Reliability of the virtual elevation method to evaluate rolling resistance of different mountain bike cross-country tyres

Abstract

Although a low rolling resistance is advantageous in mountain bike cross-country racing, no studies have used the virtual elevation method to compare tyres from different manufacturers as used in international competitions so far. The aims of this study were to assess the reliability of this method, to compare the off-road rolling resistance between tyres and to calculate the influence on off-road speed. Nine 29-in. mountain bike cross-country tyres were tested on a course representing typical ground surface conditions 5 or 6 times. The coefficient of rolling resistance was estimated with the virtual elevation method by 3 investigators and corresponding off-road speeds were calculated. The virtual elevation method was highly reliable (typical error = 0.0006, 2.8%; limits of agreement <0.0005, r ≥ 0.98). The mean coefficient of rolling resistance was 0.0219 and differed from 0.0205 to 0.0237 (P < 0.001) between tyres. The calculated differences in off-road speed amounted to 2.9-3.2% (0% slope) and 2.3-2.4% (10% slope) between the slowest and the fastest tyre. The reliability of the method and the differences in rolling resistance between the tyres illustrate the value of testing tyres for important competitions on a representative ground surface using the virtual elevation method.

Influence of wheel rim width on rolling resistance and off-road speed in cross-country mountain biking

“…Chung (2012) originally proposed the virtual elevation method to determine C r and C d A. Maier et al (2018) later evaluated the method for off-road use and showed it to be reliable (with a typical error of 2.8%) for estimating C r in off-road conditions. Briefly, elevation profiles are calculated from recorded speed and power output data.…”

“…The correct values can be determined by making iterative adjustments to the unknown C r and C d A while optically verifying the resulting elevation profile. As Maier et al (2018) have suggested, we estimated both the C r and the C d A when analysing the C d A tests and used a fixed C d A for each rider for the subsequent C r estimations. Three investigators blinded to the wheel set analysed all tests using a cycling performance software (Golden Cheetah 3.1, www.goldenchee tah.org); their estimates were then averaged for further analysis.…”

“…We compared 3 wheel sets, each equipped with the same tyre model: the first with an inner rim width of 25 mm (25 baseline ), the second with a width of 30 mm and the same tyre stiffness k as the baseline set (30 samek ), and the third with a width of 30 mm and the same tyre pressure p as the baseline set (30 samep ). Baseline tyre pressure for 25 baseline and 30 samep was 0.275 psi • kg −1 of total system mass (Maier et al, 2018). The vertical tyre stiffness of 30 samek against small-obstacle impacts (e.g., roots and rocks) was experimentally matched to 25 baseline .…”

“…Rolling resistance, aerodynamic drag, gravity, and inertia impede the motion of a cyclist (Olds, Norton, & Craig, 1993). Rolling resistance is particularly crucial in mountain bike crosscountry competitions because of the rough terrain and comparatively low speeds (Bertucci & Rogier, 2012;Bertucci, Rogier, & Reiser, 2013;Maier, Müller, Schmid, Steiner, & Wehrlin, 2018).…”

“…Various mechanical properties of a mountain bike wheel influence the wheel's rolling resistance. Maier et al (2018) have reported differences in C r of up to 15% between tyres currently used in international competitions, while Macdermid, Fink, and Stannard (2015) have identified the importance of low tyre mass, low tread surface area, low tread depth, and high tyre volume for low C r . Bertucci et al (2013) have also documented the disadvantage of rough tyre tread and the same group has reported how C r depends on the interaction between tyre pressure and ground surface (Bertucci & Rogier, 2012).…”