ADDING FAT TYRES TO YOUR ROAD BIKE

More com­fort and more speed is just a tyre switch away, but there are tricks and traps.

Australian Geographic Outdoor - - Outdoor Bike Lane -

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).

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