Clutching theory

[devilrmk]

Please tell us what you have learned about clutching. Asking questions about theory is OK also.

Keep posts about clutching THEORY not what one setup works with one sled.

EDIT BELOW (by milehighassassin)

I am adding this post that is compliments of dgreet77 This is great info!

I'm working on trying to consolidate some basic clutching info to post up just to give beginners a place to start. I've got a bunch of stuff down that should do such a thing, but was wanting to make sure it was all correct. Not only this but anything you could add would be great.

Namely something that I would like to get more info on is how the spring rates affect things. EX higher first number on drive spring = higher engagement, etc. Also how to fix slippage when using heat to determine which clutch is slipping more.

Anyways here is what I have. Let me know. Thanks!

Primary Springs​

• Use to control engagement
• Higher numbers = stiffer spring
• Higher first number = higher engagement
• Higher numbers generally = more weight needed
• Too high last number = incomplete shift
• Too low last number = too quick shift

Primary Clutch​

• Use to control RPM’s via weights
• Use to control engagement via spring
• If hotter than secondary belt slipping more in primary

Flyweights​

• Use to control RPM’s – more weight = less RPM’s
• Heel (close to pin) weight affects beginning of shift (slow speeds)
• Toe weight affects end of shift (high speeds)
• More heel weight = better low speed (crawling)
• More toe weight = better high speed

Secondary Springs​

• Use to control backshift
• Softer spring = more up-shift = more MPH
• Stiffer spring = better backshift
• Stiffer spring keeps clutch in “lower gear” longer

Secondary Clutch​

• Is torque sensing
• Use to control Backshift
• Up shift = clutch opening =increasing speed = shifting up/out
• Backshift = clutch closing =decreasing speed = shifting down
• If hotter than primary belt slipping more in secondary

Helix​

• Use to control upshift
• Shallower angle (smaller number) = faster upshift and slower backshift

 

[devilrmk]

I will start it off. First of all I don't know much about clutching.

I hear you guys talking about steeper helix angles. Correct me if I am wrong, the bigger the # the steeper the angle, correct?

 

[krom]

Aaen has written what is considered to be the bible on clutch tuning, it is recommended reading by the OEM's for certified techs:
AAen's clutch tuning handbook


buy it and leave it someplace you can read it over and over again

 

[AKSNOWRIDER]

generally speaking, the heavier the primary weight/spring combo, the less sensitive the clutching is to elevation changes.......also the shallower the helix angles, the faster the back shift,but the slower the upshift also...

 

[2XM3]

Yup, get his book, read about 4 times..only change 1 thing at a time and you will start to get a good feel of how it all works

 

[XC700116]

Well, While I agree that everyone that is going to change anything on their clutching needs to read Olav Aaen's book lets get this conversation going and take the information base that is on this site and share it.

Here's some basic things to add to it Taken from Olav Aaen's Book

Keep in mind the are general and some things do and do not apply to different styles of clutches. For instance Team Clutches do not use a torsional spring only a compresion sping. Also there are effects of where weight is added to a flyweight weather heal or toe wieght is added also effects the shift.

Primary Clutch

Spring
The Pressure spring is listed by 3 properties: Enguagement Load, Full Shift Load, And Rate. Enguagement Load is the load the flyweight needs to overcome the spring and enguage the belt. Full Shift Load is the load the flyweigh needs to overcome at the end of the shift when the machine runs at top speed. The Rate of the spring controls the shift RPM during the shift and influences wheather the RPM increases, decreases, or stays the same during the shift.

The Flyweight
The flyweight has 2 properties used for tuning, the weight and the curvature. The weight influences how much RPM is needed to overcome the loads of the primary spring and the secondary clutch. Light weights need higher RPM and Heavy weights lower RPM. The curvature determines Enguagement speed and the agressiveness of acceleration.

Secondary Clutch

Spring
The secondary spring has a torsional and compression rate. Depending on the installed length and tortional load, the total spring load acting though the Helix determines shift out point, low ratio, and shift curve. Increaseing prtention load by moving the spring to different holes in the helix to give more angle pretension increases the RPM where the belt starts moving into the secondary sheave. The rate of the spring influences how the RPM changes during the shift. Higher rates will increase RPM at top speed. Springs are usually listed by tortional rate.

Helix
The Helix cam has only one property that we are interested in, cam angle. Angles are listed as either straight or progressive. In most cases cams start out with a larger angle to give harder acceleration at the beginning of the shift, at the end of the shift the angle is reduced to keep the RPM higher. By changing the angles the shift curve can be changed, small angles have higher shift speeds, larger angles have more aggressive shift out.

 

[LRD]

An explanation of the effects of a compression only sec spring as to starting spring lbs, finish lbs and rate would be great.

Thanks

 

[XC700116]

An explanation of the effects of a compression only sec spring as to starting spring lbs, finish lbs and rate would be great.

