The fun of riding an electric bike is seeing the road roll by without breaking a sweat. The battery pack and motor do most of the work, but some other components help control that power.
Glancing down at your display and seeing the speedometer showing speeds that might be faster than you could pedal on your own. But how does it work?
An eBike Speed Sensor is a proximity switch that counts a bike wheel rotation. The sensor sends a signal to the display which calculates the bike speed using the wheel diameter. Additionally, speed sensors indicates movement of the bike allowing power to be applied during pedal assist mode. On most mid-drive conversion kits, the speed sensor is located on the bike chainstay with a magnet on the bike spokes.
Did you Know You Can Convert a Regular MTB to Electric?
Bafang a leader in DIY electric bike conversions has a mid-drive kit that will fit on most regular bikes. I’d recommend the BBSHD kit, it’s a complete setup with a huge capacity battery and a 1000 watt motor. Why this setup?
- Heavy duty 1000 watt motor (built for cargo bikes)
- +50 volt 17.5 Ah battery for speed and distance
- Easy installation with provided tools.
If you’re considering a conversion, do what I’m doing get the BANFANG BBSHD 1000w eBike Conversion with Battery
How Does an Electric Bike Speed Sensor Work?
The digital signal sent to the electric bike display has two primary functions.
1. A simple speed calculation is made using the wheel diameter times that weird pi (3.1416) number, which provides the wheel circumference. Example: a 26 in tire X 3.1416 = 81.86 inches traveled in one rotation. Then using the time interval between signals, a rotation per time is calculated. For example, it takes 776 rotations per hour to go 1 MPH. A great illustration would be to visualize the bike wheel turning at nearly 20,000 rotations per hour or 323 RPM (rotations per minute) would equal 25 mph
2. The Speed Sensor also detects movement, think of this function as an “OKAY” signal. When the display senses wheel movement from the speed sensor it ALLOWs power to go to the motor. Mostly used during PAS (Pedal Assist System) the movement signal turns on the power while pedaling because the sensor detects movement.
To a lesser degree, the speed sensor helps in calculating the distance remaining in the battery capacity.
RIDE e MTB Pro Tip: With eBikes growing in popularity learning all the terms and definitions can be a bit overwhelming. Let me help with this comprehensive glossary. 👉 eBike Glossary
Why is a Speed Sensor Needed?
As a rider, we need to understand how fast the bike is going to keep up with traffic. Also, no one wants an electric bike to take off when the bike is stationary. The speed sensor is needed to control these electric bike functions.
Differences Between Speed Sensors and Cadence Sensors on Electric Bikes
On a hub motor eBike the cadence sensor gets a signal from the bike crank. Which feeds the signal into the display when using the pedal assist system. Hub motors have speed sensors, they are located internal to the hub motor housing.
RIDE e MTB Tip: Are you thinking about building an eBike? It isn’t very hard – Heck I’ve got step by step instructions with videos along with all the tools. Check it out 👉 How to Build a Mid-Drive Electric Bike
Common Speed Sensor Problems
The most common problem with speed sensors is the gap between the magnet and sensor being too wide. The recommended gap should be 4 to 6 mm. (less than ÂĽ inch) Test the spacing by moving the magnet (on the wheel spoke) past the speed sensor and check for an indicator light.
Another common problem is the wire connections are loose or broken. The connection joints are designed to be water resistant so in turn, they are tight. Broken wires can happen pretty easily since the wires are close to the tires and spokes. It’s easy for a branch to get caught in the tire/spokes and get swiped by the wires damaging them
Sometimes when installing and connecting the wires the tiny pongs in the connector can get bent or damaged.
Speed Sensor Error Codes on Banfang Systems
Many Banfang displays will show an error. My display calls out the speed sensor.
The common error codes are:
| Error Code Number | Speed Sensor Error Description |
| 21 | External Speed Detecting Fault |
| 22 | Battery Management System Failure |
| 30 | Link or Communication Error |
Test a Speed Sensor
After setting the wheel size within the display. The best test is to use the bike and check for error codes. If you want to check how accurate the speed is, use a GPS app like Google Maps on your phone.
Another test is to spin the rear wheel and watch for the indicator light on the pick up unit located on the chainstay.
How to Attach a Phone to an Ebike?
I love biking, and I also like playing music while riding. A couple of years ago I found a biking phone mount and I’ve been recommending it for a while. It’s called Quad-Lock. The system has two parts, the first is a sturdy handlebar mount and a low-profile phone cover.
The system securely “Locks” your phone on. I’ve crashed mountain biking – YES over the handlebars and my phone has remained securely connected.
Check out the mounting system at Amazon with this shortcut link 👉 Quad-Lock Phone Mount
Can You Bypass the Speed Sensor to Go Faster?
Yes, there are a couple of ways to trick the speed sensor system into thinking the bike is going slower, so more power can be applied to go faster.
1. If you mount the magnet to a slower rotating item. For example, the pedal crank rotates at a fraction of the speed of the wheel. If you glue the magnet to the inside of the crank and mount the sensor opposite on the frame, you’ll “trick” the sensor.
2. If your electric bike display has a setting for the wheel size you can adjust the size smaller. For example on a 26-inch wheel bike setting the display at 20 in.
Neither of these options is a good idea though. If you desire to make your bike, go faster the best way is to better understand the gearing, motor, and battery options and jump into a custom high-power e-bike build.
Frequently Asked Question
Why is my eBike Going Slow?
I would recommend three things;
1. Is the battery fully charged?
2. Check tire pressure, low air pressure in tires will add considerable rolling resistance. (wide fat tires can be slower as well)
3. Less obvious can be the environment, a slight increase in the grade (hill) and wind resistance will dramatically affect electric bike speed.
More Than Leg Power
A speed sensor provide an essential component in an eBike system. An accurate link from the tire motion to the display and application of power is critical.
Hi David Humphries here the guy doing all the pedaling behind the scenes with this blog. I’ve been in the MTB world for a while and recently started getting into eMTBs. You can check out more about me HERE and on my other passion project – DIYMountainBike.com
Sources
1. “Setup and troubleshooting BBSxx speed sensor” Electric Bike Forum Accessed August 20, 2022. https://electricbike.com/forum/forum/knowledge-base/motors-and-kits/bbshd/34500-setup-and-troubleshooting-bbsxx-speed-sensor
2. “Speed Sensors vs Torque Sensors: E-bike Sensor Technology” HAIDONG. Accessed July 29, 2022. https://hd-ebike.com/blogs/speed-sensors-vs-torque-sensors-e-bike-sensor-technology