The BSA A7-A10 Forum

Technical => Clutch, Primary, Gearbox => Topic started by: Greybeard on 06.05. 2015 13:36

Title: A larger gearbox sprocket?
Post by: Greybeard on 06.05. 2015 13:36
Yo Dudes!
My Plunger A10 seems under geared; she pulls away like a train but at about 55 I'd really like to have another gear, by 60MPH I feel I'm straining the old girl. She has 18 gearbox teeth and 42 rear wheel teeth which I believe is standard. I see that Draganfly offer larger gearbox sprockets at 19, 20 & 21 teeth. I am not using modern lower profile tyres. What opinions or experience do you have of changing the gearbox sprocket to make a longer legged drive ratio?
Title: Re: A larger gearbox sprocket?
Post by: duTch on 06.05. 2015 14:53
 Isn't 19T standard...? Even at that I've also thought to go up a tooth, engine sprocket might be better, but I think they only come in one size- unless one was to muck around and adapt one
Title: Re: A larger gearbox sprocket?
Post by: a10gf on 06.05. 2015 15:13
Quote
Isn't 19T standard...?
Believe so too.

Quote from my a10 plunger webpage, testing some sprockets. My opinion, 21 not nice in town, needed lots of clutch (= lots of clutch wear).
Quote
was happy with 21 for highways, but 20 seems like a better compromise acceleration\relaxed 4th gear rpm.

Got a 21 somewhere, giveaway to anyone who wants it (and confirm it's no good in practical use :O)

18 must be good for the daily wheelies show down the neighbourhood :O)
Title: Re: A larger gearbox sprocket?
Post by: Greybeard on 06.05. 2015 15:36
Quote
Isn't 19T standard...?
Believe so too.

Quote from my a10 plunger webpage, testing some sprockets. My opinion, 21 not nice in town, needed lots of clutch (= lots of clutch wear).
Quote
was happy with 21 for highways, but 20 seems like a better compromise acceleration\relaxed 4th gear rpm.

Got a 21 somewhere, giveaway to anyone who wants it (and confirm it's no good in practical use :O)

18 must be good for the daily wheelies show down the neighbourhood :O)

A long time ago I once dropped the clutch a bit sharply and lifted the front wheel off the ground! Though that may have been before I changed the sidecar ratio rear sprocket that was on the machine when I got it. Perhaps the 18 tooth gearbox sprocket is also for a sidecar.

I've just been having a read of part of your travel notes; fascinating!  *good3*
Title: Re: A larger gearbox sprocket?
Post by: RichardL on 06.05. 2015 16:36
According to Bacon, no 18/42 combination offered stock in any model or year and no 20 or 21 tooth gears available back then. I guess those came later as aftermarket pieces. It seems to me that your bike must have been 19/42 originally and someone changed to the 18, or, if your sidecar rear was 49, Bacon shows "A7ST s/c" with 18/49 for 1949-1954, which might explain something if your bike started life as an A7. BSA might have just dragged the gear ratios along without making it a matter of record that they also applied to A10's, as well.

For the most part, in all of Bacon's books, I think he just recites from the records without adding special information. Kind'a like me, right now...and, no, I am not a librarian in real life or on TV, but I am just reading from the books.

Richard L   
Title: Re: A larger gearbox sprocket?
Post by: Butch (cb) on 06.05. 2015 16:46
18" or 19" wheels?
Title: Re: A larger gearbox sprocket?
Post by: Greybeard on 06.05. 2015 16:48
18" or 19" wheels?

19" wheels
Title: Re: A larger gearbox sprocket?
Post by: Topdad on 06.05. 2015 16:50
My s/w arm model was the same , changed to a 21 and the bike seems happy enough with it will pull nicely even 2 up so some cog on the plungers different . greybeard I'd look at changing but first check all the other sprockets and take it from there, Bob
Title: Re: A larger gearbox sprocket?
Post by: Greybeard on 06.05. 2015 16:54
Thanks TopDad Bob,
I await further illumination from the Forum Elders.  *respect*
Title: Re: A larger gearbox sprocket?
Post by: Topdad on 06.05. 2015 16:56
What time is it in Oz???  *smile*
Title: Re: A larger gearbox sprocket?
Post by: Greybeard on 06.05. 2015 17:08
What time is it in Oz???  *smile*

