As part of our series on myths in cycling, let’s look at wheel size and how it affects speed. Even though tires have grown wider in recent years, many bike makers have stuck with 700C wheels. It’s a size that is familiar to road cyclists, and there is also a fear that smaller wheels won’t roll as fast.
When we developed our Rene Herse / Compass tires, we tested all aspects of how tires perform – especially on rough roads. During that testing, we found that the common wheel sizes – 26″, 650B and 700C – all roll at the same speed.
At first sight, it makes sense that a larger wheel might roll better over obstacles and road irregularities: With a larger wheel, the bump becomes comparatively smaller, effectively smoothing out the road. And we all know that a wheel rolls faster on a smoother road…
Why doesn’t it work like that in real life? The difference between common wheel sizes is relatively small: Only about 10% between a 700C and a 26″ wheel (above). The approach angle – the angle at which the wheel is hitting an obstacle – is almost the same for both wheel sizes. Even when hitting a rock that is 4″ (100 mm) tall at its corners, the difference in approach angle is less than 2° – not enough to make a difference.
We tested this in our famous ‘rumble strip’ testing: The rumble strips of a brand-new highway provided a uniform ‘rough surface,’ allowing us to test different setups on rough roads. (Testing on real gravel roads does not work well, because there is too much variability depending on how many large and small rocks you hit during a run.)
We tested otherwise identical 38 mm-wide tires (Schwalbe Marathon HS) in three wheel sizes: 700C, 650B and 26″. We measured the power required to pedal the bike at 32.2. km/h (20 mph). Basically, that is like riding the roughest parts of a course like Dirty Kanza at race speeds.
We tested on a day with no wind and with constant temperatures. We did three test runs with each setup. This allowed us to perform a statistical analysis – ensuring that we are reporting real results, and not just ‘noise’ in your data. The columns above show the averages for those three runs. The dark part of each column is the ‘confidence interval.’ If differences between wheels fall within the dark parts, they are not statistically significant. This means they may be caused by noise in the data (such as small changes in rider position, wind, temperature, etc.).
Looking at the first three columns, there are very small differences between wheel sizes. They fall within the dark bars – they were not statistically significant. This means that they are too small to tell whether they were real or not. (And in real life, they are also too small to matter.)
The fourth column shows what happens when you reduce the air pressure slightly. We did that to check whether small variations in tire pressure might affect the results – it’s impossible to inflate tires repeatedly to exactly the same pressure. The differences were small – not enough to affect our results. (This is why we tested with stiff tires. With supple tires, pressure makes a bigger difference, and lower pressures roll faster on rough terrain.)
We also tested each setup on the smooth pavement next to the rumble strips. This allowed us to confirm that our tires really were identical in their construction, and only differed in their wheel size. We wanted to make sure that the tires in the three wheel sizes were not slightly different in some way that made them roll slower or faster. There was a little variability – the inevitable ‘noise’ – but the differences were not statistically significant. Even on the smooth pavement, the three wheel sizes required the same power.
The conclusion: Even on very rough surfaces, the three common wheel sizes (700C, 650B, 26″) offer the same performance. They roll at the same speed on smooth pavement, too.
If large wheels don’t roll faster, why do many mountain bikers report that 29ers with their large wheels feel faster than smaller-wheeled bikes? I suspect that this has more to do with the handling – how easily the front wheel gets deflected. On a mountain bike, a wheel with more inertia will climb out of ruts better and roll over bumps with less deflection, and that probably is a good thing that allows you to go faster.
On the road, larger wheels also make the bike more stable – but here this isn’t always good. The best road bikes offer nimble handling that makes them easy to place on the road. With wider tires, the bike becomes more stable – too stable for many riders’ tastes.
To keep the nimble handling of a racing bike, you need to keep the rotational inertia of the wheels the same – by reducing the rim diameter to make up for the taller (and heavier) tire. That is why 650B wheels are popular for all-road bikes.
Rotational inertia affects not only the handling, but also how the bike feels when you rise out of the saddle and rock the bike from side to side: If your wheels have too much inertia, the bike becomes harder to rock – and that may actually slow you down.
Conversely, racing bikes have stuck with large 700C wheels even though smaller wheels would be lighter and – in theory – ‘spin up’ faster. I suspect that you need some inertia to push against when you rise out of the saddle. As with the bike’s handling, you probably want just the right amount of rotational inertia from your wheels.
What does this mean in the real world of high-performance all-road bikes? We’ve already shown in the first part of this series that wider tires roll no slower than narrow ones. So there is no need to get a second, ‘go-fast’ wheelset with narrow tires for your all-road bike.
However, you do want to go to smaller wheels if you want to keep the nimble handling that makes a road bike so enjoyable. And avoid heavy tires: Wide tires have more rubber, and the effects of a lightweight tire are much more pronounced than they are on a narrow-tired racing bike.
In practical terms, here are the wheel sizes that I like best:
- 700C x 28 mm
- 650B x 42 mm (with aluminum rims)
- 650B x 48 mm (with carbon rims)
- 26″ x 52 mm (with aluminum rims)
When you calculate the rotational inertia for all these wheels, you’ll find that they come out the same. That is why bikes with these wheels feel very similar.
How your bike handles also depends on personal preference and on your terrain. If a road bike doesn’t feel stable enough for you, you can go up a wheel size to gain stability. That is one of the reason why we offer our Rene Herse offers tires in many sizes.