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Old 05-16-2012, 11:50 PM
TaterBall TaterBall is offline
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Talking Changing final drive pulleys and belt sizes

You guys are a wealth of knowledge. I have read threads on here until my eyes are bleeding, and still have questions, Hope you guys can help. A little information to set the stage: 2002 heritage softail, with a wide tire kit and the belt is rubbing on the lower frame. I am looking at going to smaller pulleys to get the belt to ride higher off of the frame, and keep around the stock ratio. it has the stock 32 trans pulley and the 70 rear wheel pulley, and a 135 tooth belt and stock 25/36 primary. it has a 240/40-18 rear tire. I have located online a final drive ratio calculator which I have provided the link here to it. http://www.scootersperformance.com/t..._rpm_calc.html.
A question to you guys is how accurate do you believe this calculator is?

I am looking at going to a 30 trans and 66 rear wheel or a 31 and a 66 rear wheel, and they both will use a 132 tooth belt according to the calculator.
I have read in threads a few months ago and cant seem to find them again that a change in the trans pulley is equal to 4 percent per tooth, and the rear is equal to 2 percent per tooth, and also read trans at 4 percent and rear at 3 percent. both of these sets of numbers dont seem to me to be the same as the calculator. Also that a tooth change in the belt amounts to about an 1/8" per tooth on the position of the rear axle.

What is you guys honest opinion if a change to a 30 or 31 trans, and the 66 rear with a 132 belt will fit my application with what I have stock on it now? The calculator says it will.

Whew that was a mouth full.
Trying to do the math before I tear it apart and it all be wrong.
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Old 05-17-2012, 04:54 AM
Smdugan40 Smdugan40 is offline
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http://www.masterdrives.com/masterbo...ing%20Info.pdf
This is a link to some basic formulas you can uses for various calculations and you can double check that online app you posted
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Old 05-17-2012, 05:03 AM
Smdugan40 Smdugan40 is offline
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IIRC from my school days a simple formula would be:
D x rpm = d x rpm
D = dia of larger
d = dia of smaller
Also IIRC
T x rpm = t x rpm
T= teeth of large gear
t = teeth of small gear
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Old 05-17-2012, 08:08 AM
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suiii15 suiii15 is offline
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Simple math would say that formula is incorrect. According to the transitive property if you divided both sides by rpm then according to your math T=t and D=d, which is not going to be true.
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Old 05-17-2012, 04:29 PM
Smdugan40 Smdugan40 is offline
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Not sure how I'm wrong but like I said IIRC which means I could very well be recalling it incorrectly but as an example:
If you have a 12" pulley turning lets say 1000 rpm
12 x 1000 = 12,000 and it's attached via belt to a 10" pulley 12,000/10 = 1,200 meaning the 10" pulley is turning at 1200 rpm
Or let's say a 31 tooth gear is turning at 2000 rpm:
31x 2000= 62,000 so a 31 tooth gear turning a 66 tooth gear 62,000/66 = 939.3939 so the 66 tooth gear is turning at almost 940 rpm.
If I'm incorrect I have no problem admitting it.
It won't be the first time nor the last lol
Just please let me know where I'm going wrong so I can learn from my mistake and not make the same mistake twice thanks
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Old 05-17-2012, 07:56 PM
TaterBall TaterBall is offline
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Thanks guys, that is good input. But can you tell me is the calculator spitting out correct information that a 30 tooth trans and a 66 tooth rear is only 17 rpm higher than the 32/70 that is on it now? And also it says that a 132 belt will work where the current is 135? Is this information correct?
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Old 05-17-2012, 08:08 PM
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You wrote T x rpm = t x rpm. that is not possible. If it was then 70tooth x 1000 rpm = 33t x 1000rpm. 70,000 does not equal 33,000.
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Old 05-17-2012, 09:49 PM
Smdugan40 Smdugan40 is offline
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Suii i think there is just a misunderstanding of what how i was explaining the formula
This formula is for figuring either the teeth if u know rpm or rpm if you know number of teeth
A 77t gear turning at 1000 rpm would turn a 33t gear 2121.212121 rpm if T x rpm = t x rpm the inverse would be to divide by which ever number you do know on the other side of the equation to get the number you don't have on that side of the equation
you need to know 3 of the numbers in the drive train to find the missing tooth count or rpm
So to answer your question with this formula tater you would need to know the rpm of the 32 tooth gear or the rpm of the 70t gear and the rpm of either the 30t gear or the 66t gear
Multiply the 30t gear by its rpm than divide by 66 to get rpm of the 66t
Than multiply the 32t gear by its rpm and divide by 70 to get the rpm of that combo
Subtract your final rpm of the first gear combo from the rpm of the second gear combo to know the difference between gear combos
Use the 32/70 combo to fill out the formula
32t x 1000 = 70t x 457.142857
32,000= 70 x 457.142857
70 x 457.142857=32,000 therefore the equation is indeed equal
As towards your belt question my memory is much fuzzier but you should be able to divide the number of teeth on your big gear by the number of teeth on your smaller gear to get your ratio so:
66/30= 2.2
70/32= 2.185
So to my fuzzy memory it appears that you would need a different belt to mesh correctly with the different gear set I'm sure someone with much more actual motorcycle experience can give you a better answer as to exactly which belt/tooth combo would work I am simply generalizing basic gear sets
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Old 05-18-2012, 07:22 AM
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So a better way of putting it would be
T x r = t x R
You cant represent the same variable with multiply numbers.
I'm still not sure how this would work since you need to already know the tooth count of the pulleys to make this work and that is what the guy was looking for, tooth count.
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Old 05-18-2012, 09:48 AM
Smdugan40 Smdugan40 is offline
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Or u need to know tooth count of 1 gear and rpm of both gears
It can also tell you final rpm at rear wheel based on what the tooth count is of your gears and the rpm of the driving gear
Guess I misunderstood the question thought it was "what is the final driven rpm if you have a 30t drive gear and a 66t driven gear and the same if you have a 32t drive gear and a 70t driven gear"
My bad on misunderstanding what was being asked
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Old 05-18-2012, 09:48 AM
 
 
 
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