Changing the gearbox sprocket is not a job I'm looking forward to so I can wait a while. Does the gearbox have to come off to do this job on a plunger engine or can I remove the clutch, strip out the gearbox in situ and change the sprocket?  *dunno2*
Title: Re: A larger gearbox sprocket?
Post by: Topdad on 06.05. 2015 22:45
From memory you can 't  change the gear box drive sprocket on a plunger engine  ,think you can still do the internals . From distant memory g/box casing held by 4studs To engine and sprocket is on the wrong side ie between g/box end and primary inner moulded to d/side crankcase. But more likely due to that to be other cogs don 't you think?  Can't wait to hear what is right? Bob
Title: Re: A larger gearbox sprocket?
Post by: duTch on 06.05. 2015 23:18


 From memory *conf*, I think you need to pull the whole unit, then split 'em....that's why I carved a hole behind my clutch and fitted a plate from same as A65/50 and Trihards- haven't utilized it much yet but bound to soon...

 Geez GrayB., given the speedo drive is upstream from the final drive, you wouldn't even be doing 55/60mph with an 18T...  *eek* probably 56.842105...with 60mph reading- mines the other way 'cos of ummatched speedo/18"x4.00 tyre with 2.040 rolling circumference.

 I feel the same of mine with  the 19T, but've been meaning to ask what anyone who uses a 20T reckons, thought it may be a bit tall...maybe not so, might stock up.

 
Quote
What time is it in Oz???  *smile*
...=   GMT +10hr.. *????*

 Bob rounded me up there- got sidetracked measuring wheel...but what he said, except not sure what you mean Bob?
Quote
But more likely due to that to be other cogs don 't you think
...
 
Title: Re: A larger gearbox sprocket?
Post by: KiwiGF on 07.05. 2015 01:10
On my 56 swing arm GF I went up one tooth on the engine sprocket, with the rest standard, that seems to be optimum for my riding, and for the NZ national limit of 100 kph, going up two teeth according to a mate with similar bike is not so good for hills etc as it causes much more changing down to 3rd. Speedo is of course unaffected by changing the engine sprocket.

I have slightly more power than std (I hope....) as I have flat top pistons and 356 cam.

You might want to keep your gearbox sprocket as it is and just change your engine sprocket by say 2 teeth, as it's less work?
Title: Re: A larger gearbox sprocket?
Post by: Greybeard on 07.05. 2015 09:32
...You might want to keep your gearbox sprocket as it is and just change your engine sprocket by say 2 teeth, as it's less work?

Sounds like a plan. I'll investigate engine sprockets.
Title: Re: A larger gearbox sprocket?
Post by: muskrat on 07.05. 2015 11:27
G'day GB
I'm pretty sure there is only one size engine sprocket for the plungers. Both A10 and A7 use 27 tooth part# 67-2050.
For the g/box sprocket the whole lump must come out and G/box removed from the motor. 18 is std for A7 and 19 for A10. The only change you can make is going to a 20 or 21.
The only other way to change the gearing is to turn off the rear sprocket from the drum and weld (or scotch key) another of a smaller size on. I have done this on my cafe and A65.
Cheers
Title: Re: A larger gearbox sprocket?
Post by: duTch on 07.05. 2015 12:45

 Like I said, that's my first choice partly because of what Kiwi said,
Quote
Speedo is of course unaffected by changing the engine sprocket.
,  but if you have an 18T, your speedo will be out of whack anyway, so sooner or later you'll have to change the G/box sprocket anyway, might be worth splitting the difference...go up oneT on each(one at a time)??
Title: Re: A larger gearbox sprocket?
Post by: Greybeard on 10.08. 2015 16:30
After reading through the previous comments I began to doubt my counting of 18 teeth on my GF gearbox sprocket; it's not easy to see, let alone count its teeth.
I put the long pipe nozzle head from a can of WD40 onto a can of gray primer paint and carefully spray painted one of the tooth valleys so I could see when it came around again. I definitely counted 18 teeth! I am planning to do some work on the 'ole girl this winter so I will get myself a 20 tooth gearbox sprocket.

