Forget 28mm, much wider tyres are coming

Following extensive lab testing, Cyclingnews has this week concluded that the future of road bike tyres could very well be far wider than those we're seeing in use today. 

It's well known that over recent years, the industry's preferred tyre size has been growing. Where 23c was once deemed the standard, Tadej Pogačar raced 30c tyres at the 2024 Tour de France and the 2024 edition of Paris Roubaix featured 35c tyres, so wide that they'd be illegal in a cyclo-cross race. 

When Pirelli launched a 40mm P Zero TLR race tyre last year, the Cyclingnews tech team posed the question: Should everyone just cut to the chase and switch to 40mm tyres now?

Our findings, which combined weight, rolling resistance data from a pedalling efficiency rig, and aerodynamic data from a wind tunnel, concluded that while time triallists and certain road racers might not benefit from making the switch, 40mm road tyres would be faster for 'most people', especially given the lower speeds amateurs hold and the proliferation of badly-surfaced roads. 

The tests also uncovered a potentially enormous saving for racers at cobbled Classics such as Paris-Roubaix. 

The testing, which was done by the same team that took 11 superbikes to the wind tunnel, covered six different Pirelli P Zero TLR tyres ranging from 26mm to 40mm, the Hunt CGR40 gravel wheels and the crazy-wide Zipp 303 XPLR SW, and spanned two days of testing at the well-renowned Silverstone Sports Engineering Hub. 

The team sought to understand whether the reduction in rolling resistance would be offset by the increase in aerodynamic drag. They tested the theory across two surface types, at two speeds, and seven yaw angles (the direction at which the wind hits the bike). 

The results, which are available to Cyclingnews subscribers, showed that wider tyres could yield a saving of more than 80 watts in rolling resistance drag alone.  

Of course, most aero race bikes can't fit a 40mm tyre – at this stage, barely any endurance road bikes will fit them either – but a specific example tied to Israel-Premier Tech's use of gravel bikes at the 2024 Paris Roubaix, but with 32mm tyres, showed a missed opportunity to save over 40 watts. 

But whether or not everyone should 'cut to the chase' as posed earlier on, things are a little less clear-cut. 

Since tyre size, wheel width and frame clearance all work in sync, it's perhaps unsurprising that things take time to change. A 35c tyre isn't very aerodynamic on a wheel designed for 28c tyres, and you won't get very far running a 40c tyre in a frame designed for 30mm. 

The data, the inferences and the conclusions reached, however, do suggest that road tyres are going to continue to grow, and that 40mm might merely be a stepping stone toward the eventual limit.