Purists look away, this is a restomod. I’ve been struggling with a bouncy speedo needle for some years now. Managed to improve it a bit over time but never managed to fully get rid of it. During that time I’ve changed the speedo cable, cleaned up the speedo and re-sleeved the angle drive and used a bracket to hold it down into the gearbox, greased the whole system etc etc. Nothing really worked that well and the bounce still resurfaced. The source of the bounce is either the angle drive (NLA) the pin gear that connects to the angle drive or the drive gear that sits on the end of the main shaft in the back of the box. Possibly a combination of all three. So the options were to pull the box find and change the gears (expensive option) or look elsewhere to bypass the box. I just couldn’t stand the bounce any longer, the one instrument we always look at. It was time to deal with it.
So in comes the Dakota digital electronic to mechanical cable drive (ECD-200BT). For a signal source I created 6 simple small brackets that attach to the gearbox side of the driveshaft using the drive shaft screws. Each of these brackets has a small magnet glued atop. The sensor is attached atop via a bracket to the frame. The inboard brakes obviously don’t make things easy here. Using a powered 3 wire hall type sensor allows for a relatively large gap (6-10mm) to the magnets. This is needed since the engine/gearbox combo sits on rubber mounts and might move relative to the frame.
So it’s 6 pulses per wheel rotation, the ECD needs a ppm (pulses per mile) baseline and that was approx 4720 ppm with my current tire size. Setup is done via Bluetooth and an app. Dead easy. Installation of the unit was the usual combo of lotus yoga and scratches since it involved digging into the binnacle and laying new cables and relays etc but proved well worth it. A test drive was needed to verify and slight adjust the ppm (using a GPS speed app and the DD app again). My wife was doing the calibration while I was driving 🙂. So now I have accurate speed and a rock steady needle. Love it, Real happy with that. This solution also allows for flexibility and recalibration to different tire sizes.
A technically better solution would be to use a tone ring as a reluctor and a passive 2 wire sensor but that would mean that the gap between sensor and reluctor is really small (<1mm) so it will not be possible to hang the sensor off the frame. I’m also thinking of using the brake disk itself as a reluctor and machine shallow grooves into the circumference and then mount the passive sensor off some of the screws that hold the gearbox to the engine. It would require a custom bracket but seems possible. Might go for that at some stage. That would be speedo 3.0
the ECD is a really well build unit, cost is 330 USD, the Ford clip-on cable works well with a few simple modifications to fit it to the smiths speedo. Hall sensor, brackets and magnets are a few quid only. should I ever want to go back to the original setup it’ll only take the effort to reconnect the old cable on both ends, so fully reversible.