Sunday, 23 June 2013

When is 700x23C not 700x23C?

Playing around with different tyres, wheels and a digital Vernier caliper throws up some interesting results and it's quite surprising just how UNfixed tyre width actually is, even though a tyre is stated to have a width of 23mm. Here is an example, results like these can have a considerable effect on exactly what will fit with what, purchase with care!

I have a Mavic Aksium Race wheel which has 20mm rims. The wheel was fitted with a Michelin ProRace3 Grip 700x23C tyre which was inflated to 100psi. Using my caliper I measured the width of the tyre at 4 evenly distributed points, in theory the tyre should be 23mm in width, being a 700x23C tyre.

The 4 measurements were 24.21mm, 23.13mm, 23.32mm and 23.62 mm. Averaging these values I came to a figure for tyre width of 23.57mm, about 2.5% out, not bad!

So far so good. I then tried a Continental GP4000S tyre of the same size on the same rim, inflated to the same 100psi and took the same measurements. The figures for this tyre were 23.41mm, 23.49mm, 23.24mm and 23.12mm. This produced an average tyre width of 23.315mm, about 1.36% out, even better!

This was where the fun started, I removed this Continental GP4000S tyre from the Mavic rim and installed the tyre onto another wheel inflating to the same 100psi.  This wheel had a rim width of 23mm so I'm fitting a 23mm tyre onto a 23mm width rim.

Out with the Vernier caliper again and the same 4 measurements were taken. This time the results surprised me, they were 25.25mm, 25.15mm, 25.23mm and 25.11mm. Averaging these values the new tyre width has become 25.18mm, which is now 9.05% wider than the stated 23mm tyre width, this IS significant.

So, what is the point of stating tyre widths without also stating factors such as rim widths which can make such a difference to the numbers and therefore the fit? Am I riding a 23mm or a 25mm tyre when using this wheel? :-)


  1. When is 26x1¼ not 26x1¼? When it's 26x1.25!

    I kid you not. For many years my father was exasperated that his (very obsolete sized) 26x1¼ tyres were virtually impossible to source, yet 26x1.25 tyres were widely available but did not fit his wheels.

    Apparently there is a technical explanation for this though I can't remember what it is (something to do with bead diameters). Still, apparently ¼ is not always the same as 0.25!