Thanks

they are about the same function just a different way about getting it in the case of the team clutches. Start and finish loads and the rate affect it in the same manner. ie higher rate and loads will raise RPM required to shift the clutch.

 

[Barrett]

you should never use the sec spring to tune rpm. if rpms are not correct change the flyweight. If rpms are not correct durring backshift or ON/OFF throttle situations, then the helix angle/sec spring can be changed to correct the problem.

 

[XC700116]

you should never use the sec spring to tune rpm. if rpms are not correct change the flyweight. If rpms are not correct durring backshift or ON/OFF throttle situations, then the helix angle/sec spring can be changed to correct the problem.

Agreed for the most part, I think what your getting at is not to use the sec spring to change your peak running RPM, however adjusting your finishing angle can work for your peak RPM but this is for during the shift than at full shift.

For example if your sled runs too high of an RPM during the shift but settles down to your desired RPM at top end then you should be increaseing the angle or lowering spring tension, likewise if your running too low RPM during the pull and then at top end finally reaches your target RPM you can choose either a lower angle helix or a stiffer spring.

It's a balancing act to achieve the disired outcome without creating a situation where the clutches are fighting each other. Attack each symptom individually and only make one change at a time. Up shift, Backshift, peak RPM, engagement.

 

[winter brew]

I agree with tuning RPM with the primary and backshift characteristics with the secondary.....that's a little oversimplified, but a good guideline.
Another VERY informative and easy thing to check is sheave temperature. Make 1 pass on a deep hill. Lay your hand on the sheave surface of both clutches.....if 1 is noticably hotter/cooler than the other, you have some work to do, and that temperature tells you which clutch needs some attention. The hotter clutch is slipping more than the other. Finding a "balance" between both clutches is the key to clutching and when you have proper RPM, a straight shift, solid/quick backshift with little waiver in RPM and both clutches are runnign the same (or close) in temperature, you are making the setup more efficient. The result is cooler running and more power to the track
Now getting there is the fun part.

 

[Barrett]

set engagment speed with primary spring and or notches in the flyweight
set running rpms with flyweight
set rate of upshift with helix angle selection
set backshift with helix angle/sec spring selection

Realise that any change effects almost every other area of the shift curve. It is a balancing act, but is scientific, not VOODOO.
The 2 components that have the GREATEST EFFECT are 1 the flyweight, and 2 the helix angle. the springs do very little.
The primary spring basically only effects engagement speed, the top rate of this spring has a neglegible effect.
The secondary spring basically just creates initial side pressure untill the helix can provide the side force from torque feedback through the driveline. The 1 area the sec spring does have significant effect is when off the throttle. In this situation it is the only thing backshifting the secondary, which is why stiffer sec springs feel more "revy" in on/off throttle stituations.
Do-Not try to use the springs to fix a shift curve that is not strait. use helix angles, or mass placement on the flyweight.

 

[XC700116]

I agree for the most part except I do not agree that the finishing rate or top rate of the primary spring is neglegable, altough not a primary tuning point it does need to be at a resonable tension and with the wide variety of springs available tuners do need to pay attention to it. too heavy and you will never shift the primary out all the way and create heat, too light and you will be shifting it too soon and will create incresed heat due to the clutches starting to fight each other.

The same case can be made on the secondary spring as it does help control the aggressiveness of your helix angle selection and can be a great place for fine tuning to avoid the heat issue and develop backshift without hindering upshift. Like you say it's a balancing act not voodo.

And I also agree that your greatest effect is derived from Helix angle and flyweight mass and profile and that those are the place to start your tuning and correct major difficiencies.

Also I will be the first guy to admit that I am in NO WAY a master clutch tuner, 99% of my experience is with Polaris equipment and I am still gaining a complete understanding of the world of clutch tuning and especially with the Team Secondary as I've only been running them for about 4 years now. With the constant evolution of clutching technology and products we all need to keep learning.

 

[XC700116]

Also if there's anyone out there that can shed some more light on the Flyweight Profile, Mass Placement and the application of these concepts PLEASE SPEAK UP This is one area where I personally am just starting to understand in more depth and there are others here that are way more educated in this particular area and I would love to learn more as I'm sure others would as well.

Some areas that would be helpful in that discussion are how different mass placement effects shift profile for different applications. For instance Big Bore torque monsters seem to like more heal weight whereas turbos do not.

Lets keep this discussion moving and expand on things as much as possible, while making sure the basics are covered.

 

[ruffryder]

Weights and the flyweight roller interface.

Weights - consist of mass and specific profile
Roller - force interface between weight and clutch

Force pushing clutches into each other, direction defined as "x - axis"

So generally, you spin the clutch, the mass on the weights causes them to want to straighten out against the rollers (the whole rock on a sting thing...) and therefore once the force is greater then that due to the spring, the clutch will move.

