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Rebound Stack Stiffness

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Hello @all!

I am new here and I was looking for a thread to tell you I am new but I can't find one. 

So I will introduce myself in this thread. I am a german engineer, 32 years old. My english isn't perfect but I am always trying my best. 

I am riding sportsbikes since I am 18 years old and I was riding different german bikes from 16.

 

You might wonder why I am riding a Suspension question in the dirtbike section, while I am riding sportbikes...I am reading silently different threads around here but at this point I decided to sign up cause I need your help. And what I've recognized is that a lot of people in the dirtbike section have experience with revalving forks and shocks.

So in regards to my forks...I have to stiffen up the rebound stack - by the way the ports of the pistons do not restrict any flow. They are wide open ;-)

So my Rebound Stack is this:

3x22 x0.15

20x0.15

18x0.2

16x0.2

14x0.2

11x0.25

+BackingPlate

 

Alright... I know there are several options to stiffen the stack. But I wanted to hear your advice...Should I use a bigger clamp shim. More face plates...and so on

 

I am looking forward for your replies and suggestions.

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On rebound stacks of 0.1mm the face shim often contacts the base plate , this adds alot of damping at high speeds , on yours that's not going to happen , you could add 13.2,12.2 to the taper, you could add a face shim or add a larger clamp , all with give more damping , if you add face shims or a bigger clamp it does a similar thing. Your English is better than most people . I recently went to Amsterdam and they spoke English better than my home town .....

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thank you for your reply. 

I guess that sooner or later a bad english phrase appear... :-P

Yeah...Every additional shim would make it stiffer in any way. I don't want to go with thicker shims to prevent destroying the shims.

So the question is still there how...which effective way makes the stack stiffer?

 

As an additional information: I have a clicker range of 25klicks. to compensate the energy of the spring i have to go down to 7 clicks out with fresh oil.

The goal is to keep it at 10 klicks out, but...I am not 100% sure whether the needle design/assembly allows it.

 

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It would really help if you told us what bike you are riding. Is it for street use or track. Clearly you have a 25mm cartridge and the valving looks similar to Ohlins. I'm guessing 8mm ID shims. Why do you believe you need more rebound damping?

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I am riding a 2003 Yamaha R1 with a 2008 Yamaha R1 Fork. 

Street Riding: Commuting and also aggressive "in the twistys". Is that correct explained?

Also with luggage and/or passenger. So all kinds of everything. Based on that my forkspring choice is a 9.75N/mm (with 8mm installed preload). This gives me the option to be comfortable while commuting alone and with 12mm additional preload on the adjuster I can make my bike fit a lot of other "heavier" situations.

I think you will get the explanation,right!?

 

@Terry Hay Your guess was good. But it is a 24mm Catridge from KYB. I inserted my own pistons to increase flow and to put all the damping on the shims.

So...7 clicks out (with warm oil) is the starting when I ride alone and commute. When it comes to heavier situations and more energy is stored into the springs the rebound goes down to 3klicks out (warm). Here is the problem...when the oil breaks down I have to go in further - but there isn't a lot.

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Set up below is using a crossover, based on your riding type it might work good

16x22 x0.10
18.10
20x0.15
18x0.2
16x0.2
14x0.2
13x0.2
12x0.2

+BackingPlate

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If you at 7 and want it to be 10 out I would just add another 2 x 22.15

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I'm sorry, I don't provide valving specs on forums but if you find your rebound control changing as you ride, this is a problem. Look to the things that will alter with temperature first. I would suggest you change your oil and review your tyre pressures before changing your stacks. Maxima oil provides the greatest temperature stability among the popular oils but is too sticky for my liking. Ohlins oil works well and is slippery. The new Motorex is even more slippery but I haven't tested its temperature capability at this stage. It may be possible that the shims have fatigued and need replacing in order to keep things more stable. For me your stack has too much high speed. This will require greater distortion (flexing) of the low speed shims.

