- Rougher trails require bigger tyres for secure traction and
- And larger volume tyres don’t play nice with narrow rims.
- The trend is towards wider rims, where understanding ID, comes into play.
Suspension travel in millimetres. Bike weight in kilograms. Cost, in Rands. These are the three numbers that riders debate most furiously when new mountain bikes are discussed.
But some numbers are perhaps even more important, and generally misunderstood. One of those is ID. And no, we aren’t referring to your unique 13-digit personal identification number.
The ID number of value to mountain bikers, is something called internal diameter (ID), which can be confusing. A simpler way of understanding it, is ID being the internal width of your rim, in millimetres.
Why is this ID number important? Because it has a huge influence on tyre shape and performance.
Any experienced rider will advise a newbie mountain biker that the most meaningful single component upgrade, guaranteeing improved trail performance, are tyres. The rubber compound, tread block pattern and inflated shape directly impact your mountain bike’s traction, braking and cornering stability.
But what if you have great tyres, how does rim width possibly make them better or worse? To understand the issue, it is worth remembering that mountain bike wheel technology shadowed road bike development for many years. The cycling obsession with lightweighting meant narrow rims were preferred, without accounting for the potential benefits of using a bit more material to make a wider rim, which is only negligibly heavier.
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Why wider, is better
For a long time, mountain bikers rode comparatively narrow rims, as riders were unconvinced that larger tyres could be greatly beneficial. Over the last decade, that perception has changed. Dramatically.
As mountain bike suspension components and frame design have improved, riders are challenging themselves by riding increasingly more technical trails. That has created a demand for better and wider tyres, too.
Whereas mountain bike rims were once between 19-23mm in width, today, it is rare to find a rim narrower than 25mm. Exhaustive testing by most mountain bike wheel and tyre brands have found agreement that 29-30mm is the ideal width for modern large volume mountain bike tyres.
On the wider rim, larger tyres inflate to their ideal shape, as intended by engineers. On a narrow rim, your risk a lightbulb-type tyre shape, which rolls and squirms under severe side loads, when cornering at high speeds or pinging through a rocky section of trail.
The same logic that makes bigger tyres better for trucks, tractors and your double-cab bakkie when driving off-road, apply to mountain bikes. If you want the have improved braking and cornering traction, bigger tyres are the way. And a bigger tyre, requires a solid foundation, hence the need for those rims with an ID number of 29-30mm.