Wheel size/calibrating cycle computer

plug1n

What is a good (i.e. any working) way to calibrate a cycle computer?

I just put on some Michelin Krylion 700x25s for the winter and managed to do a whole ride puncture free.

But these tyres are big - much bigger than a Bontrager xlite that was also 700x25 and really prone to little cuts through the tread, averaging 1 flat/50 miles in the last 2 weeks for me.

When I measure the fitted Krylion side-to-side, I get 26.5mm on a micrometer.

I have no problem with the Krylions being so big but would like to allow for the possible ~1/2% difference in circumference.

Thanks

whyamihere

Measure the circumference of the tyre when fully inflated. Most computers I've used use the circumference as the calibration number, so if you measure it yourself it will be more accurate than the numbers given in the instructions.

Gussio

whyamihere wrote:

Measure the circumference of the tyre when fully inflated. Most computers I've used use the circumference as the calibration number, so if you measure it yourself it will be more accurate than the numbers given in the instructions.

A way of measuring the circumference of the tyre is mark the tyre with a piece of chalk and line up with a chalk mark on the ground. Roll the bike forward until the mark has travelled through a complete revolution and mark the corresponding point on the ground. Measure the distance between the two points on the ground and...hey presto...you have the number that needs adding into the computer.

Anonymous

Does it really matter that much if your comp. says you've done a few hundred yards or so more at a speed a teeny bit more than you actually have?

plug1n

@whyamihere, thanks - I had somehow skipped the bit that those numbers you plugin are the wheel circumference.

Checking the Krylions with a tape, I get 2139 whereas the Cateye booklet suggests 2105.

This is about 1.5% difference or 1 minute for me over a bumpy 25 mile ride, definitely worth knowing about.

Gary D

Gussio wrote:

whyamihere wrote:

Measure the circumference of the tyre when fully inflated. Most computers I've used use the circumference as the calibration number, so if you measure it yourself it will be more accurate than the numbers given in the instructions.

A way of measuring the circumference of the tyre is mark the tyre with a piece of chalk and line up with a chalk mark on the ground. Roll the bike forward until the mark has travelled through a complete revolution and mark the corresponding point on the ground. Measure the distance between the two points on the ground and...hey presto...you have the number that needs adding into the computer.

As Gussio has said above. If you want to be really (read: obsessively) accurate, then make sure you sit on the bike with your weight on the bars and you will be measuring the true rolling radius (or circumference) of the tyre. You could also measure 2 or 3 revolutions of the wheel, if your tape measure is long enough, and then get an average.

Not that I would ever go to these lengths you understand!!

Gary.