elephant35e
Member
- Nov 14, 2012
- 7
- 0
i have a yz125 and i heard its stock is 13/49 gearing. how much speed will 14/52 gearing decrease?
Last edited:
how much speed in mph will adding sprocket teeth decrease?
- Thread starter elephant35e
- Start date
Need to know engine rpm and transmission gear ratios before we can start the math. And actual tire size will help, a 100/90-19 has a smaller diameter than a 110/90-19. Unfortunately there are other variables involved in the mph number you are asking for.
However for 13/49, you rear wheel will turn 0.26530612244 rotations for every rotation of the countershaft. With the 14/52 gearing you will get 0.26923076923 rotations from the tire for every countershaft rotation. Roughly a 1.46% gain in speed. Why so small? You are going larger on both ends, general rule of thumb is 1 tooth on the countershaft is 3 teeth on the rear sprocket. Your scenario supports this common theory. You will only be adding rotational weight on both the engine and the wheel and will likely need a longer chain which means even more weight to get spinning. The bike should accelerate less quickly, but it may not be enough to notice. Stay with a 13 on the countershaft and the 52 out back and you can expect a 6.1% speed reduction. These numbers are purely mathematical and do not take into consideration torque multiplication or coefficient of drag.
The formula to calculate this yourself is: 1-((old countershaft/old sprocket)/(new countershaft/new sprocket)) --extra parentheses for those who forget about order of operations
However for 13/49, you rear wheel will turn 0.26530612244 rotations for every rotation of the countershaft. With the 14/52 gearing you will get 0.26923076923 rotations from the tire for every countershaft rotation. Roughly a 1.46% gain in speed. Why so small? You are going larger on both ends, general rule of thumb is 1 tooth on the countershaft is 3 teeth on the rear sprocket. Your scenario supports this common theory. You will only be adding rotational weight on both the engine and the wheel and will likely need a longer chain which means even more weight to get spinning. The bike should accelerate less quickly, but it may not be enough to notice. Stay with a 13 on the countershaft and the 52 out back and you can expect a 6.1% speed reduction. These numbers are purely mathematical and do not take into consideration torque multiplication or coefficient of drag.
The formula to calculate this yourself is: 1-((old countershaft/old sprocket)/(new countershaft/new sprocket)) --extra parentheses for those who forget about order of operations
elephant35e
Member
- Nov 14, 2012
- 7
- 0
- Thread starter
- #3
oh! my gearing was already 14/52, so i just wanted to know how much speed difference.
didn't know i would gain speed!
but here are things you asked for:
engine rpm - makes power at about 11,200
Gear ratio: 1st 27/12 (2.250)
2nd 31/17 (1.824)
3rd 29/19 (1.526)
4th 24/19 (1.263)
5th 25/23 (1.087)
my yz125 is a 5 speed, so that's why there's no 6th gear.
don't know about tire size.
didn't know i would gain speed!
but here are things you asked for:
engine rpm - makes power at about 11,200
Gear ratio: 1st 27/12 (2.250)
2nd 31/17 (1.824)
3rd 29/19 (1.526)
4th 24/19 (1.263)
5th 25/23 (1.087)
my yz125 is a 5 speed, so that's why there's no 6th gear.
don't know about tire size.
OK, I will be dealing with a couple of best guesses here. First, I forgot to ask for the primary gear reduction, the best guess that I have is 64/19 (3.368). Then I will use a standard 100/90-19 tire and use "perfect world" numbers to get a circumference (knob height, tire pressure, and soil type are just a few possible factors that I am ignoring).
So, we will be dividing 11200 engine rpm by the primary reduction of 3.368 to get an input shaft speed of 3325 rpm. We now apply the 5th gear transmission ratio of 1.087 to learn that the output shaft is spinning at 3059 rpm. Factoring in a 14/52 final drive gives a wheel speed of 823.58 rpm. After doing the math for wheel diameter (90 mm sidewalls on a 19 inch wheel, 26 in total diameter, 82 inch circumference), the theoretical top speed is 63.92 mph.
With everything else remaining the same, but swapping to a 13/49, the theoretical top speed becomes 62.98.
BTW, the larger cogs could have the positive effect of smoothing acceleration and deceleration, extra rotating weight isn't always a bad thing, just ask the woods guys.
So, we will be dividing 11200 engine rpm by the primary reduction of 3.368 to get an input shaft speed of 3325 rpm. We now apply the 5th gear transmission ratio of 1.087 to learn that the output shaft is spinning at 3059 rpm. Factoring in a 14/52 final drive gives a wheel speed of 823.58 rpm. After doing the math for wheel diameter (90 mm sidewalls on a 19 inch wheel, 26 in total diameter, 82 inch circumference), the theoretical top speed is 63.92 mph.
With everything else remaining the same, but swapping to a 13/49, the theoretical top speed becomes 62.98.
BTW, the larger cogs could have the positive effect of smoothing acceleration and deceleration, extra rotating weight isn't always a bad thing, just ask the woods guys.
- Nov 25, 1999
- 7,926
- 43
The 13/48(.2708) vs 14/52(.2692) ratios are nearly the same. Simplisticly, 60 mph * .0016 (.02708 - .02692)= .096mph or less than 1/10 of a mph. This is assumes that your top speed is 60 mph which is close to the FruDaddy's calcs.
You will move the power delivery some, not in relation to engine rpm but in the speed that it occurs. Speed in dirt bikes is all relative unless you are trying to street legal your bike. Being "quick" or "fast" has more to do with transitioning from braking/accellerating and back. The "fast guys" have the ability to get on the power sooner and the brakes later which really makes the quicker rather than faster. However, this makes their lap times less therefore their overall speed "faster".
You will move the power delivery some, not in relation to engine rpm but in the speed that it occurs. Speed in dirt bikes is all relative unless you are trying to street legal your bike. Being "quick" or "fast" has more to do with transitioning from braking/accellerating and back. The "fast guys" have the ability to get on the power sooner and the brakes later which really makes the quicker rather than faster. However, this makes their lap times less therefore their overall speed "faster".
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