Stack theory HSC vs LSC

I'm starting to delve into the suspension side of things a bit more now. I have a 2010 Gas Gas EC250 with Ohlins TTX front and rear. I'm very happy with the front, apart from the occassional hs deflection. The rear however has been hard to live with. The TTX shock imo complicates matters by offering the CSC adjuster on top of the typical LSC and LSR. The CSC from what I understand is a bleed for both low speed compression and rebound, with very little effect on high speed circuits, which is handled though the shims and valving.

 

Spring rates are set at 0.46 up front, and 5.4 at the rear. Sag figures are Front: Race 60mm. Rear: Static 35mm, Race 105mm. Fork springs are running 3mm preload, and shock is about 11-12mm preload to get those figures. I have tried altering the preload and sag figures, but have not yet tried going up a rate on the rear with less preload. I will be doing that shortly.

 

The back end has always given me some grief. I'm not sure the best way to describe it. If going softer on the clickers (comp or csc) the bike feels like it hooks up a bit better, is more compliant over technical rocks, roots, trail trash at lower speeds, however will then wollow, get busy, and feel harsh when picking up the pace. It will also kick the rear end out when hitting square edges and/or fallen logs if moving fast. However, if I then firm it all up, there is a noticable reduction in the ability to get traction on loose skatey surfaces, its a pig to battle when bush bashing or when fatigued and riding slow, but really works well when pushing hard and hitting things aggressively. I guess what I am looking to achieve is the mystical balance of both. Some initial plushness for when fannying about, but enough resistance to keep it up in the stroke.

 

Being that I know what I want, I now have to try and nut out how to achieve it. I think going up a rate on the spring to a 5.6 with less preload, more static and similar race sag will be a step in the right direction. I have also removed some face shims, and thickened up the smaller end of the hsc stack, which felt better via seat of the pants, but when watchin video seems to be using too much travel. Looks a bit saggy in the rear, and also wallows down when landing from a drop or after jumping a log. Again, the increased spring should help here. Since making the valving change I have also ended up further in on the CSC so added more low speed compression back in, which kind of negates the purpose of removing the low speed face shims, and also leaves me wondering if the high speed is now firmer, or softer than before. Obviously the LSC stack will flex sooner (especially with less bleed) and also engage the HSC more than it was before, but the hsc is also firmed up.  

 

So I'm wondering what the right direction of travel here is. Do I want more LSC control, with less HSC? It should give less wallow, keep the bike up in the stroke, and allow quick movement over trail trash, rocks, roots, square edges.. this will likely make the bike feel quite firm and still have that harsh feeling when riding slowly when tired. Or do I go the other way and look at reducing the LSC to give initial plushness when riding slow, but increasing the HSC stack to reduce the amount of stroke being used. This should work in conjunction with the higher spring rate.

 

I'll try and get some video together for those interested in helping, and maybe a pitcure or two. For any tuners out there who would like to share some wisdom, the shock model is GG1882. The setup is for trail work at a brisk pace. If I had to guess probably similar to a B class scrambler.

 

Fundamentally, whats the difference in setup between an MXer and an Enduro bike, in regards to LSC vs HSC?

 

Anyone?

Jakobi,

 

I've been working on my suspension for a while now on my KTM 450 but I am by no means a pro at it.  I've been able to do both the fork and shock at home.    Some of my findings are this:

 

I used to think that by adding low speed and removing high speed shims that the bike would stay up better, and then collapse into the stroke to absorb the square edge hits like curbs, logs, rocks, roots etc.   Well long story short, I realized that the shim stack works as a whole together with one end affecting the other.   I can say that I think the LS shims are involved a lot more with the HS action than I originally thought.  

 

I noticed through testing on my own that when I removed more low speed shims it helped on the square edge stuff as well.  The bike really didn't ride differently in height, that is controlled more by the spring stiffness and preload on the spring.  For example in my fork, the stock "LS" stack was

2 24x.1

1 14x.1

8 24x.1

2 22x.1

2 20x.1

18

16

etc.

 

I am currently at:

5   24x.1

1  11x.1

24

22

20

18

16

14

9x.30

 

So  in essence, I was able to get compliance I wanted for roots and rocks by reducing some of the high speed shims, but even more of the low speed shims.  

