Increasing the gearbox sprocket size on A50

[Andy Edwards]

Hello.  I really would like to finally get my 1962 A50 sorted so that I am happy with it.  It is in standard trim with the 17 tooth gearbox sprocket and the bike just feels uncomfortably busy at anything over 50 mph.  I live in flat North Wiltshire and will only ever ride solo.  I am quite happy to cruise at 50, particularly as it only has the 7" front brake, but I would like to reduce engine RPM at that speed.  Does anyone have experience of using a larger gearbox sprocket?  I see that there are 18 and 19 tooth sprockets available as well as the standard 20 tooth sprocket for the A65. I guess that if I did increase the sprocket size I would also need a longer chain.

[Greenfield]

Hi Andy
Have a look at the tech feature in last January's 'Star' where I fitted a 19T sprocket to my '68 A50. I only changed by one tooth and didn't have to replace the chain, just adjust it. Its worth checking what size the rear sprocket is. The overall ratio for a '62 A50 would have been 5.118 with a 17 gearbox sprocket and 42 rear sprocket.

[DAVE BRADY]

Hi,

I have gone up one tooth on 2 A65s and an A10 and it does make the bikes a bit nicer rev. wise at normal speeds.
Two teeth up makes things a bit lumpy at low revs and needs a bit more clutch slip to pull away especially on hills.
A standard '62 A50 with 42/17 sprockets gives a ratio of 2.47.  A '68 I believe has 47/18 sprockets giving a ratio of 2.6.
Up one tooth on each changes this to 2.333 on a 42/18 '62 and 2.47 on the '68.
Interesting that the extra tooth on the '68 gives the same ratio as the standard '62.
Using an 18 tooth sprocket on a '62 gives 2.33.  This is close to standard '68 A65 at 2.35 with an extra tooth giving 2.238.

Dave.

[JulianS]

Remember that the speedo, being driven from the layshaft on the pre 1966 A50/A65, a change in the gearbox sprocket size will reduce the accuracy of the speedo.

[Andy Edwards]

Thanks for the replies. I'll have a look at the article in the "Star" and ponder the different ratios.  I hadn't considered that changing the sprocket size would affect the speedo reading.

[chaz]

only because the speedo drive is on the back wheel, , if it was front wheel or engine gearbox like modern bikes it wouldnt be different.

[idie]

I don't see how changing the the gearbox sprocket can change the speedo, the rear wheel will still do the same revolutions per mile. If you changed the wheel diameter that would alter it. On the pre unit twins and singles it would as a larger sprocket would slow down the layshaft which drives the speedo. Isn't the rear hub on the early A65/50s the same as the last of the pre units. If so wouldn't a set of brake shoes from an A10 fit.

[JulianS]

The 1962-65 A50 and A65 have the speedo driven from the layshaft, not the rear wheel, like the pre units, that is why changing the gearbox sprocket changes the speedo accuracy.

[idie]

Sorry not reading properly thought it was a half width hub. As most speedos read fast you may find that it will be accurate. Ride past one of the advisory speed checkers you get on the side of the road to see how accurate the speedo is.

[Andy Edwards]

Thanks for the input, I'll try an 18 tooth sprocket. There are a few other jobs to complete yet including an engine rebuild so it will be a few weeks yet before I can do a road test.

[Mike Farmer]

 

I think!!!!!

If you change the sprocket size, as stated the speedo will be adrift. But the maths are easy. simple?percentages.

Example If you change a 20 for a 21 you have made an overall 5% increase. Your speedo will now under read by 5%. So when you are reading 50mph you are actually doing 52.5 mph.

Change a 20 for a 19 you will over read by the same factor.

So the speedo difference aint gonna be a bustin lot.

Mike

[idie]

If the speedo is driven from the layshaft in the gearbox.  Increasing the number of teeth on the gearbox sprocket will slow down the lay shaft and the speedo will read slower. This is what happened on my Shooting Star. I had the speedo recalibrated to correct it.

[Mike Farmer]

 

You are abso right.

Mike