This post explains what rolling resistance is, what affects it and how it can be minimised.
1. What is rolling resistance?
2. What affects the rolling resistance?
….2.1. Tyre’s mass (“weight”)
….2.2. Tread pattern
….2.3. Sidewall and rubber compound quality
….2.4. Aerodynamic drag
….2.5. Air pressure in the tyre
….2.6. Tyre (wheel) diameter
….2.7. Tyre width
….2.8. Road conditions
Rolling resistance, often referred to as “tyre rolling speed” is the amount of mechanical drag that tyre produces while rolling (with the weight of the rider and the bicycle). Every tyre creates some drag, it is just a matter of how much.
Rolling resistance is affected by the following factors:
Has effect when accelerating, or climbing: the heavier a tyre, the more drag it will produce. This is not a critical factor in practise, for two reasons. First, rider’s weight is a lot greater than the weight difference between a lightest and a heaviest tyre, so tyre’s “weight penalty” is often negligible. Second, apart from starting from stationary, accelerations achieved on a bicycle are relatively small. “Cycling Internet” is full of texts glorifying the famous “rotating mass” (of the wheels), but it can be calculated how little effect extra 100-200 grams of tyre weight have on acceleration of even a light rider (as nicely calculated in this article: Wheel performance – rotating mass and aerodynamic drag).
Of course, the wider a tyre (tread pattern and material being the same), the more mass it has.
Knobby tyres will produce more drag than slick tyres. The greater grooves (or knobs) a tyre has, the more rolling resistance it produces (all else being equal). The difference is easily felt and measured. Slick tyre grip on (wet) pavement is explained in the post about Slick tyres. Of course, when riding in mud, sand, or snow, knobs and deep tread pattern will provide better traction and they are beneficial in such conditions.
Tyres with easily flexible sidewalls produce less rolling resistance. Greater percentage of energy used to bend the tyre at the contact patch with the ground is returned once that part of the tyre is rolled off the ground. Tyres with fine sidewalls are usually more sensitive to sidewall damage (sharp rocks etc.), but for riding on paved roads this is usually not a problem.
Sidewall quality can often be deduced from the number of (casing) threads per inch (“TPI”) that manufacturers usually note. More layers of thinner casing threads usually results in lower rolling resistance. This is where bicycle tyre manufacturers often resort to “cheating” – making a tyre out of two, or three layers of thicker threads, and noting an aggregate of both (or all three) layers’ threads as “TPI”. Good quality tyres often have three or more layer casing, with over 100 threads per inch for each layer (so “over 300 TPI”). This effect makes a relatively large and measurable difference.
Tyre compound is mostly a manufacturers marketing departments’ playground. Every now and then a “new, revolutionary compound” is made – with “better grip, lower rolling resistance and longer wear”. Call me a retro-grouch. 🙂 Based on TPI and tyre’s intended use (racing, winter, touring etc.), compound is usually well matched and there aren’t vast differences.
The simple part: the wider a tyre and the more pronounced tread pattern (or knobs), the more air drag it creates. However, since rider attributes to about 90% of the bike’s total air drag, tyres’ effect is not that significant. Yes, aero drag increases exponentially with speed increase, but the rider still remains the most significant factor. In addition to this, especially on wider rims, a wider tyre (within some width limits) will make a more aerodynamic shape than a narrower one, hence reducing the air drag.
The further a tyre’s pressure is from optimal (whether too high, or too low), the more rolling resistance a tyre will produce. Pressure being too low slows down more, while pressure being too high decreases comfort and grip more than the rolling speed. For explanation of optimal pressure, read: To what pressure should I inflate my bicycle tyres?.
The greater a wheel’s diameter, the faster it will roll over bumps, potholes and road irregularities. A wider tyre will also be “taller”, increasing a wheels (outer) diameter. Detailed explanation of dimensions: Bicycle tyre sizing and dimension standards.
If pressure, tyre compound, sidewall quality and tread pattern are the same, a wider tyre will produce less rolling resistance. This will be explained using picture 1.
As it is shown in picture 1, total height of the narrower tyre (A1) is compressed by about 1/3 at the point where the tyre contacts the ground, while the height of the wider tyre (B1) is compressed by a fraction of its total. A wider tyre, at the same pressure, can bear more weight than a narrower one.
The other important factor is the length of the contact patch (A2 and B2). A wider tyre deflects by a smaller margin of its longitudinal cross section, so a smaller section of sidewalls is deformed, which results in less resistance. Contact area is the same size, but a narrower tyre gets deformed more, becoming less round, which increases rolling resistance.
In practise, narrower tyres are usually inflated to a higher pressure than wider ones (for the same rider and bicycle weight), so they are not slower in real life, but a common misconception is that (good quality) wider tyres are (significantly) slower.
The rougher a road, the slower a narrow tyre is (in addition to being less comfortable). Due to higher pressure necessary to prevent pinch flats, narrow tyres tend to bounce more.
Same goes for off road riding – here, tyre width is important, as well as having knobs if the surface is soft (sand, snow, mud).
When choosing a bicycle tyre width, the main criteria should be bicycles intended use. Rolling resistance is mostly affected by tyre’s type and quality. After all the important factors have been explained, a simple(r) rule of thumb list will be given:
- More comfort is desired – wider tyres (with good quality sidewalls). There is a point of diminishing returns here – after which adding more width gives very little gain in comfort, and mostly adds weight. For good quality pavement 28 mm tyres are all one needs. Poor pavement: 37 is the point after which there’s very little gain in comfort. For off road 40, or even 50+ mm makes sense. Small 26″ wheels benefit from tyres of 40+ mm width, even on pavement, since a narrow tyre makes the wheel’s effective diameter smaller, and since 26″ wheels are rather small to begin with, they roll better with tyres on the wide side (slick profile for pavement, of course).
- Off road riding (mud, snow, or sand) – wider tyres with a knobby tread pattern.
- Riding on poor quality paved roads – wider tyres with a slick profile are often both more comfortable and faster.
- Paved road riding – tyres with a slick profile, good quality sidewalls and of width to match the desired level of comfort. Anything narrower than 25 mm brings significantly less comfort, with hardly noticeable (and hardly measurable) speed benefits.
From author’s opinion and experience, tyre’s quality and tread patter have a lot more effect on the “rolling speed” than tyre width. Depending on riding conditions, one can choose an optimal width, thread pattern and tyre quality.
Related post – Slick bicycle tyres: