EthansDad
02-16-2011, 11:59 AM
Hey All,
Been reading and learning on this forum for a couple of years now and wanted to share some advanced clutch tuning info I have not seen on here before. As an engineer in my day job, I really like a hard challenge. I can truly say after 3-4 years of working on CVTs and doing as much research as I can in my spare time, I still don't know much. I guess you have to be careful what challenges you want to take on! But some of these fundamentals have helped me get a better grasp of what's going on under the cover and how to tune for max performance.
If I have missed a major point, or glanced over the obvious for someone, please let us all know. I'm a big fan of open knowledge and don't claim to know half of anything as this is a hobby for me. I also can do a write up like this on spring selection if someone wants to hold me to it.
Advanced CVT Tuning Fundamentals
Basic operations of the CVT
I think enough has been written on basic operations of the CVT. The goal of tuning simply stated is to keep the transmission shifting such that the motor is always operating at peak HP. If you are not, you are wasting HP and all the expensive stuff you put on your motor does not matter. how you do that is easier said than done. Personally, I got tired of buying a ton of parts and trying them seemingly random and wanted a better model for parts selection. here is a quick overview of the model I'm currently using.
An Ideal CVT setup
let's take a perfect world look at CVT operation. getting these exact results is a never ending quest, but with some luck you should be able to get close. First, let's look at design goal #1 - choosing CVT operational RPMs.
https://lh3.googleusercontent.com/_axc6dhltIEE/TVv-IfHCAcI/AAAAAAAAAF4/vOvfzrKOrdM/s800/Picture%205.jpg
Figure 1
Take a look at figure one. we are looking at HP VS RPMs on the graph, much like what you'd get from a dyno run. based on this, at what RPM would you want your CVT to maintain? If I was using the lower HP motor, I'd want the CVT to always maintain 8k rpms. If I upgraded my motor to the second (higher) curve and did not change my clutch, I'd still make the same HP as the older motor since I'd still be shifting at 8k rpms - All my motor upgrades are wasted money. I'd need to change the clutching such that the system now shifts at 9k rpms to realize my motor upgrades. this is why knowing at what RPM you system operates is so important.
https://lh4.googleusercontent.com/_axc6dhltIEE/TVv-HWL_i4I/AAAAAAAAAFs/eje_hU36fEM/s800/Picture%202.jpg
Figure 2
now take a look at figure 2. this is the most important graph for CVTs and is a bit hard to understand at first glance what is going on here. What we are looking at on the axis is speed (in mph) VS motor RPMs. the two diagonal lines represent the low range and high range of the system. I'll talk a bit more about overall ratio, but for this graph the low range is the initial state of the CVT (has not started to shift yet) and the max speed the bike would get if you did not shift. think of taking all the weights out of your variator and running the bike. it would rev to top RPMs, but the MPH would be very slow since the initial CVT ratio has not changed. the high ratio is when the system is all the way shifted out, or the final state if you will.
now looking at figure 2 again, take a look at the dotted line. this is the goal, IMO of CVT setup. per the numbers on figure 2, what is happening here is the motor revs, but does not engage the clutch until around 5k-5.5k rpms. this would be your stall speed. after that, the bike accelerates (but does not yet begin to shift) until the motor reaches max hp RPM, which in this example is 9k rpms. the bike continues to accelerate and the CVT continues to change (upshift) the ratio until the final ratio of the CVT is reached - all while constantly maintaining peak HP rpms of 9k! Only when the final ratio is reached will the rpms climb up again, here in this example at around 100mph, the rpms climb from 9k to 11k.
Again, this figure two is the most important for using as a model for tuning, so let's look at it yet another way - how would a manual transmission look on the same graph?
https://lh3.googleusercontent.com/_axc6dhltIEE/TVv-ILA74mI/AAAAAAAAAFw/_2OewLNFqlI/s800/Picture%203.jpg
Figure 3
in figure three, we see the same graph as figure 2, but now with a manual transmission, and the ratio of each gear represented in a diagonal line on the graph. the common sense view to this is if I kept my car in 2nd gear, it would only reach 50mph no matter now much gas I gave it. Another interesting look here is how the CVT keeps the motor at peak HP rpms MORE OFTEN than a manual transmission. when you shift your car from 1-2nd, the rpms drop, then need to climb back up again. with a CVT, they just stay a peak HP rpms, therefore delivering more power to the ground more often.
