I have been a member of the site for a very short time, but have been engineering and teaching CVT clutching for the past 15 years at the factory level. Many of you have read my tech articles in SnowTech magazine. I enjoy researching and writing about these incredibly complex mechanical devices.

I have read many prior threads on this site involving CVT clutch and belt issues. These issues also exist in the snow industry. To understand how we got here, a short history lesson is in order.

The modern day CVT flyweight clutch was first developed around 1970. The first flyweights were hand-made to develop the cam surfaces we see today. The attempt to share understanding of the working CVT clutch system was first published by Olav Aaen. The first manual was nothing more than a couple of pages, but was popular and first published in 1975. In fact, so little was known about the CVT primary clutch that Olav would sell out every weekend at races in the snowbelt. He paid for all his travels just selling the manuals.

Olav Aaen Is credited with the first patent involving the CVT flyweight design (patent 3,939,720). It was written while Olav worked as an engineer for OMC. The ‘720 patent is very revealing as to the understanding of the technology at the time. If you take the time to read the whole patent, you quickly realize that the patent was written about devises that controlled the flyweight, not on how to design a flyweight. When talking to Olav about his “720” patent, he said there were just too many variables that weren’t understood at the time.

The Aaen clutch tuning manual has just been updated (40th anniversary). The manual currently sells for $35 on his site (Aaen Performance - High Performance products for Snowmobiles and Polaris ATV's), and will be the best money spent for those who want a baseline understanding the CVT clutch system.

The Aaen manual is a great starting point, but for hardcore performance guys the manual really only discusses the end results of changes made to the system. To truly understand how the system works you need to get into the physics of the system. For many years I avoided trying to do this. The CVT system seemed very complex, and frankly nobody (including myself) would touch the subject. “Experts” in the field were just happy to know how to tune them and the science became one of those black arts that was learned over time making the old guys very valuable in the high performance snowmobile business.

I have this manual and it does help one to understand the CVT system...

Money well spent IMHO...

 

The above diagram shows the three major components of the CVT system: Primary clutch, Secondary clutch, and drive belt. This diagram contains great information, and I’ve high-lighted key points of interest.

The most important point of the diagram is how the “Contact Patch” changes between the sheaves of the primary clutch and the drive belt. The contract patch changes by a “factor of 5” (1.67 to 8.50 inches squared).

Understanding how the contact patch changes as the clutch shifts is key to understanding how the tuning components are designed to apply the proper shift force based on a continuously changing contact patch.

The diagram emphasizes the importance of the Primary clutch vs. the Secondary clutch and how the two clutches interact with the Drive belt. You may have noticed, I’m emphasizing the contact patch between the Primary clutch and the Drive belt at this point. The changing contact patch in both the Primary and Secondary directs us to the proper force curves needed to properly design the two clutches.

The contact patch is only even between the Primary and Secondary clutches at the one to one shift ratio. The contact patch is always smaller in the Primary clutch up to the one to one ratio, and smaller in the Secondary clutch past one to one.

By understanding the contact patch, you quickly realize the focus should be on the Primary clutch due to the smaller contact patch for a greater period of time. The majority of Drive belt issues all begin in the Primary clutch. If the Primary clutch is poorly calibrated the belt will slip and heat will quickly delaminate/degrade any belt.

Always remember “It’s called the Primary clutch for a reason”. If you can’t get the power through the Primary, it does you no good to be working on the Secondary.

I’m going off line for now, but will add to the thread if others show interest. Thanks. Randy

 

I welcome any and all "training" and discussion with the clutching experts on the forum.
Thanks, and I look forward to more. These things are a whole 'nother deal than a gearbox and a rear dif ratio.

 

Great info and post . I have that manual it has helped me a lot . Randy ,Please continue this thread great info

Frank

 

awesome stuff keep it coming..

 

Don't stop there.

 

Please continue.

 

I have used this guys stuff in my sleds before. It has worked very well.

Please continue, IMO the clutching tech on this forum is very far behind sled forums tech.

 

You had me at CVT!

 

First CVT that I ever owned was on a 1974 Rokon motorcycle. It worked really well and the only tuning I did was clock the secondary spring and change the weights in the primary to raise the engagement rpm. Maybe I didn't clock that spring, just replaced it. Hard to remember that far back!
Dwight

 

OK, it looks like there is enough interest so I’ll move forward.

From our discussion yesterday you should have learned that the contact patch between the Drive Belt and Sheave faces in the primary clutch is a major design concern for the Engineer. This lack of contact should be further investigated as to understand the force requirements needed to properly transfer engine torque.

If the force applied to the belt is insufficient, the belt will slip and generate heat. The heat will build to the point that the Drive Belt will simply delaminate/fail.

Above is a diagram I’ve used for years. I did not generate the data, rather I found it in an SAE paper on CVT design written years ago for a European auto manufacture. Of course I failed to document what the paper number was. The Diagram shows the force requirements needed to properly transfer engine torque for engines ranging from 57-100 ft-lbs of torque.

