Hey guys,

Ages ago I dropped my GS and cracked the plastic round the gauges. (Like everyone else  ) . Its just got duct tape round it at the moment to keep the weather out but I was going to try and build up some custom digital gauges at some point over the summer when I've got some time. I can cope with the signals coming from the little switches all over the place but anyone know what's inside the tacho and speedo cables? Is there something rotating inside or pulling inside or what?

thanks...

rotating, i dont know EXACTLY how it works, but i had my speedo open and there like an upside down metal bowl that the needle is attached to, and inside the bowl theres something that the cable is attached to which works (i think) off centrifugal force so when the inside cable spins faster and faster it grips the inside of the bowl more and more causing it to rotate with it and turn the needle to the appropriate speedo on the gauge face. theres also a spring pushing against the needle to 1. keep it from spinning the same speed as the cable and 2. to bring the needle back when decelerating.

anyone correct me if im wrong i didnt look super closly at it.

ok for the tach and the speedo, there is a magnet that spins inside of an aluminum bowl, the eddy currents from the magnet react with the aluminum bowl, and causes it to turn. there is a small spring that counteracts the turning to make it a smooth movement  

I never gave it much though, but now that you ask, I'm really curious myself.

Quote from: jeremy_nash on January 05, 2010, 09:51:53 AM
ok for the tach and the speedo, there is a magnet that spins inside of an aluminum bowl, the eddy currents from the magnet react with the aluminum bowl, and causes it to turn. there is a small spring that counteracts the turning to make it a smooth movement  

WOOOO man i was close

Quote from: crazyfish on January 05, 2010, 08:16:45 AM
anyone know what's inside the tacho and speedo cables? Is there something rotating inside or pulling inside or what?

A spinning cable is correct.

In days gone by when everything was spinning cables, at head-on collisions one thing I believe accident investigators would look at is the speedo. The spinning cable going from spinning to dead-stop would snap. Other elements in the system would fail due to the sudden change in momentum forces. And the idea was, the speedo would indicate the vehicle's speed at the time of the head-on. Of course, it's by no ways fool proof as the needle could also jump to almost anywhere on the dial and indicate any old speed.

The older bicycle speedos worked the same way. A device that looks like a small electric generator from a toy was lowered onto the front wheel and it had a cable attached to a dial. These days, such speedo rely on a small item placed on the wheel which get noted each time it goes by the reader. You tell the Computer the circumference of your wheel and it calculates distance/speed based on how many times the wheel turns within the given time frame.

The older ways with mechanical means are really quite ingenious.

Michael

I believe the investigator's interest in the speedometer after the crash was to identify the spot at which the needle would slap the gauge face, leaving a mark as a witness to the speed at impact.  Google "needle slap"

Thank you for clarifying.

Michael