Bicycle Quarterly : Performance of Tires

Posted on June 13, 2012

Source : Off The Beaten Path

Bicycle Quarterly Volume 5 Issue 1

Bicycle Quarterly Volume 5 Issue 1

Perhaps the most influential topic in Bicycle Quarterly has been our research on the resistance of tires, first published in Vol 5, No. 1 (above). It is not a coincidence that 5 years after that issue was published, professional racers are adopting wider tires and lower pressures. How did all this come about?

My research started after I read a big test of racing tires in the German magazine TOUR in late 2005 (below). As part of their testing, TOUR measured the rolling resistance on a steel drum in a lab setting. They found that the “fastest” tires had half the resistance as the slowest. In real-life terms during a ten-kilometer time trial, the slowest tire (Continental Grand Prix 3000) would take 34 seconds longer than the fastest (a hand-made clincher with a cotton casing). TOUR noted this but did not critically evaluate the relevance of their results: 34 seconds in a short time trial is the difference between winning and 20th place.

Reifen 1005-2

Reifen 1005-2

Most of us don’t spend much time at “time trial” speeds, where wind resistance predominates, so I did a quick calculation to see what this meant for real-world riders. I found that the differences were even more significant. Furthermore, TOUR tested only high-end racing tires. What about the wider tires most of us used? Were we giving up significant speed? We had to find out!

The first thing to check was whether we could replicate TOUR’s results on real roads. We thought of a variety of tests, and concluded that a rolldown test had the most promise. Mark Vande Kamp suggested an old soapbox derby track that was perfect: It started steep and then gradually changed to a very gentle slope.

Illustration of Bicycle Quarterly Rollout Testing

Illustration of Bicycle Quarterly Rollout Testing

The first step was to get two sets of tires that TOUR had tested: The Deda Tre Giro d’Italia were the fastest in the test, and the Continental Ultra Gatorskins were among the slower tires. On a calm morning, we headed to the soapbox track. Each of us had mounted one of the sets of tires on our bikes. I rolled down the slope while Mark timed me. Then we switched wheels, so I could roll down with the second set of tires, on the same bike. We repeated this several times, and the results were clear: The Deda tires were significantly faster than the Continentals, just as TOUR had predicted.

The next step was to gather all the tires we wanted to test. It was a long list, and we eventually pared it down to 18 tire models. (We later tested additional tires.) To make the testing session more time efficient, we borrowed as many wheels as we could find, so we did not have to mount tires between tests. Then we waited for a perfectly calm day. In Seattle, this usually happens when the weather changes. During sunny days, the wind blows from the north, whereas it switches to the south on rainy days. In between, there are hours of perfect calm. From riding many randonneur brevets, we also knew that around sunrise, there was the greatest chance of no wind. Finally, a day came with a promising weather forecast. Long before dawn, we loaded up two Burley trailers stacked high with wheels, a floor pump, clipboards and everything else we needed.

Testing at Woodland

Testing at Woodland

Then we started our testing. Mark rode up the hill. He held onto a stepladder on the start line, so he would smoothly roll down the hill without wobbling or having to clip into the pedals. He used the same position during each roll-down: hands on the hoods, elbows locked. (We later confirmed in the wind tunnel that this position can be maintained with great consistency.) Two timekeepers recorded Mark’s time in the lower section of the run. We ran each tire/pressure combination three times to see how consistent our data was. We also ran the same tires on different wheels to check whether the wheels influenced the results. (They did not.)

We tested and tested all morning. Different tires, the same tires in different widths, the same tires at different pressures. Clinchers, tubulars, racing tires, touring tires, new tires and well-worn tires with thin tread.

We tested the same baseline tire three times during our test session – first, in the middle and last – to see whether conditions had changed. We were lucky: the morning was completely calm. It also was overcast, so temperatures did not change. (We later found that temperature greatly affects the rolling resistance of tires.) One morning was not enough to complete our testing, so we spent many more days on the test track. Sometimes we went out at 4:30 a.m. only to have a light wind spring up. All we could do was go home and hope for better luck next time.

At the end, we had pages and pages of measurements that we formatted for analysis. During our analysis, we found some interesting results:

  • The speed differences between our tires were even greater than those tested by TOUR. The fastest tire, the Deda Tre, rolled 20% faster than the slowest, the Rivendell Nifty-Swifty. A 20% difference in on-the-road speed is huge!
  • Wider tires roll faster. A Michelin Pro2 Race in 25 mm width was faster than the same tire’s 23 mm version, which in turn was faster than the 20 mm version.
  • Very high tire pressures don’t roll much faster. Above an “adequate” tire pressure, the tire’s speed increases only very slightly with higher pressures. This contradicted the tests performed on steel drums, including those by TOUR.
  • Tires should not be tested without a rider on the bike. Most of the energy is lost in the rider, as vibrations cause friction in the body’s tissues (suspension losses). That is why testing in the lab can be misleading. In the lab, higher pressures roll significantly faster, but on the road, the suspension losses increase with higher pressures and cancel the advantage of the reduced tire deformation.

One important question was still open: Did the results represent real differences in tire performance, or was there too much noise in the data? After all, even slight changes in rider position, a tiny gust of wind, or other factors might influence the results. To check this, Mark, who has a Ph.D. with a Minor in Statistics, did a sophisticated statistical analysis. He found that our results were “statistically significant.” (Basically, he compared the data from the three runs of the same tire with the data from different tires. The variations between runs with the same tire were much smaller than the variations between different tires.) This means we really did measure differences in tire performance. (Many studies skip this step, but it’s crucially important.)

Our research had profound implications:

  1. Tire resistance is much more important than previously thought. For most riders, changing the tires is by far the biggest improvement they can make to their bikes’ performance. (Aero wheels will improve your bicycle’s speed by less than 2%, whereas tires can make a 20% difference.)
  2. On steel drums, wider tires were slower because they had to run at relatively low pressures. Once we had shown that the high pressures served little benefit, it became clear that on real roads, wider tires are faster, period.
  3. The secret to a fast tire is a supple casing. Compared to the casing, all other tire factors are relatively unimportant… In the past, many considered a high pressure rating as a sign of a “good, fast” tire. In fact, tires with high pressure ratings tend to need sturdier casings that make the tire slower.

Like most research that calls into question long-held beliefs, our findings first met with much skepticism, but over the years, they have become widely accepted. We used our research to help Grand Bois design supple, wide tires that offer more comfort and better performance than most other tires. Switching to these new tires, my  times in randonneur brevets decreased significantly. (I had been riding some of the slower tires in our test!) In addition, I no longer had a hard time keeping up in pacelines. (In a paceline, the air resistance is significantly reduced by drafting, and most of the resistance you have to overcome is rolling resistance.)

Today, even professional racers no longer worry about tire pressure. Their tires now are wider and run at lower pressures than those of most “weekend warriors.” And for the rest of us, this research has made it possible to build bikes with comfortable 42 mm tires that are as fast as racing bikes.

There still are some questions about tires that remain to be answered: whether clinchers truly are faster than tubulars; at what point the advantages of wider tires turn into disadvantages (it is unlikely that a 100 mm-wide tire rolls faster than a 50 mm-wide one), and whether tire deflection due to pedaling causes additional resistance when running very low pressures (in our tests, the rider coasted). We have been doing more research, and will report on it in future issues ofBicycle Quarterly.

Further reading: