This is my first thread. I have been lurking for a couple weeks so I guess another person got pulled in. It’s great fun and I think I am getting the feel for the general personality and intelligence of the board. Good folks.
I’m a good backyard mechanic and do most of my own repairs. I will even do my own welding and such. I can usually puzzle this stuff out.
So. Everyone that has a car has one, but I doubt many people know how it works. The analog speedometer. Basically a spinning cable to a dial.
I don’t think that it could use friction against a spring because something is going to where out.
The only other thing I can think of is a small hydraulic pump or motor pushing against the spring/needle.
An interesting dissection of the word speedometer yields speedo-meter. Not usful, just fun to pronounce it that way.
Anyway here’s a clue as to what’s going on in there. In my aunt’s car the last 3/4 of the little stick in the speedometer had broken off leavin gjust a nub. And now, rather then pointing at her speed it spun around. And the faster she drove the faster it spun.
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No, that doesn’t make much sense there are all!!! Why would it do that?
Hmm, ok, I think it measures the number of rotations of one of the wheels, or rather front axle. And then divides that over time. You can do that with a simple counting machine and not electronics, right?
The spinning cable has a magnet on the end, which rotates inside another magnet called a “drag cup”, which is in turn attached to the needle. The faster the cable spins, the harder it pulls on the drag cup and the farther the needle goes. The needle pushes against a spring, which returns it to zero when the cable is not spinning.
Since there is no direct mechanical contact between the 2 magnets, there is nothing to wear out. The little wires inside the cable can break, since they are constantly being flexed, and at some point the cable won’t work anymore.
On cars with electronic dashboards, nothing moves at all - a capacitive sensor on the transmission counts the rate at which a tooth on some rotating part passes by, and converts that into a speed signal.
ElvisL1ves is right; to elaborate, it’s usually a spinning magnet and a non-ferrous disc; the spinning magnet induces a magnetic field in the disc, which in turn tends to oppose(repel) the force that caused it(Lenz’s law), this repulsion causes the disc to rotate against the srping, raising the needle.
Nearly all automotive speedometer cables are gear driven by the transmission. Some older cars did use a front wheel pickup and some newer cars use an electronic trigger in the transmission, but 99% of the cars on the road use the tranny/cable setup.