Setting up a bicycle speedometer means mounting the sensor on the front fork, aligning a spoke magnet within 4 mm of it, and entering your exact tire diameter into the display.
For the full breakdown, see our best Bicycle Speedometer guide.
What You Need Before You Start
Gather the wired kit (sensor, magnet, display, wiring), small zip ties, a flathead screwdriver, and a pair of pliers. Most kits include the display bracket and O-rings. A tape measure for the wheel-size step saves a second trip inside.
Sensor and Magnet Alignment
The sensor mounts on the front fork, typically on the right leg (opposite the chain side) and about 6 inches above the hub. Use zip ties to fasten it to the inside rear corner of the fork leg, with the sensor body facing downward or toward the wheel — the metal/sensor side must face the magnet.
The magnet clamps to a front-wheel spoke at the same vertical height as the sensor. It passes the sensor tip without contacting it. Tighten the magnet’s screw with the screwdriver so it won’t shift during rides.
If your bike has disc brakes, move the sensor to the opposite fork side to keep it clear of the rotor.
Wiring and Display Mount
Run the wire up the rear of the shock absorber or alongside the brake cable to the handlebars. Secure it with zip ties above and below every flex point (steering stem, fork crown) but leave about 10% slack so steering turns don’t strain the wire.
Mount the display bracket on the handlebar center using the provided O-rings or zip ties. The face should point upward and stay legible while you’re in a riding position. Insert the battery with the positive (+) side facing out.
Turn the unit on, enter your wheel size (see the calibration section below), and select miles or kilometers. Spin the front wheel — a non-zero speed reading confirms the system works.
Calibration: Getting the Right Wheel Size
Speed and distance calculations depend on one number: the exact circumference of your wheel and tire together. The tire sidewall shows a diameter marking (like 26×1.95 or 700×32C), but that’s the rim size and tire width, not the true rolling circumference.
Two ways to get it right:
| Method | How to Do It | Accuracy |
|---|---|---|
| Roll-out measurement | Mark the tire at the ground, roll one full wheel revolution (with your weight on the bike), and measure the distance between marks in millimeters. | Best — accounts for rider weight compressing the tire. |
| Wheel-diameter code | Check your speedometer’s manual for a chart that maps your tire marking (e.g., 700×32C) to a manufacturer-specific code. | Good — uses the manufacturer’s standard table. |
| Online calculator | Look up a tire-size-to-circumference chart (common on cycling sites). | Fine for most riders — matches the calculator values well. |
Enter the circumference in millimeters or the code into the display.
FAQs
Does the speedometer work on the rear wheel instead?
Most wired speedometers are designed for the front wheel — the wire is cut for that distance and the sensor’s orientation assumes the fork. Rear mounting requires a longer cable and exposes the wire to chain and derailleur parts, which can snag it. Stick with the front unless your kit explicitly supports rear use.
What if the speedometer still shows zero after installation?
Check the magnet-to-sensor gap first — anything wider than 5 mm kills the signal. Make sure the magnet faces the sensor’s metal side (not the plastic back) and that both parts sit at the same spoke height.
Will changing tire size or pressure mess up my calibration?
Recalibrate by measuring the roll-out distance again after a significant tire or pressure change, or accept the small speed error.
References & Sources
- Polar. “Speed Sensor BT Smart accessory manual.” Details sensor placement, magnet alignment (4 mm gap), and alignment verification via red LED.