XC riders and roadies think so but now science has a definitive answer.

XC riders and roadies think so but now science has a definitive answer. Do wider stance rims lower your rolling resistance?

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By Max Glaskin

Choosing the bits for a new bike is exciting because the variety of options for almost every component means you can really customise your perfect trailblazer – money permitting, of course. Yet now, scientists say that one of the hottest trends – while it may improve your cornering – won’t actually make your bike roll any faster.

For decades the internal width of XC rims was around 20mm. Then fat bikes and Plus-size tyres needed rims over 25mm wide and XC competition bikes soon followed. Now some of them have grown to 40mm across. The theory is that it reduces one of the forces that slow you down – rolling resistance.

Every bike in the world suffers rolling resistance. It’s due to the continual deformation of the tyre as it rolls onto the ground. It absorbs your energy – energy which would otherwise be used for speed. So, for example, the lower rolling resistance of a 29er means it moves at anything up to 3% faster than a bike with 26in wheels if the same rider puts in the same grunt.

A similar idea has grown up for wider rims. The theory is that the cross-section of the tyre on a wider rim is bigger and deforms less. So riders who want to slash seconds from their XC times have been choosing the newest, wider, 30mm specification to reduce rolling resistance. Trouble is, nobody had tested the theory.

Four scientists from Switzerland’s elite sport research institute smelt something fishy. They followed their noses and grabbed a Scott Scale 900 SL 29er with DT Swiss XM 1501 Spline One wheels and tubeless Racing Ralph 2.25in tyres. Three lab rats of similar build and age volunteered to ride the bike on exactly the same XC course at exactly the same speed. Again and again. For nine days. Dull, eh?

Citation: po.st/RRCitation

Secretly the scientists swapped out the wheels between runs so they never knew when they were on 25mm or 30mm-wide rims. Power and speed data were collected for the dozens of runs, then cleaned up, washed down and analysed.

The upshot? “The effect on flat and uphill off-road speed was found to be trivial,” came the damning conclusion. In other words, the influence on flat and uphill off-road speed of the 25 and 30mm rim widths seems negligible, so don’t bother basing your choice on the idea that wider rims will give you faster times. Instead, say the scientists, “choose the rim width that offers the best bike handling”.