Another question would also be what’s the difference between wheel speed and vehicle speed?
In: 620
Analog systems used a speedometer cable which had a gear on one end and a speedcup on the other. The gear is meshed to a gear on the output shaft of the transmission for cars and usually on the front wheel hub for motorcycles, and this spins the whole cable. The speedcup at the other end is a small electromagnetic device, the magnets are spun by the speedometer cable and they in turn move the needle in the speedo. This may be an electromagnetic device but it requires no external power nor does it use digital signals, it’s completely analog. In the case of cars, the output shaft goes to the differential, which goes to the wheels which move the car forward. So the gear on the speedometer cable is calculated to be such that it gives the proper speed reading, assuming none of these gear ratios from the point it takes its measurement and the wheels change. But wheels do change, whether by an owner changing their rims, or more commonly putting on tires with a slightly different side profile. When you stray from what the stock setup is, this in turn causes the speedometer reading to no longer be accurate.
Newer electronic systems use no moving parts and digital signals for the reading. For all modern cars and most modern bikes the speed reading is taken from the ABS sensors at the wheels. In both cases the reading is not a true speed reading but a wheel speed, or calculated wheel speed, reading. The vehicle speed is the speed of the vehicle moving through space. The wheel speed is how fast the wheels are spinning. The wheel speed does not always match the vehicle speed in the following cases, when the wheels lock under braking, hence ABS sensors need to read wheel speed, when they lose traction under acceleration or slippery conditions, when they’re airborne, and in the case of cars, when they’re turning the inside wheels spin slower than the outside wheels.
Cars and bikes need to know what the wheel speed is in order for systems like ABS, Traction Control and dynamic stability control to work. All those systems apply or release the brakes, or give or cut power to the wheels, in order to maintain traction and stability, and they determine that by detecting when the wheels are spinning at different speeds from one another.
Latest Answers