A cyclist’s greatest foe is not the rain, steep hills, a heavy bike, or an unrelenting headwind. Rather, it is a tiny but measurable force that constantly acts to slow us down, even when we’re travelling downhill: friction. Friction takes many forms and can be found in a variety of places, the most familiar of which is air friction. Some riders will spend thousands of dollars improving the aerodynamics of their body and bike to minimise the effects of air friction (or drag), and with good reason, since it is the major force acting to slow a speeding cyclist. Friction also acts on the chain, cassette, and bearings to resist the efforts of the rider but these effects are minor when compared to what is going on between the road and the tyres.
Any rider knows that the amount of friction offered by the road depends upon the nature of the surface, where smooth roads are easier to ride on than rough. Similarly, tyres can be fast or slow depending on how much friction (or rolling resistance) they offer when in contact with the road. There’s not much a rider can do about the surface of the road, but you can choose a tyre with a low rolling resistance in order to save energy and improve your efficiency.
A detailed discussion of rolling resistance requires a few mathematical equations, some elaboration on the effects of hysteresis, and a deeper understanding of the laws of physics, but I don’t think there is any need for that when the concept underlying it all is a simple one: the amount of friction offered by a tyre depends upon how much of it is in contact with the road. Soft tyres are slow because they flatten and allow more contact with the road; pumping them up improves their rolling resistance because it reduces the size of the contact patch.
The rolling resistance for any given tyre (referred to as Crr) can be calculated by measuring the force required to drive the wheel at a certain speed eg 50km/h. Searching the web will yield a few collections of data but I’ll save you the effort and point you towards the data sets prepared by Al Morrison. The man has been meticulously cataloging Crr for numerous tubulars and clinchers since 2006; his last data set from 2010 comprised over 100 tyres and provides some interesting insights on the nature of rolling resistance. The difference in Crr between the fastest and slowest tyres in his data set is 2-fold (see Figure 1 for some example), which is equivalent to around 20 watts, depending on rider weight and the target speed. Tubulars comprise 5 out of the top 6 tyres, however the top 10 is evenly shared with clinchers, and indeed, a clincher claims second place in his tests, demonstrating they can be just as fast as a tubular. It turns out that rubber contributes significantly to the rolling resistance of a tyre, so thicker tyres with more rubber (ie training tyres versus race tyres) tend to be slower. As a tyre wears, the rolling resistance starts decrease too, as demonstrated by some of Morrison’s data (Figure 2). Another way to decrease rolling resistance of a clincher is to swap standard butyl tubes for latex tubes (Figure 3). For those riding tubulars, then pay attention to how much glue you use because light gluing slows the tyre down; multiple coats of glue on both the rim and tyre improves the rolling resistance (Figure 4).
Judging from the frequency of his testing, it looks like Al is due publish his next data set soon. It is worth noting that the values calculated for Crr apply to test conditions only; Al notes that Crr may be up to 100% greater out on the road, but the relative differences between different tyres appears to be conserved. For more detail, take the time to read through Al’s data.
One aspect of rolling resistance that has received extra attention in the last couple of years is the effect of width, where wider tyres have measurably lower rolling resistance (when all other things are equal). The clever folks at Roues Artisanales published an article on this phenomenon some time ago with a graphic that shows how the contact patch of a wider tyre can be smaller than a narrower tyre, but it is still somewhat counter-intuitive. There is lab data to support this notion (see this article, and another article here) though it is not quite as freely available as Al Morrison’s work. The clearest demonstration of the benefits of a wider tyre can be found in the pages of Bicycle Quarterly from 2006 where rolling resistance was tested in a very simple manner: by measuring how long it took for a rider to coast down a hill. In these tests, wider tyres were faster, a satisfying real-world demonstration that defied the predictions of lab testing at the time, as noted by the author. The influence of tyre width on rolling resistance suggests that wider rims may lower the rolling resistance of a tyre, however there is little data to support this idea, and thus most of the benefits lie elsewhere, such as road feel, handling, and perhaps aerodynamics.
