How wide will road tyres get?

20/04/25 18:42 Filed in: Gear

Remember the adage: all things being equal, the wider tyre has lower rolling resistance. Also remember the corollary: Things are never equal.

What are you supposed to make of those two statements?

Every time you move up one size of tyre, from a 20 mm to a 23 mm to a 25 mm to a 28 mm to a 30 mm, the amount that tyre has to distort to make contact with the road decreases. If the thickness of the rubber and the carcass of the tyre are identical across all the sizes, then each step sees a reduction in energy losses. Within one model of tyre it would extremely unusual for all those sizes to be offered. And if several were on offer, the larger ones tend to have a thicker layer of rubber on top, or (sometimes and) a heavier carcass. Both of which increase rolling resistance.
And then there is the matter of aerodynamic resistance. If all of those tyres are put on the same wheel, then only one can be optimally aero, and all the others are less slippery.
No matter what, increased size brings increased weight. The wider rim necessary to provide good support and good aero efficiency to the wider tyre also adds weight and complexity.

Rolling resistance is always important on a bicycle with so little power to motivate the bike, but aero resistance becomes key from around 20 kmh upwards. By 40 kmh the aero factor is so great you can almost forget about rolling resistance.

Wider tyres operate at lower pressures with lower rolling resistance when used on wider rims, but add mass and aero drag (especially when we exceed the optimal tyre size for a given rim). Increasing size reduces one aspect but increases the other aspect. Our optimum then depends on the bike's velocity.

All of that is a way of saying that they cannot become too wide or weight/aero will suffer more than rolling resistance can gain for a net loss in speed. I think that limit has to be somewhere around 30mm tyres.

There is also the question of what feels "good" too, but I haven't ridden enough good, fat tyres to know what to expect from a 30 mm racing tyre. My 28 mm tubeless training tyres surprise me by feeling quite sharp (neither heavy nor slow). But I know the measured rolling resistance puts them well behind a good racy 25 mm tyre.

Perhaps that 30ish mm limit is a temporary one? As technology improves the wider tyres could surpass the narrower ones. Regardless, the practical limit can be observed by looking at motorbikes. The biggest and most powerful litre sport bikes use about a 120 mm front tyre. Those have 150x the power of a bicycle on tap. We never need go there. Even a low powered sport bike has nearly the same front tyre - a 100 or 110 mm section. These tyres are dictated by mass (of the bike & rider combo) more than anything else.

Footnote: looks like the UCI will step in and rule a max size for road bikes. Like the minimum mass they specified around 30 years ago, if they do it will hang around a long time.

Michael Hanslip Coaching

If you want to go faster, you have to pedal harder

How wide will road tyres get?