My reasoning for wanting more pivot end weight is so that I can run less weight on the finish end. It makes the pressure more uniform from bottom to top. It also allows for a flatter helix angle resulting in less slippage and heat build-up. As joe mentioned, the back shift is also better if you don't have as much weight out on the end.

i understand the concept here. people have been under tucking weights for 15-20 years now, but let's not confuse the average sledder. where you desire to place your arm weight is completely a personal preference. there is no "right" or "wrong" to it. ramp grinding is a very common practice with people who know what they are doing, it's not for everyone! i'm not a 100% sure about this less end weight, better back shift issue. it will not give you a faster backshift. it will allow the engine to lose rpm faster though.maybe i miss understood the wording in that part of the post ???

idooski did you weigh the stock arm alone, with absolutely nothing else in it, i.e. bushings, just the bare arm??? mine were 39.2 grams. i personally don't think 9 tenths of a gram in an arm weight is that huge of a deal, it's a cotter pin. could just be different factory casts? just curious :D

Believe me I'm not trying to confuse the average sledder, but it is another tuning tool that deserves some air time.

Why are the TRA III arms curved. Do you know? If it doesn't help, why the change? I was working on curved arms way before they came out with them. I had no marketing plans, but it proved to me that I wasn't beating my head against the wall.

I agree that changing arm weight and weight distribution along the arm is a personal preference, but that applies to almost every aspect of performance tuning to an extent, doesn't it? Just like what helix or spring or belt you want to run. I'm not sure I'm sold on the idea, but it definitely changes the characteristics of the machine and I feel it's worth my time to investigate the possibilities.

I guess in the true sense of the theory lighter arms or less pin weight shouldn't make the sled backshift quicker. But it does. It allows the spring in primary to sense less pressure quicker which allows the primary to open quicker, which in turn allows the secondary to react quicker to the negative torque signal it's getting from the track. Is that not a quicker backshift?

I weighed the arm bare except for the bushings. In the set I weighed the other day, all weighed 40.2gms. I just weighed another set in which all arms weighed 39.4 grams. One set is much older than the other. Probably a difference in molds.

.9 gms can be huge in a TRA depending on where it's at. It can be an even bigger problem if it's only one arm that is heavier.

From what I understood, the curved arm was used to make room in the smaller TRAIII clutch. There wasnt room to use a full length arm.


Now, my added twist. I dont know quite how to translate this into TRA terms, but a trick I have used allot on P-85 clutches is to "tuck" the weight. This is done by grinding the "heel" (the spot on the weight where it rests agianst the clutch in a idle poistion). This moves the start point on the arm towards the pin, and adds explosive holeshots. usually one can add 2gram of weight. Can something like this be done on a TRA?

And oh yea, there are 2 differnt Alum arms, just as you have discovered. I think they changed over in the CK3 sleds and newer, but am not exactly sure as to when.

Just remember, this is all just my opinion on how moving weight around works. :)

From what I understood, the curved arm was used to make room in the smaller TRAIII clutch. There wasnt room to use a full length arm.

Why did the arm have to be curved? Couldn't the engineers at Doo have just made a shorter straight arm? Tooling would have been cheaper.

I dont know quite how to translate this into TRA terms, but a trick I have used allot on P-85 clutches is to "tuck" the weight. This is done by grinding the "heel" (the spot on the weight where it rests agianst the clutch in a idle poistion). This moves the start point on the arm towards the pin, and adds explosive holeshots. usually one can add 2gram of weight. Can something like this be done on a TRA?

IMHO, what you just described about what you did with the P-85 is exactly the same principal applied differently due to the use of dissimilar parts to do the same job.

When the tuners at Doo refer to tucking weight, they refer to using taller ramp profiles to keep the center of mass closer to the axis of rotation. They do this with the use of taller ramp profiles tucking
the roller in toward the center. This lessens the axial force applied against the ramps. ie - it takes more RPM to exert the same axial force.

