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Discussion Starter · #1 ·
RockShox Deluxe Shim Stack Tuning
The RockShox deluxe select+ is stock on a wide range of bikes. The shock has 10 clicks of rebound adjustments and an open/closed climb switch. This thread looks at tuning the shock shim stacks.
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Compression damping
The compression circuit has no bleed and runs a conventional shim stack on a flat valve face. RockShox uses five different shim stack tunes on the compression circuit (LC, H1, L, M, H). This thread evaluates the LH380 tune, which runs the H compression stack.
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Three slots on the compression valve face feed the rebound ports. The slots are 0.5 mm deep producing about 64% of the rebound port hole area resulting in the slots controlling the flow into the rebound ports.

Rebound clicker circuit
The clicker circuit feeds flow through the shock shaft to the low speed valve on top of the shaft. The low speed valve has a single 19x0.15mm shim preloaded 0.2 mm by the lip on the outside edge of the valve. The low speed valve allows flow in the rebound direction and no flow in compression.
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At the midrange 5 click position the low speed shim stack controls the flow from zero to 5 in/sec. Above 5 in/sec the clicker needle throat limits the flow. At 3 clicks out the needle throat limits the flow at shaft velocities of 3 in/sec.

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Preload on the low speed valve shim stack requires 30 lbf of shock shaft force to crack the shim stack open. That creates the "pedaling platform". On a 3:1 link ratio suspension the 30 lbf cracking force requires about 10 lbf of force at the wheel. The cracking force creates suspension packing problems since the rebound stroke will not respond to wheel force changes less than 10 lbf.

Main piston rebound
RockShox uses four different rebound shim stacks (L, M, H, LN) on the select+ shock. The LH380 tune runs the "L" rebound stack. The rebound valve face lip preloads the shim stack 0.25 mm. The linear stack (LN) runs a slightly smaller face shim diameter that sits just inside the lip resulting in no preload for the linear stack.

The rebound ports are feed by three 0.5 mm deep slots on the compression valve face. The slots restrict the entrance flow making the main piston behave like a spring loaded check valve feeding a fixed orifice restriction. Stiffer shim stacks increase the blow-off pressure, but do not produce much difference in the rebound damping force curve shape.
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Discussion Starter · #2 · (Edited)
Rebound damping from combined main piston and low speed valve
The combined flow through the low speed valve and main piston set the rebound damping force. At low speed, all of the flow is through the clicker circuit and the low speed valve flow resistance sets the circuit pressure. As speeds increase, the low speed valve eventually produces enough backpressure to blow-off the main piston shim stack. Added flow through the main piston creates an increase in shock shaft velocity.

The rebound damping force curve is computed from Shim ReStackor outputs by simply adding together the flow through the low speed valve and main piston circuits evaluating each circuit at the same pressure drop.

The RockShox Select+ LH380 tune with the clickers set at the midrange 5 click position blows-off the main piston "L" stack at a shaft velocity around 10 in/sec. When the main piston blows-off the additional oil flow increases the shaft velocity, which results in a reduced rate of damping force increase.
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Shock performance "sweet spot"
The usual assumption in shock tuning is the factory spent some time test riding different shim stack configurations to optimize the suspension performance and arrive at the final configuration used in the production bike. By legend, the shock performance "sweet spot" has all of the clickers set midrange so lighter or heavier riders only have to twirl the clickers a couple of clicks to perfect the suspension performance.

The Specialized rider app for a 2019 Turbo Levo running a RockShock Select+ with the clickers set at the five click midrange position gives a rider weight is 85 Kg with a recommended shock pressure 217 psi for 30% race sag. Running that setup through the suspension response calculations of ReStackor gives some clues on how the suspension is supposed to work.

Suspension motions popping the wheels back to the ground after a rock impact produce suspension rebound velocities around 62 in/sec on a one inch stroke. Deeper strokes produce even higher wheel rebound velocities.

With the rider on the seat, the increased weight of the combined chassis plus rider produces slower rebound velocities of 42 in/sec when recovering from a four inch suspension bottoming stroke. Shorter strokes produce even lower rebound velocities.
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Those basic suspension calculations show the shock damps two different suspension motions:
  • Shaft velocities less than 50 in/sec damp chassis plus rider rebound motions
  • High speed motions above 50 in/sec damp wheel rebound motions
Replotting the above data against the suspension wheel position gives some additional insights. Chassis rebound motions are underdamped and overshoot race sag on return from any stroke depth. Rebound strokes deeper than three inches from race sag clang into the full extension stop with a final shock shaft velocity of 11.6 in/sec. That shaft velocity on a 3:1 link ratio suspension gives a chassis velocity of 34.8 in/sec when the suspension tops out causing the chassis to jump 1.6 inches into the air. Magazines call that playful.
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Discussion Starter · #3 ·
Here's the problem
On rough terrain, the bike bucks on high speed descents because there is no rebound damping holding the back end down. To fix that, the shock needs stiffer rebound damping in the 20 to 50 in/sec shaft velocity range to control chassis motions and keep the wheels on the ground.

