Bicycle Mechanics - Having trouble with computer

I'm having trouble setting up the wheel size (distance in mm of one rotation of the tire) in my new cycling computer. The manual that has the oh-so-handy reference chart skips my wheel size (700x23c)! Anyone else out there with this problem? I have tried to measure the distance by the 'roll-out' method, but can't seem to get the same number twice.

Am I putting too much energy into this?

If I'm not mistaken, a 700c wheel is 622mm in diameter. Add the tire width and multiply with pi and you have the circumference: (622 + 23 + 23) * 3,14 = 2097mm. You might want to lower that number to adjust for tire compression caused by your weight.

/Csson

Do a roll out. Mark a spot on the ground, with the valve stem at the 6:00 position, then roll it one revolution as striaght as you can, mark your second spot, then measure. If you don't have a metric tape, the conversion is 25.4mm to 1".

Follow the manufatuers instructions regarding inputing the numbers.

Or do a three revolution roll out multiple times, average, then divide by three. This seems to help minimize inconsistencies in the roll out.

According to the chart that came with my computer it should be 210. Rollout measurements might be more accurate, but can you set your computer to a fraction of a cm. You dont need to know your exact speed, but only how fast today's speed compared with yesterday's or the day before's. Set your computer the quick and easy way and spend more time riding.

My computer takes the distance in mms. This is what I do:

Find a quiet, dry stretch of sealed road, splash some water on top of your front tyre, ride straight for 15-20 meters or so, leaving wet spots on the pavement. Measure the distance from one spot's leading edge to the next, do the same for a couple of other spots. Average your measurements and input the result in your computer.

I am not so worried about the speed, but I like my riding distance to log in accurately.

--J

(edit: typos)

what thetray is trying to say is that an accurate rollout is useless if the computer doesn't contain that wheel/tire size. for example, shimano flightdeck does not contain 700x23. so i just set it to 700x25

Question?what type of computer are we talking about. I have at least three computers, and they all have diff settings for 700x23 wheels.

deliriou5-My flight deck has 700x23 in it, I found it under 2105. (Head unit sc-6501) Im having a problem switching from Mode 2, to Mode 1 while riding. I can switch by hitting a on the back of the head unit, but cant seem to switch with my flight control buttons.

Question?what type of computer are we talking about. I have at least three computers, and they all have diff settings for 700x23 wheels.
I have a Supergo HRM2000. It's default setting for a 700x25 wheel is 2124. I mainly want to make sure the mileage is accurate on the odometer. In the past, I drove the routes I wanted to take ahead of time to know how many miles, but the computer is a little shy on the mileage.

According to Polar documentation, the 700x23C has a circumferance of 2096 mm. It is most likely the default setup on your computer.

deliriou5-My flight deck has 700x23 in it, I found it under 2105. (Head unit sc-6501) Im having a problem switching from Mode 2, to Mode 1 while riding. I can switch by hitting a on the back of the head unit, but cant seem to switch with my flight control buttons.

oh... i have the sc-6500... the previous generation.... they don't have 700x23. go figure... it's the most common bike tire size! :rolleyes:

The rollout is useless if your computer only accepts tyre sizes instead of the accurate circumference of the tyre in mms (2100 mm or whatever). If you can input the mms, IMHO you should do the rollout. It is more accurate than using a conversion table (700x23 = 2100) or calculating the theoretical circumference with pi.

--J

None of this stuff is really accurate. Unless you ride on the bike when you measure the circumference, weigh the same and don't wear down the tires, there will be variations in radius and circumferences (changing with 6x radius). The numbers discussed are good enough and the tolerances are minor for most users.