Steve Ball said:I got a 'snakebite' flat in the front tyre after hitting a bit of roadwork
this morning. The tyre had at least 105psi in it. The tyre's a Continental
GP Attack and I notice its labelled "700x22C". All my tubes say "700x23C". I
assume the millimetre difference is ignorable and nothing to do with my bad
luck this morning. Or not?
This got me poking around Sheldon Brown's website, where I read that many
experienced cyclists run different pressures and even different tyres front
and rear, with the aim of maximising traction at the front and minimizing
rolling resistance at the rear. There's a table here:
http://www.sheldonbrown.com/tires.html#pressure
but it talks in terms of 'wheel load', without saying roughly what
proportion of a rider's weight is on the rear wheel (more) versus what's
taken by the front (less). Anybody have any idea?
But, given that the rear takes more weight, the front less, it's suggested
you inflate the rear harder, the front softer, which works in nicely with
less rolling resistance (due to less deformation) at the rear, more traction
(dues to greater contact area) at the front.
I've always read my tyre walls (well, _sometimes_ always read them) and put
100 psi in both but, given that I only weight 65Kg, they may well be
over-inflated, especially the front. All of which makes this morning's flat
more puzzling...
Comments from the gallery?
"hitting a bot of roadwork" can easily produce deformation forces large
enough to pinch a tube no matter what tyre pressure you use. Certainly a
700x22C or 23C is small enough that even a large rock could do the
job. Especially damaging are thing your run into rather than over. There
is a small road bridge on Wattle Street (Canberra) where the roadway
had subsided about 4cm exposing a right angle steel edge. I always bunny
hopped over it because that kind of front-on contact is really punishing
on road bikes (less on MTBs because the suspension absorbs a fair amount
of the impact).
If you want the last word on rolling resistance, talk to the MTB cross
country riders who have been tinkering with tyre pressure and profile to
a far greater extent than the blacktop folks, and yes, they do run lower
pressure on the front under many circumstances.
The most extreme version of 'wheel load' imbalance is loaded cycle
touring, where rear panniers and racktop luggage make up a lot of the
weight. On my touring bike I have always run a wider tyre on the back
than on the front (eg 32mm on front and 37mm on back for light touring,
or 42/47 for heavier loads). The point is that a narrow tyre will
require greater pressure as the load increases, and it is time to stop
when you get to the pressure on the placard or thereabouts. An easy way
to think about this is that the at-rest surface area in contact with the
road is equal to the load on the wheel divided by the tyre pressure. As
you add load you have to increase pressure, but if a tyre was 25% wider
then it could handle 25% more load at the same pressure. (This
simplistic arithmetic leaves out several factors but is good enough for
this kind of calculation).
I'm not sure I believe the viewpoint that reducing pressure results in
better traction for road tyres. Each tyre is designed with an optimum
profile. Tyres that corner well present a good profile to the road under
sideways stresses. This profile will be negatively impacted by over or
under inflation. We can all detect this in the extreme case (like when a
front tyre has a slow leak) as the underinflated tyre does not handle
predicatably.
So my opinion (and I'm sure there will be others) is to run tyres at the
optimum pressure. Maybe run a wider tyre at the back. In fact you'll
discover that if you do that you may even run it at a lower pressure
than the front one, certainly not a higher pressure. Have a look at the
Continental Attack/Force combination as an example of this kind of thinking.
