Modern Athlete Magazine Issue 96, July 2017 | Page 45

BIGGER
Advice
Ma
is Better… Or is it?
What’s with all of the wide rims these days, and why have things changed to a ‘bigger-is-better’ approach? To answer these
questions, we need to go back to the beginning. – BY THE GENTLEMAN RACER
T
his current phenomenon isn’t only confined to mountain bike rims, as
road bikes have been growing of late too. In an earlier time, road bike
tyres were 20C – approximately 2cm in width – and later the industry
23C standard, while MTB’s were 2.0-inches. That was just the way
things were, the way of the world, so to say. However, with mountain-
biking branching out into ever more ‘interesting’ genres and road cycling becoming
redefined – ever heard of all-road? – tyres have grown more voluminous.
But why did tyre volumes increase in the first place? After all, it was a widely-held
belief that a wider tyre will have more rolling resistance due to the fact that more
of it is in contact with the trail or tarmac. Also, a bigger tyre will be heavier as well
as slower, so the status quo of thinner tyres remained for a long while. However, a
small technical revolution was happening in the background, with more advanced
tyre carcass designs meant that bigger tyres didn’t automatically mean a massive
increase in weight. The pivotal point was that if you make a wider tyre taller at the
same time, its rolling resistance becomes comparable to a smaller-volume tyre.
Interesting Results
Exhaustive testing by tyre manufacturers started to show that a 2.25 MTB tyre, in
its sweet-spot in terms of pressure, could actually roll faster than a higher-pressure
2.0 tyre. With this larger volume-casing and a lower pressure, there were many
handling advantages, too, as the tyre would conform to the trail surface rather than
‘ping’ off every little obstacle in its path.
Riders soon began to embrace the idea, and nowadays most of the bikes at the
start of your local races will be kitted with 2.25 width XC tyres. Also, the pro’s
tackling the Spring Classics in Europe have been riding larger-volume tubbies to
address the same problems, giving more grip, comfort and control. This really
gained momentum with the invention of the Gran Fondo (Italian for big ride). Riders
wanted more comfortable, predictable-handling bikes for these long pedals over
varying surfaces, such as
rough tarmac, dirt and
even shale.
grown, but the rims they’re attached to have not. So why is this a problem? Surely
a tyre’s bead just clips into whatever width rim it’s given? The answer is yes, and
thus a 2.7-inch tyre will fit a 19C-width rim, but that’s not the problem. The problem
arises when you ride a corner on that big tyre with its skinny rim.
Believe it or not, the best way to understand why this is a problem is to think of the
difference between a mushroom and a cupcake. A mushroom has a big head (like
the tyre) on a thin, flexible stalk (like the rim). What happens when you grab the
head of the mushroom and yank it side-to-side? That skinny stalk doesn’t do much
to resist that movement, does it? That is exactly what happens with a large-volume/
narrow-rim arrangement. A large, grippy tyre cannot generate much cornering
power when its base is moving underneath it. Worse still, this movement increases
the chance of ‘burping,’ when cornering forces rip a tyre’s bead away from the rim,
causing instant tyre pressure loss.
A cupcake is always the better option. The cupcake-shaped tyre-rim cross-section
has a much more stable base for the tyre to work off. That stable, wide base
means the tyre’s performance is dramatically improved. The wider rim provides a
further increase in volume, too, so one can lower the pressure even further, safely
increasing grip without the ‘wallowing’ associated with a ‘mushroom’.
Bigger is better…
So like most things in the modern world, going bigger is best, and so far, every
single sceptic I’ve encountered has converted after the first ride! But there is an
actual practical limit to what size tyre one can run on a rim. Remember this, and if
in doubt, ask your local bike shop to advise you.
PS: Wait till I tell you about 41mm-wide trail rims and 2.5-inch tyres being the new
MTB tyre norm!
As a result, road tyres have
grown right alongside their
MTB cousins, with many
local shops not having sold
a 23C-width tyre in ages.
Instead, a 25C tyre has
become the new norm,
and many of the newer
road frames are now even
being fitted with 28C tyres.
Growing Pains
So all is well in the tyre
world, because bigger
is better… but there is a
rather interesting problem
that all this growth
unearthed: Tyres have
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