 

On my shock,  the stock valving is this:

 

6   44x.25

2   42x.2

40

38

36

34

32

30

28

20x.30

 

I changed this to:

4  44x.20

2 42x.20

40

38

36

34

32

30

28

26

20x.30

 

Yes, I added a 26 above the clamp but I took out a ton of LS it appears by getting rid of the .25 shims, and reducing the number of shims in total. 

 

I love the change. I am 170 lbs, ride at a similar pace to yours, trails, enduro, street, light moto, woods, etc.  Absorbs curbs, roots, rocks,etc.  And it rides up nice in the stroke without all the see saw movement like you described when you let your clicker out.  It even feels firm initially but will cave in when it is time to do so. 

 

In my findings, and some will disagree with this, I feel that while some bleed is good, too much bleed, be it a clicker out too far or too big of a bleed hole drilled in a piston, will make the bike soft and wallow.    My personal preference after all of the testing I have been doing is to get a stack where I can run the clicker in tighter which will make the stack lift sooner.  I think that the free flow bleed works to a point however if there is too much free flow the suspension will be soft until it gets to the point where the flow capacity is maxed out.  Then the stack must lift.   I feel there is a "window" of free bleed, and the larger that window the more you will feel the transition between the bleed and the stack. 

Edited by Bailey28

Hey Bailey! Long time no see chief! Hows the back/neck going? You're back on the bike so thats gotta be great news!

 

Indeed you are correct. The LSC and HSC can't be isolated as such, even with a multi stage stack. The face shims on the LSC still need to flex before engaging the HSC stack, so if running more LSC you are indeed making the whole stack stiffer.

 

Now you've just added another element to the mix. Reducing both stacks. I think I'm going to have to go with some trial and error and just make the changes and evaluate to see if its the right direction. I'm not sure if the bike is kicking over the square edges due to blowing through and hitting the bumpstop, or because the HSC is just too hard and its not reacting to the trail trash. By removing face shims I'll essentially soften the whole stack, and then running less LSC bleed should keep the action nice and firm. The CSC adjuster just adds complexity having another adjustable bleed that effects both circuits. It seems that by initially reducing the comp stack (face shims), I then ended up tuning the difference back out with the CSC which is what adds the confusion.

 

Based on everything I have been thinking though, I have ordered the next rate higher spring, and will try that with my current setup before making any other changes. Then I may look at removing another couple face shims from the stack.

 

Welcome to the world of tinkering.

Edited by Jakobi

I crashed on the GG300, then sold it to my neighbor who still rides it.  I missed it so I bought a nice 07 450 EXC and plated it for the street.  Really, I do about 50/50 street  and trail riding with it.  I've been to the track once since I started again and did one enduro since returning.  I ride senior C pace. 

 

I realized that when talking about revalving, there is a small window of valving setups that will work for the individual rider and situation.   No one set of valving is going to cover it all, so I needed to figure out what I want the bike to do. 

I try to stay with single stage stacks as  don't want to complicate things with a crossover (except for rebound, where it seems that it is needed).

 

I started out by taking out HS shims in the tapered portion of the stack and adding in LS face shims.  Doing that made the bike blow through the stroke but still had a harshness on smaller stuff.     With a good tapered stack I find that I can reduce the number of face shims so I can run the clicker in more and still have a good taper to the stack.

 

I experiment all of the time and by far spent more on buying oil, shims, valves, etc and personal time investment than if I just sent the suspension off to someone to "get done".  I have sent off before and didn't learn a thing.  Now I at least know enough, and keep a journal of all my changes to find a direction to go in. 

 

I try to find a good balance on the bike and have the characteristics of the fork action match the shock action.  By keeping the oil, springs, sag numbers, tire pressure, fork height, chain tension, oil height, bladder pressure, etc. the same after each tinkering session,  I can see what changes VALVING ONLY made. 

Another philosophy of mine I want to mention is "Working the stack".   By that I mean - how fast do I have to hit that obstacle/root/rock/curb/ etc. to "work the stack".     Did I just put together a valve stack that only a AA pro could lift off the face of the piston?  Or did I build a stack that rides half way down in the stroke with no one on the bike...

 

I tried the RT gold valve..Set up their way it was beyond stiff... even at an honest senior C level.  10 44x.25 above the crossover?? Wow... Ouch.. It was good for heavy sandy moto, but that was about it... There was little support here and on KTM Talk so I abandoned it and went back with the stock valving.. Honestly, I couldn't see RT being much of a help. Their roots/woods suggestions were the same as their desert racing settings..  How could that be?

