CTS Explained

[mjslim]

I thought you guys in the Husky forum might be interested in this. I did a little 2D geometry layout on the computer to try and help myself understand the dynamics behind Husky’s new CTS concept, where the countershaft sprocket shares the same center of rotation as the swing arm. As many of you know from reading tests, the new 449/511’s are sometimes said to “stand up in the corners” or have the rear end ride high when accelerating through turns. Husky’s own literature claims “less rear wheel squat under acceleration” or something to that effect. I wanted to understand how the CTS causes this to happen.

Just looking at the CTS setup, it is not intuitively obvious as to how you would have less “squatting” with the shared axis. I could understand how the chain tension stays constant through the range of rear wheel travel, but not the reduced squatting. If you do a simple 2D layout, however, it is pretty easy to see. The answer can be found by looking at the top run of the chain relative to the swing arm pivot as the swingarm goes through its range of motion. Do that for the CTS (shared axis) versus a conventional (non-shared axis) setup and you can see the difference.

With the CTS setup, the distance from the swingarm pivot to the top run of chain is constant regardless of where the swingarm is in its range of travel. This constant distance is simply the radius of the countershaft sprocket, and it is the same at 11 inches of rear wheel travel as it is at zero inches of travel.

With a conventional setup, the distance from the swingarm pivot to the top run of chain increases as the swingarm goes through its range of travel. This distance is roughly twice the distance at 11 inches as it is at zero inches of travel!

What this means in engineering terms is that under acceleration, i.e. when there is tension in the top run of the chain, there is a significantly smaller torque acting to compress the rear suspension in the CTS setup versus a conventional setup, hence less squatting. Or to put it another way, the further you are into the rear suspension travel during a turn, the more your chain is trying to compress your rear suspension with a conventional setup; the CTS minimizes this effect and therefore rides higher in the rear, all things being equal. So it more a matter of what the CTS doesn’t do that makes it unique when it comes to handling – the chain has less compressive effect on the rear suspension under acceleration due to the fixed chain offset distance.

[Krieg]

Awesome explanation is layman's terms! Thanks! ?

[KAM1]

Thanks, I feel smarter when things start too make sense. Ignore any spelling mistake I ain't that smart.

[mjslim]

Awesome explanation is layman's terms! Thanks! ?

Thanks. Its kind of tough to explain something like this when you can't post a sketch.

Now you know why these bikes are getting such mixed reviews in the magazines - nobody's used to the new feel yet, since virtually every dirt bike made up until now has the separate swing arm pivot and countershaft axis setup. Once the pros figure out how to go fast on them then I think the opinions will change.

[Ride]

It is a good explanation but i think flawed to some extent. I have been into and researched lots of different alternative suspension designs for years and played around with a lot of CAD stuff as it is part of my living. One thing you are missing / left out is that instead of the torque being in a line like it is with a CTS (draw a line from the CS center to the axle center), the standard way of having three points (CS center, rear axle center, and swingarm pivot) it now has leverage over one side of the triangle and tries to collapse the triangle (torque effect). As the torque lines are not straight (like with the CTS) it can and does induce torque into the system. Think of it this way, you play tug-of-war with two people and it is a straight line force, add a third person (point) off center and now not only are the two outside people pulled off and at an angle but the middle third person is trying to get shoved into line with the rest. If these three points are not movable (like in a standard bike) the toque is redistributed as rotating or collapsing the points.

in their words...

The advantages of the CTS system compared to the traditional systems are huge. During the suspension movement along the whole stroke, the chain is not subject to any change in length. The tension of the chain remains constant regardless of the swing arm position, while the chain and sprockets benefit from reduced wear. An important positive effect on the riding characteristics is the marked reduction of the impact of alternating the load on the transmission. This aspect has been studied by a spin-off company of the University of Padova called Dynamotion which has proved that the CTS improves traction in the various stages of acceleration and consequently during also the vertical movements of the suspension on rough terrain.

The result is better traction during acceleration and surer, more confident gear changes. More in general, the values obtained by adopting the CTS have a much more uniform trend in all conditions, leading to improved ridability and stability of the motorcycle.

IMHO it works, and works well. For my type of riding I have no issues with handling and actually have grown to love how it handles. It was odd at first, the rear seems to never move up and down yet all the bumps / trail rash just goes by. Several people ahve ridden my bike and were instantly fast on it and none of them remarked about anything odd (Well Ken does not like my trials front tire ) At first i thought the bike just had great rear suspension but know I realize it is more than that and is the CTS. The rear stays planted, controlled and drives forward with more traction than any other bike I have ridden. It is a great loose rocky rooty hill climber and with a trials rear tire to add tot he traction and the super smooth motor it feels unstoppable. I have only had to adjust the chain twice in 700 miles and it will be interesting to see if the chain and sprockets wear longer. Seems they should.

