I just got a bike computer wired, and when I set the wheel size, am I supposed to do it by rim size or tire size? When I ride by a speed checker, it says on the computer 2mph faster than what the speed checker says.
Setting computer
- Thread starter fischer550
- Start date
Yup... outside circumference... Most instructions say to make a chalk mark on the tire at the 6 o'clock position and a mark on the ground or sidewalk, then roll the bike one full turn of the wheels so the chalk mark is at the 6 o'clock position again and make another mark on the ground, then measure the distance between the 2 marks on the ground. This is an accurate way to get the circumference, but you can also measure the total diameter of the tire and multiply by pi ... or 3.14
You can look on-line to find the circumference for most tires but at best you get a general number that is "in the ballpark" of the number you are looking for. In my experience they are generally several millimeters off at best.
As has been mentioned, the "roll out" method is best. Personally I stretch out a 25" tape measure and line the front tire up at the zero mark with the valve stem at the 6 o'clock position. Then roll forward along the tape until the valve is once again at the 6 o'clock position. This assumes, of course, that the magnetic pickup is on the front tire. I then use an on-line conversion tool to covert the inches figure I get to millimeters, which is what most bike computers require.
Using chalk or tape to mark the outside edge of the tire would be more accurate, however, so I picked up a good idea here!
As has been mentioned, the "roll out" method is best. Personally I stretch out a 25" tape measure and line the front tire up at the zero mark with the valve stem at the 6 o'clock position. Then roll forward along the tape until the valve is once again at the 6 o'clock position. This assumes, of course, that the magnetic pickup is on the front tire. I then use an on-line conversion tool to covert the inches figure I get to millimeters, which is what most bike computers require.
Using chalk or tape to mark the outside edge of the tire would be more accurate, however, so I picked up a good idea here!