Remarkably, while this diagram was generated for the auto industry, it is exactly consistent with the engine torque range Polaris currently builds their UTV engines. This graph that compares Force needed vs Shift ratio at two different engine torque levels. It gives the Engineer requirements; without a target, we would just be guessing on what is necessary to properly design the force-producing elements within the CVT Primary & Secondary clutches.

In my next post, we will actually calculate force generated by a flyweight within the RZR 1000 Primary clutch. In the meantime, does anyone have a dyno sheet for this engine? I will use your actual data if someone provides the HP vs. Torque curve. If not, we will use the 100 ft-lb line in the above graph as our target. Don’t be afraid to ask questions.

Get your calculators out and blow off the dust. Thanks Randy

 

OK, it looks like there is enough interest so I’ll move forward.

From our discussion yesterday you should have learned that the contact patch between the Drive Belt and Sheave faces in the primary clutch is a major design concern for the Engineer. This lack of contact should be further investigated as to understand the force requirements needed to properly transfer engine torque.

If the force applied to the belt is insufficient, the belt will slip and generate heat. The heat will build to the point that the Drive Belt will simply delaminate/fail.

Above is a diagram I’ve used for years. I did not generate the data, rather I found it in an SAE paper on CVT design written years ago for a European auto manufacture. Of course I failed to document what the paper number was. The Diagram shows the force requirements needed to properly transfer engine torque for engines ranging from 57-100 ft-lbs of torque.

Remarkably, while this diagram was generated for the auto industry, it is exactly consistent with the engine torque range Polaris currently builds their UTV engines. This graph that compares Force needed vs Shift ratio at two different engine torque levels. It gives the Engineer requirements; without a target, we would just be guessing on what is necessary to properly design the force-producing elements within the CVT Primary & Secondary clutches.

In my next post, we will actually calculate force generated by a flyweight within the RZR 1000 Primary clutch. In the meantime, does anyone have a dyno sheet for this engine? I will use your actual data if someone provides the HP vs. Torque curve. If not, we will use the 100 ft-lb line in the above graph as our target. Don’t be afraid to ask questions.

Get your calculators out and blow off the dust. Thanks Randy

Here is a borrowed link to a dyno sheet on an XP 900 I hope it helps.

http://www.rzrforums.net/rzr-xp-900/79382-dyno-session-xp.html

 

I'm gonna need more popcorn.....opcorn:

 

I have the manual but am very interested in any knowledge on the subject! Thanks heelclicker

 

subbed.

 

Great info here. Thank you. I am looking forward to seeing more of what you come up with and share.

 

Now that we have developed the target, the engineer can now start doing force calculations. Before the calculation can be made, the engineer has to determine some key dimensional characteristics about the primary clutch, flyweight, and primary clutch spring.

Prior art flyweights looked nothing like the flyweights shown above. In the past all Polaris flyweights pretty much looked the same. In fact, if the weights were not marked with their designation such as a 10, 20, K, MH…etc., you could not tell them apart. It took a trained eye to tell the difference and to most people a flyweight was nothing more than a flyweight. These same conventional flyweights have been used by Polaris for 40 plus years.

Above you will see a diagram that represents the current line of flyweights used in virtually 100% of all Polaris UTV and many ATV’s sold today. It’s a remarkable line up for sure, and for us old timers this demonstrates the high level of engineering involved in flyweight design over the past 7 years. Polaris has shifted its focus to tailoring the flyweight to the exact engine and bike combinations.

You will also notice that Polaris is standardizing its flyweight designations. Their flyweights are now designated as a “series”, i.e., 25, 26, 27, 28. In years prior, you would have a 10MH, an S55H. It was very confusing. Just for everyone’s info, the 25 series is used in the RZR 570; the 26 series is used in RZR 900 & 1000; the 27 series is used in the Ranger 900, and the 28 series is used in the ACE 370.

Polaris is up to the 30 series. The 29 and 30 series (not shown here) are basically the same as the 26 series without bushings, and with varying material densities to create a heavier flyweight.

This is all for now and next we will identify all key dimensions needed to make force calculations.

 

Thank you for the great information!! I have a burning question though, how does elevation effect clutch performance?

 

Clutch performance does not change. What changes is the power produced by the engine. Air density changes as elevation increases.

To give you an example of the power loss due to elevation gain, here are two data points. At sea level (considered zero feet), the 1000 RZR makes a claimed 108HP. At 8000 ft of altitude (West Yellowstone) air composition changes by 11%. in other words your 108HP engine now only makes 96.1HP. There is simply nothing you do about it unless you put more air back into the system (turbo?).

The clutching has to be re-calibrated due to the loss of engine power. At sea level Polaris clutches the 1000 RZR with the 26-61 flyweight; at elevation Polaris calibrates the same RZR with a 26-55 flyweight. No other changes are made to the springs or helix.