major sub systems - driver, driven, belt
some new terms here, but same old parts we know. The driver or sometimes called the driving clutch is what controls the shift. this is often called in our world of ATVs a variator. the driven clutch is the rear pulley that houses the clutch and bell and torque spring. key thing to understand here is that the driver (variator) controls the shift speed and that the driven (rear pulley / torque spring) controls the backshift of the system and overall system efficiency. if you just upgraded your system (new pipe / cylinder, etc) and need to move your rpms of your CVT up from 9k to 10k, you will do this in the variator, period. if you manage to change this around with torque springs, you've done so at the cost of system efficiency loss. example, I go from soft torque spring to heavy torque spring and now my system shifts 1k rpm higher - yes, but now you are burning off that extra HP in heat inside your CVT cover to overcome a heavy spring and that power never gets to the ground!! changing the shift RPM will come from varying roller weights, variator types (roller weight ramp angle), variator sheave angles (angle of the pulley faces) and the spacing of the variator sheaves (often called shimming). The belt width, groove angle and length will also play a part. Can write more on this later, but key take away - start with your variator to change your shift speed, then come back around to finding the best torque spring for your setup.
CVT ratios explained
I've seen some questions and posts on here before relating final gearing to CVT setup. here is the deal - they are absolutely married to each other. look at the ATV as a system that starts at the crank shaft of the driver all the way to the rear wheel that applies power to the ground. Your system will have one overall intial ratio, and one overall final ratio with the ratio in between handled via the CVT - much like figure 2 but for the whole system, not just the CVT.
example - my motor has a peak RPM of 13k. my CVT is setup to reach over drive, or a final ratio of .75:1. I use the low speed tranny gears and 19/30T final drive. I also use 16" tires. what is my vehicles max theoretical speed?
answer:
-13k at Driver is actually increased (.75:1) such that the driven (rear pulley) is spinning at 17.33k RPMS
-the tranny then reduces by 9:1 such that the counter sprocket is spinning at 1925rpms
-the final gearing is 19/30, so the rear axle is spinning at 1219rpms.
-the 16 inch tires are also spinning at 1219 rpms or move the ATV forward 60,990 inches per minute.
-if you don't feel like doing the math on this one, use this link:http://wahiduddin.net/calc/calc_speed_rpm.htm
-Overall Answer - they ATV's final gearing could reach 58MPH - likely too high for MX setup.
-data used: 13k rpms, Transmission ratio = tranny 9:1 + CVT final .75:1 or 6.75:1. final gearing is 1.578:1 for the 19/30T combo
Why this is important - you can, by accident, design a system that is not practical at all and is wasting HP. the key goal here on overall system ratio - START with picking your top MPH and go backwards from there. for youth MX, 45mph is a good target, maybe less, maybe more you pick. if you add up your system ratio, using the final CVT ratio in the equations above, what is it? you might find the bike is capable or setup to reach 65mph! very nice, but will never use it on the mx track. a lower overall system ratio is needed to use every bit of power to get the bike from 0-45mph as quick as possible. if the rest of your final drive is leaning towards higher speeds like 65mph, and you are trying to bring it down with just the CVT, you have a hard job ahead of you. need to get the final ratios in line so your CVT will not be overworked and inefficient in shifting from 0-45mph on the track with a final drive that is designed for 0-65mph.
Using a tach to test
This write up likely raises more questions than it answers. there is no easy "bolt this on and you are good" answers here. if you don't like tinkering, find a good shop that will do it for you. for those that do like tinkering - take another look at figure 2 and get your tach out.
with a little practice you can measure not only stall, but also the constant shift RPM. can also find where your RPMs start to become non-linear or basically map out your power curve of your bike. I have yet to find a way to get the max HP rpm of a bike with only a tach, need to get that from your motor builder - but generally I can guess it within 200-300rpms by watching when the rate of rpms starts to tapper off and return to linear (1 bit of throttle push = 1 bit of rpm increase is linear -1 bit of throttle push for ~5 bits of rpm increase is non-linear).
biggest problem I see with most is their CVT leaves the initial ratio before peak HP rpms is reached. kind of like starting your car off in 2nd gear. can play with the variator and other parts to get what you are after with time and practice.
