Ariel Owners Motorcycle Club

The gears inside the angle drive?
Or just the gears inside the hub....

Sorry - just the gears inside the hub.

Bringing this topic back to life as others were looking for the correct speedo gearbox for their WNG's. These gearboxes all seem to look the same but come with different ratios and also a difference in the gears depending on whether they are fitted to the left or right hand side of the wheel. The output needs to rotate clockwise as a Smiths chronometric will only work correctly when the cable is turning in that direction.

I have been looking at my speedo gearbox which had the part number 25791 stamped into it, with a gear ratio of 8:14 (8 teeth on input pinion, 14 teeth on output pinion), to make sure the ratio was OK for a Smiths chronometric 1600 revolutions per mile speedometer (The WNG S433 and most other chronometrics work on this value). Looking at the WNG parts lists the correct one is part number 5242-38 with a reference number SM/25224, a diagram of which I found on the Internet (see below). It is a long bush right hand geared (this means that when fitted to the right side of the wheel the output turns clockwise). They are shown as coming in two ratios 8:12 and 5:12.

Calculating the required ratio to give 1600 revolutions per mile traveled by the front tyre (with the 43T/14T hub gears) results in a required gearbox ratio of 8:12 (it reduces the input turns by that ratio). So the input pinion has 8 teeth and the output pinion 12. So my 8:14 ratio is a little bit out and gives 1400 revs per mile which means the speedo would under read by about 12%.

These,or similar Smiths angle drives are also fitted as rev counter drives on most old English diesel engines,and a lot of the ones on ebay are from engines,despite what the sellers claim. The engine ones are generally 1.1 or 2.1 internal ratio...............from where comes the story of how Laurie Bond managed to con Joe Craig at Norton into believing his rotary valve engine was producing phenomenal horsepower.

I have two front wheel speedo gearboxes and one set of gears if someone is interested.

There is a guy in Australia who makes this design of speedo drive from scratch, to what ever specification required. I can find out his contact details if anyone is interested.
Cheers,
Bevan

On Burney I fitted a gearbox that later turned out to be one for a 16H.
It was NOS and I was happy.
But..a 16H type is meant for left hand mounting and as a consequense it turned the 'wrong' way for a W/NG.
So until I found/bought a correct one I took out the cable and had many happy miles oblivious from any exsisting speed limitations !
Few months ago I found out that the complete gear + cover plate has gone AWOL because it pushed itself out as a result of the different running direction.
Another lesson learned.

This is my next 'new' one. maybe I just keep it on my desk to gaze and pamper.

Great Info! Thanks for sharing!

rein.heerkens wrote:I have two front wheel speedo gearboxes and one set of gears if someone is interested.

PM sent

Resurrecting this thread again, I've been out for a 56 mile run (Google maps) and recorded only 49.6...

Watching the town speed warnings (the ones that flash your speed up) it appears the speedo under reads by about 12%.

Sounds like a gearing problem.