 

Great stuff....

 

Great info. How does sheave mat'l affect performance?

 

Excellent question. The aluminum material used for a cast one is probably a 300 series aluminum (at least it was the last time I did one). I also build billet primary clutches so I'm familiar with the coefficient of friction between the belt and differing material properties.

Cast aluminum material is much more porous then billet. Making a cast sheave is a better choice for a CVT primary clutch. The only time I recommend billet is when more arms (more than the standard 3) or some other function is needed.

This is where aftermarket belt manufactures see gains in performance. By changing the durometer of the belt, you can improve the coefficient of friction between the belt and sheave. Softer (stickier) durometers always give better rear wheel horse power numbers but for a much shorter time/duration.

IMO, Changing belts to a softer compound only masks a greater problem with the calibration. I always try to change the calibration with the tuning components available. Usually the harder stock belt is much more consistent.

FYI. We don't sell O.E.M. or aftermarket belts so we have no skin in this game.

 

Yesterday we discussed the different flyweights Polaris uses in their UTV/ATV line up. As you can see there a tremendous differences in the shape and sizes based on the differing engine/bike combinations.

After years of high performance work in many areas of design, I have come to the conclusion that the CVT flyweight is the most important part of any power transfer assembly. From the end of the crankshaft to the contact of rear tires to the ground, the CVT flyweight is the area of highest importance to the powertrain engineer. An improperly designed flyweight will result in low power transfer to the rear wheels, and more importantly will result in power transfer losses seen as elevated belt temperatures due to belt slippage in the primary clutch.

To understand how important the flyweight is to the overall machine, just think of them as the “Engine Control Management system” (ECM). No other single component controls the efficiency of the power plant (i.e., Engine) more than the flyweight. I’m sure I just ruffled the feathers of a few of my electrical engineer friends with that statement. :shhh:

Shown above are three of the flyweights we looked at yesterday. We have taken the flyweights (25, 26, and 27 Series) and put them in a two dimensional schematic (X,Y) for further analysis. The three dimensional (X,Y,Z) analysis is not need to make the calculation need to calculate force applied to the belt.

The flyweights are shown statically hanging from the pivot pin. As you can see the flyweight has a very complicated shape and we have to figure out how to get some key measurements from it. The key dimension the engineer needs is the “Center of Mass” (COM) location of the flyweight. Since this thread is named “Clutch Design 101”; a single COM point will used when performing calculations of complex shapes such as the flyweight. You can also use multiple COM points, but that method is very advanced, and both methods give the same end result. The multiple COM method teaches the engineer how effective each section of the flyweight is during operational rotational.

By hanging the flyweight from the rotational pivot, the “plumb line” naturally generates the COM point in the “X” direction. Half of the mass is on one side of the plumb line and the other half on the opposite side. Getting the COM point in the “Y” direction requires some specialized measurement equipment. We have shown the proper COM location for the three flyweights above, and its location from the center of the pivot.

Next we will put the flyweight in the clutch and generate more critical data. Randy

 

Me too....unable to see the pics. I thought it was just my computer.

 

Thank you for all the info....

 

Glad to hear it's showing the "black art" of clutching is really more of a science than some may think. Cindy

 

It is... When I took my cover off to check my belt... I was like WTF... Looked at the belt and put it back together... Now I'm glad I can get a book to read on all this for down the road performance or maintenance...

 

In our last get together, we discussed the Center of Mass location (COM). This point on the weight is critical in determining the force generated by the flyweights. Today 3D cad programs can easily generate the exact COM location of any complex shapes such as the flyweight.

Our focus will now shift to the CVT Primary Clutch. The primary clutch exists for the purpose of harnessing the energy of the flyweight. I know some would argue with me, but the flyweight is the only positive force-producing element in the entire CVT drive. All other components (springs and helixes as well as the clutches themselves) subtract from the energy produced by the flyweight.

The primary clutch is designed to precisely place the flyweights. In the attached diagram we have now placed the flyweight in the primary clutch. The very first key dimension within the clutch is the “Heel Pad”. The heel pad places the flyweight in the proper position that dictates the start of “Operational Rotation” of the flyweight.

By examining the diagram above, you will notice that the COM of the flyweight no longer hangs directly in line with the rotation pivot. The heel pad is critical as it sets the angle in which the flyweights COM is untucked from the rotational pivot location. The heel pad distance is measured from the center of pivot location, and is one of the key dimensions that has a large influence of the overall performance of the CVT system as a whole.

We have shown the heel pad location at a -8.5mm in the X-direction from the rotational pivot. In years past, Polaris had standardized on a -7.0mm heel pad location, and for the first time in 40 years, has changed it to the -8.0 to -8.5mm location with the new RZR clutch which came out in approx. 2010.
Next we will get more key dimensions needed from the primary clutch.

A Memorial Day Salute to all my fellow Veterans! Randy out.

 

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