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Discussion Starter · #1 ·
So for all the board members, and particularly Napkin, here's a link to a document that explains why the SV650 rear suspension is progressive. I've tried to explain it in words but had no luck. I have a bad feeling that these words and sketches won't have much luck with some folks, but I'm willing to give it a shot.

It's not math based or anything.

http://docs.google.com/Doc?id=dfjcvtc2_76dh9jndcc
 

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Discussion Starter · #3 ·
Napkin insisted it wasn't progressive because the shock compresses close to the same amount per inch of rear axle deflection (app. 0.45" (plus or minus a few hundredths of an inch) of shock compression per inch of axle lift). As you can see in the referenced paper, it's the geometry of force application that makes it act progressively.
 

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Napkin insisted it wasn't progressive because the shock compresses close to the same amount per inch of rear axle deflection (app. 0.45" (plus or minus a few hundredths of an inch) of shock compression per inch of axle lift). As you can see in the referenced paper, it's the geometry of force application that makes it act progressively.
Well, you have to admit, his logic makes sense. It's interesting that they stick that whole crazy linkage in there and the shock winds up compressing exactly the same amount per unit of suspension travel at both the end and beginning of the stroke, something that would not happen in many, many other configurations- and that all those other configurations would take up considerably more room.

You are certain that throughout the whole stroke, the ratio is the same within a few hundredths of an inch? That is hard to wrap ones head around.
 

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Discussion Starter · #5 ·
The travel ratio is the same, the force ratios are not. On mechanisms like this it isn't very easy to picture what's happening. There are numerical methods (equations describing all the various movements) but they are not simple either. (Well, it's all just trig for any given position, but it takes differential calculus to describe the general case).

It takes a knowledge of how the equations work to understand the linkage. Graphical methods are better, but they do take a bit of imagination and study to understand. You have to imagine what each bit does as it travels through its motion.

Problems like these come up in engineering courses on dynamics.
 

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Appreciate the sketches. One thing I don't understand is why A1 becomes shorter at full compression.

On the dogbones, the changes in angles that you mentioned in your last post of the other thread were fairly minor. Does this mean the change in the force required to compress the shock (as you approach full compression) with shorter dogbones is equally minor?

Not asking you to run any calculations. That's more trouble than it's worth. I'm just curious for a ballpark guesstimate of what you think the actual effect might be. As I said before, my seat-of-the-pants feeling is that the raising link has affected the actual ride very little. Still, that could be just because I haven't put the bike into any situations since the swap in which bottoming out was a real possibility.
 

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Discussion Starter · #7 ·
You need to look at the sketches more to see why the lever arms get shorter. They vary as the sine of the angle between the relevant pivot points change (e.g., swingarm dogbone pivot - to cushion lever dogbone pivot - to cushion lever chassis pivot). They both change. The dogbone effective lever arm gets shorter quicker because of its steeper angle to start with. The shock effective lever arm starts before 90 degrees, so it actually gets a bit longer before it starts to get shorter.

Very true, changing the dogbone length a little (as in raising or lowering the rear ride height 1") makes very little difference in the geometry. Using longer or shorter shocks makes more of a difference, relatively, but still not a great amount.

Actually, I had calculated the changes to be more than they are with different shock lengths. When I resurrected those numbers I realized I had made an error. With longer shocks the biggest hazard is running out of shock stroke and bottoming the shock. With shorter shocks you have less compression difference before the apparent spring stiffness begins to climb rather steeply. Still, I'd avoid changing shock length too much.
 

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Andy,
you don't understand very simple and basic 101 about motorcycle suspension.
if you have linear spring on shock and shock travels same distance for every inch at wheel travel it is impossible to be anything than linear.
only thing that can compress shock specified distance is pressure dependent on spring rate.this can not change.you can not change spring rate.
you are simply imagining things.
 

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Well, you have to admit, his logic makes sense. It's interesting that they stick that whole crazy linkage in there and the shock winds up compressing exactly the same amount per unit of suspension travel at both the end and beginning of the stroke, something that would not happen in many, many other configurations- and that all those other configurations would take up considerably more room.

You are certain that throughout the whole stroke, the ratio is the same within a few hundredths of an inch? That is hard to wrap ones head around.
back in time when they came up with this linkage systems they were experimenting with progression.most bikes today are linear.there is small variance at very end of travel.why you think racers extend shocks instead changing dog bones?
 

