Another question would also be what’s the difference between wheel speed and vehicle speed?
In: 620
Fun fact: when your vehicle is rolling down the street, under normal conditions, the top of each tire is moving forward at twice the speed of the car, while the bottom of each tire is not moving at all.
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.
I am not seeing many answers explaining Wheel speed vs Vehicle speed. Wheel speed is calculated from wheel RPM x Circumference of the wheel, but vehicle speed is the actual speed at which your vehicle is moving as measured from an external device such as a radar gun or GPS.
Unless your vehicle is using speed measurements by external sources, it has no idea what speed it’s actually doing. Internal measurements only assume that there is a perfect contact between wheel and road surface with no slippage. It simply measures the RPM of the wheel, multiplies it with circumference and gives out the estimated linear speed of the vehicle.
If your tires are rotating in a puddle and your vehicle is not moving forward, old school analogue speedometers will show some speed on the speedometer but actually your vehicle is stationary. If your vehicle is skidding out on the road, your vehicle speed will be non-zero but as the wheels are locked under braking speedometer will show 0 speed. This of course can be managed by myriad of electronic sensors and control units to have better tuned algorithms to counter wheel slippage and the resulting mis-representation of speed on speedometer
Rarely are things explained like someone is five on this site. Am I the only one who notifies this?
Early cars used a small gear that ran off the transmission output shaft. A steel cable in a sleeve plugged into the gear and was run to the back of the speedometer. The speedometer has magnetic witchcraft that moves the needle. The gear on the transmission could be changed to adjust for different speedometers, wheel sizes, and final dri e ratios.
In the 80s, manufacturers replaced the mechanical steel cable with a hall effect sensor, usually mounted in the same spot where the mechanical one went. It outputs a voltage or pulses as it spins. This was wired into the computer and the computer then output a voltage to the speedometer. Adjustments for wheels and ratios were programmed into the computer.
When 4 wheel ABS came along, each wheel got a hall effect sensor, where the computer could monitor each wheels speed and correct for loss of traction as well as run the speedometer. The same sensors are used for traction control and vehicle stability control systems.
Some mechanical or electronic bit in the car “knows” how big the wheels are suppossed to be. Based on how long it takes the wheel to make one full revolution the math is fairly straightforward to turn that into how fast the car is driving.
If you spin the wheels on the car on a slick surface where the car doesn’t actually move the speedometer will still tick up based on the speed of the wheels. The system assumes the car is actually moving rather than just peeling out.
Also, if you change the tires on your car and put a drastically different size on it can cause your speedometer to read incorrectly unless you get it recalibrate to compensate. This can be a common problem for the people who put the giant tires on their lifted truck. The bigger the tires are the slower its going to read and then you get pulled over for speeding even though your gauge doesn’t say you are.
u/Vast-Combination4046 explains it very well.
Race cars use pitot tubes, same as airplanes. This measures the speed of the chassis going through the air, rather than the number of times your wheel is rotating per minute. Advantage of such a system is that it shows your true speed even if your tire is slipping. Disadvantage is that it has to be recalibrated for every air pressure and humidity setting – easy for a race, not so much for your daily driver.
It’s pretty simple honestly. You have a sensor that spins and for however many times it spins there is a corresponding speed. It factors in things like what gear you’re in, what size tires, the transmission gearing, tear end gear all that stuff, but simply put it just spins a sensor that counts the number of spins.
Wheel speed vs vehicle speed is actually a really cool aspect that’s becoming a lot more prevalent especially in racing. It’s goal is to help with traction ultimately. Let’s say you have a Corvette with a big supercharging making 1,000 horsepower, cool stuff! Corvettes are rear wheel drive which means the engines power is delivered to the rear wheels while the front wheels just roll and turn.
Let’s say you want to race someone at a track because street racing is stupid. You know that grip is the key to racing so you turn on traction control and here we go. Now vehicle speed is easy it’s how fast the entire vehicle is moving. Wheel speed doesn’t equal vehicle speed though. Let’s say you take off and the tires start spinning from all that power. Your back tires are going to spinning faster since they don’t have traction and are being powered by the engine compared to your front tires which are just rolling at the speed of the vehicle. The cars computer can see this and regulate power (variety of ways often pulling back timing) to try and keep the front and rear wheels spinning at the same speed meaning you have traction. The same works for braking, you don’t want your brakes to lock up and skid, that’s not the fastest way to slow a car. So while braking the car is looking at each wheel and if one has a rapidly dropping wheel speed the ABS module will cut and reapply braking quickly and as repeatedly as needed to ensure the tire never stops rolling and stays at that maximum strength braking force.
There are other things that can be useful to measure wheel speed but that’s the basic gist. Something like drifting prioritizes the rear wheels spinning much faster than the front wheels to make the car slide on purpose. So you’d see the rear wheels doing 100mph or whatever but the front doing 40mph and the vehicles absolute speed would be the 40mph of the front wheels.
Latest Answers