Rear hub speedo drive

[IanCordes]

I will have to check what drive is fitted on my roadster. I have a feeling it indicates a little fast, but my recollection was that they all did back in the day. That is a massive difference though, 1.5:1 and 2.4:1. Other driven pinions were available as well as 14T, so maybe they were different on the roadsters, perhaps?

Ian

[IanCordes]

I have just checked the wheel/tyre diameter on my roadster, which, as you rightly said John, is 18" x 3.25", and a trials wheel shod with 18" x 4.00" tyre. The roadster diameter is 24 3/4" and the trials 26 3/4" . Over to you John to do the maths!

Ian

PS I hope you don't mind us carrying this on in your thread Stuart? I think your question is pretty-well answered, and this is relevant to the thread heading.

[John Wakefield]

Thanks, yes obvious that the trials bike will have less turns per mile than the trials bike which would require a differently calibrated speedo, the gearing number
usually around 1600) relates to turns per mile, Smith did do speedos with different calibrations but I dont expect Greeves bothered too much with matching the correct speedo to the drive ration. They would probably have bought one type for all thus getting a better price on quantity.

Attached is an article from The Classic Motorcycle March 1994.showing how to calculate speedo drive ratio

[IanCordes]

Thanks for that John. Remind me to study that on a cold and even rainier evening; just 'Not tonight, Josephine'.....

[Stuart Baskill]

I have just checked the wheel/tyre diameter on my roadster, which, as you rightly said John, is 18" x 3.25", and a trials wheel shod with 18" x 4.00" tyre. The roadster diameter is 24 3/4" and the trials 26 3/4" . Over to you John to do the maths!

Ian

PS I hope you don't mind us carrying this on in your thread Stuart? I think your question is pretty-well answered, and this is relevant to the thread heading.

I don't mind at all.The more information that appears the better for us all. I was surprised how easily my speedo drive came apart.

[Stuart Baskill]

Thanks for that John. Remind me to study that on a cold and even rainier evening; just 'Not tonight, Josephine'.....

As a matter of interest the speedo drive that I have been playing with is off my 1955 sold,1956 model 20T that I have owned for almost 50 years so it is possibly original.It has 44 teeth in the wheel and 14 teeth on the speedo drive. The threads on the speedo drive are 1/2" and 7/16" diameter,thread type unknown at present. Rotation is the same,in or out. The split pins fitted are 3/32" and 1/16" diameter..

[John Wakefield]

What number teeth are on your bevel gears in the unit Stuart?

[IanCordes]

Good news that you have tackled the job Stuart. The thread to speedo cable would be1/2" x 26tpi BSCy, The other one should be 23/32" x 26 tpi BSCy, a bizarre choice, which makes repairing or cleaning up threads a bit tricky, when it could have been made at 3/4" BSCy, for which taps & dies are more readily available. I was thinking that my spare body with the damaged threads could be taken down to 11/16" BSCy to make it usable, although that would entail making a new nut.

[Stuart Baskill]

What number teeth are on your bevel gears in the unit Stuart?

The shaft driven from the hub has 8 teeth and the output to the speedo cable has 12 teeth. Ian is correct with 23/32" thread on gearbox,I must go to Specsavers !!!. Also the 14 teeth gear wasn't fitted on my gearbox originaly as it was missing but I think it is the correct size.If the speedo shows 95MPH I may be wrong !!!! AGAIN.

[John Wakefield]

Thanks Stuart that makes the gearbox ratio the same as Ian's, looks like this is the standard ratio (1.5 : 1) for Greeves. If the large hub gear is 44 then the small one must be 14 to mesh correctly. The centres are fixed with no adjustment.

[John Wakefield]

The critical factor here is the number of turns of speedo cable per revolutions of rear wheel.
On my East Coaster the cable turns 2.1 turns to one rev of rear wheel. using the formula in my previous post the speedo gearing number should be 1714 but the speedo fitted to the bike (the original one fitted by Greeves) is 1600, so its running fast, 10 mph across the range in fact. It would appear that Greeves had not bothered to buy in the correctly calibrated speedos for the roadsters and possibly used the same ones that they fitted to the Anglian which had a speedo drive driven from gearbox. Thus they could get a better price on a larger batch from Smiths and adopted the attitude that one type fits all, the bonus being that by reading fast the lads of the day could see 80 mph on the clock (when actual speed was only 70). This of course would sell bikes, and being fast would ensure that the rider was well within the speed limits.

[ian640]

This is off the original subject which Stuart posted about, but if it helps, the number which is stamped on the end cap will indicate the specification; ratio, clockwise or anti-clockwise etc. All mine which appear suitable for Greeves, are stamped BG 2504/01. Whether Greeves did fit different specification drive units, depending on 18" or 19" wheels, trials or roadsters, I don't know. I doubt they changed the drive unit. The ratios could be more easily varied by changing the number of teeth on the drive ring in the hub (44 in a TES for example), and the driven pinion, 14 teeth on a TES. Checking the parts book for your model will tell you the spec.

Ian

Any idea if BG 2504/01 would be the correct drive for my TFS? I've come across a drive for sale and wanted to check if it's the correct one.

I don't have a parts book for the machine.

If the drive I've found is correct then all I need is a speedo. Anyone any ideas which one of those is the correct one, or would work? There appear to be several D-shaped one on eBay at the moment.

Refitting one with a binnacle might not work as the bike has telescopic forks and a handlebar cross brace. I just want a working speedo from the period - not too bothered about originality.

Thank you.

[tony]

Ian the Griffon Enduros have Ceriani forks with braced bars and use the Smiths SSM5002/00A grey faced speedometer with a bracket attached to the handlebar clamps. The speedometer is housed in a round tube which the speedometer slides into.











There is a drawing for the speedometer housing and the bracket can be purchased on line, though as it is in USA probably cheaper to make one or get one made. The larger Smiths are more readily available and of course they do reproduction ones which are at least 1/2 the cost of an original and probably more accurate.

[John Wakefield]

The most important number is the gearing number usually in the 1600 series. You will need to check the turns of the speedo cable in relation to the turns of back wheel and then do the maths using the equation. The number you arrive at is the gearing number that the speedo needs to have.

[tony]

All the American spec bikes I have seen have the SSM5002/00A speedometer - TFS Trail, Ranger, Anglian Enduro, Griffon Enduro (250/380) 18 inch rear with the usual Smiths set up, except the Griffon which uses a CEV drive.