Vaguely useful spreadsheet to calculate speed from different sprocket set-ups...

[t20racerman]

Hi all

For my race bikes I made up a gearing/speed chart to help me work out the correct gearing for each course I was racing on. As my BSA speedo doesn't work, but the tacho does (both chronometric) I thought it would be useful to modify my chart to work out how fast I was going at different rpm! I've also changed the drive sprocket from 21 to 22 teeth for higher cruising speed at less rpm.

I doubt anyone will find it useful, but if you think you might you can get the spreadsheet from here:

http://www.t20suzuki.com/cz/BSA_gearing.xls

It gives rpm/speeds for both standard 21 and 22 teeth engine sprockets, and for different rear sprockets too. My bike has GT750 Suzuki rear wheel which has many rear sprockets cheaply available - hence the tables for 41,42 and 43 teeth rear sprockets.
You can modify it and put your own data in if you want.

Meanwhile, I'm saving up to get my speedo renovated!  
Adrian

Edit: link works now!

[Beezageezauk]

Cheers Adrian,

Excellent piece of work.

As my road going (touring) A10GF seems a bit revvy on standard gearing I was going to post a question about the difference in RPM it would make by gearing up from a 21 to 22 tooth engine sprocket.  You've just answered my question.

Going by your figures I guess the bike could take a 22 tooth sprocket ok and a 23 tooth would be a bit too high.

Many thanks for your efforts and good luck with your speedo..

Beezageezauk.

[RichardL]

Not "vaguely" useful, "very" useful. I, too, have a 22-tooth cush sprocket.

[MG]

Ha, great minds think alike, Adrian!  

I've made something very similar a while ago, but also included the internal gearbox ratios (i.e. the single pinions). I don't have any webspace to make it downloadable, but if anyone wants to have it, just pm me your email address.
Orange cells are input fields, green ones are displayed results.

Cheers, Markus

[t20racerman]

Glad one or two of you found it useful. Nice piece of work too MG, that is how it should be done! 

Going by your figures I guess the bike could take a 22 tooth sprocket ok and a 23 tooth would be a bit too high.

Beezageezauk - 23 would be WAY too high. 22 makes it much better on those long fast roads, but it does make it feel a bit gutless at times. When I last had the bike on the road I went up to 22, then down to 21 again 'cos I missed the 'oommph'! However, it is now gas flowed, bigger carb etc and pulls it very nice indeed.

Adrian

[MG]

To make life easier (mainly for myself  ), here's a direct download link to my Excel sheet:

http://dl.dropbox.com/u/40922180/RPM_speed_A10_A7.xls

Cheers, Markus

[brackenfel]

Thanks Adrian,
That is most useful... My bike (62 GF, iron head) curiously has a 20 tooth engine sprocket! I counted it several times , just to make sure.. Otherwise it's standard. It is "buzzy" but pulls well  I got an "as new" 21 on Ebay a while back, I might try it next time I have to look at the primary area.
Is it usually possbile to get away with a tooth or two without a new primary chain??

Thanks,
Adrian (another one!!)

[t20racerman]

Hi there

I went up a tooth size without needing a new primary chain. There was plenty of adjustment and it still has a fair bit left.  I suppose though that a brand new chain might be somewhat tighter than my one. I'm sure you will find it fits well enough but you'll have to trial it and see to be sure.

Good luck,
Adrian

[brackenfel]

Thanks for confirming that Adrian.. I'll give it a go sometime over the Winter maybe..

Cheers,
the other Adrian!!

[Neil_EngUK]

Thank you for the spreadsheet. Is massively useful, but are my adjustments correct?
I want this bike to cruise well, with a bit of extra grunt.

I have a 16 inch wheel with a fat tyre (curtesy of the previous owner who had a thing about choppers), and the external diameter of the tyre is about 24".
This makes for a metric diameter of 60.96 cm and a circumference of 1.91 metres.

Engine sprocket = 24T
Clutch basket = 46T
Gearbox sprocket is 19T (but going to upgrade this to 21T)
Rear wheel drive sprocket is 42T

The spreadsheet subject to the above amendments shows that at 120km, the bike should be chugging along at just over 4,000 rpm.

I am rebuilding the A10 1961 GF as a bobber, and de-chopping it etc.
I know its not the usual thing, but as the bike is a bitsa (with a good triumph clutch), I intend to change only the knackered gearbox sprocket and the clutch basket, as the other sprockets are in VGC.

What do you think?
Would folks recomend going up to a 23T gearbox sprocket or going to 21T? :)

Many thanks
Neil

[Rocket Racer]

Thank you for the spreadsheet. Is massively useful, but are my adjustments correct?
I want this bike to cruise well, with a bit of extra grunt.

I have a 16 inch wheel with a fat tyre (curtesy of the previous owner who had a thing about choppers), and the external diameter of the tyre is about 24".
This makes for a metric diameter of 60.96 cm and a circumference of 1.91 metres.

Engine sprocket = 24T
Clutch basket = 46T
Gearbox sprocket is 19T (but going to upgrade this to 21T)
Rear wheel drive sprocket is 42T

The spreadsheet subject to the above amendments shows that at 120km, the bike should be chugging along at just over 4,000 rpm.

I am rebuilding the A10 1961 GF as a bobber, and de-chopping it etc.
I know its not the usual thing, but as the bike is a bitsa (with a good triumph clutch), I intend to change only the knackered gearbox sprocket and the clutch basket, as the other sprockets are in VGC.

What do you think?
Would folks recomend going up to a 23T gearbox sprocket or going to 21T? :)

Many thanks
Neil

Even with the 21T gearbox you're looking at third gear able to take you to 108mph @ redline, so using 4th as an overdrive.  I suspect every time you hit a decent hill it'll need chopping down to third.
With a 23T 3rd gear capable of 118mph and 100kph being a lowly 3000rpm below any decent power in top 

a 21T engine and 21T gearbox allows top to hit 114mph and 5000rpm is over 80mph, so on the cam from there to the redline 
Gearing taller isnt always a better idea if the bike needs chopping back a gear everytime there's a hill or a headwind.. Just my 10 cents.

[Topdad]

Hi RR, think you're on the money, mine has a 21T engine sprocket and when first used with engine just run in hills needed a cog down however as the bikes worn in that isn't needed now . My bikes engine is a DA10 number with iron head ex valves opened up by SRM to max with a spitfire cam followers and 8.to1  pistons  she now cruises nicely even 2 up without that revvy feeling ,Bob.

[bsa-bill]

nice one Marcus, I love spreadsheets(doesn't make me a bad person) I just find the challenge of achieving results via formula  kind of satisfying -  yeah I  know