...and a new chain!
Title: Re: A larger gearbox sprocket?
Post by: Klaus on 10.08. 2015 18:53
I have 21 T gearboxsprocket on my RR but with a tuned engine. Easy run with 4000 revs and scratched 75 miles. But at top gear 5500 is the end.
If you get one teeth more at the gearbox, is that the same as you count 3 more at the rear.
The racer is more tuned and lighter with fearing, so l get a 22 T sprocket.
All sprockets are old ones, welding up with blanks.

cheers Klaus
Title: Re: A larger gearbox sprocket?
Post by: bsa-bill on 10.08. 2015 20:43
I went along this route when building up the RGF and someone was good enough to post a chart of engine, gearbox rear chain sprocket sizes and radio's.
I have ungraciously lost it, but it should be in here and maybe the member who posted it might recall where it is or some one with better search skills might find it, it  might not apply in the same way to plungers though
Title: Re: A larger gearbox sprocket?
Post by: muskrat on 10.08. 2015 21:17
MG did this one     http://dl.dropbox.com/u/40922180/RPM_speed_A10_A7.xls
Adrian did this one    http://www.t20suzuki.com/cz/BSA_gearing.xls
Richard posted this one   http://victorylibrary.com/graphics/A10-ratio-chart.jpg
Cheers
Title: Re: A larger gearbox sprocket?
Post by: bsa-bill on 11.08. 2015 09:46
I found it - had it stored up in a cloud  *smile* (hows that for getting down with the kids)
nice little  sheet allows you to change the number of teeth on any of the four sprockets, wheel circumference and gives a final ratio, also speed at revs down the side

Right the problem is it's a Spreadsheet, needs Exel or open office would do it to, but how do I put it into a reply as a executable file ?
 
Title: Re: A larger gearbox sprocket?
Post by: Greybeard on 11.08. 2015 10:06
Just paste a link to it here.

This should work: https://www.dropbox.com/s/juonla0zuzjust9/Test.xls?dl=0
Title: Re: A larger gearbox sprocket?
Post by: bsa-bill on 11.08. 2015 10:36
I don't have a link as in website for the sheet, it's been a download at some time in the past but can't find it.
I've pasted the gearratiocalculato sheet into the test sheet but not sure how to proceed further can you get it from your dropbox.
If I try to save it it seems to do so but only to my PC.
Title: Re: A larger gearbox sprocket?
Post by: Greybeard on 11.08. 2015 13:35
I thought I'd dig out the rear sprocket that was on my GF rear wheel when I bought it; it has 49 teeth. So my machine used to have a final drive ratio of 18 to 49. I reckon someone must have hitched up a gigantic double adult sidecar; or used the bike for mountaineering!

I'll put the sidecar rear sprocket on the 'For Sale' posts
Title: Re: A larger gearbox sprocket?
Post by: bsa-bill on 11.08. 2015 14:44
Cheers guys, bear with me this saved from ods as xlx file but I find usually works

ah you need to click on the "enable editing" box - hope that's it

Title: Re: A larger gearbox sprocket?
Post by: Topdad on 11.08. 2015 15:23
Hi Bill ,just checked out mine and assuming  ( dangerous for me ) that clutch, gearbox and rear are 43,19and 42  ,to which I have added a 21 tooth engine sprocket I get a reading of approx. 2250 revs at 50mph which equates very much to what I reading through my standard sensory monitor ( ie my big backside .) Means cruising at 60 is very comfortable , thanks Bill.
Title: Re: A larger gearbox sprocket?
Post by: Greybeard on 11.08. 2015 15:44
Thanks BSA-Bill,
That is very interesting. Makes me think that if I change from an 18 to a 20 tooth gearbox sprocket my current 65mph should become 73mph; which would be nice!
Title: Re: A larger gearbox sprocket?
Post by: Tomcat on 09.01. 2016 23:11
I thought I'd dig out the rear sprocket that was on my GF rear wheel when I bought it; it has 49 teeth. So my machine used to have a final drive ratio of 18 to 49. I reckon someone must have hitched up a gigantic double adult sidecar; or used the bike for mountaineering!

I'll put the sidecar rear sprocket on the 'For Sale' posts



Wow Greybeard, I like your outfit! Ive never been a sidecar fan, but your's is one I'd love to get my hands on!  *smile*
Title: Re: A larger gearbox sprocket?
Post by: Rocket Racer on 01.02. 2016 01:41
I thought I'd dig out the rear sprocket that was on my GF rear wheel when I bought it; it has 49 teeth. So my machine used to have a final drive ratio of 18 to 49. I reckon someone must have hitched up a gigantic double adult sidecar; or used the bike for mountaineering!

I'll put the sidecar rear sprocket on the 'For Sale' posts

I suspect artists licence as the door should have been on the outside  *doh*
and more probably towed.

Unless running a chair I tend to stick with stock gearing, as BSA had a good idea what they were doing  *whistle*
Over gearing typically bites you when you have headwinds and hills to deal with and effects your acceleration when passing.

If I was doing both sprockets I tend to fit B series sprockets which come in more modern 520 chain width good enough for most high performance moderns and giving more chain options.