gearing calculations--a lil brain teaser.

[FROSTY:Me]

A bike is running 15/47 gearing as standard.
The owner has access to 46 and 45 tooth rear sprockets and 14 and 16 tooth fronts.
To gear the bike up by 4%-or closest to it which combination of sprockets would YOU suggest??
Likewise to gear the bike DOWN by 5% what would you do
A lil test of KB brains

[cowpoos]

I've been think about getting sky for a while now...but I'm still undecided...I wanna see the superbike races but really theres not much more that would interest me I don't think.
so I'm still pondering...

[Jantar]

A bike is running 15/47 gearing as standard.
The owner has access to 46 and 45 tooth rear sprockets and 14 and 16 tooth fronts.
To gear the bike up by 4%-or closest to it which combination of sprockets would YOU suggest??
Likewise to gear the bike DOWN by 5% what would you do
A lil test of KB brains

Original ratio = 0.319

To gear up by 4% 15/45 Ratio = 0.333

To gear down 5% 14/46 Ratio = 0.304

[FROSTY:Me]

aww jant--spoil my fun
Ok calculate all variables with sprockets available then

[Flyingpony]

Original ratio = 0.319
To gear up by 4% 15/45 Ratio = 0.333
To gear down 5% 14/46 Ratio = 0.304

Second that.

[Flyingpony]

Ok calculate all variables with sprockets available then

14/45 Ratio = 0.311 -2.519%
14/46 Ratio = 0.304 -4.638%
14/47 Ratio = 0.298 -6.667%

15/45 Ratio = 0.333 4.444%
15/46 Ratio = 0.326 2.174%
15/47 Ratio = 0.319 Original

16/45 Ratio = 0.356 11.407%
16/46 Ratio = 0.348 8.986%
16/47 Ratio = 0.340 6.667%

Percentages are a change over/under original.

[The Stranger:Me]

So you have worked out the ratio using the number of teeth.

NOT arguing, however when I did the my CBTA at Ride Rite the guy taking it stated that this was incorrect. He stated that that approach was an approximation. He said that the correct way was as a ratio of the radius of the sprockets.

Just put it forward it as a thought.

[vifferman]

Never mind the calculations - 15T is about the limits of how small you want to go on a front sprocket for a 525 or 530 chain, at least. The smaller the sprocket, the more stress on the chain having to change direction traversing it. Yes, front sprockets are cheaper, but (a) rear sprocket changes give you smaller graduations of gearing changes, and (b) a larger rear sprocket is easier on your chain (and probably robs less power) than a smaller front sprocket.

[vifferman]

So you have worked out the ratio using the number of teeth.

NOT arguing, however when I did the my CBTA at Ride Rite the guy taking it stated that this was incorrect. He stated that that approach was an approximation. He said that the correct way was as a ratio of the radius of the sprockets.

True, but it's not much of an approximation, so it's close enough, and easier to measure.

[FlyingDutchMan]

So you have worked out the ratio using the number of teeth.

NOT arguing, however when I did the my CBTA at Ride Rite the guy taking it stated that this was incorrect. He stated that that approach was an approximation. He said that the correct way was as a ratio of the radius of the sprockets.

Just put it forward it as a thought.

Actually, its dead on. If the front sprocket turns one tooth, the rear sprocket turns one tooth - chain don't slip (if they do, you're in the shit). The amount it turns is determined angular distance between each tooth, which can be directly determined by the number of teeth.

[TwoSeven]

A bike is running 15/47 gearing as standard.
The owner has access to 46 and 45 tooth rear sprockets and 14 and 16 tooth fronts.
To gear the bike up by 4%-or closest to it which combination of sprockets would YOU suggest??
Likewise to gear the bike DOWN by 5% what would you do
A lil test of KB brains

First of all it depends on the internal gearing of the bike, the rpm its going to run on average, whether acceleration is desired over speed and the driving force (power being applied to the rear wheel).

You cant just knock up a sprocket change (of +/- 5%) and expect it to solve the problem, the tradeoffs being given away may end up being bigger penalties than the benifit that is being expected.

What is the bike and scenario you are trying to solve the gearing for ?

[The Stranger:Me]

Just changed the YZF750 from 15/41 to 15/47.
Pretty wicked.
5500 at 100kph in top so should topout at about 235kph (assuming redline), awesome pickup in any gear and great for wheelies and burnouts.

[TwoSeven]

So you have worked out the ratio using the number of teeth.

NOT arguing, however when I did the my CBTA at Ride Rite the guy taking it stated that this was incorrect. He stated that that approach was an approximation. He said that the correct way was as a ratio of the radius of the sprockets.

Just put it forward it as a thought.

I suspect he is using pitch circle diameter. I think tho, that given a certain chain type, the PCD can be worked out by the number of teeth on the sprocket.

[far queue]

Never mind the calculations - 15T is about the limits of how small you want to go on a front sprocket for a 525 or 530 chain, at least. The smaller the sprocket, the more stress on the chain having to change direction traversing it. Yes, front sprockets are cheaper, but (a) rear sprocket changes give you smaller graduations of gearing changes, and (b) a larger rear sprocket is easier on your chain (and probably robs less power) than a smaller front sprocket.

My bike came standard with 15/41, which was OK on the road but way too high off road. I now run a 45 on the rear all the time and vary the front between 13, 14, 15 depending on the riding I want to do. 13/45 is great off road, gives me good engine revs at low low speeds and close gear ratios, but top speed suffers a lot on the road. 15/45 gives me good road speeds, and wider gear ratios which is fine on the road, but low speed off road suffers. 14/45 is a compromise between the two. I regurlary change the front sprocket, it's only a 10 minute job.

Also the chain is 525. The 14 & 15 sprockets are 525. The 13 & 45 sprockets are 520. You can run a 525 chain on 520 sprockets with no problem as the only difference is chain width, and in fact it helps to clear mud from the chain & sprockets.

[nadroj]

Remembering back to my cycling days - by multiplying the number of teeth on the front sprocket by the diameter of the rear tyre (rolling diameter) then divide that by the number of teeth on the rear sprocket will give you the distance covered by the bike per revolution of the front sprocket. Hence by knowing the final drive ratio of the gearbox in any gear the top speed of any gear can be calculated at redline. How good is my memory?