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15 tooth sprocket

A larger front sprocket makes the final gearing taller and increases the peak speed in each gear. A larger rear sprocket does just the opposite. Gearing will certainly affect MPG, but so will your riding habits. The guy whose constantly goosing his throttle may only get 20 MPG where as the guy who takes it easy may get 30 MPG in the same environment. The amount and frequency of twisting your wrist can a very profound impact on your MPG.

qadsan has it right...this explaination may help some more. Increasing the size of the front sprocket makes it nearer to the size of the rear sprocket, making the gear ratio closer to 1:1. Decreasing the size of the front has the opposite effect, of course. The rear sprocket is the opposite, with a smaller rear sprocket bringing us closer to 1:1. The greater the difference in diameter, the more torque multiplication (and rpm at a given speed in a given gear). 15 teeth on the front makes for great highway cruising, like an overdrive. A thirteen tooth sprocket up front makes for shoulder-dislocating acceleration (on uncorked xr650R's). I love the feel of the thirteen, and I put up with slower top speed and less mpg in trade for the big-bore two-stroke like acceleration.

I do alot of dual sport riding where we well cover 300 miles a day, I already have a IMS 4.6 gallon tank and have jetted the bike for best overall performance and milage. The more miles I can get out of my bike the less fuel I have to carry on my back. I think I will give the 15 tooth front sprocket a try and see how it works. It still doesn't make sence to me, but physics was never my strong point. Thanks for your input.

It still doesn't make sence to me,

Think of it this way. Picture your front sprocket as a 3 inch diameter wheel. The circumference of a 3" wheel is 9.42 inches (3 * 3.14). Now install a larger 4" front sprocket, which has a circumference of 12.56". Everytime the drive shaft makes one complete revolution, the outside of the 3" front sprocket travels 9.42" where as the larger front 4" sprocket covers 12.56" for one revolution. If each sprocket makes one revolution in the same amount of time, the larger sprocket will have moved the chain (which in turn rotates the rear wheel) a greater distance and at a faster rate than the smaller sprocket. This is why your peak speed increases in each gear with a larger front drive sprocket.

OK, now for the rear sprocket. Think of your rear sprocket as an 9" wheel, which has a circumference of 28.26". If you install a 12" rear sprocket, it will have a circumference of 37.68". A 3" diameter front sprocket will have to make three complete revolutions before a 9" rear sprocket makes one complete revolution. If you install a larger 12" rear sprocket, the front 3" sprocket will have to make 4 complete revolutions before the 12" rear sprocket makes one complete revolution. As you can see, the rear wheel speed will decrease with a larger rear sprocket because of its greater circumference.

I hope this helps a bit

Teacher... will this be on the test?

Impressive Qadsan, you explained in a few words what I have had physics professors spend a whole lecture on!

Maybe you can simplify some Human Physiology for me?

Cardiac Output and Vascular Resistance’s affect on blood pressure is just physics/hemodynamics!

Keep up the good work!

JT

It still doesn't make sence to me,

Think of it this way. Picture your front sprocket as a 3 inch diameter wheel. The circumference of a 3" wheel is 9.42 inches (3 * 3.14). Now install a larger 4" front sprocket, which has a circumference of 12.56". Everytime the drive shaft makes one complete revolution, the outside of the 3" front sprocket travels 9.42" where as the larger front 4" sprocket covers 12.56" for one revolution. If each sprocket makes one revolution in the same amount of time, the larger sprocket will have moved the chain (which in turn rotates the rear wheel) a greater distance and at a faster rate than the smaller sprocket. This is why your peak speed increases in each gear with a larger front drive sprocket.

OK, now for the rear sprocket. Think of your rear sprocket as an 9" wheel, which has a circumference of 28.26". If you install a 12" rear sprocket, it will have a circumference of 37.68". A 3" diameter front sprocket will have to make three complete revolutions before a 9" rear sprocket makes one complete revolution. If you install a larger 12" rear sprocket, the front 3" sprocket will have to make 4 complete revolutions before the 12" rear sprocket makes one complete revolution. As you can see, the rear wheel speed will decrease with a larger rear sprocket because of its greater circumference.

I hope this helps a bit

So if you want to be the fastest thing around, put the 12" sprocket on the front and the 3" sprocket on the back. Hold on tight

LOL then your top speed will only be about 10mph LOL

I got it now. If I would of just pulled out the old mountain bike and took it for a spin around the block and play with the gears I could of saved myself some time with my backwards logic. I do appreciate your efforts.

LOL then your top speed will only be about 10mph LOL

Nope... 200mph!

You've got it backwards.

Someone care to do the math? You can actually figure out the top speed of that combo... in thoery anyway. And if the bike had the balls to pull that. But then it'd have to be a 1800cc or whatever.

You can also look at it this way;

(for my simple mind)

a 14 counter raised to a 15, is the same or similar as a 3 tooth larger rear.

So... a;

14/50 combo is same as a 15/53-54.

or

15/45 is the same as 14/41-42

Or if you have a 50 tooth rear, and want more top end... add a larger counter (by one tooth).

For the same scenario as above... you can also install a 47 rear instead of changing countershaft.

NOW... remember... change length changes too!!! Large changes may require a chain. Hence the counter shaft sprocket is usually easiest.

I run a 16/48 for Baja and all street.

and a 15/50 with a 10 cup paddle for dunes (5 teeth difference)

uhhh... where was I?! Man... all these numbers and arithmetic. I'm losin' it. And it's easy as 1-2-3 math! hehehe... DOH!

WOW! a 16 tooth? Didn't know they made them that big! I'd love to see what your top end is! (Probably wet my pants though!)

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I know this was covered in old post, but I wasn't paying attention. If I run a 15 tooth sprocket on my pig, what is the differences over the stock 14 tooth gearing. My friend tells me he gets about a 3 mile increase per gallon and is faster on the top end. Maybe I'm backwards in my thinking process, but wouldn't it give you the opposite effect? It seems like you would run at a higher rpm and be slower on the top end.