Jubilee speedo

Andy,

Go onto NOC Shop spares. Search "speedometer gearbox".

One is what you need. 2:1. Check the BG5330/247 2:1 matches.

Peter

Andy Sochanic advises that the lightweights originally had 19:10 ratio gearboxes to the BG5330/247 design.

I know that for the last 20 years the NOC spares scheme has been selling 2:1 ratio for Lightweights. Of course this doesn't make it right.

But if it is wrong, your 2:1 (or 20:10) gearbox will rotate 5% slower than the19:10 gearbox. So your speedometer will read 5% lower.

By law speedos have to read faster than actual speed. So perhaps the 2:1 reduction will make your speedo more accurate. Knocking your speedo reading down 1.5 mph at a true 30 mph, or down 3 mph at 60 mph.

Andy also advises 19:10 gearboxes are very hard to get, with the correct spindle hole.

With thanks to Andy

Peter

Just to muddy the waters:

As Peter notes above, " By law speedos have to read faster than actual speed".In my dotage, I had always thought the legal requirement was "accurate to +/- 10% @ 30mph", which doesnt tie in with that.

So I had a look at The Road Vehicles (Construction and Use) Regulations 1986, (https://www.legislation.gov.uk/uksi/1986/1078/regulation/35/made) which didn't directly specify the accuracy required (although it does require the ability to read in both MPH and KPH if registered on or after April 1st 1984, so not an issue for most Nortons) but it did refer also to an EU Directive (don't you just love'em!)- "a vehicle may comply with Community Directive 75/443 or with ECE Regulation 39." (https://www.legislation.gov.uk/eudr/1975/443) which contains the following gem:

“4.4. The speed indicated must never be less than the true speed. At the speeds specified for the test in 4.3.5 above and between these speeds, there shall be the following relationship between the speed indicated on the dial of the speedometer (V1) and the true speed (V2)"

A formula follows, which I dont seem to be able to reproduce in this comment, so look it up for yourself, if you're that interested!

So that clears everything up? No, me neither.....

Apologies, Peter - if I misled you.

A 2:1 speedo gearbox will rotate FASTER than a 19:10 gearbox, by approx 5%.

Dommi & Commando with 19" rear wheels use a 2/1 drive.

A Navi was fitted with an 18" wheel, which means it will turn more times in a mile than does a larger wheel, so the Speedo Drive was reduced to 19/10 to slow the speedo cable down.

Hence, I repeat, the 2/1 drive, if fitted to a Navi (or any 18" wheel)  will read approx 5% fast on the original speedo - if in good working order. The original speedo should show "1600" on the dial - indicating 1600 turns to a mile. Other ratios are available. Two examples shown:

PS - the blackface one is my Navi, and the White face one is on my Electra. On the Navi - the leading "1" is missing, as it now actually reads 102,765 miles.

(Its currently resting, as my clutch 'clicks' when I pull in the cable & locks. Embarrassing at T-Juctions or traffic lights.....)

Michael,

The equations basically means the speedo is allowed to display a speed that is up to your actual speed + 10% + 4KM/h

So a reading of 70MPH could actually be as low a 61MPH

It is not allowed to read slower than actual.

Tony