 

Knowing what valve stacks my particular bike wants with me on it, riding over what I normally ride over, is the key to making me happy. 

Edited by Bailey28

Thanks for the advice Bailey. Basically summed up the direction that I am moving in too. Less face shims, more progressive/beefed up HS stack, less bleed. With the new spring on the way I guess I should test that out first, before making any valving mods. Totally with you re removing variables. Always use the same pressure, same oil etc. Just getting into it now.

 

Back to the orginal question, MX stacks vs Enduro. Obviously MX is stiffer overall, but do they tend to run different tapers as well?

 

Glad to hear you're back in the saddle!

Thanks for the advice Bailey. Basically summed up the direction that I am moving in too. Less face shims, more progressive/beefed up HS stack, less bleed. With the new spring on the way I guess I should test that out first, before making any valving mods. Totally with you re removing variables. Always use the same pressure, same oil etc. Just getting into it now.

 

Back to the orginal question, MX stacks vs Enduro. Obviously MX is stiffer overall, but do they tend to run different tapers as well?

 

Glad to hear you're back in the saddle!

 

I would say "generally" MX/SX stacks are stiffer than enduro/woods ones.  Many enduro/woods setups have 2-stage shim stacks in the base-valve and a mid-valve with either a check-plate or shim setup with a lot of float.  Whereas many MX/SX setup have a single-stage base-valve and a mid-valve with less float.

 

Riding enduro/woods/trails generally lets you run softer stacks.... you can argue it will be more compliant, comfortable and less-tiring.  However due to rocks/roots (square-edge stuff) on the trails I find running an early crossover (after a few face shims) in the basevalve (as well keeping your clickers further out, like a bleed shim) and more float in the mid-valve the ticket for having good  compliance over rocks (doesn't deflect or pummel your hands), but still have enough bottoming resistance for larger hits.

 

Whereas in MX/SX the course is usually rock-free and "smoother" and most importantly you're looking for bottoming resistance.  So you can afford to run a much stiffer stack.

 

You can't have it all, since in reality LS and HS damping work together, they are never truly separate stages.  So if you have a lot of LS, you'll also have a lot of HS damping and the other way around as well.  What the crossover does is lessen the stack's stiffness at lower suspension velocities, while not affecting the higher velocity damping (as much)... till the 1st stage deflects enough to touch the 2nd stage and then it acts like a single-stage stack.

Edited by bmwpowere36m3

Thanks for the explaination. I've been busy reading as much as I can and soaking it all in, and think I'm getting a grasp on the theory. The next step will be paying more attention to others actual stacks and the mods made, as well as implementing a couple of changes myself to get some more real experience on how much effect each change makes.

 

The first shuffle for me was definitely a step in the right direction.

My experimentation on a 2013 300 EXC has led me to a low speed stack close to stock, but then makes a definite cross over into a very stiff high speed stack. A very fast mate of mine with worked over (for serious enduro) CC forks, on a 250 SX is comparable. His shock works, but is very 'dead' in its feel. TTX fitted KTM's I've ridden feel soggy initially (to me), but firm up well.

Great!  like I mentioned, I'm learning every time I get into the garage and make changes.        I am hesitant to try the dual stage stacks as the crossover adds another dimension to the works.      I found that I actually need the crossover in the rebound stage when I built a seemingly light but single stage rebound stack.  It packed on small stuff however it was a dream on larger hits. 

 

My bike came with a two stage fork stack but a single stage shock stack from the factory.   Stock EXC KTM used:

 

2   24x.1

     14x.1

8   24x.1

taper from here

 

This is what I had. It was unbearable at anything but motocross.  Fork didn't move. 

 

Fork moves well at the current 5 24x.1, 11x.1 24 22 20 18 etc.  I know this looks like a two stage stack, but I don't think that 11x.1 in the middle is doing anything that I can feel. 

 

The next recipe I want to try is this:

(3)  24x.1

  11x.1

   22

   20

   18

   16

   14

   12

   8x.3

   14x.1

   18x.1

 

I think  this is radically different with the crossover thrown in as well but it might be even more plush on square edge. 