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@mog interesting point and it won't change the highspeed area

@Terry Hay well the maxima 5w was the first oil i was using and the ohlins 1309 was the next after it. but in terms of the oil getting older both oils behave very similiar. motorex might be an option, but i haven't test it too. silkolene rsf5w might also be an option...The shims are straight and have been new when I build the cartridge. I also checked whether I have any blowby but everything is sealed very good.

My mind is openend for any ideas...And yes....the highspeed area might be too stiff and I could try to soften this area and go up iin stiffness on the low speed area

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On a single stage stack if you alter the high speed part you still add low speed as well , if you add low speed you still add high speed ,they are tied together by the stack design ,to alter one independent of the other is impossible , and if you want to add more low speed but keep the high speed the same ,it requires restackor to know you doing it correctly

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The two are undoubtably linked but the level of influence that each provides can easily be altered. We possibly get a little lazy with our terminology when we discuss valving with people new to the topic. Most will undoubtably see the large shims as low speed and the small as high speed. Restackor is a nice guide but far from "necessary". Ultimately each rider will want what they want. This makes a good test process more essential as it is the sum of all the parts that yeilds the overall result.... not just a shimstack calculation.

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so...testing is everything.

I went today forward and inserted 2 shims into the stack. Both new shims are over the clamp shim:

3x22 x0.15

20x0.15

18x0.2

16x0.2

14x0.2

13x0.15

12x0.15

11x0.25

+BackingPlate

 

Holy Sh**.... the rebound (cold) is now at 20klicks out. Oil is Silkolene RSF 5W. It might go down to 18 warm...but at the moment the stack is too stiff.

I hadn't expected such a dramatic change!?

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You got more than you bargained for. You stiffened the stacked where it was already stiff. There is much more than "overall stack stiffness" going on here........Good luck.

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So you added the 12x.15 and the 13x.15?

That makes the 14x.2 much stiffer.  Instead of bending rather easily over the 11mm clamp now it is well supported by the new shims.  At low speeds it will behave like its got a 14mm clamp shim.  At high speed it might deflect the 11/12/13/14mm shims but not in the top half of the stroke.

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@Terry Hay testing is the way figure out what works and what doen't work. So I made a step Forward and got a result. not the result i've expected but thats the Situation right now.

By the way...nothing is destroyed and with a Little elbowgrease (and Money spend for oil) I can realize a lot of other Setups...

So my plan is to carefully ride the bike on the Weekend to see how it develops in a warm Situation. But I don't expect that the oil will come to me a lot. It has a very high VI and will not dramaticly Change from cold to hot.

in regards to the Suggestion from @turbo dan you are right. testing the Rebound manually...the character of the Rebound stroke is that the highspeed (coming from a deep stroke) has become slower compared to the first stack. the low Speed Rebound area (going only into a light stroke) seems to be nearly out of adjustment. when I go to 20 klicks out the flow is quick enough at low Speeds (maybe due to the fact that the needle allows the Bypass?!)

Hopefully you are getting the right Picture of me....I am willing to test different stacks and I will invest the time and the Money into this. And if a stack doesn't work or doesn't feel good...I am not sad or in Anger...I just start to think about how to go on AND I am telling you my experience and my thoughts to discuss the next steps.

In the best case this is an experience for you guys and me...?!

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OMC

Your attitude is perfect.....there is no bad result if you have learned something. As you ride, be aware that rebound is only a consequence of compression. If your fork doesn't compress, it won't rebound. We tend to notice the "reaction" rather than the action. Most rebound problems stem from poor compression management...not all, but most...

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Yes...i am Aware of this. for testing Rebound visually (without driving) I open the compression completly to see whether it Pogo's or not.

And will start riding with the compression open. this is the normal procedure...i gonna Close compression until i have no brakedive and/or bumps are not "hurting". I think you get the concept. I also have a travelindicator and marked real bottom out...so everything is ready...

honestly i don't expect a good Rebound behavior and i realized a mistake while opening this thread: I should have given you a characteristic of my Rebound at first.

I guess that could have been more helpful.

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If you want to have the reb adjuster at a particular position, using "restackor" seems a good approach to me.

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