With the P-85, you grind off the heel and what happens? You have moved the center of mass of the flyweight away from the axis of rotation(crankshaft). The center of mass is where all the centrifugal force is applied. This increases the amount of force applied against the roller by the flyweight.

In the TRA when you move the weight of the arms toward the pivot, you are effectively moving the center of mass of the arms away from the axis of rotation which in turn because of the way the arms are located in the clutch at full rest moves the center of mass away from the clutch center. At full rest the roller is closer to the axis of rotation than the pivot point.

I'm not very good sometimes at putting my explanations down on paper so if it's not clear what I'm trying to say go ahead and fire those questions at me and I'll see if I can make myself clearer. :D

What I am refering too isnt grinding the tip of the weight at all. We have been grinding the part where the weight rests against the clutch. It is a flat part only about .400 away from the pin. This is where the stop for the weight rests against the clutch. If anything, weight would be more tip heavy, but it is a very marginal amount. I remove a wedge as wide as the weight, .020 tall at the fat end, and about .375 long. We would be talking about .1grams if that.


I will post some pics later on, showing what I am talking about here LOL.

Now, from what little I know about the TRAIII, the arms are shorter center of pin to center of pin. And so are the ramps. They are shorter (in the direction of adjuster pad towards the centerline of the clutch). From the discussion I had, the curve was used to change CG of the weight thru the shift, as to have as much thrust against the ramp at the end o the shift as the begining, where typical TRA falls away thru the shift.

What I am refering too isnt grinding the tip of the weight at all. We have been grinding the part where the weight rests against the clutch.

I believe I understand what you are saying and I am still picturing the same thing. I never thought that you were grinding the tip or doing anything for that matter that was altering the weight of the flyweight on purpose.

This is what I am picturing. You let me know if I'm right or not. You are grinding some mat'l away on the flyweights near the pivot end at the point where they are in contact with the rollers at full rest. Then you are blending back into the profile.

When the clutch spins, the flyweights are thrown from center to the outside by centrifugal force putting pressure on the rollers converting the centrifugal force to an axial force and closing the clutch on the belt.

By grinding where you did on the weights, you did change the center of mass, but it's negligable. The significant thing that you did is effectively move the center of mass away from the axial center of the clutch. As the radius from the axis of rotation to the center of mass on your weight increases the centrifugal force increases directly. So when you launch, the weights are farther frm the axis at engagement resulting in more pressure sooner.

Now, from what little I know about the TRAIII, the arms are shorter center of pin to center of pin. And so are the ramps. They are shorter (in the direction of adjuster pad towards the centerline of the clutch).

I agree.

From the discussion I had, the curve was used to change CG of the weight thru the shift, as to have as much thrust against the ramp at the end o the shift as the begining, where typical TRA falls away thru the shift.

That is exactly what I'm trying to say. By curving the arms with the convex toward the O.D. of the clutch (away from the axis of rotation) they have immediately increased the force against the ramps without changing anything else.

The farther away from center you get the center of mass of your arms, the more force will be exerted against the ramps.

Two of the ways of doing that have been mentioned and they are 1) Build a curve into the arm, and 2) move the center of mass of the arm toward the pivot point (which is always farther from the axis than the roller).

There's another 2 cents :p

idooski, just as i suspected, i didn't understand exactly what you were talking about previously. not that you aren't very articulate, it's more that i'm not. we're on the same page now, thanks. :D

I usually don't understand what I'm talking about either! ;)

So don't feel bad about questioning me. I welcome it. If I'm wrong, I'm wrong and I expect you to let me know about it. If I'm right, I'm right. We all get lucky sometimes. :p :p

Actually the grinding I do on the weights is on the back side of the weight, not on the rollerside of it at all. It isnt a matter of changing the weight, but in what position the weight rests at idle.

I completely understand that you are not changing the weight.

I guess what I would like to know is in what way are you changiing the position of the weight? Like I said before, I am not very familiar with the P-85 clutch. It sounds like you are creating some 'slack' before the actual engagement. Got any pics Dave? I'm curious about what you are doing.

here are some pics of what was actually changed. These are identical weights, 10-60 Polaris weights. I have highlighted with a black marker to show what was actually ground. This little bit changes allot performance wise.

