If you want to buy a spare or replacement wheel for your bike, you need to know its size. This is a standard maintenance operation for your vehicle and you can follow two methods, both simple, to successfully complete the measurement process: you can measure the diameter of the rim and that of the tire. Sometimes you also need to know the circumference of the wheel, and this is a fact that you can achieve in many ways.
Steps
Method 1 of 3: Standard Technique
Step 1. Make the bike stand upright by leaning it against a wall or kickstand
This way you can proceed without the risk of the vehicle falling over. If you are working alone, use a retractable metal ruler, which is sturdier than a plastic one and leaves you one hand free.
Step 2. Measure the distance, in inches (1 inch = 2.54 cm), from where the wheel touches the ground to the center of the hub
This way you found the radius, half the diameter. You have to calculate the values in inches because, in most cases, bike wheels are categorized according to this unit of measurement. Multiply the length of the beam to find the diameter. With the exception of BMXs, most adult bikes have wheels with a diameter of between 26 and 29 inches.
Step 3. Measure the flat part of the wheel, from one side of the tread to the other
This is the width of the wheel and its value can change a lot, depending on the use for which the tire is intended. Generally speaking, a wide tread is used on rough terrain, while a thin tire allows you to reach higher speeds on smooth asphalt.
Step 4. When buying a new tire, remember that the measurement is traditionally expressed by first indicating the diameter and then the width
For example, a tire that says “26 x 1.75” means it has a diameter of 26 inches and a tread width of 1.75 inches.
Method 2 of 3: ISO technique
Step 1. Check if the wheel dimensions of the bike are expressed according to the European designation ISO (International Standard Organization)
This method uses the metric system (millimeters) to organize the size of bicycles. At this point you have to use a meter that expresses millimeters or remember that an inch corresponds to 25, 4 mm and proceed to the appropriate calculations with the help of a calculator. To convert a value from inches to millimeters, just multiply it by 25, 4.
Step 2. Make sure the bike is leaning against a wall or on a kickstand
Measure the distance from the center of the wheel to the inner edge of the tire. Again, you have to double the value to find the diameter. Most of the wheels on adult bicycles that comply with the ISO system have a diameter between 650 and 700 mm.
Step 3. Measure the tread width
Using the same method described in the first part of this article, measure the width of the tire from side to side. Remember that tires of different widths can be used on the same bike, even if there is not much variation.
Step 4. When you buy a tire whose dimensions are expressed in the ISO system, the width is indicated first and then the diameter
For example, a wheel size 53.3 x 700 has a width of 53.3mm and a diameter of 700mm, starting from the inside edge of the tire to the opposite inside edge.
Method 3 of 3: Calculating the circumference
Step 1. Measure the circumference or perimeter of the wheel to properly calibrate the speedometer, odometer, GPS tracker or computer
Just like the odometer and speedometer on cars, even the bike's will not be accurate if you change the size of the wheels without calibrating the instrumentation accordingly. Whether you have to buy an on-board computer or calibrate the existing one, because you have mounted wheels of different sizes, then you need to calculate the circumference of the tires.
Step 2. You can easily proceed with the calculation by multiplying the diameter by pi
The circumference of each circle is quickly determined, if the outer diameter is known (from one outer edge to the other). The value of pi is equal to 3.14, so the circumference of a 26-inch wheel is calculated by multiplying 26 x 3.14 = 81.64 inches.
If you already know the width and diameter of the wheel, you can find its circumference by consulting various online tables
Step 3. Measure the circumference with string
If you don't know the diameter of the wheel, then you can still measure the circumference by wrapping a string around the outer edge of the tire. Mark or cut the string where it meets the starting end and measure the length of this segment to get the circumference.
Step 4. Put a drop of fresh paint on one spot on the tread
Push the bike forward, being careful to follow a straight path. You should do at least two full rotations, in order to get two paint marks on the ground. Measure the distance between two consecutive marks on the ground to determine the circumference of the wheel.
Advice
- The size of the tire is usually printed on the shoulder of the tire and indicates the diameter and width, for example 27 x 1, 5.
- When measuring the diameter of the wheel, avoid it from spinning, because you would get a less accurate measurement.
- When using the standard measurement method, the diameter value should be an integer. If you get a decimal number, round it to the nearest integer.
