders their race.”

Bowed but not broken, Poertner and the Zipp team spent the next year tweaking the 303 for its redemption. Part of the process was field testing on the roughest cobble sections with none other than Fabian Cancellara. In addition to getting the wheel itself right (Spartacus won the race on 303s the next year), the Zipp team made some extraordinary discoveries about the influence of tire pressure on performance.

“We found that he rode his fastest when we reduced the tire pressures down near 60 psi. Basically the lower we went, the faster he went until he broke his wheels. We also discovered that the pumps on the team truck were off by as much as 12 psi relative to each other, so we had to really get every single detail right in terms of optimizing speed and tire pressures.” Poertner was fascinated, but the call to make the next great leap in aero design beckoned, and further investigation went to the back burner.

That changed when Poertner left Zipp in 2013 to buy ownership of Silca. Relieved of concerns about things like Strouhal numbers and centers of pressure, he could return to the questions provoked by his findings on the pavé.

A fortuitous opportunity to answer those questions arrived last year when Butler University began repairs to a stretch of road on its campus near the Silca headquarters in Indianapolis. “They used a machine to chew up the old pavement, which created a perfectly uniform peak-to-valley measurement of 8mm in the road surface. We couldn’t have asked for better measurable conditions.” Later the University repaved the road with asphalt, allowing Poertner to test the performance of tires inflated to different pressures under relatively controlled conditions.

He found that on the most freshly compacted asphalt, the optimal tire pressure for a 170-pound athlete was 110 psi. Performance decreased above that. None of this will surprise those who’ve taken time to study rolling resistance in tires. Most people have seen the curve denoting the theoretical change in a tire’s coefficient of rolling resistance (Crr) as a function of tire pressure. Manufacturers have told us for years that beyond a certain point added pressure really doesn’t help. But in laboratory roller tests the data has only indicated that there are no improvements in performance. What Poertner and others (in the case of the graph above, Tom Anhalt) have found is that things actually get much worse past a certain tire pressure, placing new emphasis on being careful not to overinflate. The obvious difference between laboratory and actual road data is that rollers are smooth steel drums and the road has varying degrees of texture. There was obviously something else going on beyond rolling resistance. Anhalt referred to the overinflation mark as “breakpoint pressure.”

For now, Poertner has dubbed the phenomenon “impedance,” which he admits isn’t the most accurate term. It does lend to a good approximation for what happens over rougher surfaces, though. A good paradigm is to think of each pebble and divot in the road like a tiny valley or mountain. The tire has to negotiate each change in elevation. A tire made hard by too much pressure has to go up and over each one like a microscopic Hawi climb. A “squishier” tire simply gives way and rolls straight on as if it wasn’t there. What’s surprising is just how quickly the effect occurs. Like Cancellara over the cobbles, Poertner found that the fastest pressure over the torn-up pavement was about 65 psi, significantly lower than what conventional wisdom would recommend considering that it’s nowhere near as rough as the legendary road to Roubaix. In fact, once he established the trend between grades of pavement, he found that just about every amateur athlete out there is running their tires too high. Poertner was gracious enough to allow TRS the first peek at his experimental data. The graph below shows a clear relationship between pressure and the amount of power to maintain a constant speed over different road surfaces. With increasing roughness, the breakpoint pressure occurs earlier, meaning that the optimal tire pressure is much lower than most people think.

“Keep in mind that 110 is the highest pressure you want under the most perfect imaginable conditions, with asphalt that hasn’t even been driven on yet. It starts dropping quickly from that. I tell people and they think I’m crazy. But then they’ll actually try it at 90 or even 85 psi, and they come back and say that they are having much better results.”

For those who are curious, Poertner now consistently runs his tires at 70 psi. The key point is that whereas we used to consider rolling resistance the main challenge to speed, there are now two considerations which compete in holding sway over your tire pressure decision. It’s a competition in which rolling resistance typically comes out on the losing end. Keep in mind that rough pavement demanded similar pressure to cobbles. “When we plotted it out, we found the line defining the relationship between pressure and impedance is much steeper than rolling resistance, which means you start bleeding off energy due to impedance much more quickly than rolling resistance. It turns out that it’s much better to be 10 or even 20 psi lower than the ideal tire pressure than 10 psi higher.” Below we present another pretty picture illustrating the point. Poertner has extrapolated his findings into neater lines to establish the new paradigm at the tire pump.

It’s therefore beneficial to err on the side of Poertner’s impedance than rolling resistance. This is enough of a paradigm shift in itself, but there are further implications for tire selection, which takes Poertner back to his familiar stomping grounds in wheel construction.

“Here’s the next thing you have to think about. As tire width increases, tire pressure decreases. So a wider tire performs better in terms of rolling performance.” That leads to a new competition, this time between rolling resistance and aerodynamics. Again, conventional wisdom and past experience say that aero trumps all and a narrower tire is better. Again, conventional wisdom is wrong.

“Narrow tires and wheels still work better on high-quality indoor tracks. When Bradley Wiggins broke the hour record he ran on very narrow and thin tires. He also ran them at 300 psi and used Argon gas on the most efficient surface in the track cycling world. He made his run under extreme conditions. Even on high quality concrete tracks, wider tires start to perform better and we find optimal pressures quickly move toward 110-120psi on excellent concrete surfaces. Again, that’s how quickly the performance drop-off occurs.”

While he hasn’t processed his data into a presentable format for the masses, Poertner’s findings portend new questions for every answer they offer. If wider is better, is a lot wider a lot better? And if a wider tire is what the world needs, then do we need a wider wheel to hold it? More accurately do we need wider wheels, wider forks, and wider chain and seat stays to hold them? Who is going to manufacture all this stuff, and how will the bike designer, the wheel builder, and the tire maker negotiate the new standards?

“It is an insane process. Everyone in the bike industry is so secretive. It’s just part of the culture and it’s not going to change. So there will be lots of discussions. I remember when wheels went from 19mm to 23mm. It was a very gradual process. And then we went from 23mm to 25mm. Now we’re seeing 28mm wheels. Where does it stop? I don’t know.”

It does seem, however, that this is a new beginning for tire performance. Demand drives supply, and as it seems new bike and equipment designs are offering fewer aerodynamic gains, consumer attention will turn toward areas they perceive there’s a potential boost. Having spent years receiving no more scrutiny than the proverbial kick from cyclists and triathletes, tires may be on the cusp of a return to the spotlight. The discussion will likely be a spirited one.

“I see discussions in forums all the time in which people express beliefs that are grounded on nothing more than personal preference. People like to believe they have the right answer, and they don’t want to accept new data.” Poertner is no stranger to that phenomenon, having fought the aero-versus-weight battle in his early years at Zipp. Now that data is coming to light in this debate, it’s likely attitudes will shift quickly and early adopters will gain a minute or two on their dogmatic competitors. That ought to break the ideological impedance, and set the industry and sport alike on a course to see just where this developmental path ends.

photo credit: Resist Nomad via photopin (license)