Speedo- how does it work- faults?

I’m still trying to track down my erratic, or non functioning speedo faults.
Am I correct in assuming that if the speedo shaft is turning the disc inside the dome cover linked to the needle, then the speed needle should turn?
I can’t work out why the disk is turning and the magnetic field is not enabling the needle to turn- needle is free, and will turn if I manually rotate the dome assembly.
I`ll pull the current one out of the bike and see what that one does. On my last ride the gear drive stripped the plastic gear.

The speedo arrangement is actually very simple.

The wheel driven gearbox drives a flex drive cable, which rotates a magnet called a drag cup. This drag cup pulls another cup round with it that’s called the slave cup (if such a term is permissible these days).

This slave cup is pulling against a controlling spring.

The faster the wheel rotates, the faster the drag cup rotates, and thus the further round the dial the needle goes.

On the FF, there is a side shaft driven by a worm gear via a crown gear that turns the mileometer and trip meter.

There are only four possible sources of fault in the system. The wheel gearbox, the flex drive, the speedo itself and the mileometer drive.

I can send you an Aus spec yellow dial speedo and KM mileometer (if a Km mileometer isn’t an oxymoron) that I know are serviceable if you like?

Gooday Chris, thanks for your advice. I think I’ve had a breakthrough, and possibly fixed the problem. I had two different speedos, one was making a growling noise and operating sporadically, another one appeared to be OK.
I examined the noisy one closely and found what was making the noise- it was the slave cup, which had slipped down the shaft. When it was rotated at above 15kmh it was forced down onto the housing. I drove it with a drill to about 3000 rpm, equiv to about 90 kmh and it was very noisy. I then assembled the better speedo with cable and drive unit and spun it up with the drill, and it was quiet, steady needle, with no issues.
I’ve now reassembled the good unit with new cable, cleaned and greased drive unit onto the bike. I’ll have to wait a few days until I can test ride the bike as I’m on 24 hour contact Duty shift till Friday morning, but hoping that I’ve finally fixed the problem.
If that hasn`t fixed it I’ll certainly discuss your option of Km parts, thanks!

Hi, Greg,

That sounds promising. Please keep me posted.


Hi Chris, scrolling through posts, realised I hadn’t updated you. Speedo now fixed and reliable.
I believe the original problem was speedo drive unit on axle which worked ok when cold, but once hot would drive ok and then catch, causing the wild fluctuation.
Also learnt that the speedo itself is quite robust, and also that the amount of needle preload, of the tiny return spring, can cause a 20kmh variation in indicated speed for the same road speed and revs.

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