Author Topic: gear ratios explanation?  (Read 4055 times)

Offline snowbeard

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gear ratios explanation?
« on: 12.09. 2007 19:22 »
well, I apologize for re-dominating the conversations here, just as soon as someone posts something newer than mine, eh?  ;)  I'm really trying to conserve my requests, but I've been trying to figure this out for awhile...

so I found the great chart here http://victorylibrary.com/graphics/A10-ratio-chart.jpg but I don't know quite how to use it?



So from what I gather, all our bikes (A10's anyway) will have the same top end running gear ratio, it's only how you get there that changes?  fourth being 1:1 with the drive sprocket, whatever it's tooth number may be, but still a straight run.  so that means that no matter what box you use, you'll still hit the same top end (excluding variable front sprockets for now)

I have recently confirmed that I have the SC.T2 scrambler gearbox that should be on a 1957 Spitfire Scrambler (it is on one, I mean ';-)  and I am wondering if I should consider changing anything?  I haven't gotten the bike up to actual road speeds but maybe once, so I don't really know how it feels, but it seems to be a common theme to change the drivetrain around here.

I expect my best bet is the larger front sprocket, 21 or 23 tooth. but I'd be happy to hear a little more about what I've got and what to expect when I get it out on the open road (someday)

thanks!!
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\'57 BSA A-10 Spitfire Scrambler
Spitfire Starting Video
\1960 Super Rocket (basket)
\1981 Suzi GS650
\1988 BMW K100LT in Lisbon!!

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Offline Brucie64

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Re: gear ratios explanation?
« Reply #1 on: 11.07. 2009 11:58 »
Hi Snowbeard,

Just reading through some old posts on Gearbox issues. I also have a scambler SC.T2 gearbox fitted to my spitfire and after getting it on the road I quickly realised that in its original format the gearing is totally unsuitable for road use.

Probably the easiest remedy as you suggest is to up the teeth on the gearbox sprocket, I have recently bought a 21 tooth gearbox sprocket and just wondered how you got on, did you fit one too?
Bruce
Spitfire
UK

Online BSA_54A10

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Re: gear ratios explanation?
« Reply #2 on: 11.07. 2009 12:27 »
Exactly correct.
All that you are changing is the gap between the gears and the actual first gear.
Because the speedo drive come from the gearbox you have to be a bit carefull what ratios you use or the speedo will not be correct.
To change the actual speed that the bike will do in top gear you need to change either the engine sprocket or the counter shaft sprocket or rear wheel sprocket or a combination of all 3.

So what you now need to do is go for a long ride and find out what speed range the engine is best balanced at ( fewest uncomfortable vibrations ) we call this the sweet spot and it will be different for every bike.

Say for example your bike smooths out at 30 mph then gets rough again by 50 mph.
You would then say your sweet spot was at 40 mph.
But you most often ride in the 50 mph  to 60 mph range.
So your cruse speed would be 55 mph.
In which case you would need to raise the overall ration by 15 mph or 37 %.
Raising or lowering the over all ratios is what that little calculation down the bottom is all about
Bike Beesa
Trevor

Online RichardL

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Re: gear ratios explanation?
« Reply #3 on: 11.07. 2009 12:47 »
Snowbeard,

It is interesting to note that the Scrambles gearbox has lower ratios than Standard for 1st and 2nd with only 3rd being a higher ratio.  Maybe it's interesting to me because I've misinterpreted the "Scrambles" purpose. I would think, perhaps wrongly, that the higher 3rd gear might be a useful thing in the hills and mountains of Colorado (but I've forgotten exactly where it is you might be moving). 

Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Offline snowbeard

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Re: gear ratios explanation?
« Reply #4 on: 11.07. 2009 15:43 »
now that I've been riding the bike for a year plus, but with no tach and speedo, I am starting to think it may have been changed at some point.  first is good for maybe 10mph, just a launch to get started really. second will take me up to say 35 at high(ish) rpms, third is cruising gear for around town, 30-35 with decent revs, not lugging the engine, nor screaming it, and will go up to about 45-50 tops.  I've had it to 70-75 in fourth with no troubles, but that is to be expected.  all this is estimated tho.

never having experienced a true known scrambles box, I have no comparison.  what would you say your ranges are Brucie?  how is it unsuitable?
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\'57 BSA A-10 Spitfire Scrambler
Spitfire Starting Video
\1960 Super Rocket (basket)
\1981 Suzi GS650
\1988 BMW K100LT in Lisbon!!

