Off The Beaten Path

12 Myths in Cycling (1): Wider Tires Are Slower

When we started to publish Bicycle Quarterly 15 years ago, it seemed that most of the technical aspects of bicycles were well-established. And yet, as we tested many different bikes, we started to question many of the things we had accepted as ‘facts.’ To celebrate our 15th anniversary, we’ll look at some of these myths. We’ll explain why we (and everybody else) used to believe them, and how things really work. Let’s start this series with the biggest one:

Myth 1: Wider Tires Are Slower

For almost a century, cyclists ‘knew’ that narrower tires roll faster. Some people realized that in theory, wider tires are faster due to their shorter contact patch, which deforms less as they roll. But the thinking was that in practice, the lower pressure at which wider tires must run limited their performance. If you wanted to go fast, you chose narrow tires.

That is what we thought when we started testing tires almost 12 years ago. And yet, as long-distance riders, we wondered whether the narrowest tires, pumped to the highest pressures, really were optimal for us. What if wider tires were a few percent slower, but their greater comfort reduced our fatigue? Remaining fresh toward the end of a long ride would help us put out more power, so we might go faster in the end. What we needed to know was how much speed we would give up by going to wider tires.

Real-Road Testing

So we started by testing 20, 23 and 25 mm tires (same tire model). Imagine our surprise when the 20 mm were slowest, and the 25 mm fastest. This wasn’t what we expected! And yet, when we repeated our tests with a different methodology (power meter vs. roll-down), the results remained the same. There was no doubt that the narrowest tires are slower than slightly wider ones.

Then we tested wider tires, and realized that, once you go wider than 25 mm, the performance of tires doesn’t change as they get wider. Since then, we’ve tried to figure out how wide a tire can be before its performance begins to drop off.

We’ve used the results of our testing to develop our Compass tires, which are optimized for performance and comfort on real roads. And since we now have very similar tires in widths from 26 to 54 mm, we could do controlled testing of all these sizes. We found that they all perform the same. Even on very smooth asphalt, you don’t lose anything by going to wider tires (at least up to 54 mm). And on rough roads, wider tires are definitely faster.

As we did more research, we realized that cyclists used to know this. When pneumatic tires were first invented, the fast-riding ‘scorchers’ used wide tires, because they rolled over road irregularities better. And in the 1920s, Vélocio, the editor of the French magazine Le Cycliste, discovered that as long as wide tires had supple casings, they rolled as fast as narrow ones. But all this was forgotten in later decades, as racers went to narrower and narrower tires.

Why did it take almost a century to rediscover this? There are two reasons why cyclists used to believe that narrower tires were faster:

1. Laboratory tests on steel drums eliminate the rider and thus the suspension losses. If you look at hysteretic losses alone, narrower tires run at higher pressures and thus flex less, meaning they absorb less energy.

We tested on real roads, with a rider on the bike, and found that the increased vibrations of the narrower tires caused energy losses that canceled out the gains from the reduced flex. These suspension losses are mostly absorbed in the rider’s body. Imagine a bean bag that drops on the ground without bouncing back – all the energy is absorbed by friction between the beans. The human body works similarly. Studies by the U.S. Army found that the more discomfort vibrations cause, the more energy is being absorbed.

2. Placebo effect: The faster we ride, the higher the frequency at which our bike vibrates, because our tires encounter road irregularities at a higher speed. However, narrower tires also increase the frequency of the vibrations they transmit. Basically, a bike with narrow tires feels faster even though it may actually be slower. Inflating your tires harder is a simple way of tricking your brain into feeling that you are going faster, but if you have a bike computer, it’ll tell you that you haven’t actually increased your speed. Conversely, wide tires vibrate less, and thus feel slow to most cyclists.

So for almost a century, narrow tires felt faster, and they tested faster in the laboratory. There was little reason to question whether they actually were faster. It took Bicycle Quarterly‘s real-road tests to show that a vibrating bike (and rider) is absorbing energy that reduces the bike’s speed.

What all this means is that you can have your cake and eat it, too. If you run wider tires at lower pressures, you increase the flex of the tire (negative), but you reduce the suspension losses (positive): the two effects cancel each other, and your speed remains the same.

This also explains why supple casings make such a huge difference in tire performance: They are easier to flex, so they absorb less energy. And they absorb vibrations better, which reduces the suspension losses. So they use less energy on both counts. Talk about a win-win scenario! And of course, since they absorb vibrations better, they are more comfortable, too.

Aerodynamics

What about the aerodynamics of wider tires? Many riders believe that wider tires will be slower, because they have more wind resistance. We tested this in the wind tunnel and found that the difference between 25 and 32 mm tires was too small to measure reliably in a real-world scenario. The German magazine TOUR built a sophisticated setup with a motorized dummy rider and found that a 28 mm-wide tire had the same wind resistance as a 25 mm tire when the wind was coming from straight ahead. With a crosswind, the wider tire was very slightly less aerodynamic. Even then, the wider tires required only 5 watt more – on real roads, the reduced suspension losses probably make up for that.

We tested our tires on smooth pavement at 29.5 km/h (18.3 mph), and found no speed difference between narrow and wide tires. If you ride much faster, then it’s possible that wider tires roll a little slower, but the difference will be so small that it’ll get lost in all the other factors that influence your bike’s speed. On the other hand, if you ride slower, then the advantage of wider tires will be even greater.

Spinning up

Wider tires are a little heavier than narrow ones. The difference is smaller than many cyclists imagine – air doesn’t weigh anything – but a wide tire has a little more rubber and casing. Won’t this make the wider tires harder to accelerate? The answer is “No.” The reason is simple: Bicycles don’t accelerate very quickly. Even a professional bike racer’s power-to-weight ratio is far less than that of the slowest economy cars, and those don’t exactly push you back in the seat when you floor the throttle. Bikes don’t accelerate fast enough for small changes in wheel weight to make a difference. That is why professional sprinters can use relatively large wheels (which inherently are heavier) and still win races.

The UCI requires a minimum wheel size of 55 cm, yet racers use 700C wheels that are 10 cm larger than required. If wheel weight mattered as much as most cyclists imagine, then pros using the smallest wheels would win every race. And yet, even though many have tried smaller wheels, all have returned to 700C wheels – probably because the larger wheels handle better due to their optimized rotational inertia. (But that is a topic for another post.)

Conclusion

What this means for us riders is that we can choose our tire width freely, without having to worry about performance. Of course, this doesn’t mean that a wide ‘touring’ tire will perform as well as a narrow ‘racing’ tire. Casing performance determines 95% of a road tire’s speed, and to get good performance, you need a supple high-performance casing. (The other 5% come from the thickness of the tread.)

Tire width influences the feel of the bike, but not its speed. If you like the buzzy, connected-to-the-road feel of a racing bike, choose narrower tires. If you want superior cornering grip and the ability to go fast even when the roads get rough, choose wider tires.

Further reading: