Author Topic: 63 A10 SR engine sprocket  (Read 1744 times)

Offline worntorn

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63 A10 SR engine sprocket
« on: 18.08. 2015 04:35 »
Looking in the  primary inspection opening, I have  counted the teeth on the engine sprocket and it is standard at 21 teeth. All other sprockets are standard as well. After some reading here and riding the bike a couple of hundred miles or so, it seems that a 22 tooth engine sprocket would be worth a try on there.
My question is- - will this be a two bump or a four bump sprocket? I would like to leave the primary cover on for now, don't have a spare gasket should I ruin that pulling the cover off just for a look.
Thanks
Glen

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Re: 63 A10 SR engine sprocket
« Reply #1 on: 18.08. 2015 07:12 »
All swinging arm sprockets are of the 'two bump' type and sprockets with 22 and 23 teeth are available new.
Priory Magnetos Ltd - A10 spares, magneto and dynamo refurbs. www.priorymagnetos.co.uk

Offline chaterlea25

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Re: 63 A10 SR engine sprocket
« Reply #2 on: 18.08. 2015 13:03 »
Hi Glen,
I am running a 23 tooth engine sprocket on my SR, I tried 22 and 24 tooth as well
It would pull the 24 no problem solo but it was too high when two up (on hilly going)
I settled on 23 which was fine even on Swiss alpine climbs *smile*
Standard 70 link primary chain will fit with 23 tooth
My SR engine is very well "sorted" though >:D >:D >:D

Regards
John
1961 Super Rocket
1963 RGS (ongoing)

Offline worntorn

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Re: 63 A10 SR engine sprocket
« Reply #3 on: 18.08. 2015 15:10 »
Thanks for the replies, I'll order a two bump 23 tooth today and report back on how that works out. The engine feels strong now that the Carb is sorted. On leakdown it showed just 3% loss on one side, 4% on the other, test done at 100 PS I.

Glen

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Re: 63 A10 SR engine sprocket
« Reply #4 on: 18.08. 2015 22:47 »
All swinging arm sprockets are of the 'two bump' type and sprockets with 22 and 23 teeth are available new.

Sorry Andrew, but I think that you will find that the first swinging arm A & B models had a four lobe engine sprocket.
1954 only, I think.
                   

Offline chaterlea25

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Re: 63 A10 SR engine sprocket
« Reply #5 on: 18.08. 2015 23:33 »
Hi Again Glen,
I forgot to mention that the 23 tooth engine sprocket I bought had quite a pronounced shoulder on the inner face, which would or could alter primary chain alignment  *problem*
In my case because I was modifying the clutch at the same time it all worked out ok, but it would not have worked with the previous clutch adaptor

Regards
John
1961 Super Rocket
1963 RGS (ongoing)

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Re: 63 A10 SR engine sprocket
« Reply #6 on: 18.08. 2015 23:38 »
All swinging arm sprockets are of the 'two bump' type and sprockets with 22 and 23 teeth are available new.

Sorry Andrew, but I think that you will find that the first swinging arm A & B models had a four lobe engine sprocket.
1954 only, I think.
                 

I think you're correct, but you come across them so rarely I'd forgotten about them. I've got a drawer full of the two lobe type and just a couple of the four lobe type.
Priory Magnetos Ltd - A10 spares, magneto and dynamo refurbs. www.priorymagnetos.co.uk

Offline Viking

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Re: 63 A10 SR engine sprocket
« Reply #7 on: 25.08. 2015 12:20 »
I changed from 21T to 23T om my 1958 A10 SR with RGS tuned engine.

It made a very pleasing change.
It was too low geared with the 21T sprocket.

The engine changed from irritating vibration around 50-60 mph (80-95Km/h) to a cruising type behavior, and settle down to a nice and torque manner. It goes well on the motorway as well. 

3rd gear is then usable in town, up to 50-60 Km/h

Now the set is: 23T engine, 43T clutch, 21T gearbox, 42T drum. 19” wheel

Giving a total 3.56 over all gear ratio. Calculated top speed 103 MPH
 
Using a 21T engine sprocket gives (with no other changes):
A total 3.90 over all gear ratio. Calculated top speed 94 MPH.

I can warmly recommend the change to a 23T sprocket.

Offline worntorn

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Re: 63 A10 SR engine sprocket
« Reply #8 on: 25.08. 2015 21:52 »
Viking, that is good to hear.
 I take it you have also changed your gearbox output sprocket as well, since the stock item is a 19 tooth?

With the maximum rpm at 6500 according to BSA, I  get a theoretical maximum speed of 146 MPH with 3.56 gearing. Of course the engine doesn't have power to pull 6500 with high gearing, but I'm wondering how you calculated a theoretical maximum of 103 MPH? What RPM did you use?

According to tests of the day, the Late Super Rocket (same as RGS Tune) did 109 MPH with the standard 4.53 gearing and 116 MPH with the same gearing but mufflers removed.
Not that I intend to try for that, I have already pushed the old girl hard enough showing 85 MPH on my incorrect speedo . This  corrected speed at 85 speedo was 96 MPH by GPS. With a bit more road, I think it would have gone even higher.