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Offline Brucie64

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Re: gear ratios explanation?
« Reply #5 on: 11.07. 2009 21:53 »
Hi Snowbeard,

Me too, I have don't have a speedo or tacho also so can only estimate what speed I am doing by judging the traffic around me. I would however say that in its present SC.T2 gearbox form I would be lucky to get 45-50mph in fourth gear before the engine revs and vibration become to much. I still have bags of power available but wouldn't want to rev the idiot out of the bike hence my idea to increase the gearbox sprocket to 21 teeth as compensation.

Bruce
Spitfire
UK

Online chaterlea25

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Re: gear ratios explanation?
« Reply #6 on: 11.07. 2009 23:21 »
Hi All,
It is more usual (and easier) to alter the overall gearing at the engine sprocket
"A"series sprockets were available up to 24 teeth,
Altering the engine sprocket does not affect the speedo drive ratio,
Spinning the clutch faster with a bigger sprocket gives the clutch an easier life too!
A lot of folk neglect to take in the overall diamater of the back wheel, ie tyre size, as this has a big influence on the gearing!!!!

With a 23 tooth engine sprocket and otherwise standard(????) sprockets on my SR (STD box)
70mph = 4000rpm, it will pull top from around 30 mph no problems, rear tyre is 100/90x19
I have a 24 tooth engine sprocket that I must try sometime
Regards
JOhn O R
1961 Super Rocket
1963 RGS (ongoing)

Offline raymo

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Re: gear ratios explanation?
« Reply #7 on: 12.07. 2009 22:25 »
Hi All,
It is more usual (and easier) to alter the overall gearing at the engine sprocket
"A"series sprockets were available up to 24 teeth,
Altering the engine sprocket does not affect the speedo drive ratio,
Spinning the clutch faster with a bigger sprocket gives the clutch an easier life too!
A lot of folk neglect to take in the overall diamater of the back wheel, ie tyre size, as this has a big influence on the gearing!!!!

With a 23 tooth engine sprocket and otherwise standard(????) sprockets on my SR (STD box)
70mph = 4000rpm, it will pull top from around 30 mph no problems, rear tyre is 100/90x19
I have a 24 tooth engine sprocket that I must try sometime
Regards
JOhn O R

 this is exactly what I used to do on my olf Golden Flash.. 19T engine sprocket fro trips to the english lake..  and a 23 fro the rest of the time.. it would do 70mph max with the 19T fitte but in the twisty lakland lanes I had a good range of gears... nor sitting in 2nd and 3rd all the time :-)

cheers

Ray
DA10 Sr ( 1961  I think :-) )
B25S Starfire 1969
T500 ( the suzuki 2T Beast ) 1972
XLV 750Rf Honda current ride
ok so only the Honda is working :-)

Offline Brucie64

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Re: gear ratios explanation?
« Reply #8 on: 15.07. 2009 21:22 »
Now I'm stumped  *sad2*, I decided to bow down to popular concensus and look at changing the engine sprocket instead of the gearbox sprocket for the reasons listed by J0hn O R.
I removed the primary chain case tonight for the very first time and was suprised to find a wee 16 tooth sprocket connected to the crankshaft. I thought that the smallest I might find on a spitfire scrambler would be a 17 tooth jobby!. This gearing I suppose explains why the bike rev's itself to bits whilst only bobbing along at around 40-50mph.

Here's the conundrum......with a 18" back wheel and not being sure of the size of the gearbox sprocket (but estimate it must be around 16 or 17 tooth), what size engine sprocket would give me a decent turn of speed whilst still allowing me to bobble along in top gear most of the time?.

