wheel circumference???

Quoted post said:

Originally posted by Prowler
hi, i have just bought a cateye cordless 7 wireless computer and am having touble finding the circumference of my wheels in any of the charts provided. They are 700x22c and am getting a value of 2086mm by doing a roll out - but this appears to be wrong comapred with the other values on the cateye manual. Would anyone know what the official estimate is so i can figure out if my roll out is anywhere close?? Any info would be much appreciated!!!! Thanks, Rik.

My roll out on my 700C X 28 = 2,154.5 mm

I looked on line at the Cateye Cordless 7 manual at:

<http://www.cateye.com/tech_support/manuals/computers/FR7CL(7)v1_E.pdf>

Your roll out measurement is exactly what they have listed for 700 X 20C. Your roll out measurement seems to be in the right area.

i reckon u r close. i have 700x23 and mine is 2099. remember u must have tyres pumped up (sounds stupid but u would be surprised!) and for more accuracy sit on the bike while rolling out the distance.

Quoted post said:

Originally posted by drewjc
i reckon u r close. i have 700x23 and mine is 2099. remember u must have tyres pumped up (sounds stupid but u would be surprised!) and for more accuracy sit on the bike while rolling out the distance.

ya pump em and then roll out should give you a very accurate answer, theres variation among tires so go with the number you get

It doesn't really matter what the manual says. Your measured roll-out is what is on the bike and if you set the computer to that value it will be accurate. Even if you're off a bit, the cycle computer will be more accurate than a cars odometer.

For greater accuracy, you should do the rollout while seated on the bike. Make sure the tires are inflated fully and steer straight. For the ultimate in accuracy, roll the wheel out 10 revs and measure the total length and divide by 10 - you need a long tape for that.

TD

Quoted post said:

Originally posted by tyler_derden
For the ultimate in accuracy, roll the wheel out 10 revs and measure the total length and divide by 10 - you need a long tape for that.

TD

I went to a track & field stadium and tested my bike computer against the track lengths there. Also a good way to check accuracy.

same problem these days: my rollout gave about your value, (2079) but having already done the setting in the past I know that this is only a good starting value, which leads to overestimates of the distance, and therefore the actual speed - I suppose because when you ride the bike the radius decreases of at least a couple of millimeters. So I will correct the circumference with the data from the milestones along the road, from the difference among the data returned by the cateye -say over 10 km- and the actual distance indicated by the milestones (you have to make a simple proportion).

Myskin said:

same problem these days: my rollout gave about your value, (2079) but having already done the setting  in the past I know that this is only a good starting value, which leads to overestimates of the distance, and therefore the actual speed - I suppose because when you ride the bike the radius decreases of at least a couple of millimeters. So I will correct the circumference with the data from the milestones along the road, from the difference among the data returned by the cateye -say over 10 km- and the actual distance indicated by the milestones (you have to make a simple proportion).  Â

I'll bet in the 8 years since, the OP sorted his problem. The proper way to do a rollout is with yourself on the bike. You can roughly estimate the weighted rollout by doing an unweighted rollout and the subtracting 4mm from the value measured. Using that number would produce a rollout for you of 2075mm. Given that 20mph with your old rollout would read as 19.96mph, and there would a 203ft difference in the distance measurement over 20 miles.