Tyre-road rolling resistance is a major factor in the performance of a vehicle. By investigating the rolling resistance, a better understanding of the efficiency of different wheel diameters will develop. A major issue in the mountain biking world is the relative merits of using 26in. versus 29in. wheels and the resultant effect on cyclist performance. As rolling resistance is indicative of the behaviour of a vehicle over specific terrain, it can be viewed as an objective parameter to compare the relative performance of these two wheel sizes. The aim of this study was to evaluate the rolling resistance of four mountain bikes as affected by wheel diameter and terrain type, cyclist mass, tyre inflation pressure and suspension type using coast-down tests. The following major conclusions were drawn: average rolling resistance of the 26in. diameter wheel was higher than that of the 29in. diameter wheel; a sand surfacing had the highest rolling resistance coefficient; terrain surface showed the largest effect on rolling resistance coefficients measured, followed by the cyclist mass, wheel diameter and tyre inflation pressure; and the best combination for maintaining momentum after traversing over an obstacle was high tyre inflation pressure, low cyclist mass and full suspension 29in. wheel diameter option.