linear/digressive

[GDI70]

http://www.youtube.com/watch?v=YMnZTodTCZ0

[Sun Devil]

Take it away DaveJ......hahaha. JK.

[cardinale199]

goog explanation vid

[mog]

so how about we discuss this some more , we know as he said we want digressive compression damping , but for rebound would linear work well ? or do we still want digressive ?

[grayracer513]

That's an interesting question. The variations in suspension input under compression are almost limitless, as the input force is a function of speed and surface conditions. With rebound, however, even though the conditions and needs may change, the input force is limited to what the springs provide. It seems, in theory at least, as if it is much simpler to have the compression side be adaptable to a wide range of situations than to have the rebound figure out what the difference between one condition vs. another.

[kawamaha]

so how about we discuss this some more , we know as he said we want digressive compression damping , but for rebound would linear work well ? or do we still want digressive ?

as he explained digressive compression damping its a good thing for example when you hit a small obstackle at high speed.

but for rebound you don't have such a high impact, only the controlled force of the spring. so if I look at rebound stacks, mostly two stage (progressive) stacks seems to be the right way....

[Vietze]

I've seen preloaded reb stacks on WP Supermoto forks and on a Sachs shock, probably also supermoto.

Being not into supermoto I cant figure out why one would need there a stack like that...

[mikerides33]

Just wondering guys- what he shows is a quick hit or "spike" and I suppose that is linear but then blowoff occurs followed by a regression. Why would it be called digressive? I remember you guys discussing the terminolgy but this seems pretty cut and dry. Why isn't it called regressive after you blow off?

[Vietze]

well, cause it's not regressive.?

Progressive, degressive, regressive...

There terms all refering to the slope.

You might know this stuff from mathematic lessons, first derivative gives the slope of the function.

So, progressive means the slope gets steeper the more you go further the x-axis.

degressive means the slope gets smaller the further you go on the x-axis, but never gets negative.

Regressive means the slope gets negative at some point.

Here I made a quick illustration how I'm thinking of it:

Uploaded with ImageShack.us

[mog]

i dont think we would like a regressive stack.

[grayracer513]

I shudder to imagine such a thing.

Although I think I had a bike like that once long ago...?

I think Mikerides was thrown off by the misspelling of the word as DIgressive, which is not the same thing at all as DEgressive.

[mikerides33]

Thanks Gray...and thanks for the illustration Vietze. We certainly would not like a regressive stack a shown in the 3rd.

[DaveJ]

Okay…here’s my two cents.

First of all, the term progressive just means that you get more of something as an input occurs.

So progression can be related to resistance and speed, and/or force and distance, and/or the ratio of axle movement to the rod movement.

So a linear graph that shows ANY positive change in hydraulic resistance (conversion) as related to speed, would be called progressive.

And the compression of any spring is progressive – it gets harder and harder to compress the spring the more you compress it. In most cases, usually a linear return.

AND…the rising rate on the rear suspension due to the linkage or shock interface would also create a progression. Call it a “ratio progression”, if you like.

So… all modern day dirt bikes have progressive springs, and progressive hydraulics as well as a progressive rising rate on the rear. And the air chamber in the fork, as well as the nitrogen charge in the shock, is progressive as well. For that matter, so is the hydraulic bottom out, the shock bumper, the spring perch..AND…the frame of the bike, (which is one giant spring). By the way, you don’t need a linkage to have a progressive rising rate.

Now…digressive simply means that you have left the path of a true linear line. And linear means you rise an equal distance for what you move forward (X2Y2). Deviant from that line, be it below or above the line, you have digressive performance. That said, obviously somewhere and somehow, someone starting calling a given curve shape “digressive” and now we have things like “digressive shocks”. In this case, they are trying to say that a return rate that climbs much faster than a linear line provides a more suitable response and therefore more control. In my opinion, this is a good thing for forks on a dirt bike, a good thing for shocks in a road going car, but a bad thing for the rear shock on a dirt bike. More on that later.

And now, regression.

Regression simply means that you have a diminishing return – you now are getting less and less of something as you moved further into your comparison. So…the high speed adjustor on your shock would allow you to control regression of damping return as the rod speed enters into a higher speed velocity.

The Dell Taco, (DDT) as noted once upon a time, would digress higher than a stock mid-valve (above the linear line) and would then have more regression (moving back towards the linear line) as faster rod speeds were encountered.

What we could do is refer to things as having a positive digression, or a negative digression, respective to whether the return is above or below the linear line or how it moves away or towards it.

That would mean that a stock mid-valve would have a negative digression for low-speeds and a positive digression for high speed…AND the Dell Taco would be the opposite; positive digression for low, and negative (returning to that direction) digression for high speed. Same planet - different worlds.

By the way, is not possible for any current suspension to have a regression (a negative change) that is greater than the positive change in velocity. In other words, that third chart on the above graph would never exist with our current hardware, (stock or not). The best that one would see would be a flat line, but never a line that curves downward. You can’t have an increase in velocity with a greater and greater loss in damping performance, (conversion). By the way, this scenario WOULD be possible if you had a burst valve, but all mid-valve springs and shims, and their closely related valve bodies, cannot offer burst capabilities. So…there. ?