Offline Viking

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Re: 63 A10 SR engine sprocket
« Reply #9 on: 26.08. 2015 09:46 »
Gearing on MY 58 SR.
I was told by SRM and my local BSA specialist, that A10SR use 19T gearbox sprocket for Side Car use and 21T for standard, but for using the Sidecar the recommendation was to keep the gearbox at standard ie 21T and use a smaller engine sprocket ie 16-18T ( where 21T sprocket is the standard for solo ).

So MY gearing is. ( now running without a side car ) 23T engine, 43 clutch = 1,870 ratio
Gearbox 21, rear 42 = 2,00 ratio
Over all ration in total = 1,870 x 2,00 = 3,74
Using 19” rear wheel/rim giving a theoretical max speed of 98MPH @ 6500 rpm.
(Giving it a 19T gearbox sprocket, result in a max speed of 89MHP)
“Test of the day”  was carried out on “special” prepared motorcycles.
(ie the tuning department might have selected very good units, with extra high compression etc.)   

Calculating the matter, I use the spreadsheet developed by Steve Nott www.stewenott.com

So basically I recommend the 23T engine sprocket. To adapt the gearing to modern roads..

Offline duTch

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Re: 63 A10 SR engine sprocket
« Reply #10 on: 26.08. 2015 11:52 »
 
Quote
...too much stuff... *conf*...

 All good and well, but are we to assume that the SR has a RRT2 box?  If so, when I had my RR in the late 70's I was told (I don't always believe what I'm 'told'), but on a few occasions that the RR's went faster in third than they did in fourth.... *????*
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easy
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Re: 63 A10 SR engine sprocket
« Reply #11 on: 26.08. 2015 12:58 »
That may be the case duTch. Tall gearing and a stiff head wind or a bit of a hill. Not a big gap from 3rd to 4th.
Cheers
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Offline worntorn

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Re: 63 A10 SR engine sprocket
« Reply #12 on: 26.08. 2015 15:57 »
Gearing on MY 58 SR.
I was told by SRM and my local BSA specialist, that A10SR use 19T gearbox sprocket for Side Car use and 21T for standard, but for using the Sidecar the recommendation was to keep the gearbox at standard ie 21T and use a smaller engine sprocket ie 16-18T ( where 21T sprocket is the standard for solo ).

So MY gearing is. ( now running without a side car ) 23T engine, 43 clutch = 1,870 ratio
Gearbox 21, rear 42 = 2,00 ratio
Over all ration in total = 1,870 x 2,00 = 3,74
Using 19” rear wheel/rim giving a theoretical max speed of 98MPH @ 6500 rpm.
(Giving it a 19T gearbox sprocket, result in a max speed of 89MHP)
“Test of the day”  was carried out on “special” prepared motorcycles.
(ie the tuning department might have selected very good units, with extra high compression etc

Calculating the matter, I use the spreadsheet developed by Steve Nott www.stewenott.com

So basically I recommend the 23T engine sprocket. To adapt the gearing to modern roads..


Someone else mentioned that SRM says some strange things. From the BSA Instruction Manual for the A series, 1958, the factory gearing for the A10SR is listed as 19 tooth gearbox for both solo and sidecar. Clutch is 43 and rear sprocket is 42 same as yours and those stay the same for both solo and sidecar.
The change for sidecar is done at the engine with the solo 21 tooth engine sprocket changing to an 18 tooth for sidecar (now that would be low geared!)

For speed at 6500 with your 3.56 gearing, I get:

6500 rpm divided by 3.56 = 1826 rpm. So at 6500 engine rpm, with 3.56 to one overall gearing, in top gear the rear wheel would in theory be turning 1826 rpm. The distance around the rear tire is 84.5" according to Avon and Dunlop.
So 1826 revolutions x 84.5 "=154,297" travelled per minute. Dividing by 12 to get feet per minute gives 12,858 feet travelled per minute. 12,858 feet per minute is 771485 feet per hour. Dividing by 5280 to get MPH , the speed at 6500 rpm with a 19" wheel and 3.56 gearing is 146.11 MPH!
That is a tall geared A10, obviously it handles it.
  On edit I used the ratio from you first post, 3.56. Your second post shows 3.74 , seems right. Using 3.74 instead of 3.56 I get a theoretical top speed at 6500 of 139 MPH.

Glen

Offline duTch

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Re: 63 A10 SR engine sprocket
« Reply #13 on: 26.08. 2015 19:08 »
 ...and if you could fold a fag paper in half 50 times, how thick would it be ?
 ( hypothetically, but it's physically impossible to do,)

 
Quote
....84.5" according to Avon and Dunlop.

  Have you measured yours...? I think one of my 18" rears is 2130mm- but that could be a front 21 will check tomorrow
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easy
Australia

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Re: 63 A10 SR engine sprocket
« Reply #14 on: 26.08. 2015 20:58 »
...and if you could fold a fag paper in half 50 times, how thick would it be ?
 ( hypothetically, but it's physically impossible to do,)

You'd be somewhere beyond the moon... but not far *roll*
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