The BSA A7-A10 Forum

Technical => Clutch, Primary, Gearbox => Topic started by: worntorn on 18.08. 2015 04:35

Title: 63 A10 SR engine sprocket
Post by: worntorn 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
Title: Re: 63 A10 SR engine sprocket
Post by: beezermacc 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.
Title: Re: 63 A10 SR engine sprocket
Post by: chaterlea25 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
Title: Re: 63 A10 SR engine sprocket
Post by: worntorn 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
Title: Re: 63 A10 SR engine sprocket
Post by: trevinoz 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.
                   
Title: Re: 63 A10 SR engine sprocket
Post by: chaterlea25 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
Title: Re: 63 A10 SR engine sprocket
Post by: beezermacc 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.
Title: Re: 63 A10 SR engine sprocket
Post by: Viking 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.
Title: Re: 63 A10 SR engine sprocket
Post by: worntorn 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.
Title: Re: 63 A10 SR engine sprocket
Post by: Viking 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..
Title: Re: 63 A10 SR engine sprocket
Post by: duTch 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.... *????*
Title: Re: 63 A10 SR engine sprocket
Post by: muskrat 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
Title: Re: 63 A10 SR engine sprocket
Post by: worntorn 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
Title: Re: 63 A10 SR engine sprocket
Post by: duTch 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
Title: Re: 63 A10 SR engine sprocket
Post by: morris 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*
Title: Re: 63 A10 SR engine sprocket
Post by: worntorn on 26.08. 2015 22:35
Just sayin' that if the spreadsheet shows. 6500 rpm = 108 MPH for a 19" rear wheel , 3.50 tyre and 3.74 to one gearing, the spreadsheet is wonky by a large factor.
108 MPH with that high gearing is just a tad over 5,000 rpm. Unless my arithmetic is wrong, which does happen now and then.

Glen
Title: Re: 63 A10 SR engine sprocket
Post by: worntorn on 07.09. 2015 02:53
I changed the engine sprocket to the 23 tooth today and it makes things much nicer at highway speeds. The bike doesn't have quite as much urge thru the gears as with the 21 tooth, but still feels quite lively and it is very smooth running. It also stays in each gear a tad longer and to a higher speed, which I like.

The old seat, which looked OK but was wrong for the bike, has been replaced with an RK Leighton item. The old seat was the correct  for the earlier bikes, the Leighton one is a good 1" or so wider and is correct for this bike. It is very comfortable, well worth the £.
I also dropped the footpegs an inch so that the footpeg arms run approximately level.  This extra inch of leg room is nice. It's very comfortable ride now  with the cowhorn bars, soft contoured seat and lots of legroom to the pegs. I feel like I'm in a comfortable old TV room chair when I ride it.
Would it be possible to run a small TV from the E3L? I wouldn't necessarily have to use a clicker with it, though it would be nice.  *smiley4*

Glen
Title: Re: 63 A10 SR engine sprocket
Post by: worntorn on 10.09. 2015 06:31
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

Hi John

The 23 tooth sprocket I received also had a substantial shoulder. I was on the lookout for this (from reading your post) so it was no problem. I cut the shoulder back on the lathe and kept trying it until I had alignment.
Very hard material, as it should be!

Glen
Title: Re: 63 A10 SR engine sprocket
Post by: chaterlea25 on 10.09. 2015 21:09
HI Glen
Glad to have been of help,
I have seen several clutch chainwheels well chewed up because of misalignment  *sad2*

Like below !!!!

Regards
John
Title: Re: 63 A10 SR engine sprocket
Post by: East_Coast_BSA on 15.10. 2015 05:18
The stock/current gearing on my 59' SR is 21/43 on the primary and 19/42 on the Final drive.  I was thinking about bumping up the gearing.  The bike is very responsive for around town riding, but if I have to get on the highway (which I like to avoid) it gets "buzzy" around 50-55 MPH.  I filled the handlebars with silicone RTV and that did help, but I'd really like some more gear.  I see that Draganfly sells the 23t gear and the 70 pitch chain for about half of what SRM gets.  Is there a difference in quality between the two?  I've had good luck with both places.
Title: Re: 63 A10 SR engine sprocket
Post by: Viking on 15.10. 2015 08:16
I changed the engine sprocket to the 23 tooth today and it makes things much nicer at highway speeds. The bike doesn't have quite as much urge thru the gears as with the 21 tooth, but still feels quite lively and it is very smooth running. It also stays in each gear a tad longer and to a higher speed, which I like. 

I found exactly the same. The SR is under geared with a 21T engine sprocket.
3rd gear is newer used.

After changing to a 23T, 3rd gear is fine for town riding ie 30MHP. At 50 MHP the engine is pleasant smooth and “semi slow running” Seat vibration at 50 MPH is gone. 
The bike is designed for winding roads in the 50ies.
Most old roads are straighten op and leveled out, changing the need and use for a low geared bike.
Today the roads are more for cruising.
Motorway use is still a challenge. Cruising hour after hour at 80-90MPH is hard work for the SR and “oil boiling”
With a 23T sprocket. 70 MPH is easy and comfortable, without “cooking the oil”
If easy cruising at 100 MPH is a high priority, buy a “Japasaky” or “Ducaguzzy”

Do it the Don way:

The best twin for overall use  

"A wonderful stabile and reliable machine"
    
http://www.dailymotion.com/video/xpcvc1_don-s-road-test-bsa-a10-golden-flash_sport