How Does a Speedometer Work?

[Elcipse_(~)=(~)]

DodgeNitroBIRM,
I am writing a paper for my college intro to Physics paper and i was wanting to thank you for discussing the mechanics of a Speedometer. My paper is to be a page long and about how speedometers and odometers work.

[drakkie]

A small addition to this thread. It is not uncommon nowadays to have the Engine ECU calculate the speed at the wheels using multiple sensor signals, which are allready present anyway. IIRC the latest Delphi & Bosch Systems have the ability to do so.

[Fuerte100]

I went to howstuffworks.com and found this (for mechanical, or "eddy" speedometers)

Let's say a car is traveling along the highway at a constant speed. That means its transmission and driveshaft are rotating at a speed that corresponds to the vehicle speed. It also means that the mandrel in the speedometer's drive cable -- because it's connected to the transmission via a set of gears -- is also rotating at the same speed. And, finally, the permanent magnet at the other end of the drive cable is rotating.

As the magnet spins, it sets up a rotating magnetic field, creating forces that act on the speedcup. These forces cause electrical current to flow in the cup in small rotating eddies, known as eddy currents. In some applications, eddy currents represent lost power and are therefore undesirable. But in the case of a speedometer, the eddy currents create a drag torque that does work on the speedcup. The cup and its attached needle turn in the same direction that the magnetic field is turning -- but only as far as the hairspring will allow it. The needle on the speedcup comes to a rest where the opposing force of the hairspring balances the force created by the revolving magnet.

What if the car increases or decreases its speed? If the car travels faster, the permanent magnet inside the speedcup will rotate faster, which creates a stronger magnetic field, larger eddy currents and a greater deflection of the speedometer needle. If the car slows down, the magnet inside the cup rotates more slowly, which reduces the strength of the magnetic field, resulting in smaller eddy currents and less deflection of the needle. When a car is stopped, the hairspring holds the needle at zero.

and this for an electrical speedometer:

An electronic speedometer receives its data from a vehicle speed sensor (VSS), not a drive cable. The VSS is mounted to the transmission output shaft or to the crankshaft and consists of a toothed metal disk and a stationary detector that covers a magnetic coil. As the teeth move past the coil, they "interrupt" the magnetic field, creating a series of pulses that are sent to a computer. For each 40,000 pulses from the VSS, the trip and total odometers increase by one mile. Speed is also determined from the input pulse frequency. Circuit electronics in the car are designed to display the speed either on a digital screen or on a typical analog system with a needle and dial.

Source: Howstuffworks "Eddy-Current Speedometer"
Beginning of article: Howstuffworks "How Speedometers Work"

[drakkie]

Exactly. That's basically the way all rotation sensors work. Which sensor to get the signal from is another thing. Nowadays the ABS-sensors are used a lot or data from the ECU's injection/ignition part.

[VtecMini]

So does that mean that the accuracy of any mechanical speedo is COMPLETELY reliant on the... erm... "springiness" of the hairspring? Does the temperature of the hairspring not therefore affect the accuracy of the speedo?

Or did I completely miss something?

[SIMPLETON]

most car and motorcycle are off a bit. On motorcycles its very common for them to read 5-10% higher than the actual speed

[Matra et Alpine]

Originally Posted by VtecMini

So does that mean that the accuracy of any mechanical speedo is COMPLETELY reliant on the... erm... "springiness" of the hairspring? Does the temperature of the hairspring not therefore affect the accuracy of the speedo?

Or did I completely miss something?

Nope. Your 100% right.
Just that the quantity of the variation is limited by the choice of the spring material and compliance.
However, it IS the reason why the manufacturers have always been asked by goverments to make the speedo read OVER so that when any individual variation occurs they dpeedo won't read significantly lower than the real speed.

[VtecMini]

Originally Posted by Matra et Alpine

Nope. Your 100% right.

Yeah? Booyah!

So how long since have mechanical speedos been redundant then? I'm guessing electronic ones have taken precedence now, right?

[Matra et Alpine]

Does ANY new car still ahve mechanical ?
I don't know but would doubt it as electronic is cheaper

[Fuerte100]

Originally Posted by Matra et Alpine

Does ANY new car still ahve mechanical ?
I don't know but would doubt it as electronic is cheaper

Yeah I guess that is the situation but I bet that the mechanical one is more reliable. Especially if someone sends out an EMP (yes, i know that would fry your car )

[Matra et Alpine]

Originally Posted by Fuerte100

Yeah I guess that is the situation but I bet that the mechanical one is more reliable. Especially if someone sends out an EMP (yes, i know that would fry your car )

As someone who has had to fit and repair speedo cables and drive gears on many cars and Halda take-offs .... nope ... electronic MUCH more reliable ( well as long as it's not made by Lucas or the Italians )

[Sledgehammer]

So if the drive shaft rotating magnets, creating eddy's controls the speedo, why does the car register a slow speed, >5 mph? wouldnt the eddy spikes be to slow to accuratly measure? Are their simply so many "toothed needles" to not worry about slow rotation speeds?