Mountain bikers employ lots of technical strategies to help them cover ground faster: lighter bikes, specialised tread patterns for maximum traction, suspension to negotiate uneven surfaces, and powerful brakes that work in muddy conditions.
However, the amount of resistance encountered by a mountain biker travelling at a given speed will be higher than that of a road biker thanks to increased air and rolling resistance.
The increased air resistance arises from the more upright riding posture while the increased rolling resistance is due to the heavier tyres and knobbly tread patterns, which sap more energy as they roll along and deform over the ground.
Given the lower average speeds attained on a mountain bike and much greater power losses from rolling resistance, many mountain bikers assume that trying to minimise aerodynamic resistance is pretty irrelevant because a) it’s low anyway due to lower speeds on a mountain bike and b) any gains that could be made will pale into insignificance when compared to rolling resistance losses.
A new study on mountain bikers by French researchers suggests otherwise.
In the study, the researchers set out to quantify the typical aerodynamic resistance of mountain bikers in the seated position by testing a group of them in the lab using techniques garnered from wind tunnel testing, and how much this contributed to overall power losses.
In particular, they wanted to find out what proportion of the total resistance losses was as a result of aerodynamic resistance and how much was due to rolling resistance. They also compared the rolling resistances of two types of mountain bike tyre (smooth and knobbly) across three different field surfaces (road, sand and grass) with two pressure inflations (29psi and 58psi).
This was done by using ‘coastdown’ (deceleration) measurements, where the rate of decline in speed of the bike and rider when freewheeling from a given speed can be converted into energy losses and thus rolling resistance.
In a nutshell
The first finding was that mountain bikers had an ‘effective frontal area’ of around 0.38m2. Effective frontal area is a measure of the amount of aerodynamic resistance an object encounters as it travels through air and is calculated from the drag coefficient (how ‘slippery’ its shape and surface is) and how much surface area is presented to the air in the direction of travel.
By comparison, some elite time trial riders have dipped under 0.20m2 for this measure. Of greater importance, however, was that as a result of this effective frontal area, the riders were expending eight-35 per cent of their energy overcoming air resistance.
Over very rough ground with knobbly tyres at 29psi, the rolling resistance losses were maximised, which meant that they accounted for 92 per cent of energy loss with aerodynamic losses accounting for just eight per cent. Using smoother, more highly inflated tyres on the road, however, resulted in much lower rolling resistance losses, higher speeds and (proportionately) much higher losses from aerodynamic resistance – accounting for over a third of total resistance.
What these findings show is that despite the lower average speeds, aerodynamics can still be important for mountain bikers, especially on less rugged terrain with smoother tyres. The UCI has already banned skin-suits for downhill riders so there is a limit to how much riders can change about their kit.
Mountain biking is highly technical and weight shifts, balancing the bike and riding position to maintain grip and stability will have more overall affect than tucking down to reduce frontal area. However, in sections where this is possible riders need to be aware of the benefits.
This article was first published in the July 4 issue of Cycling Weekly. Read Cycling Weekly magazine on the day of release where ever you are in the world International digital edition, UK digital edition. And if you like us, rate us!