This is a view of the area ground, but from the end to see how much was ground.

Now this is a picture of the profile side of the weights. I used the marker to show where the roller starts and finishes on the profile. As you can see, the roller contacts allot closer to the pin than the stock weight. The modified weight is on the bottom.

Now, this area ground is where the weight rests against the stop in the clutch. This is a pic of the side of the weight with a "stop" drawn on paper and showing where the weight is in a rest position.

This is a pic of the same thing using the "Tucked" weight. I put the weight on the same pin location, and rested it against the same "stop". As you can see, grinding this .02-.03 of an inch moves the location of the weight about .200" at the tip. mind you, this is the posiont both weights would be in at holeshot. As you can see, there is allot more leverage with the tucked setup, and it usually requires 2gr-4gr increase too. How often to you get the chance to increase the amount of leverage AND weight in one change? Just a change in the rigt direction asking me.

I like to remove material off of the sliding sheave face on the flyweight style clutches instead of modifying the flyweight.
Where you remove the material that is concentric of the pin on a "P" weight is almost negligable to mention when talking of forces.

The action of tucking the flyweight is substantial. Tucking moves the c of g towards the clutch shaft center.
Because the c of g of the flyweight is closer to the center, the engine has to produce more rpm to create force for the flyweight to push. This makes for higher engagement and/or a revvy feel when you roll on the throttle.
You use less of the meat of the torque curve when tucking the flyweight.

Buuuuuuuut....To actually get tucking to work you must move the spider closer to the engine. Unless you have a series of a "P" primary where when the flyweight is installed, it rests against the spider when you have one of these clutches and you tuck the weight, you increase the belt clearance a few thou.
I have never had a clutch as such. I know there are some as Ive seen them, but all the "P", "AC" & Comet I've own[ed]/ played with, I had to move the spider to make the tucking work.

Tucking the weight is opposite of what idooski and myself would like to play with regarding TRA arms.
Pull out a Polaris tuning book or Aaen or Cutler clutch book.
Looking at a Polaris flyweight>>> Look at an "S" series flyweight. Notice how regardless of "S" model, they are all full tip weights?
Ok look at an 10M series weight. Notice the modification where the tip is cut? The tip is not full.

Ok now have to use imagination here...Imagine that you have an "S" flyweight and a "M" flyweight and they weigh the same. for lack of better numbers, lets say both series weigh 50 grams.
You have the full tip "S" installed and you measure 5000 rpm engagement.
You remove the "S" and install the "M" flyweight and now you will measure 4700 rpm.

The reason for the lower engagement on the "M" is because the c of g is farther away from the clutch shaft center. The engine has to produce less rpm to overcome the primary spring with the "M" flyweight.

Now with a TRA arm that is heavier, it will have more mass towards the pin. The c of g will be farther away from the clutch shaft center. ...remember bucket of water example back in that other post? As much as possible, I am getting away from the character of a mag arm. I do not want the "revvy" feel of a mag arm. Because of the mag arm having the c of g near to the roller, when at high shift ratios the mass is great out there. This keeps the primary sheave pushed in longer. Time and time again I have been at high speeds going down a hill, go thru the bottom and up the top, I get an rpm loss. The flyweight stays out there and "hangs".

Now lets quit using the term "heavier" and speak about what we want. We want an arm that has the c of g towards the pin, farther away from the roller. This will give the primary sheave an easier time to return back to open when the engine rpm drops.

i already know this because I have used lead sinker material ty-rap to the tra arm. he he he...
i removed the setscrews from the threaded pin and added as much lead to the arm. But to reach the correct rpm, I had to add more lead...
used my fathers sinker-makin stuff. lol


....did I do ok for describing what I see? :D

Yes, when tucking, the spider has to be reshimmed, but very well worth it.

just pm'd ya.