I'm all in for the CTS and hope the next gen of 2 strokes includes it as i think it would make a huge difference there.

When i got this beast I had many thoughts it would be a overly complex turd. I am finding out after over 700 miles that the things they did like the CTS and the air box really do bring new and better things to the table. The crank mounted clutch also works great and has excellent feel. The top mounted linkage seems a big overkill and can't really see how it is much of an advantage other than the bottom of the bike is very smooth and nothing to hang up on / has a lot of ground clearance.

Good discussion, thanks. ?

[Ride]

BTW what i find even more fascinating is the linkage front ends where dive, no dive, or anti dive are all possible via geometry. LOTS of possibilities there.

Back in the day when there was a WROKS class it was awesome the stuff they came up with and tried. Decoster said not long ago that RM linkage front end was still tot his day the best front end he has ever rode and said it was like cheating.

Honda had a go at it to...

these all worked great but forks are EZer / cheaper to build and all the pivot points on the linkage would ware out and the front end becomes loose.

Oh, also Boysen Link did a similar thing to CTS many years ago, it made the swing arm pivot float and hence eliminating the chain torque through keeping the points aligned...

(also note the square section rear spring, people were WAY into the next big thing then)

[mjslim]

Ride,

Didn't quite understand your explanation of why you thought my reasoning was flawed, but I think you are basically saying the same thing when you talk about the "triangle", just not in engineering terms that I'm used to. My analysis was based on classic free-body diagrams, looking at lines of actions of forces and the resulting moments at the extremes of swingarm movement. Your basic "mechanical engineering 101" type stuff.

And btw those are some cool pics of the linkage style front suspensions, I remember reading about Decoster riding that funky looking Suzuki when I was a kid. Brings back memories. You are quite the moto-historian to be able to whip out those pics in a moments notice. I'm impressed! ?

[huskyfrk]

unless the most popular of riders takes to a "springer" front end, the american public will avoid it in droves... their mindset just won't accept it. nowdays few riders, except those over 50 + have ever ridden a different front end. whether it be a DKW or Sachs, greeves. even the early ones were very good, with the technology and R & D, new versions they would be great. but the public are like sheep. Look at the RAD front end on the old yamaha street bike. long gone now. worked quite nice too...

[SeVoN]

What this means in engineering terms is that under acceleration, i.e. when there is tension in the top run of the chain, there is a significantly smaller torque acting to compress the rear suspension in the CTS setup versus a conventional setup, hence less squatting. Or to put it another way, the further you are into the rear suspension travel during a turn, the more your chain is trying to compress your rear suspension with a conventional setup; the CTS minimizes this effect and therefore rides higher in the rear, all things being equal. So it more a matter of what the CTS doesn’t do that makes it unique when it comes to handling – the chain has less compressive effect on the rear suspension under acceleration due to the fixed chain offset distance.

Bikes with a conventional swingarm pivot ABOVE the countershaft sprocket height will actually RISE as you accelerate. Seems counter-intuitive, but it's true. You have to be very deep in the travel for this effect to be negated (when the countershaft sprocket, swingarm pivot and axle are all in-line).

Bikes with the countershaft sprocket and swingarm pivot at the same point (like Husky's CTS) lose out on the anti-squat effect. The designers can still minimize squat under acceleration with this setup through suspension linkage, swingarm angle, etc. But true anti-squat isn't possible.

This is a good book which explains the effects of chain torque on suspension action: Motorcycle Design and Technology Handbook

[Ride]

unless the most popular of riders takes to a "springer" front end, the american public will avoid it in droves... their mindset just won't accept it. nowdays few riders, except those over 50 + have ever ridden a different front end. whether it be a DKW or Sachs, greeves. even the early ones were very good, with the technology and R & D, new versions they would be great. but the public are like sheep. Look at the RAD front end on the old yamaha street bike. long gone now. worked quite nice too...

Yep. Unfortunate. Even this new 449/511 upset a lot of people for being so different. Sheep is right.

[Ride]

Ride,

Didn't quite understand your explanation of why you thought my reasoning was flawed, but I think you are basically saying the same thing when you talk about the "triangle", just not in engineering terms that I'm used to. My analysis was based on classic free-body diagrams, looking at lines of actions of forces and the resulting moments at the extremes of swingarm movement. Your basic "mechanical engineering 101" type stuff.

And btw those are some cool pics of the linkage style front suspensions, I remember reading about Decoster riding that funky looking Suzuki when I was a kid. Brings back memories. You are quite the moto-historian to be able to whip out those pics in a moments notice. I'm impressed! ?

sorry man did not mean it that way. Was more adding to what you said and making the point about the three points verses two in the system.

[robertaccio]

great posts, all the internet passion plays can get annoying, science is golden, thanks.

