The 25mm Thing. (originally published December, 2013...reworked slightly now ;)
You’re probably aware that the latest ‘big
thing’ in cycling that isn’t to do with disc brakes is the shift towards
running larger section tyres. This flies in the face of pretty much all previously
accepted racing wisdom and tradition and may be hard for some to accept, but
it’s based on hard science. For a given tyre pressure, a 25mm tyre will roll
better than an identical one in 23mm section.
So. According to various sources, the long
and short of it is that 25mm tyres run at the same pressure generate less rolling
resistance than 23s. Simple! Well...in that sense it is. However, we’re not
talking like 20w savings or anything like that, really just percentage points.
There are a few important points that should be considered:
·
There is variation in rolling
resistance within models and even batches of tyres due to the manufacturing
processes involved- so- it is possible that a ‘good rolling’
23mm could generate less rolling resistance than a ‘poor rolling’ 25 of the
same model.
·
A good quality 23mm tyre would
almost certainly roll better than a lower quality 25mm tyre.
·
A 25mm tyre designed for
touring, or designed to be puncture resistant first and foremost (think Gatorskin
and the like) will not roll as well as an out and out race 23mm tyre (think
Conti GP Supersonic etc) by design. Think back to my posts on TPI etc, there
are a lot of contributory factors to rolling resistance.
·
Tyre pressure is critical. You
will note that I always say ‘at a given pressure’, because a 23mm tyre pumped
up to 100% of recommended pressure,
eg 120 PSI, will roll better than a 25mm tyre only pumped to say 80 PSI (or
about 67%) of recommended pressure.
·
A 19mm track tubular tyre
pumped up to recommended pressure of ~200 PSI will roll better than a 25mm road
clincher pumped up to 120 PSI. Not the most realistic of comparisons, but worth
mentioning.
It is quite clearly not correct to just say ‘25mm tyres are faster, end of’ as a blanket
statement. There is however more behind the shift towards wider section tyres.
If a 25mm tyre rolls better at a given pressure, then logically it is possible
to lower the tyre pressure and, overall, experience no respective loss in
rolling resistance. Lower tyre pressures equal more comfort (which is always
nice), and also mean more grip (due to the carcass being able to deform more to
the surface of the road-more tread in contact with the ground=more grip). Lower
pressures also, considering Boyle’s Law (I think), could mean safer riding for
those deciding to risk using carbon clincher rims on big climbs. Allow me to
elaborate/go off on a potentially incorrect tangent:
·
Every beaded (i.e: clincher)
tyre has a pressure at which the outward pressure will exceed the force of the
bead keeping it on the rim-and will jump off the rim/blowout. Carbon clincher
rims suffer from poor heat dissipation under breaking and you have no doubt
read the many horror stories of people descending big cols and blowing out/off
whatever they were running on their carbon clinchers in the process. Blaming incidents
on these on equipment is unfair. It’s not the equipment’s fault-it’s the choice
of equipment! Tyres for example are engineered with as much headroom between the
recommended pressure to absolute max ‘this-will-now-blowoff-the-rim’ pressure
as feasible bearing in mind the other characteristics required for its
application. Going back to Boyle’s Law: ‘for a fixed amount of an ideal gas
kept at a fixed temperature, pressure and volume are inversely proportional.’
So, if temperature increases for a given volume, so must pressure; hence the
known issues regarding long heavy breaking on carbon clinchers. Or at least
that’s how I interpret it/remember GCSE physics (please correct me if I’m
wrong). So if you insist on taking your carbon clinchers to the Alps/Pyrenees
etc then can I suggest also packing some 25mm tyres? You’ll be able to run them
at a lower pressure (without going any slower as explained previously), but
more crucially will also therefore give yourself perhaps 5-10PSI extra safety
headroom. Which could mean the difference between you getting to the bottom of
a long descent upright, or not.
Whether or not 25s would have increased in
popularity without the concurrent advances in wheel technology pushing wider
more aero rims or not is another point worth considering. Wider rims are more
aero (or at least, react better in a wider range of wind yaw angles. A.K.A
‘real world not wind tunnel’ riding and so require less energy input from the
rider to continue riding in a straight line) and wider tyres fit this wider
profile better, ensuring a smoother transition from tyre to rim for the air-so
it’s win/win. Would a 25mm tyre be more aero on a ‘standard’ narrower profile
rim? I can’t answer that, but what seems obvious now is that it would roll faster
than a 23mm version of the same model at the same pressure.
That’s the key to the whole ‘25mm thing’,
they are indeed faster-but only in
certain situations. If your bike has the clearances to take 25s (sadly my race
bike doesn’t...or at least not with any safety margin), then next time you need
to replace your tyres, try the same model in 25mm. Go as fast for more comfort
and grip, or go faster for roughly similar comfort/grip? I’d take that for
something as simple as swapping tyres.
Thanks for reading. I hope that helped you
better understand ‘the 25mm thing’, and if it did maybe consider sharing it
with others who might be interested? I think it’s fascinating personally, and
would like to think I’ve helped people better understand it!