Good luck, and hope this helps.
-EA
Been reading and learning on this forum for a couple of years now and wanted to share some advanced clutch tuning info I have not seen on here before. As an engineer in my day job, I really like a hard challenge. I can truly say after 3-4 years of working on CVTs and doing as much research as I can in my spare time, I still don't know much. I guess you have to be careful what challenges you want to take on! But some of these fundamentals have helped me get a better grasp of what's going on under the cover and how to tune for max performance.
If I have missed a major point, or glanced over the obvious for someone, please let us all know. I'm a big fan of open knowledge and don't claim to know half of anything as this is a hobby for me. I also can do a write up like this on spring selection if someone wants to hold me to it.
Advanced CVT Tuning Fundamentals
Basic operations of the CVT
I think enough has been written on basic operations of the CVT. The goal of tuning simply stated is to keep the transmission shifting such that the motor is always operating at peak HP. If you are not, you are wasting HP and all the expensive stuff you put on your motor does not matter. how you do that is easier said than done. Personally, I got tired of buying a ton of parts and trying them seemingly random and wanted a better model for parts selection. here is a quick overview of the model I'm currently using.
An Ideal CVT setup
let's take a perfect world look at CVT operation. getting these exact results is a never ending quest, but with some luck you should be able to get close. First, let's look at design goal #1 - choosing CVT operational RPMs.
https://lh3.googleusercontent.com/_axc6dhltIEE/TVv-IfHCAcI/AAAAAAAAAF4/vOvfzrKOrdM/s800/Picture%205.jpg
Figure 1
Take a look at figure one. we are looking at HP VS RPMs on the graph, much like what you'd get from a dyno run. based on this, at what RPM would you want your CVT to maintain? If I was using the lower HP motor, I'd want the CVT to always maintain 8k rpms. If I upgraded my motor to the second (higher) curve and did not change my clutch, I'd still make the same HP as the older motor since I'd still be shifting at 8k rpms - All my motor upgrades are wasted money. I'd need to change the clutching such that the system now shifts at 9k rpms to realize my motor upgrades. this is why knowing at what RPM you system operates is so important.
https://lh4.googleusercontent.com/_axc6dhltIEE/TVv-HWL_i4I/AAAAAAAAAFs/eje_hU36fEM/s800/Picture%202.jpg
Figure 2
now take a look at figure 2. this is the most important graph for CVTs and is a bit hard to understand at first glance what is going on here. What we are looking at on the axis is speed (in mph) VS motor RPMs. the two diagonal lines represent the low range and high range of the system. I'll talk a bit more about overall ratio, but for this graph the low range is the initial state of the CVT (has not started to shift yet) and the max speed the bike would get if you did not shift. think of taking all the weights out of your variator and running the bike. it would rev to top RPMs, but the MPH would be very slow since the initial CVT ratio has not changed. the high ratio is when the system is all the way shifted out, or the final state if you will.
now looking at figure 2 again, take a look at the dotted line. this is the goal, IMO of CVT setup. per the numbers on figure 2, what is happening here is the motor revs, but does not engage the clutch until around 5k-5.5k rpms. this would be your stall speed. after that, the bike accelerates (but does not yet begin to shift) until the motor reaches max hp RPM, which in this example is 9k rpms. the bike continues to accelerate and the CVT continues to change (upshift) the ratio until the final ratio of the CVT is reached - all while constantly maintaining peak HP rpms of 9k! Only when the final ratio is reached will the rpms climb up again, here in this example at around 100mph, the rpms climb from 9k to 11k.
Again, this figure two is the most important for using as a model for tuning, so let's look at it yet another way - how would a manual transmission look on the same graph?
https://lh3.googleusercontent.com/_axc6dhltIEE/TVv-ILA74mI/AAAAAAAAAFw/_2OewLNFqlI/s800/Picture%203.jpg
Figure 3
in figure three, we see the same graph as figure 2, but now with a manual transmission, and the ratio of each gear represented in a diagonal line on the graph. the common sense view to this is if I kept my car in 2nd gear, it would only reach 50mph no matter now much gas I gave it. Another interesting look here is how the CVT keeps the motor at peak HP rpms MORE OFTEN than a manual transmission. when you shift your car from 1-2nd, the rpms drop, then need to climb back up again. with a CVT, they just stay a peak HP rpms, therefore delivering more power to the ground more often.
