Sprocket and ratio theory

1

123petey

XS650 Enthusiast
Messages
55
Reaction score
41
Points
18
Location
Salem, Oregon
Hey all - I have a question that I’ve been struggling with and haven’t found much valuable information. So stock sprockets on the XS650 is 17/34, 2.00 ratio. If you change to 16/32, maintaining 2.00 ratio, what is the result? I’m assuming the engine is spinning faster, so more gas consumption, but possible more power since these bikes like the higher RPMs? I know stock ratios on any bike are the sweet spot for economy and power, generally speaking. So anyway, back to the question - can you lower both gears, maintain ratio, but access more usable power at the cost of fuel or are you just making the motor work harder for the same results as stock?
 
If you don't change the ratio, you're not going to change anything (RPMs, power output, fuel consumption). Going smaller on the front isn't recommended anyway. It's harder on the chain because it has to make a tighter turn around the smaller sprocket. The usual routine on these is to leave the front alone and go down just one or two teeth on the rear. I know it doesn't sound like much of a change but it really makes a difference. I went down one on the rear (to a 33T) on my '78 Standard with an 18" rear wheel and it's really nice. Drops the cruising RPMs by a few hundred RPMs but doesn't rob the take-off power in the lower gears. I went two down (to a 32T) on my '83 Special with a 16" rear wheel and I think it's even better. I had tried the 32T on my '78 18" wheel but didn't like it.
 
So just to put this up front, I’m not trying to be difficult or argumentative. I’m just really curious about this because I’ve never thought about it. I’ve changed one or the other and understand gearing ratios and the effect. But I have to respectively disagree - if you change both down or up, they are counteracting each other to make a zero sum difference at the wheel but the engine output shaft is definitely spinning faster or slower while still maintaining the same wheel revolutions. And I know this is a stupid question because no one will do this but I got stuck on this and can’t get it out of my head until I solve it. I do appreciate the response and let’s keep the conversation going
 
123petey said:
So just to put this up front, I’m not trying to be difficult or argumentative. I’m just really curious about this because I’ve never thought about it. I’ve changed one or the other and understand gearing ratios and the effect. But I have to respectively disagree - if you change both down or up, they are counteracting each other to make a zero sum difference at the wheel but the engine output shaft is definitely spinning faster or slower while still maintaining the same wheel revolutions. And I know this is a stupid question because no one will do this but I got stuck on this and can’t get it out of my head until I solve it. I do appreciate the response and let’s keep the conversation going
Click to expand...
A ratio is a ratio, both sides stay the same 1/2" is the same as 4/8" the wrench will fit either bolt no matter how you measured it.
What DOES change is the amount of force on the chain and it's speed. As the sprocket set gets smaller the chain goes slower and has higher stress on it.
 
Why not just go up a tooth or two on the rear if you are looking for "more power"? If you maintain a 2:1 ratio it stays the same no matter if there are 17 teeth or 100 teeth.
 
I’ve already stated that I’m not looking to “do” anything. I appreciate everyone’s efforts to help me understand how to gear up or down, I fully understand how that works. I understand ratios, and ratios don’t change. 1/2, 2/4, 4/8, all the same. It’s a little surprising that no one else is realizing that there has to be some difference when maintaining a ratio but changing sizes of sprockets? This is a ridiculous example but if I have 1 tooth on the front, and 2 on the back, you are saying the output shaft will be spinning the same revolutions as a 17/34 in relation to the output as the rear wheel to the ground? The rpms have to be higher to obtain the same speed, right?

I’m seriously not trying to be difficult or obtuse I’m just thinking out loud and looking for some outside knowledge
 
123petey said:
I’ve already stated that I’m not looking to “do” anything. I appreciate everyone’s efforts to help me understand how to gear up or down, I fully understand how that works. I understand ratios, and ratios don’t change. 1/2, 2/4, 4/8, all the same. It’s a little surprising that no one else is realizing that there has to be some difference when maintaining a ratio but changing sizes of sprockets? This is a ridiculous example but if I have 1 tooth on the front, and 2 on the back, you are saying the output shaft will be spinning the same revolutions as a 17/34 in relation to the output as the rear wheel to the ground? The rpms have to be higher to obtain the same speed, right?

I’m seriously not trying to be difficult or obtuse I’m just thinking out loud and looking for some outside knowledge
Click to expand...