My guess is somewhere between 21 to 23 tooth maybe but I'm defo no expert. Is this about right?
Bruce
Spitfire
UK

Online RichardL

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Re: gear ratios explanation?
« Reply #9 on: 15.07. 2009 22:12 »
Bruce,

Looking in Bacon's Restoration book, the 57 A10 Spitfire Scrambler is listed as having had a 27-tooth engine sprocket, a 54-tooth clutch sprocket, a 19-tooth gearbox sprocket and a 49-tooth rear sprocket. Are you sure you are counting teeth accurately? The only reference, I see,  to any 16-tooth sprocket would have been an option for the gearbox, and yes, 17-teeth for the gearbox was also available. The smallest sprocket availabe for the engine appears to have been 17 teeth.

Richard L.
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Offline snowbeard

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Re: gear ratios explanation?
« Reply #10 on: 15.07. 2009 23:12 »
interesting. I can see why Bacon's 27 tooth on a scrambler would give higher revs for lower speeds, but that doesn't explain why your 16 tooth would only make 50mph?  seems you would be running super fast with that, at the cost of a harder start in first and less torque?  all in all it depends on what you have on the gearbox too, you can count that without disassembling anything, but it's a little hard to see.  just mark one tooth and count as you turn the wheel thru to that tooth again!

so a response that I got in another forum when asking after this sort of thing was

Snowbeard, according to the later parts manual ('59-62), the Spitfire scrambler came with a 21T engine sprocket (as did all solo A10's) but a 16T countershaft sprocket whereas other A10 models came with a 19T countershaft sprocket. If this is the case on your machine, it would explain the high revs, but you would have to take the entire primary off to fix the problem. The upside of this is that 19T countershaft sprockets should be readily available. /quote

then when I finally got to count my teeth I got 21 tooth engine gear, 43 tooth clutch sprocket, 19 tooth drive gear and 46 tooth on my rear sprocket,

to which was replied:
Snowbeard, it looks like you have the standard A10 solo gearing which is a good compromise between acceleration and top end. With that gearing you should be turning about 4200 @ 65 mph and 6400 @ 100 mph.  /quote

I also have not seen a 16 tooth cush drive gear offered, not to say it won't happen.  is your crankshaft drive gear a cush drive? two lobed? or four?  just checking on what might have been done to 'er.
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Spitfire Starting Video
\1960 Super Rocket (basket)
\1981 Suzi GS650
\1988 BMW K100LT in Lisbon!!

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Online RichardL

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Re: gear ratios explanation?
« Reply #11 on: 15.07. 2009 23:37 »
My first questions would be: Should we consider that the Bacon book is wrong? Perhaps it is, but the only A10 sprocket sequences ending in 46 tooth rears are 23, 43, 19, 46 and 22, 43, 19, 46 for '62 Spitfire and '63 RGS, respectively. Where a 16 tooth cush sprocket would come from, I just can't see from his charts.

I think you might have gotten it backwards in saying the 27-tooth engine sprocket leads to high revs and low speeds. It should be the opposite. That's a 2:1 ratio versus a 2.53:1 ratio for the 17/43 combo.


Richard L.
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.

Offline Brucie64

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Re: gear ratios explanation?
« Reply #12 on: 16.07. 2009 09:45 »
Seems like I have opened up a right can of worms here!

Question (forgive my ignorance), is the cush drive gear the same thing as what is described in this topic as the engine gear or are they two seperate entities?. The 16T sprocket I am talking about on my bike is sitting behind the cush spring like device and is connected to the clutch sprocket via the primary chain. My 16T sprocket is the two lobe variety (is this the engine sprocket everyone's talking about?)

I also agree (in principal) with Richard L that the bigger the sprocket on the engine side the lower the revs should be for any given speed, this is why I thought with mine only having 16T it would be revving the bo***cks out of the engine at relatively moderate speeds (40-50mph) in top gear.

Bruce
Spitfire
UK

Offline Brucie64

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Re: gear ratios explanation?
« Reply #13 on: 16.07. 2009 09:56 »
I have found a photo of someone else's primary chaincase on the net which clearly shows which sprocket I am referring to. Is this the engine sprocket and if so mine's definately 16T.
Bruce
Spitfire
UK

Online RichardL

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Re: gear ratios explanation?
« Reply #14 on: 16.07. 2009 12:05 »
Plan on signing up for the world-wide 2017 DISTINGUISHED GENTLEMAN'S RIDE on September 30, 2018. Watch website at https://www.gentlemansride.com for details.