Engine Sprocket Size

[Mosin]

I have just discovered that my 1960 A7SS has been running a 21t engine sprocket as opposed to the standard 18t sprocket. It seemed to run well enough with the 21 tooth item, but since this is now worn out I thought I would go back to the standard unit with the replacement. Would I be right in thinking that I can expect to see a somewhat lower top speed, but improved acceleration and more torque?

Maths really isn't my strong point, so please try to keep any explanations in layman's terms. lists of ratios etc make my brain hurt!

Thanks 

[Swarfcut]

 In a word, Yes it will be quicker off the mark. Torque at the crank stays the same, but the spread will come on lower in the rev range thr'o the gears. Fuel consumption will be worse, more engine revolutions per mile..........Sweet spot will come in at a lower road speed in top gear.

 Swarfy.

[berger]

if your ss is anything like mine you won't need first gear with an 18 tooth stick a 20 on it , pub tomorrow because i can

[Black Sheep]

21T is rather overgeared. Standard 18T sprocket gives better hill climbing and more comfortable against a headwind.

[Triton Thrasher]

Would I be right in thinking that I can expect to see a somewhat lower top speed, but improved acceleration and more torque?

Were you doing top speed?

I will say that raising gearing is usually a mistake.

[Brian]

I have to agree with TT, I have built many A10 engines and a few A7's and have found over the years the standard gearing is best. It is a lot less harmful for an engine to rev slightly higher than to have it "work" or "labour". I once bought a plunger A7 very cheaply due to the owner over gearing it resulting in multiple seizures.

[Colsbeeza]

Hi Mosin,
My Flash was fitted with a 19-tooth sprocket, all others being standard sizes. It seemed to be overrevving at town speeds, so I fitted a 21-tooth. Pulling up my steep driveway with the 21-tooth I could use a smaller sprocket and requires a bit of clutch slip, but at say 60mph it sounds to be overrevving for that speed which was a bit of a surprise. I do have access to small country roads here, and gearing is good at 40-50mph.
For speeds above 60mph I am questioning what gearing I have in the gearbox, so need to check that out, and also put a tacho on it to tell me what revs it is actually doing.
Given the countryside in Cumbria, you may find an 18-tooth ( sidecar gearing?) is too small. The difference will be very noticeable.
Col

[groily]

I've played with the gearing on numerous of mine over the years, A10 included. In the end, I've always ended up close to standard, even if the actual sprocket combinations at crank and 'box aren't per book.
My overall take has been that an extra tooth on the 'box (about 5-6% increase typically)  is usually OK on 650s, especially where lower profile tyres like Roadriders or TT100s are fitted which will reduce the effective increase compared to some others.
My A was very sweet at 60-65mph with a 19 on the 'box and TT100 on the rear, although it started to buzz a bit nearer 70. Cruising about 2/3rds rpm has always seemed good to me on standard-tune engines.
Which says I broadly agree with TT and Brian on this (especially being a big bar steward with the profile of a lead brick into the wind / uphill).

[Mosin]

Thanks for all the feedback guys. I will try the 18 for a bit and see how I get on. Having failed to make it up Hardknott Pass on several occasions, I am hoping that the 18 will allow me to achieve this, even if I go back to a slightly higher gearing afterwards.

I should perhaps reassure people that even with the 21 on, I never ran the SS at more than 60/65mph and that is with a very well sorted engine and 1000 mile oil changes. It is important to take care of the ones we love!

[Black Sheep]

On standard gearing you should be able to romp up the Hardknott Pass. Mobile homes coming down against you are the real challenge.
My best ride over there was one December, 10 at night, full moon, -3 degrees. Only other traffic encountered was a big Yamaha traillie coming the other way. Magic! 

[Greybeard]

I've changed the gearbox sprocket* on my Plungie to get a longer-legged ratio. Even though I'd changed the rear wheel sprocket from combo to solo, the bike always felt like it should have a fifth gear. I can't remember the tooth count of the sprocket; 21 teeth maybe? The bike rides just right now. I needed to get the speedo regulated to show accurate road speed.

*As far as I know you cannot buy duplex engine sprockets.