The ratio is 2 to 1 ( 34T on the rear wheel and 17T on the tranny). That means the tranny output spins 2 revolutions for every 1 revolution of the rear wheel. You can boil this down to the rear has twice the circumference of the front. Without actually measuring, lets say17 inches around the front sprocket and 34 inches around the rear. Hell, make it a "V" belt drive.... no teeth... it's still a 2 to 1 ratio. Make it a cogged belt with 100 cogs on the front and 200 on the rear.... still the same 2 to 1 ratio. Use a shaft drive with 17 teeth on the pinion and 34 teeth on the ring gear. It's all the same... the ratio is 2 revolutions of the powered gear/pulley/sprocket/whatever for every 1 revolution of the driven gear/pulley/sprocket/whatever. 2 to 1 is 2 to 1.... be it 2 gazillion whatevers to 1 gazillion whatevers or 17 whatevers to 34 whatevers.... or 170 whatevers to 340 whatevers.... the result is 2 revolutions driving, that turns a wheel 1 revolution.
 
Mr 123petey - forget what gears are fitted but consider the rotational speed of the gearbox output shaft relative to that of the rear wheel.
If we decide that we want two turns of the output shaft to give one turn of the rear wheel we have a multitude of choices as to how that can be achieved.
It could be 10:20, 16:32, 17:34, 20:40, 40:80 and so on. The result would be two turns of the output shaft to one of the rear wheel. In an ideal world you would want to have as large a gear as possible to maximise chain life but this is often limited by the available space.

For any given engine speed the motor will generate the same amount of torque regardless of what gear ratios you choose. What you can do however, by changing ratio's either in the gearbox or chain sprockets, is to alter where (i.e. at what speed) maximum torque occurs. This is a compromise. By lowering the ratio's you will get improved acceleration but you will hit maximum revs at a lower speed or by increasing the ratio better fuel consumption.
What manufacturers will try to do is to get the best compromise between the two for the typical use for which the bike is intended.
 
123petey said:
. . . stock sprockets on the XS650 is 17/34, 2.00 ratio. If you change to 16/32, maintaining 2.00 ratio, what is the result? I’m assuming the engine is spinning faster, so more gas consumption . . .
Click to expand...

With all due respect, I think this is where you have tripped yourself up. Your assumption is not correct, the engine will not be spinning faster, or using more gas. With the ratio maintained, there will be no difference to how the engine works at all speeds.
 
Raymond said:
With all due respect, I think this is where you have tripped yourself up. Your assumption is not correct, the engine will not be spinning faster, or using more gas. With the ratio maintained, there will be no difference to how the engine works at all speeds.
Click to expand...

Now is where I try to confuse things by saying that it can be better to not have ratios that are evenly divided. And go one further to propose having the number of links in the chain also not evenly divisible by the number of teeth on either sprocket!
 
kshansen said:
Now is where I try to confuse things by saying that it can be better to not have ratios that are evenly divided. And go one further to propose having the number of links in the chain also not evenly divisible by the number of teeth on either sprocket!
Click to expand...

That is correct, and the idea is to avoid the same set of teeth & chain links constantly meeting up. If there is a minor fault on one tooth or one link, the effect is spread across the transmission not concentrated on one place. IIRC, the crankshaft sprocket, intermediate wheel and cam sprockets inside the classic Edward Turner Triumph twin had numbers of teeth chosen so that the same pair of teeth only meet up once on every gazillion engine revolutions. I was going to say they were prime numbers but that cannot be correct because the cam sprockets must have exactly twice as many teeth as the one on the crankshaft.
 
Look around here for the gearing spreadsheet. You can play with sprockets and see the effect. You will find 2 to 1 is 2 to 1 no matter how you get there
 
The only thing that changes say if you keep a ratio of 2 to 1 then (for arguments sake) is you have a 10/20 and go to a 15/30 is your chain would have extra length so it would travel more to make one revolution. This would make the chain and sprockets last a bit longer.
 
I understand what your saying..........visually things should change if the sprockets change size.

Doesn't in reality............the time it takes for the crank to travel one revolution, also is the same time a sprocket takes to travel one revolution. Doesn't matter what size, the front sprocket could be 100 teeth and the outside of the sprocket still takes the same time to do one revolution. ........as long as the two sprockets retain the ratio of 2:1 then the bike travels the same speed.

