Actually, Restackor allows you to put in the spring preload on the shims pretty easily. I measured the spring rate of the preload spring and the compression of the spring per click of HS compression. I showed the chart with zero preload just to see what UDI's mod accomplishes, but it's easy to run the calculation for any given number of HS clicks and as you say the low-speed and mid-speed behavior changes significantly (strongly digressive damping).
With my modded stack (damping curve very similar to Udi's) I end up running about 5 clicks of HS compression, 2 turns of LS compression on a yellow spring with two preload spacers. I weigh 165lbs and the fork is tuned for a mixture of DH race and medium size hits.
Here is a comparison between full open and 5 clicks of high speed compression on the stock shim stack.
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No, Restackor lets you preload the face shims with negative float - as far as opening threshold alone goes, you can calculate this with a reasonable accuracy if you know the spring rate and only have a single stack, but once the shims have opened at all, the force of the spring is no longer being accounted for. Don't get me wrong, it's a cool program, but it's not actually especially accurate on average (esp in this situation) and is not a substitute for a dyno. There are models you can make in that program that ARE very accurate, this particular one is not one of them.
That's a good point and something I have been scratching my head about. There might be a way around that but it involves piecing together simulations with increasing float, only using the results right around the opening threshold.
Ok anyone thinking of doing this mod hold off and if you already have sorry. The inner lip of the dust seal must hold the oil in as I now have oil weeping out of them. This means that the inner main oil seal is usless. Will get new none RS inner seals and see how I go.
nybike1971 the 2011 HS stand stack is
20 x .1
16 x .1
14 x .1
20 x .15
20 x .1
20 x .1
20 x .1
14 x .1
10 x .4
My new stack is
20 x .15
20 x .1
20 x .1
20 x .1
20 x .1
16 x .15
16 x .15
16 x .1
14 x .15
14 x .1
This is as hard as I could make the stack out of the shims I had at the time.
Havnt riden this stack yet but it still feels the same. Even with both LSC and HSC fully adjusted up I can still go thought 160mm of travel just bounceing on the fork. This is with 60psi which gives 23% sage I weight 67kg.
I feel that the LSC adjustment is not restrictive enought as the threaded needle comes fare from closeing the orifice leaving two 2.4mm slots for oil to pass.
Does anyone know if the 2010 is the same?
Has anyone ran 15wt oil I currently run 10wt.
The unfortunate fact is that the damper is pretty crap because, as you say, the LS adjuster isn't a proper needle adjustment that allows you to fully (or almost fully) close off the LS circuit, it's some blunt-nosed POS that essentially adjusts a slot (just like going back to 2004 Boxxer rebound adjusters... sigh). I think this is a large part of the reason why they tend to ride pretty harsh; in order to get any amount of LSC out of em you need to have the HSC circuit completely closed up until a fairly high force, which then makes it harder to tune the HSC so as not to be excessive or harsh. I've been thinking about trying to make a proper needle for it but at the end of the day, why the hell should I have to, it's a freaking expensive fork to begin with. More inclined to give up on it and buy something that works out of the box - or I would be if I could afford to.
Holy cow! There are a number of misconceptions here.
The preload spring simply increases the force required to lift the shims off of the face i.e. the stack cracking pressure.
The force needed to deflect the stack beyond that point depends on the combined stiffness of the spring and shim stack. The important thing to realize is the shim stack is much stiffer than the spring. For Ubis stack above the stiffness of the shim stack is about 450 lb/in that is like the stiffness of an automotive suspension spring. Way stiffer than the HS preload spring.
To understand the effect of the HS spring you can draw a simple force balance on the stack.
At the point where the shim stack cracks open the shim lift is near zero so the cracking force is defined by the spring preload.
Lifting the stack beyond that point depends on the combined stiffness of the stack and spring.
And here is the rub. The stiffness of Udis modified shim stack is about 450 lbf/in. The stiffness of the HS blow-off spring is much smaller. So, the stack lift is dominated by the stiffness of the shim stack. The HS spring makes little difference.
At the point where the shim stack cracks open the shim lift is near zero so the cracking force is defined by the spring preload.
Lifting the stack beyond that point depends on the combined stiffness of the stack and spring.
And here is the rub. The stiffness of Udis modified shim stack is about 450 lbf/in. The stiffness of the HS blow-off spring is much smaller. So, the stack lift is dominated by the stiffness of the shim stack. The HS spring makes little difference.
To emphasize the point if you had a HS preload spring with a stiffness of 75 lbf/in and set the preload to give a cracking pressure of 5 lbf the change in effective stack stiffness with and without the spring is negligible.
If you wanted to you could add ghost shims to the stack and adjust the position and thickness of the shims to mimic the stiffness of the spring. Since the face shims on the stack have a linear force vs- displacement curve, at least over the practical range of deflection, adding ghost face shims will mimic the effect of the HS spring. In most cases it makes little difference.
The change in ride when you crank down the HS preload spring is huge. That change is caused by the difference in cracking pressure. The change in HS damping is small because the stack is much stiffer than the spring at least for practically tuned dampers.
Very nice posts, Clicked. Thank you for that!
For the sake of completeness, some time ago I measured the spring rate of the HS preload spring in the 2010 Boxxer Team and it comes out to roughly 28.5lbs/in. The preload range on the HS adjuster goes from 0.3mm (full open) to 2.7mm (full closed). This is equivalent to 0.23lbs of preload force per click (out of 12 clicks). Full open preload force is 0.3lbs and full closed is 3lbs.
Definitely some cool posts there, Clicked!
So given the above, and nybike's post, does someone want to list what "ghost" shims would be needed to mimic the above configuration (my stack) except with a lot of HSC spring preload wound on (2 clicks back from fully closed). So about 2.3lbs of preload force according to the above.
It would be cool to have the adjusters running closer to the middle of their range.
I'd like to hear how it's calculated too for interest's sake, and maybe some explanation as to why it can't be matched exactly (if that's the case). I can't picture shims completely matching the effect of spring preload on the face shims.
So given the above, and nybike's post, does someone want to list what "ghost" shims would be needed to mimic the above configuration (my stack) except with a lot of HSC spring preload wound on (2 clicks back from fully closed). So about 2.3lbs of preload force according to the above.
It would be cool to have the adjusters running closer to the middle of their range.
I'd like to hear how it's calculated too for interest's sake, and maybe some explanation as to why it can't be matched exactly (if that's the case). I can't picture shims completely matching the effect of spring preload on the face shims.
Here is a screen shot of the 2010 Boxxer Team and Worldcup valve measurements and stock oil viscosity.I like that idea.
Could someone post up a good photo of the valve so I can show the valve port dimensions that are needed for the calc?
I have not measured the shape of the LS needle, so I just assumed a linear profile.
Attachments
hotdamn, just the thread I need to follow, but then it's dead.. meh! If this thread's been taken up somewhere else, please point me in the right direction. Will be much appreciated
Almost every single world cup racer on a boxxer (and many, many others) have removed their oil seals.
In place do they just opt for a foam ring?
+2 just searched and cant find the rest of this thread..
does anyone know what gets done here: http://www.avalanchedownhillracing.com/Rock Shox/Boxxer 35mm Mod.htm
He describes it pretty well.
It is a re-shimming of the factory compression damper. Not as good as the full monty but still an improvement. Sram actually does something similiar to the 2011 and up forks as far as i recall.
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