As not to hijack your post, I digress---

 

I have noticed that the single stage stacks appear in a motocross setting.  Like stated above most of the tracks are rock free and having a single stage stack allows a stiffer setup with predictability... Not to say that a two stage stack would not be predictable but my guess is that it is supposed to be softer initially. 

 

I think a single stage is linear, whereas a two stage stack could be considered progressive in that the damping would get stiffer after the face shims contact the shims after the crossover.    I would imagine that getting the right size crossover in the right place in the stack is a challenge.   I am currently hemming and hawing over what I should change my crossover in the shock rebound stack to in the near future...

 

If I  had :

 

24x.1

24x.1

22x.1

24x.1

24x.1

 

I don't think I can count the 22 as a crossover functionally, but by definition it is one?  In the above stack, I could never imagine that the face shims would ever contact the other set of 24's.   I think 1mm of the overhanging 24 shim would have to bend .1mm to touch the next 24 under the 22.. I cant see that happening...

 

But if it were

24x.1

24x.1

8x.1

24.x1

etc. 

Would this be like a bleed that always would leak oil past the valve face whenever the slightest pressure was put on it? These question are the reason I haven't yet gotten into the crossovers as part of tuning.. But instead of reinventing the wheel, I might perhaps be better off following the multitudes of advice out there where crossover sized have already been established, i.e.

 

24x.1

24x.1

14x.1

24x.1

24x.1

22

20

18

16

14

12

8x.2

14

18

base......

The 11.1 is a crossover , and the type of stack you suggested 24 then 22 then 24 is similar to what Honda ran on the base of the crf , 30.1stack then 25.1 then 30.1 , it adds bleed , you should get the restackor program as you can see these changes in detail

Technically the 8.1 shim in the second stack wouldn't be a bleed, but a crossover shim (early one at that).  A bleed shim would be right on the face of the piston and a smaller diameter than the lip around the ports.

 

If I were you, I wouldn't play too much with the crossover (diameter or thickness) right now.  If anything, I would change its position relative to the piston face.  As Mog suggested, buying a program like restackor is a great investment into seeing what actual shim changes do.  From there it’s trial and error and rider feedback.

 

Realize that trying to get a suspension working well under "slow" rocky conditions and equally as good in fast, rough conditions is not entireally possible.  It's going to be a compromise.  You need more damping for the faster/rougher stuff or you'll be blowing through the stroke and maybe even bottoming, yet for the slow/rough stuff its going to pound you to hell and deflect.

Keep it coming..

 

So basically, if the shock was setup as a compromise between mx and enduro, it would likely fall somewhere in the middle. On the plusher side for mx, but the firmer side for woods/bush riding.. That falls in line with what I'm experiencing to a tee. Not a bad setup if you can keep the bike moving fast. Reducing the LSC simply turns it to mush, so ideally I want to reduce the stack overall and then run less bleed to get the LSC just right. Overall I will end up with less HSC to soak up the roots, rocks, logs, etc. Its making sense

If I were you guys I'd invest in the Restackor software and start running simulations.

http://www.shimrestackor.com/

I could purchase shim restackor, measure everything up and get a visual of whats going on.. but for me, half the enjoyment is working through it mentally. Its the process of learning how and why. Plus.. I still don't know how much softer I would need it to be so even when making changes, they'll need to be physically implemented and tested before I'm able to determine if it was an improvement or not.

I tried to get restackor to work a few months ago and it did... for a few days then it crashed.  I gave up on it. The biggest thing with restackor was being able to see what changes would do for the stacks.. It really didn't give you a starting point or tell you what the best stacks would be.  I know that is through testing and trial/error or paying someone else to do it.

 

I've been noticing lately that if I make changes at one end, they need to be incorporated a lot of times into the other end in order to maintain the balanced feel of the bike. 

 

For example, I just revalved my shock.. I had been super happy with the fork and thought the shock could use a little tweaking. Now I'm super happy with the shock and figure that my forks need a little tweaking.   I am shooting for a set up that works well for what I ride... roots, rocks, street, very light moto in sand and hard packed, hare scrambles, woods, grass type trail stuff.   I don't SX, supermoto, jump 100' doubles, etc. 

Your crossover O/D should never be less then your clamp shim O/D.

Your crossover O/D should never be less then your clamp shim O/D.

 

Why not?

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now