what is the easiest way to work out pulses per mile ? ie four mags on prop etc

I worked this one out a while back. Not tested this so it could be wrong, but here goes:

> C - Wheel circumference (cm)
> R - Diff ratio (R:1)
> n - no of sensor triggers per rotation of prop shaft.

1 mile = 160934.4 cm
so wheel rotations per mile = 160934.4 / C
so shaft rotations per mile = (160934.4 / C) * R
so sensor triggers per mile = ((160934.4 / C) * R) * n

HTH

Oh, and you can work out wheel circumference here:

http://www.miata.net/garage/tirecalc.html



[Edited on 13/2/07 by bilbo]

Yeh- I was going to say, best to use a rolling circumference calculator. Tyres deform a lot during the rolling process and putting in a simple tyre diameter * pi will give misleading results...

Purchased the cheapest bike speedo I could find, £5.99 halfolds, and attached one magnet to the chuck in my lathe the set the circumference for the actual wheel , 1.770m (185/60/13) on the bike speedo.

I then placed a 3" lump of tube in the jaws of the chuck to replicate the prop adaptor and surrounded with magnets placing the hall sensor to catch these ones.

When running the lathe with 9 magnets on the 'prop' I recorded 1 mile travelled @ 20mph on the bike speedo, this related to 3.64 miles @ 70mph on the blade clock.

As my diff ratio 3.65 this means the miles travelled will be within 1% of actual miles travelled so jobs a good un, no speedo healer required.

Point to note, the blade clocks so I presume all bike clocks need lots of pulses to register a decent speed so you need to be able to spin something quite fast, with 1 magnet I did not believe the balde clocks were working as no speed registered but left for a few minutes it did clock a few tenths of a mile.

Regards Mark

There's a calculator on the ETB website

http://www.etbinstruments.com/

Click: Digidash2 @ the bottom of the page
Click: Technical help
Click: Digidash downloads
Click: Digidash 2 analysis s/w
run digidash2_tools.exe

Click: Digidash tool box
Click: Calibration calculator

Fill in the details and voila the number of pulses