A bike computer calculate the distance traveled by a bike by measuring the number of times the bike’s wheel rotate and then multiplying that number by the circumference of the wheel. The circumference of the wheel is set on the computer, so if that setting are incorrect, the bike computer will display incorrect data about the distance that the bike has traveled. Furthermore, because the bike computer use the circumference of the wheel to calculate other data about the ride, such as speed, an incorrect wheel circumference will also introduce error into other data displayed by the computer.
Because different type of tires have different circumferences, changing the tires on a bike will change the distance that the wheel travel in one rotation. For instance, the circumference of road bike tires is generally smaller than the circumference of mountain bike tires due to the fact that mountain bike tires has knobby tread pattern that compress under the weight of the rider. Thus, the circumference of the wheel that travel on a bike must be set to match the circumference of the tires that is installed on the bike.
Measure and Set Wheel Circumference
The pressure of the air in the tires can also play a role in the circumference of the rotated wheel. If the tire pump to a low air pressure, the tires may flatten against the ground, which will reduce the circumference of the wheel. If the tire pump to a high air pressure, the tire may expand which will increase the circumference of the wheel.
Thus, the air pressure in the tires should be set to the desired riding pressure before the circumference of the wheel are set. The circumference of the wheel can be determined in a variety of ways, such as using a chart to find the circumference of different size tire. However, the most accurate method is to perform a manual rollout test which will account for the specific tire and weight of the rider.
To perform a manual rollout test, mark a point on the floor, roll the bicycle for one full rotation of the wheel, and measure the distance between the two mark. This test should of been performed while you are sitting on the bicycle, as your weight will help to compress the tire and provide more accurate measurement. Manual entry of the wheel circumference is often more reliable than the GPS calibration of the wheel circumference.
GPS calibration measurement use satellite data to determine the circumference of the wheel, but those signal may not reach satellites if the bicycle is in an area with trees or canyons. Manually entering the wheel circumference allow for the input of the measurement that was taken during the manual rollout test. Additionally, manual entry of the wheel circumference is important for those that ride indoors on a trainer, as the GPS data will not detect the distance that the bicycle roll.
The accuracy of the wheel circumference setting is directly related to the accuracy of all other data measurement from the computer. Errors in the circumference of the wheel will lead to errors in the measurement of the speed and distance ridden. If the circumference of the wheel is measured within one percent of the actual measurement, the measurements of the speed and distance will also be one percent off from the actual distance and speed measurement.
If the user incorrectly enters the circumference of the wheel into the computer, the training data and mileage measurement will also be incorrect. The wheel circumference should be checked each time the tires is changed or the tire pressure is changed. Additionally, if bicycles other than the one that is being ridden are use, the circumference should be checked, as different bicycles often have different sized tire.
If multiple bicycles are used for training, each bicycle can be labeled with the circumference of the wheel in relation to that bicycle.
