Setting tire size in computer

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My Cat Eye instructions say to use either 212 or 213mm. However, this is only a estimate and a place to start. I then calibrate it on a rails to trails trail that still has the old RR milage markers, and check it over 10 miles. I have found the estmates to be off by over 10mm's.

c9buff said:

Hey guys,

Just got a cyclocomputer and want to input the tire size. The sidewall of my tire reads 700X26c. The instructions from Cateye say to enter the tire's circumference directly in increments of 1mm.

What number do I put in? Thanks in advance.

buff😕

You need to measure the circumference. Start with your tube stem at 6:00 o'clock, and put a mark on the ground at that point. Roll your bike forward until your wheel has made one complete revolution, and then mark the ground again. Measure between marks. (conversion - Inches x 25.4 = mm).

Tom

c9buff said:

Hey guys,

Just got a cyclocomputer and want to input the tire size. The sidewall of my tire reads 700X26c. The instructions from Cateye say to enter the tire's circumference directly in increments of 1mm.

What number do I put in? Thanks in advance.

buff😕

When you do the measurement by rolling the bike, you want the tires inflated to riding pressure and you want to make sure that you are aboard bike as you role it so you get the circumference with the tires compressed by your weight. Also roll the bike in an absolutely straight line. The difference may be small, but if you are going through the effort of a measurement, might as well get it right.

By the way, circumference in mm should be somewhere around 2130mm or so for a 700 X 26 tire. Some computers use a cm measurement, which is the unit of measure that buckybux must be using.

buckybux said:

My Cat Eye instructions say to use either 212 or 213mm. However, this is only a estimate and a place to start. I then calibrate it on a rails to trails trail that still has the old RR milage markers, and check it over 10 miles. I have found the estmates to be off by over 10mm's.

Do the rollout test as suggested, best done by actually sitting on the bike while someone helps you so that you get the true radius. The number you get will let you see if you want to round up or down to the nearest increment you enter into the computer. You see that you can't nail it exactly, your computer is granular, only allowing whole integer input. Such is life. You don't need any more accurate, do you?

Insight Driver said:

Do the rollout test as suggested, best done by actually sitting on the bike while someone helps you so that you get the true radius. The number you get will let you see if you want to round up or down to the nearest increment you enter into the computer. You see that you can't nail it exactly, your computer is granular, only allowing whole integer input. Such is life. You don't need any more accurate, do you?

Let's see. If you are able to adjust to the nearest mm, then at most you should be off by 0.5 mm. That is a 0.023% error. If the average is 2130 mm, and you are off by 0.5 mm, then over the course of a 100 mile ride, your error would be 124 feet. I think I could live with that.

Even if you could only adjust to the nearest cm, and you are off by 0.5 cm, that is a 0.23% error rate. The error in 100 miles would be less than 1/4 mile.

To look at it another way, at 20 mph, the error in the displayed speed would be 0.005 mph (for those computers that calibrate to the mm) or 0.05 mph (for those that calibrate to the cm).

I doubt that the tape measure you use to measure the roll out distance is any more accurate than that.

Thanks for the help guys.

I can live with the .5 millimeter error.

buff🙂

If you can't get someone to help you with a roll-out measurement, you can get close to the roll out number by rolling the bike by itself and then subtracting 4mm for every revolution of your measuring wheel. Remember only do the 4mm subtraction if you have the bike unweighted, and do the roll-out as others mentioned w/ your usual inflation pressure.

Also on the rollout test, you should go a distance that allows the wheels to rotate a few times. If you go a distance that gives 10 full rotations, and your tire is 2130mm in circumference and your error is nominally 0.5mm, then over the course of a mile your error can be reduced to 1.5 inches. Of course when your error gets that small, then your greatest source of error will be due to tire pressures that deviate from the pressure used during the rollout test.

Insight Driver said:

Do the rollout test as suggested, best done by actually sitting on the bike while someone helps you so that you get the true radius.

One can actually do a roll out test un-assisted. On a flat part of the road mark a straight line with white powdered chalk. Ride in a straight line going over the chalk and you should get tire markings from the tire to the road. Measure c-c. You can also use paint applying liberally but it might get messy as some may splatter on your D/T.