Now…on the matter of how much is enough and how much is too much, I think it’s important to understand that a hydraulic fluid is both a solid and a liquid, depending on what you do and do not allow it to do. The intention is to create a “hydraulic cushion” by allowing the fluid to act as both a solid AND a liquid…sort of like the cushions on your couch. So…the best cushion (firm but plush) comes from running the compression damping right on the edge of being too much. Tricky stuff. This is often why making a fork stiffer, (if done in the right way) provides for a smoother ride. And yes, sometimes when you make them softer, the rider is more harsh or rough.

That said, it’s really REALLY hard to create just enough damping, without it being too much, for all of the things that we roll over at all the different speeds. The most difficult are often the desert guys, because they ride the more diverse terrains at the greatest range of speeds. Making matters more complex is this issue of rod acceleration and rod velocity…but that’s a different subject.

The bottom line is that we hope to have a reaction that is never above or below what we need (over or under-stroking) as that leads to changes in the direction of the chassis (less forward speed) and also makes our hands hurt.

On the rebound side, we absolutely can’t have a linear return, as there is more that is involved than just one spring. For example, on the rear of the bike you have the main spring, the compression of the nitrogen, the compression of the bumper and, in severe cases, chassis deflection. Add all that up and you’re basically sitting on the equivalent of a bomb.

When we (SPI) set up a shock, we allow for nearly unrestricted ultra low to moderate low speeds, as this provides for a very subtle shock for coming out of corners or for drag racing, (maximum traction)…but then we radically hold the shock (via high speed rebound) when there is a massive amount of energy stored up, as to prevent the rider from ever getting toss over the bars or just kicked off…something I really think is more critical to our safety than a neck brace. However, not everyone agrees with this approach, including the manufactures. But…many riders that ride our stuff will say that it feels like there is a hand coming out of the ground, holding onto the tire. It plants, and doesn’t move. It’s a weird sensation, but you never again have to manage or worry about the back of the bike despite whatever you clip or come up short on.

The moral of the story? A dirt bike suspension needs to be set-up unlike any other form of transportation...and, the dynamics of what is best for the front it often very different than what is best for the rear.

?

[noslo33]

all i know is the stack that dave came up with for my kids 250f yammie rear shock can only be described one way,total awesomeness!!!wooohooo,peace and wheelies.

[mikerides33]

When we (SPI) set up a shock, we allow for nearly unrestricted ultra low to moderate low speeds, as this provides for a very subtle shock for coming out of corners or for drag racing, (maximum traction)…but then we radically hold the shock (via high speed rebound) when there is a massive amount of energy stored up, as to prevent the rider from ever getting toss over the bars or just kicked off…something I really think is more critical to our safety than a neck brace. However, not everyone agrees with this approach, including the manufactures. But…many riders that ride our stuff will say that it feels like there is a hand coming out of the ground, holding onto the tire. It plants, and doesn’t move. It’s a weird sensation, but you never again have to manage or worry about the back of the bike despite whatever you clip or come up short on.

?

This is gold right here. I had so and so (their name rhymes with shmemex tech) valve my 350 and I couldn't get going. Finally I had the SPI stacks go in with a recipe for rebound that has me glued.....pinned out of corners without fear of testing my neck brace. This is stuff that more people need to realize so that there can be less injuries. I have brought up the Stewart Daytona crash many times to people because its the example of how some say control could be acheived and some say it can not. Dave the shock is fantastic. The hand of God is holding my ass down now thankyou:thumbsup:

[jwaseman]

http://www.youtube.com/watch?v=YMnZTodTCZ0

Great post Marc!

And now, regression.

Regression simply means that you have a diminishing return – you now are getting less and less of something as you moved further into your comparison. So…the high speed adjustor on your shock would allow you to control regression of damping return as the rod speed enters into a higher speed velocity.

The Dell Taco, (DDT) as noted once upon a time, would digress higher than a stock mid-valve (above the linear line) and would then have more regression (moving back towards the linear line) as faster rod speeds were encountered.

?

Yep , Gen 1 DDT came to mind as far as regression goes. I'm glad its fixed:thumbsup:

This is gold right here. I had so and so (their name rhymes with shmemex tech) valve my 350 and I couldn't get going. Finally I had the SPI stacks go in with a recipe for rebound that has me glued.....pinned out of corners without fear of testing my neck brace. This is stuff that more people need to realize so that there can be less injuries. I have brought up the Stewart Daytona crash many times to people because its the example of how some say control could be acheived and some say it can not. Dave the shock is fantastic. The hand of God is holding my ass down now thankyou:thumbsup:

Amen there!?

[mikerides33]

Thank you too James!

[gbjim]

Cool, learn something everyday, I always thought digressive was another word for rambling; you know when people don’t when to stop.

[wblake]

When we (SPI) set up a shock, we allow for nearly unrestricted ultra low to moderate low speeds, as this provides for a very subtle shock for coming out of corners or for drag racing, (maximum traction)…but then we radically hold the shock (via high speed rebound) when there is a massive amount of energy stored up, as to prevent the rider from ever getting toss over the bars or just kicked off…something I really think is more critical to our safety than a neck brace. However, not everyone agrees with this approach, including the manufactures. But…many riders that ride our stuff will say that it feels like there is a hand coming out of the ground, holding onto the tire. It plants, and doesn’t move. It’s a weird sensation, but you never again have to manage or worry about the back of the bike despite whatever you clip or come up short on.

Great write up and very informative as always Dave....only thing comes to mind is why dont many others agree with this setup?? is there something your giving away by doing that on a shock and you dont see it offen on stock setups???

thanks again, am not implying anything just curious!!!

P.S when is the website coming back???

[Sun Devil]

Good stuff Dave!