I know its a OEM video and quick and dirty but you guys did see this no?

http://www.facebook.com/video/video.php?v=10150200360039251

Long Live Horst Leitner

The name "ATK" comes from Leitner's 1981 patented device to eliminate chain torque for improved handling. Known later as the A-Trax, Leitner originally called the device the Anti-Tension Kettenantrieb.

Besides all the tech stuff the smartest statement is that the chassis reacts differently which demands the pilots change their style/position etc to exploit this new design, all the Pros of the modern age grew up (Burson,Kamo for example) riding pro-squat bikes and all the rider schools and rider styles reflect this chassis reaction body and weighting position. When the new adaptation to this chassis is complete, and its getting there (DK solid 8th in last H&H) we will see more and more smiles from the bulk of riders. When I did my ride on the CTS bike the first thing I felt was my old ATK under acceleration and this is the feel I get from my 08 with the high back chain slider, the rear feel more "free" under torque loads.

Edited May 5, 2011 by robertaccio
update

[Russ Henry]

Doesn't the rocker arm linkage also contribute to the improved traction?

Why would they put the linkage on top if there was not a significant advantage?

[Ride]

Doesn't the rocker arm linkage also contribute to the improved traction?

Why would they put the linkage on top if there was not a significant advantage?

Doubtful as it is simply a progressive ratio link like any other. I think they did it to keep the bottom of the bike smooth (which log crossings prove works) and keep the linkage from smashing into rocks. The last few years the linkage was getting REALLY low...

Also the CTS might make the swing arm pivot slightly lower. The CTS also makes the swing arm longer which leaves more room for the linkage. I'm also finding the 499/511 has great ground clearance, feels like more than any current huskys i have ridden except maybe the 125.

BTW KTM/Berg also have the shock above the swingarm (not through)

[mjslim]

Doesn't the rocker arm linkage also contribute to the improved traction?

Why would they put the linkage on top if there was not a significant advantage?

I agree with Ride regarding the linkage.

The improved traction from CTS comes primarily from having a chain length which stays constant as the rear suspension compresses and rebounds. In a conventional setup, the chain length increases and decreases as the suspension works, transferring tension variations to the rear wheel and countershaft. This is a related but different effect from the anti-squat tendency discussed in the original post.

[36MotoMarc]

In a conventional setup, the chain length increases and decreases as the suspension works, transferring tension variations to the rear wheel and countershaft. This is a related but different effect from the anti-squat tendency discussed in the original post.

Yes because the chain starts long, gets shorter, then longer again as the suspension goes from fully extended to fully compressed. In some situations the chain torque will actually want to extend or "jack up" the rear end. It will be interesting to see how the whole thing plays out as everyone is used to the three-axis linkage feel and all the manufacturers are heavily invested in it, and public perception believes them to be necessary. I think it will be a few years before a winner can be picked. It seems like a step forward, but may work better in some arenas than others. As others have stated, its been done before. The AMP link/ATK, the Boyesen link, etc.

[erik329]

One thing you are missing / left out is that instead of the torque being in a line like it is with a CTS (draw a line from the CS center to the axle center), the standard way of having three points (CS center, rear axle center, and swingarm pivot) it now has leverage over one side of the triangle and tries to collapse the triangle (torque effect).

If I'm imagining what you're saying correctly, the triangle effect you speak of only puts more load onto a the chain slider in a conventional suspension (middle diagram shown below).

Similar to what mjslim stated, and according to basic engineering statics, the perpendicular distance from swingarm pivot to chain center is the ONLY thing that that causes a "squat" effect on the suspension. A conventional suspension is the same as CTS at normal ride height, but changes as the suspension is compressed.

[erik329]

Here's a better picture. The numbers and geometry are just made up.

[Ride]

If I'm imagining what you're saying correctly, the triangle effect you speak of only puts more load onto a the chain slider in a conventional suspension (middle diagram shown below).

Similar to what mjslim stated, and according to basic engineering statics, the perpendicular distance from swingarm pivot to chain center is the ONLY thing that that causes a "squat" effect on the suspension. A conventional suspension is the same as CTS at normal ride height, but changes as the suspension is compressed.

I don't think so. See the last drawing above. The chain is able to try the pull the swing arm up because the triangle is on that side (top) and the motor tries to pull that leg of the triangle. Conversely if the pivot point of the swing arm was above the chain or at least the straight line between the CS center and rear axle center the rear suspension would try to extend under acceleration because now that leg of the triangle is on the bottom.

[erik329]

I think we're saying the same thing, but it really has nothing to do with triangles. A torque is simply a force times a distance. The torque about the swingarm pivot is the force of the chain pulling and the distance being the perpendicular distance from the chain to the rotation point (swingarm pivot). If the chain were below the swingarm pivot, it would indeed rotate the other direction. And if it was inline with the swingarm pivot, there would be no torque on the swingarm.