major sub systems - driver, driven, belt
some new terms here, but same old parts we know. The driver or sometimes called the driving clutch is what controls the shift. this is often called in our world of ATVs a variator. the driven clutch is the rear pulley that houses the clutch and bell and torque spring. key thing to understand here is that the driver (variator) controls the shift speed and that the driven (rear pulley / torque spring) controls the backshift of the system and overall system efficiency. if you just upgraded your system (new pipe / cylinder, etc) and need to move your rpms of your CVT up from 9k to 10k, you will do this in the variator, period. if you manage to change this around with torque springs, you've done so at the cost of system efficiency loss. example, I go from soft torque spring to heavy torque spring and now my system shifts 1k rpm higher - yes, but now you are burning off that extra HP in heat inside your CVT cover to overcome a heavy spring and that power never gets to the ground!! changing the shift RPM will come from varying roller weights, variator types (roller weight ramp angle), variator sheave angles (angle of the pulley faces) and the spacing of the variator sheaves (often called shimming). The belt width, groove angle and length will also play a part. Can write more on this later, but key take away - start with your variator to change your shift speed, then come back around to finding the best torque spring for your setup.
CVT ratios explained
I've seen some questions and posts on here before relating final gearing to CVT setup. here is the deal - they are absolutely married to each other. look at the ATV as a system that starts at the crank shaft of the driver all the way to the rear wheel that applies power to the ground. Your system will have one overall intial ratio, and one overall final ratio with the ratio in between handled via the CVT - much like figure 2 but for the whole system, not just the CVT.
example - my motor has a peak RPM of 13k. my CVT is setup to reach over drive, or a final ratio of .75:1. I use the low speed tranny gears and 19/30T final drive. I also use 16" tires. what is my vehicles max theoretical speed?
answer:
-13k at Driver is actually increased (.75:1) such that the driven (rear pulley) is spinning at 17.33k RPMS
-the tranny then reduces by 9:1 such that the counter sprocket is spinning at 1925rpms
-the final gearing is 19/30, so the rear axle is spinning at 1219rpms.
-the 16 inch tires are also spinning at 1219 rpms or move the ATV forward 60,990 inches per minute.
-if you don't feel like doing the math on this one, use this link:http://wahiduddin.net/calc/calc_speed_rpm.htm
-Overall Answer - they ATV's final gearing could reach 58MPH - likely too high for MX setup.
-data used: 13k rpms, Transmission ratio = tranny 9:1 + CVT final .75:1 or 6.75:1. final gearing is 1.578:1 for the 19/30T combo
Why this is important - you can, by accident, design a system that is not practical at all and is wasting HP. the key goal here on overall system ratio - START with picking your top MPH and go backwards from there. for youth MX, 45mph is a good target, maybe less, maybe more you pick. if you add up your system ratio, using the final CVT ratio in the equations above, what is it? you might find the bike is capable or setup to reach 65mph! very nice, but will never use it on the mx track. a lower overall system ratio is needed to use every bit of power to get the bike from 0-45mph as quick as possible. if the rest of your final drive is leaning towards higher speeds like 65mph, and you are trying to bring it down with just the CVT, you have a hard job ahead of you. need to get the final ratios in line so your CVT will not be overworked and inefficient in shifting from 0-45mph on the track with a final drive that is designed for 0-65mph.
Using a tach to test
This write up likely raises more questions than it answers. there is no easy "bolt this on and you are good" answers here. if you don't like tinkering, find a good shop that will do it for you. for those that do like tinkering - take another look at figure 2 and get your tach out.
with a little practice you can measure not only stall, but also the constant shift RPM. can also find where your RPMs start to become non-linear or basically map out your power curve of your bike. I have yet to find a way to get the max HP rpm of a bike with only a tach, need to get that from your motor builder - but generally I can guess it within 200-300rpms by watching when the rate of rpms starts to tapper off and return to linear (1 bit of throttle push = 1 bit of rpm increase is linear -1 bit of throttle push for ~5 bits of rpm increase is non-linear).
biggest problem I see with most is their CVT leaves the initial ratio before peak HP rpms is reached. kind of like starting your car off in 2nd gear. can play with the variator and other parts to get what you are after with time and practice.
Good luck, and hope this helps.
-EA