Correct speedo for 1961 24dc ?

[565545]

In top gear my speedo shows only approx 35mph. I'm clearly going much faster. I have had the speedo checked, and it functions well. I have replaced the drive cable ( it is driven from under the crankcase/gearbox). I reckon that perhaps the driving cogs at the engine end of the cable could be worn? Does anybody know if I have the correct speedo , the numbers on the dial are... Smiths SN 3170/09 1632 and speed range 10mph to 80 mph. Any help much appreciated.

[Brian Thompson]

Correct Speedo for 1961 DC.

From what you describe it sounds as though you could be taking the drive for your speedo from the tachometer drive, which is mounted on the engine....This could explain why the readings you are getting are a bit off! As I understand, the drive for the speedo is usually taken from the rear wheel via a small gearbox mounted onto the rear brake plate and driven from a gear inside the hub. This is the method used on my TCS trials bike. For further reference have a look at the post on 'Speedo Drives' in 'Parts Sourcing', which includes contact details for a current supplier of new pattern replacements.

As far as further info on your Smiths speedo, you don't say on your profile if you are a GRA member or not. If you can confirm this I can supply full specs on all Smiths instuments and drives as originally fitted to Greeves (including Invacar) from 1954 to 1966, complete with drawings and illustrations.

Brian.

[John Wakefield]

Speedo drive

The drive outlet is NOT a tacho drive but speedo drive. However this is effected by final gear ratio of gearbox final drive & rear wheels sprockets, Greeves always used a speedo driven from rear hub at 3.14 to 1 ratio. You can check what ratio you have by disconnecting the cable at speedo, fix a cardboard pointer to cable & see how many revs the drive makes to one rev of back wheel. (Should be just over 3 revs) Also the speedo gearing reference number should I think be 1600 yours is 1632, I think this would make it read slow as the number relates to 1600 or 1632 revs at a certain speed.

[565545]

24dc speedo

The bike came with the 'speedo' connected to the drive underneath the engine. I did not know that villiers engines had a tacho drive. I have looked at my rear wheel hub and there appears to be a blanking piece where the speedo drive should be - I will investigate further. I am indeed a GRA member. Regards Stephen

[Brian Thompson]

24DC Speedo.

Thanks for that Stephen. I've had a look at the Smiths info I have covering Greeves fitments for the 1962 model year, and the number you gave tallies as being the correct instrument for your bike. As long as it has not been recalibrated to another spec since then, all should be well at that end (famous last words!)

As you say though, it does sound as if it's the rear hub drive gearbox that you need to find and install, along with the correct cable.

If you'd like to send me a PM (Private Message) with your email address I'd be happy to forward a copy of all the relevant Smiths specs to you. Once you have them you'll at least know what you're looking for, and if you can't find an original item perhaps the contact given in the 'Parts Sourcing' thread on speedo drives will be able to help.

One final thought; there might be a bit of further info available in the factory parts book for your bike, particularly with regard to the hub gear (that drives the gearbox) and number of teeth. Might be worth a look if you have one, or one of the members will know I'm sure. I'm afraid I can't help there as I don't have one myself, but if you contact Tim Griffin through the club (details in 'Leading Link') he might be able to provide you with a copy at modest cost.

Brian.

[565545]

24dc speedo

Brian, thanks for your help. I have looked in my norton villiers maintenance manual and there is indeed a speedometer drive'assembly'. Teeth on the little gear wheels vary in number depending on the engine type. I might have the wrong size or it may be worn; I'll have to investigate further. Regards Stephen

[Brian Thompson]

24DC Speedo.

My pleasure Stephen, and we all learn a bit more as we go along don't we! I'm not too hot on roadsters I must admit, as my interest is more with the trials bikes (I have a 1959/60 24TCS), but John certainly knows his stuff.

Let me know if you want that Smiths info somewhere down the line as it's all ready to go should you need it.

Best of luck with it, and let us know how you get on!

[John Wakefield]

The point that seems to have been missed here is that if the final drive sprockets are changed to different sizes then it effects the speedo calibration, whereas the drive ratio from the rear hub remains the same no matter what sprockets are used.
I would not think that there are any alternative ratios for the gearbox speedo drive as the gear in this has to mate with one of the gearbox cogs.

[ian bryant]

Hi
I realise your speedo problem was some time ago but did you ever get to the bottom of it as I have the same problem on my 1961 24DC
Ian

[John Wakefield]

Just to bring this subject up again. I have recently read an article on Smiths Speedo's from Classic Motorcycle March 1994. (thanks to Pete Smith for heads up on that)
It gives a formula for working out the correct speedo gearing.
With regards to '565545's' speedo Smiths SN 3170/09 1632.
SN3170/09 is the model code & 1632 is the gearing number which refers to turns of speedo cable per mile. Its this latter number thats important.
To calculate the correct gearing number there is a formula to calculate turns per mile (TPM) for a given bike.

TPM = 1680 x N divided by R

N = number of turns of speedo cable to 6 turns of wheel & R = Radius of wheel/tyre in inches.

After calculation you should get a number in the 1600 series. It needs to be within 35.

I recently did a check on my EC, N = 12.5 & R = 12.25 inches. That worked out at 1714. My speedo (like most roadster Greeves) is 1600 so it is obvious why my speedo was reading approx 10mph fast.
According to article ratios on the speedo drive in rear hup are either 1 1/4 or 2 turns per wheel. Mine works out at fractionally over 2 turns per one rev of rear wheel. I dont think Greeves put much effort into sourcing the correct ratio speedo & just bought a batch of what ever was available from Smiths at the time.