ADDING FAT TYRES TO YOUR ROAD BIKE
More comfort and more speed is just a tyre switch away, but there are tricks and traps.
WHEN IT COMES to road bike tyres, less is more – according to bike makers and Tour de France racers, at least. By minimising the width of the tyre, the contact patch – the part of the tyre that actually touches the tarmac – can be reduced, which in turn reduces rolling resistance and improves speed.
Now I don’t know about you, but I have no intentions of entering any kind of race on my roadie – but I am still keen to keep speeds up. I do, however, want things to be a little more comfortable under my hands and butt, especially over longer distances. Well, it turns out you can have your cake and ride it too – to a point.
First, a quick refresher in road bike tyre sizes. By far the most common wheel size on road and hybrid bikes is 700c, which refers to the diameter of the rim. Tyres are measured in millimetre widths, starting as low as 19mm across the tread face, all the way up to 45 and beyond.
When it comes to averages, most road bikes sold today will come stock with a 700c x 23mm tyre, while adventure or gravel bikes will come with something wider, like a 30mm. No matter what your bike comes stock with, there’s nothing preventing you (in most cases) from fitting a larger diameter tyre when you’ve worn that stock set out.
Science shows that the contact patch of a 25mm tyre is shorter and wider than that of a 21mm or 23mm tyre – in fact, engineers at wheel maker Mavic have determined that the rolling resistance of a 23mm tyre at 100psi of pressure is the same as that of a 25mm wide tyre at 80psi. That is a huge difference in pressure and the lower number will feel way more comfortable.
So a wider tyre can be run at lower pressures, which improves rider comfort and reduces fatigue. A wider tyre is less prone to punctures, too.
So how wide can you go? There are a couple of limitations to be aware of. The first one is frame and fork clearance; the tyre you want to use needs to physically fit on your bike. The majority of modern carbon bikes have a little extra room than alloy versions, as a rule of thumb, but there’s only one way to find out – and that’s by fitting it.
Another issue is brake calliper clearance. Your tyre needs to be able to pass through the brake shoes that are fitted to your bike without being deflated – of course, if you have disc brakes, you don’t have that issue.
Finally, rim width also plays a part. The trend over the last few years is for wider rims, thankfully, which allows for more flexibility when it comes to bumping up your tyre size.
Using our new Canyon Ultimate as a test mule, we were able to swap the stock 25mm Mavic tyres – which actually rode more like 23mm tyres – for Vittoria Rubino Pro tyres that measured 28mm, with room to spare. The late model Shimano Ultegra brakes also allowed the larger tyre to slip through the pads – but only just.
Our advice? Start by going one size up from your stock set up. If you have 23mm tyres, go 25s, while 28mm tyres will for the most part fit rims and frames designed for 25mm tyres. When it comes to larger volume tyres on disc brake-equipped machines, it may be possible to skip two sizes.
A note on fitting; tubes will stretch to fill the available space, so there is no need to buy new ones to go up a size or two. Your bike shop will also be able to offer advice on fitment sizing, and may even offer a ‘try before you buy’ solution of test fitting a wheel with your tyre choice already installed. And don’t forget about tyre pressures! A good track pump with a gauge is an invaluable addition to your garage.
The bike closest to camera has 700c x 25mm tyres fitted, while the other has 28mm. The two critical places to check for clearance with bigger tyres is the front forks (top right) and the rear chainstays (bottom right).