Here is where it can confuse.........the speed of the out side of the sprocket changes with different sizes.........it takes the same time for a 16 tooth sprocket to do one revolution, (crank revolution), as a 100 tooth sprocket, but the sprocket at the teeth, (one tooth), will be traveling faster to complete the revolution.......it has further to travel.

To put it another way. The circumference changes with the size of the sprocket............it takes the same time to do one revolution of the crank shaft as it does at the teeth of the sprocket......... the speed one tooth takes to do a revolution of the circumference increases as the size of the sprocket increases.........

again, as long as the ratio stays the same..... the speed/revolution transfer at the crank shaft to the rear wheel speed/revolution stays the same.......changing the amount of teeth on either of the sprockets, (change the ratio), will either increase or decrease the speed of a tooth doing one revolution. One tooth more on the front and the time it takes the tooth to do one revolution speeds up......the chain is traveling faster......this is transferred to the rear sprocket/wheel, (as long as the rear sprocket stays the same)

The crank is the constant
 
650Skull said:
I understand what your saying..........visually things should change if the sprockets change size.

Doesn't in reality............the time it takes for the crank to travel one revolution, also is the same time a sprocket takes to travel one revolution. Doesn't matter what size, the front sprocket could be 100 teeth and the outside of the sprocket still takes the same time to do one revolution. ........as long as the two sprockets retain the ratio of 2:1 then the bike travels the same speed.

Here is where it can confuse.........the speed of the out side of the sprocket changes with different sizes.........it takes the same time for a 16 tooth sprocket to do one revolution, (crank revolution), as a 100 tooth sprocket, but the sprocket at the teeth, (one tooth), will be traveling faster to complete the revolution.......it has further to travel.

To put it another way. The circumference changes with the size of the sprocket............it takes the same time to do one revolution of the crank shaft as it does at the teeth of the sprocket......... the speed one tooth takes to do a revolution of the circumference increases as the size of the sprocket increases.........

again, as long as the ratio stays the same..... the speed/revolution transfer at the crank shaft to the rear wheel speed/revolution stays the same.......changing the amount of teeth on either of the sprockets, (change the ratio), will either increase or decrease the speed of a tooth doing one revolution. One tooth more on the front and the time it takes the tooth to do one revolution speeds up......the chain is traveling faster......this is transferred to the rear sprocket/wheel, (as long as the rear sprocket stays the same)

The crank is the constant
Click to expand...

Or to put it another way, the larger the sprocket the greater the angular velocity
 
Understood. I appreciate all the input. I agree, and at the end of the day theoretically going from 17/34 to 16/32 if there would be any difference, would most likely be tiny. It’s one of those rabbit holes that when you’re alone with a few beers can be pretty steep. I still feel that there are other forces at work as the circumference changes - chain length, possible lugging that could generated, etc - but I agree, my head hurts, too, and I don’t want to think about it anymore.

Was just wanting a thoughtful dialogue on a thought that never crossed my mind before and couldn’t really find any info on it. Was definitely not trying to start an argument. Thanks for indulging
 
In other news I’m hopefully going to fire up my 700 big bore 277 rephased with the vape system for the first time today! It’s supposed to reach 115-120 degrees today so being in the garage is really not inviting...

Here’s a pic of how I mounted the system in an under seat tray.
 

Attachments

  • 6BF46666-87D7-4358-92DD-C505CE70D27C.jpeg
    6BF46666-87D7-4358-92DD-C505CE70D27C.jpeg
    146.5 KB · Views: 135
One disadvantage I could see with using smaller gears, besides the obvious of rubbing chain on something which would also be a problem for much larger gears, would be the smaller to gear the more the chain has to bend to go around it.
 

Similar threads

Muckroot
XSJohn's right side hotter theory - explored
2
Replies
21
Views
3K
xjwmx
xjwmx
FLEA
hi ratio gears
Replies
11
Views
2K
FLEA
FLEA
M
An Adventure in Firsts: '83 XS650 Heritage Special Build/Rebuild
38 39 40
Replies
785
Views
81K
Jim
Jim
G
Retarded
Replies
5
Views
879
TwoManyXS1Bs
TwoManyXS1Bs
MaxPete
Force Induction - not for the faint of heart!
Replies
5
Views
2K
MaxPete
MaxPete
Back
Top