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back in time when they came up with this linkage systems they were experimenting with progression.most bikes today are linear.there is small variance at very end of travel.why you think racers extend shocks instead changing dog bones?
I think you misunderstood me. I was suggesting that perhaps the point of the linkage was to make it linear in as compact a space as possible.
 

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sorry if it came that way.was just making point for others.
you are correct.
it all came from dirt bikes where shock was getting longer and longer for more suspension travel.they needed smaller compact package.
it is all big marketing bs :)
 

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it is all big marketing bs :)
See, that's the thing- Andy, who generally does know what he's talking about, is not the only one saying this. It is on Suzuki sites as well.

I'm sorry if this was covered in the other thread, but from where did the assertion that throughout the travel, the swingarm movement and the shock movement were always proportional?
 

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I have measured it my self.andy came with same with his math.
you don't need to be mechanical engineer to figure this out.it is really simple and anybody here can measure it.
it is simple measurement between wheel and shock travel in small increments.
tlc,your new gen bike is little different.
 

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Discussion Starter · #15 ·
LISTEN UP AND LISTEN TIGHT!

The linkage is pretty linear as far as relative motion is concerned, at least at the angles of motion the suspension operates within.

IT IS PROGRESSIVE WHEN YOU LOOK AT THE FORCE REQUIRED AT THE CUSHION LEVER DOGBONE LINK REQUIRED TO MOVE THE SHOCK A GIVEN DISTANCE.

It all has to do with the varying lengths of the effective lever arms.

STOP TALKING ABOUT RELATIVE MOTION AND START LOOKING AT THE TRIGONOMETRY OF EACH LINK.

Sorry for the shouting.
 

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Discussion Starter · #16 ·
I sent this to Napkin, I clarified a couple of points here.

Just for laughs, try this: Take a wheel of some sort. A bicycle wheel without a tire is perfect. Mount it so it rotates on its axle normally. Hang a weight (a quart milk bottle full of water does nicely) from a stout string and wind the string around the wheel. Picture an old water well bucket. As you turn the wheel back and forth the weight goes up and down. Note that since the weight is always the same distance from the axle the torque on the wheel is constant.

Now get the weight off the ground and stop the wheel from turning. Select a spoke at the bottom of the wheel and attach a string on that spoke. Hook a fishing scale on that string and pull horizontally. Free the wheel so it can turn. Note that when the spoke is at the bottom and the string is held horizontally in the plane of the wheel the fishing scale will register the weight of the bottle.

Do two things now. Without letting the wheel change position, change the angle of the string and fishing scale. You will note that as the angle increases from the horizontal or from the plane of the wheel the load on the fishing scale will increase. Now pull the string and scale in the plane of the wheel keeping the string horizontal so the wheel rotates. You will note that as the wheel rotates the load on the scale again increases. Note that the torque producing that load remains constant.

Answer the question: Why does the load at the scale change? When you can answer that question the SV suspension and its progressive nature should become more clear.
 

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Edit: Andy, The following are two equations for calculating mechanical advantage:

(1)
Mech_advantage=(distance over which effort is applied)/(distance over which load is moved)

(2)
Mech_advantage=(output force)/(input force)

Correct me if I'm wrong but it seems that you are suggesting a system whereby somehow equation (1) is giving one value and equation (2) another.
 

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Very true, changing the dogbone length a little (as in raising or lowering the rear ride height 1") makes very little difference in the geometry. Using longer or shorter shocks makes more of a difference, relatively, but still not a great amount.

Actually, I had calculated the changes to be more than they are with different shock lengths. When I resurrected those numbers I realized I had made an error. With longer shocks the biggest hazard is running out of shock stroke and bottoming the shock. With shorter shocks you have less compression difference before the apparent spring stiffness begins to climb rather steeply. Still, I'd avoid changing shock length too much.
Thanks for the clarifications. Curious myself because, obviously, I'd installed a slightly shorter GSXR shock and compensated on the ride height with a change in dogbones. Worth pursuing not only for my own info, but also because a lot of other guys on the boards have done similar things. So, regardless of whether you're able to help Napkin comprehend what's up, I appreciate the effort.

Bottom line seems to be a barely noticeable change in compression damping until the system begins to approach max travel, at which point the shorter shock and dogbones will result in some harshness. Is that a fair statement?
 
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