VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 Speedometer... How do it works ????????????

I know it works, but I don't understand how.

Outside the are a horse shoe magnet, inside a rotating steel
part, and between them, a non magnetic aluminium drum.

Are aluminium a little magnetic ???
How do it works?
It's a mystery for me.

Pictures are from a 24-25 model speedo.

Arvid

The horseshoe is a magnet and it sets up a magnetic field with the metal spider that turns inside of the speedometer drum, thus giving the correct miles per hour.

To calibrate the miles per hour (these old speedometers are usually way out of calibration) will require a special speedometer machine and a electronic magnet gun that is used to "zap" the horseshoe inside of the speedometer to make the miles per hour read correctly. With the electronic magnet gun more magnetism is added to the horseshoe or it is demagnetized some depending upon if miles per hour drum is running fast or slow.

the pictured speedometer has a fixed magnet, a few years newer speedometers used a magnet spun by the cable to replace the horse shoe magnet. the number drum was the partially surrounded by a ferrous hood to replace the spinning star like part to provide a magnetic path.


The small permanent magnet affixed to the rotating cable interacts with the aluminum number cup. As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which themselves produce another magnetic field. The effect is that the magnet drags the cup, and thus the speedometer drum spins in the direction of its rotation with no mechanical connection between them

aluminum is not attracted to a magnet but is influenced by a magnetic field

I was told by a speedo repair guy that 100 rpm on the cable should read 60 MPH on the dial.

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OK Dads
It is not easy to understand aluminium is influenced by a magnetic field.But it most be that way.

You can easy adjust the speedometer yourself.

Dismount the speedcable back of the dashboard.
Take the speedometer out of the house.Install the cable on the speedometer. Take your GPS with you in the car and you have the car speed quite accurate.
Have a friend to drive, and adjust the spring on the aluminium drum to match the GPS speed

some French dude in the mid 1800s discovered it. I'd like to know what he was doing and why when he discovered it. and who decided to adapt the principle in a speedometer.

These old drum type speedometers work on 1,000 rpm/mile.This was quoted to me by the instrument repairer who fixed my '28 truck speedometer,and my '29 Olds speedometer.

Just a small clarification on my previous post.I should have posted that for every 1,000 revolutions of the speedometer cable,the odometer/trip meter should read 1 mile travelled.

These speedometers work on virtually the same principle as the
electriciy meters in your meter box.The eddy currents induced into the aluminium spinning disc make it rotate(the speed at which it rotates depends on the power consumption of your household appliances at the time),and through a set of gears,turn the power used dials,which your then friendly? meter reader takes note of.