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rebound theory

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just wondering about rebound theory.

 

lets say you have a shock with a 5-0 spring and it is setup and working well.

 

you now increase the spring rate to 6.0. The spring rate was increased because it has a heavier force acting on it{heavier rider).

 

now the theory goes that we must increase the rebound damping for the heavier spring as it is storing more energy after compression.

 

now my thought is,the compressed spring(6.0) does have more stored energy but it is being acted on by a heavier load(the heavier rider).so if the rebounding spring is being resisted by a heavier load is there actually any net gain in rebound load?Is there a need for more rebound damping ?

 

my thought is rebound damping is not just about resisting a compressed spring for but resisting a whole load force(bike and rider and compressed spring force)so rider weight would actually subtract from the compressed load of the heavier spring.

 

there was one comment in a post along time ago and one of the tuners vaguely alluded to this.

 

educate me.

 

thanks cj

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Disclaimer.....

Not an expert.

But here's my take on what your saying.

That may be true when the wheel never leaves the ground you won't need more rebound.

Maybe with the 5.0 spring the rider hitting a single rolling whoop. He will get 3 inches of travel.

Now he hits it exactly the same rolling whoop with the 6.0 and that gets him 2 inches of travel.

The same amount of force is applied to the spring it just moves one less than the other. So maybe it this case the same rebound effort would be adequate.

Now on the other side of things.

Bottom both setups out.

The 6.0 will do a signifigantly better job of trying to catapult you when using the same rebound setup.

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I see some big guys trying to go with setups like this every so often.  They get great traction with the more open rebound setting, and the initial plushness is better with more compression bleed as well.  Then the bike pogos like mad once they come off the top of a roller or g-out, or if they get pitched forward by something and the rear becomes unloaded from deep in the travel.  

 

Loaded vs. unloaded is still low speed vs. high speed in the stacks, and still comes down to preference.  I also know light guys who want the rebound action crazy slow.  

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so rebound free bleed helps to let the tire track on small chop and then an appropriate (stiff for heavy spring) stack to catch the high speed?

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so rebound free bleed helps to let the tire track on small chop and then an appropriate (stiff for heavy spring) stack to catch the high speed?

 

I think the first part of the shimstack is activated while tracking as well.  Probably while mid-stroke and (mostly) loaded.  

 

I guarantee you notice a difference in traction going from a normal shimstack to throwing 4x40x.3 on the rebound face.  Tells me the shimstack is opening at least a little bit for traction, and the highest speeds (unloaded, deep travel) are managed elsewhere in the stack, if you wanted to have minimal effect on tracking ability that is.

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here's what i did on a 09 yz450f today. rider is 210 lbs. 6.0kg shock spring. starting clicker at 11 out.

 

(2).36.3,36.25,28.1,36.3,34.3,32.3,30.3,29.3,28.3,27.3,26.3

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From the physics side, heavier weight doesn't cancel out the stiffer spring rate. Damping is about absorbing energy, and a stiffer spring stores more energy than a softer one, so you need more damping to absorb it, period.

Also, keep in mind that rebound damping isn't just about controlling chassis pitch when the wheel is on the ground; it's also about controlling wheel movement when it's in intermittent ground contact. In the latter case, increased spring rate has no additional mass, as the swingarm/wheel/tire/etc hasn't changed one bit.

Finally note that spring rates, mass, and damping and not necessarily linearly related, especially damping. IE, 20% more spring rate doesn't mean you need 20% more damping (from a pure physics standpoint it's a square-root relationship, so 20% more spring needs about 9-10% more damping, but in the real world it's not necessarily that simple).

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This topic is exactly what I am pondering with my rebound stack right now....07 ktm 450 exc. 

 

I shimmed it this way thinking it would absorb small chop and slow it down on bigger hits:

 

36x.20  (3)

28x.20

34x.2

32x.2

30x2

28x.2

26x.2

24x.2

20x.30

no backer

 

I found that the clicker really makes a difference where it is set.  at 0-10 out the bike stays low when you push on the seat.. At 20+ out, the bike springs back super fast.  I am at 15 out...  Big hits are controlled, but I noticed that small chop was harsher than expected.     I want to try this:

 

36x.15

30x.10

34x.2

32x.2

30x.2

28x.2

26x.2

20x.40

22x.2

24x.2

28x.2

 

In theory I think  this will allow the face shim to bend real easy, but only slightly, to get the small chop plushness.  I don't know if it will cause a pogo action or not.....I don't think it will take much force for the face shim to contact the rest of the stack with the 30x.10 crossover.   I think that this would control the larger rebound movements early on in the stroke pretty well.... I added in the backer shims to limit the entire stack from flexing too far on a super large g-out or log hop situation..    Do you all think this will work? 

Edited by Bailey28

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I would worry about a 0.15 getting sucked into the port and damaging the shim

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I added in the backer shims to limit the entire stack from flexing too far on a super large g-out or log hop situation..    Do you all think this will work?

A few random comments (not an expert):

Your revised stack looks quite a bit softer than the stock stack, not just at low speeds but across the board. You've switched from 3X 0.20 face shims to 1X 0.15 face shim, which is going to be something like 7X softer (not the whole stack, just this initial portion). This will have a significant impact on the stiffness of the entire stack as well. You'd have to run it on a dyno or restackor to get a good number for this, but you'r basically removing 2 out of 11 of the shims that are in there, and going thinner to boot, so it's got to be at least 20-50% softer than what you have now AFTER the crossover closes. That's a really big difference... It might be pogo-y after big hits, if it works well now.

I'm not sure that your "backer" shims are going to do much in the way of limiting flex. If you look at the shape of the stack, you're going to need a lot of deflection of the main stack before you get any backer contact. They may eventually hit, but it's hard to say when, or if it will do what you want.

If your bike currently feels good on big hits but not as great as you want on chop, why make such a drastic change to the entire stack? Seems really risky. Why not just try moving the crossover up 1 or 2 locations? That would significant effect the stiffness of the first stage, without significantly impacting the big hit stuff.

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