The "force" is complicated though. What is the force? How do you change it? Well, that is an interesting one to talk about. First lets talk about the interface between the weight and the roller.

To push the clutches together, you need a force in the direction of the + x-axis. The primary spring applies a force in the - x-axis direction.

When the flywheel weights pushes on the roller, the force is not only in the + x-axis direction. The force is also in the direction of moving the roller away from the clutch center line (crank shaft center line also). Lets call away from center - line as + y-axis. The total force on the roller is due to the weight construction, the amount of mass, and the location of the mass. But, that is not all. There is something else that is more important (IMO) and that is the direction of the force.

If the force is ALL in the + x-axis direction (0 degrees), then ALL of the force is used to overcome the spring force and close the primary sheaths together. If the force is ALL in the +y-axis direction (90 degrees), then NONE of the force is used to overcome the spring force. Note that the x and y axis are 90 degrees apart. So, from this information, it can be assumed that if the force on the x-axis is equal to the y-axis, then the force is being applied at 45 degrees.

This is where the weight profile comes into play. The direction of the force onto the roller is perpendicular to the tangent of the surface of the weight touching the roller. So if the total force is kept constant, but the force direction (angle) is changed, then the amount of force applied to closing the clutch sheathes is also changed.

IMO, clutch weight profile is much more important then the actual amount of weight.

That is why the stock yamaha weights are like 65.2g or so for the Nytro MTX (stock, no rivets) with rivets about 70-75 or so grams, while the adjustable ones I have now are in the 55 gram range, but provide a more aggressive upshift. The weight profile creates more (usable) force with the 55 grams weights, as compared to the 70 gram weights.


One thing I would like to add is the whole, tune rpms with the primary only. That philosophy seems to only work when you are close to having the correct setup. Sometimes a too aggressive upshift in the secondary is the problem.

Hopefully some of that can be comprehended. That is my take on it..

 

[Barrett]

IMO, clutch weight profile is much more important then the actual amount of weight.


One thing I would like to add is the whole, tune rpms with the primary only. That philosophy seems to only work when you are close to having the correct setup. Sometimes a too aggressive upshift in the secondary is the problem.

Agreed. If you are trying to fix an ailing tuneup, it is sometimes best to look back to the stock helix/spring combo, and tune from there.
if you are starting from scratch, you will definately have to approach tuning a little diferently. I like to start by selecting a sec spring/helix combo that has been used sucessfully on another combo similar in HP, and intended usage to what I am doing, and start there.

 

[XC700116]

OK so now that everyone has X's and Y's confusing them I need to review my college physics books one of these days

care to add more specific info as to the effects of a flatter or more curved profile as well as a heal/toe weight bias in actual application. ie the meat and potatoes of the above explanation, heal weight does this, toe weight does that kind of a thing.

 

[ruffryder]

As to weight and location, it is important to remember that clutches are like carburetors. Carbs work on throttle position only!!! That is the input into the system. For the primary clutch it is RPM only. Generally the way it works is more mass towards the heel (where the weight pivots) is for increasing the shift force at low shift out. The tip is generally for increase the shift force at full or high shift out.

 

[XC700116]

As to weight and location, it is important to remember that clutches are like carburetors. Carbs work on throttle position only!!! That is the input into the system. For the primary clutch it is RPM only. Generally the way it works is more mass towards the heel (where the weight pivots) is for increasing the shift force at low rpms. The tip is generally for increase the shift force at high rpms.

That's pretty much my understanding as well but wasn't sure on it 100% as I'm sure there are more intricate effects involved when profile is concerned and I didn't want to shoot my mouth of on it if I wasn't sure.

thanks for clarifying and adding.


speaking of profile other than notches for adjusting engagement, I'm guessing that a flatter profile would give a little less "progressive" increase in force vs a more curved one.

ie: the flatter one has less angular advantage early in the movement than the more curved one of the same weight thereby being a less "aggressive" shift. I'm thinking this due to the above mathmatic explanation that ruffy gave.

Is this correct? Correct me if I am wrong please.

 

[Barrett]

As to weight and location, it is important to remember that clutches are like carburetors. Carbs work on throttle position only!!! That is the input into the system. For the primary clutch it is RPM only. Generally the way it works is more mass towards the heel (where the weight pivots) is for increasing the shift force at low rpms. The tip is generally for increase the shift force at high rpms.

I have to correct you here - the bold should read shift ratio , as the weight in each position of the flyweight effects a certain part of the shift curve, and should still be tuned to run at shift rpms.
ex: if engine runs 8K, it should run 8k from start to finish, even though the heal of the flyweight effects the beginning of the shift(slow ground speed) , and the tip effects the end of the shift(high ground speed).
The secondary clutch needs to be tuned to create opposing shift force such that the engine(and flyweights) needs to be at shift RPM in order to overcome it, and therefore begin shifting out the clutches(and increasing ground speed).