Aerodynamics versus rolling resistance
Before you start considering wider tyres for your bike, what you gain in reducing rolling resistance, you may lose in aerodynamic drag. Wind tunnel testing has demonstrated that wider tyres experience more drag, but the overall effect depends on the shape and width of the rim. Wheel designers such as Hed, Reynolds and Zipp typically optimise the performance of their aero rims for a specific tyre width, typically 21-23mm, so fitting a wider tyre (eg 25mm) may have a profound effect on the aerodynamics of the wheel.
Increasing air pressure in a clincher or tubular will decrease its rolling resistance (for example see Figure 5). This relationship is obvious one and accounts for the willingness of many riders to test the maximum inflation of their tyres. However, there comes a point where the reduction in rolling resistance becomes minimal and the rider has to contend with the disadvantages of over-inflated tyres, which include a harsh, unforgiving ride and a loss of traction.
Putting the theory into practice
Tyres with a low rolling resistance feel fast on the road. Such tyres also feel “supple” and “sing”, especially at high speeds. It’s a bit like bouncing a properly inflated basketball, the pitch and tone of the bounce is perfect, and so it goes for the sound of the tyres: they strike a perfect note as you roll down the road. However, there is more to the performance of a tyre than its rolling resistance. Great tyres provide plenty of traction and inspire aggressive cornering; they also roll beautifully without being unnecessarily harsh.
Keep in mind that the fastest tyres are generally lighter and have less rubber than slower tyres and therefore, typically suffer from a greater risk of puncturing. High thread counts promise a supple, fast tyre, but there is no guarantee of this. Ultimately, the decision on tyres and tubes will require something of a compromise but the data discussed above demonstrates that a switch to a wider tyre with a lower rolling resistance can have a tangible benefit for a modest price.
Switching to a latex tubes definitely improves the feel of good tyres. I have been comparing standard butyl tubes with lightweight butyl tubes made by BBB and Michelin AirComp latex tubes in Continental GP4000S tyres. I’ve no data on rolling resistance–see Al Morrison’s data since he routinely installs latex tubes before measuring the Crr for clinchers–but on the basis of the ride quality alone, latex tubes make the tyres sing and improves their suppleness. However, be prepared to pump up your tyres every day if you decide on latex tubes. Interestingly, the lightweight butyl tubes weighed virtually the same as the latex tubes (~80g), and while they were an improvement over standard butyl tubes, there was still an appreciable difference in ride quality compared to latex. If there is any difference in susceptibility to puncturing, then I’ve yet to experience it.
Deciding on the right tyre pressure requires some experimentation. I’ve found that different brands and models of tyres work best at different pressures, so I would encourage all riders experiment with both lower and higher air pressures. In assessing different pressures, take note not only of the feel of the ride in a straight line but also while turning at different speeds. As a recent example, I tried out some Michelin Pro3 tyres about 18 months ago and set them up at the same pressure as Continental GP4000 tyres, 110psi on 19mm rims. They rolled well at this pressure but I wasn’t impressed with their cornering until I dropped the pressure to 100psi, and then the tyres came alive.
They say that there are two certainties in life, but for cyclists, there are many more, such as friction, punctures, and the need for new tyres. Such a necessity creates the perfect opportunity for experimentation. It is worth noting that the gains provided by a set of tyres with a low rolling resistance are in the same order as fitting a set of aerodynamic wheels but at a fraction of the cost. The data discussed above argues that a set of tyres that have been run-in are a better choice for an important race or event; add in latex tubes and optimised tyre pressures for the best result. For those that have less expectations for their performance, switching from 23mm to 25mm tyres will save you some energy during your commute or social rides.
Acknowledgments: Thanks to BBB Parts Australia for providing BBB Superlite tubes for testing.