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As a layman, have never understood how speedos ( and rev counters ) work . I mean, what keeps the needle on say 30 mph or 1000 rpm - why doesn't it spin to the max and stay there ? guess it's some sort of electric set up but would like a ' proper ' explanation if somebody can be bothered ?
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Speedos used to work by a cable attached to the gearbox.
The cable would spin at a speed determined by road speed and drive the speedo.
So the car does 10mph, the cable spins at 10rpm and the speedo reads 10mph - assuming it's all properly calibrated.
Car goes faster, cable spins faster and speedo reads a higher speed.
Now all done by electronics, I think.
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I've no idea, mine hasn't worked for about 8 months !! on an an Espace its a digital reading and only registers 0 and 8 mph -probably cost a fortune to get repaired - so I use my Sat nav's speedo -and it passed its Mot -
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Speedo not a testable item of equipment.
Ted
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Car goes faster cable spins faster and speedo reads a higher speed.
Although the cable was connected to the speedo, it wasn't physically connected to the needle. The cable was connected to a shaft that had a magnetic disc on the end of it. Then there was an air gap and another metal disc was attached to the needle shaft. Basically the connection between the speedo cable and needle was done by magnetism.
I know on modern cars one of ABS sensors is used to send a electronic signal to the speedo. I imagine inside the speedo is probably potentiometers and the like.
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tinyurl.com/yg36qpp
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In traditional cable-operated speedometers, the spinning cable connects to a rotating magnet. This is mounted close to a metal disk at the back of the pivot the needle is mounted on. The magnet induces corresponding magnetism in the disk, which is in effect a kind of fluid clutch. It tries to turn, but is restrained by a spring. How far it manages to turn by overcoming the spring depends on the speed the car, gearbox shaft, cable, and magnet, hence the needle gives a direct measurement of the car speed.
An electronic speedo is just a meter measuring the rate of pulses from a coil sensor mounted next to a special ring on the transmission crown wheel.
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>>Basically the connection between the speedo cable and needle was done by magnetism.
>>The magnet induces corresponding magnetism in the disk,
Not quite. You're both right that the speedo cable spins a magnet, but the needle's connected to an aluminium disc (non-ferrous). Eddy currents are set up in the aluminium disc by the spinning magnet. The aluminium disc is spring loaded causing it to rest at 0 MPH. If the aluminium disc was free to spin, it would turn at (or close to) the same speed as the spinning magnet and the eddy currents would be at or near zero. The faster the magnet spins the greater the eddy currents and the greater the turning torque applied to the aluminium disc and the higher the speedo reading.
QED. 'O' level physics circa 1965.
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Impressive memory BB Tom!
Exercise bikes with magnetic resistance use the exact same principles.
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As did (I believe) retarders on coaches.
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