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Computers and Tyres

Harold Bridge


What to do when about to ride an important event and the tyres look a bit worn? In the past it has led me into buying new tyres and keeping the others as "spares".

Over the past 10-12 years I have collected a large number of half used Michelin Hilite Comps and Supercomps, both 23 and 20. The smaller ones were for my forays into competition.

Now retired and attempting to husband my resourses I have been using these old tyres up. The "fast" bike has been shod with a 20 mm tyre on the front and a 23 mm on the back. (Note: Michelin 20s are about the same size as a Continental 23).

I use 2 computers. The Cateye Micro gives true event time, provided I switch on at the start and off at the finish. It is calibrated to the nearest centimetre but reads in 10 metre increments. It runs off the front wheel and is calibrated to 209 cm. The Vetta HR1000 calibrates in millimetres but only reads in 100 metre increments. It runs off the rear wheel so that the Heart Rate monitor and computer can be used on the turbo trainer. It is calibrated to 2095 mm.

I have a circuit I ride quite frequently and every time I get off my bike at the gate both computers show 36.9 km.

Yesterday, in preparation for a mini tour I changed the tyres. I bought some Continental 3000 700Cx25 and installed them last night. An adjustment of the front mudguard was required otherwise everything was ticketyboo. I went out and did my circuit this morning and hoped to assess the correction factor required for the computer reading with the bigger tyres. The Cateye read 36.4, the Vetta 36.7. Obviously, the front wheel had the bigger change in diameter. But before the change they both read the same. According to those figures the Cateye should be re-calibrated to 212 cm and the Vetta to 2106 mm. But the 2 tyres are the same size?