Cheap-ass no cost mod
The RockShox LH380 tune runs two 18x0.15 mm shims in the rebound stack and an 18x0.20 mm shim in the compression stack. Swapping the rebound 18x0.15 mm shim with the compression stack 18x0.20 mm shim increases rebound damping by 25% at 20 in/sec. And, running the rebound clickers at seven out, produces more-or-less the same high speed rebound damping as the original LH380 tune. The mod gives stiffer rebound damping in the 20 to 50 in/sec chassis motion range, where you need it, without overdamping high speed wheel motions.

Swapping the softer 18x0.15 mm shim into the compression stack drops compression damping by about 8%. That gives a little more suspension compliance at the expense of bottoming resistance.
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Running the modified setup response calculations shows the bike can now successfully recover from a three inch rebound stroke and keep the wheels on the ground. A deeper suspension bottoming stroke at four inches still launches the chassis into the air when rebound hits full extension.
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Ride test
Success!

SDI stocks the entire rainbow of RockShox 9 mm shim ID's; $1.59 ea + $14.50 shipped. So instead of a simple shim swap you can go clinical and spec details in both the compression and rebound shim stack configurations.
 

· Elitest thrill junkie
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Good stuff. I got a bunch of my own shims from SDI and was tuning my SDS+ and had come to a similar conclusion with the high speed/low speed rebound bleed. The fully checked rebound seems to be a bit of an issue, since they are putting such hard shims over it.

2.5:1 is probably a more reasonable LR assumption, because these metric shocks seemed to be being used on longer-stroke applications relative to wheel travel. Don't think that will change the results drastically though. In general I'm finding a lot of over-damped stuff because your only aftermarket options are usually "medium tune" and these bikes are running lower LRs than years past.

That RS tuning (assembly) guide, quoted above, is a great resource. The SD is also super easy to work on, with a bleed-procedure that doesn't require special tools.
 

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Discussion Starter · #5 · (Edited)
Link ratio
Did not attempt to measure the bikes link ratio, I took Cascade Components word for it. On the up side, the link ratio curve they give produces 52.5 mm of shock travel at the bikes spec 150 mm travel limit - so that part works out.

At 30% shock shaft sag the Cascade Components curve gives a link ratio of 2.95 for the stock suspension. I rounded that off as 3:1 in the above discussions.

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If you ran the shock on a 2.5:1 link ratio suspension the effective rebound damping stiffness would increase by about 40% due to the combination of faster shock shaft speeds and the reduced spring stiffness needed for the same rider weight on a 2.5:1 link ratio suspension.

That kind of highlights buying "medium tune" aftermarket shocks is kind of a crap shoot when you don't know the link ratio they had in mind.
 

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Dougal, You have been dyno testing these things.
With the lockout system on where does the shock hit 380 nt= 85 lbf?
A Super Deluxe RC3 (Select + I suppose) LL380 on firmest compression ramps from about 65kg at 0.1m/s to about 110kg at 0.5 m/s.
 

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Discussion Starter · #13 ·
Any insights on how larger negative chamber (megneg) affects rebound speeds near sag?
Negative air chamber
Andrextr stepped through the effect of negative chamber volume on the shock spring force curve. Increased negative chamber volume linearizes spring force through the initial portion of the stroke. In the limit of an infinite negative chamber, the spring force curve is progressive through the entire stroke. Andrextr points out the spring force progression of a large negative chamber adds mid-stroke support in comparison to small chambers where spring force is kind-of flat around race-sag.
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Dougal measured the spring force of a RockShox Deulxe and showed the curve is pretty linear around race sag. MegNeg would make it more linear. But, the difference in spring rate at race-sag would be small - like Dougal said.

However, as Andrextr pointed out, the MegNeg chamber will move spring force toward a linear curve. Dougal shows the extreme example (black line) of a linear spring force curve. Linearizing spring force makes the force stiffer at mid-stroke. Sort-of-like adding a mid-stroke token - whatever that is. Correcting for the force difference between the linear (black line) and progressive (blue line) requires closing the rebound clickers by about two clicks.

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Ideally, the stock damping curve was tuned to match the suspension weight, spring rate and link ratio to perfect suspension response across the entire range of suspension stroke depths.

Assuming the stock shock perfected that tuning, the MegNeg chamber will make the spring force stiffer at mid-stroke and that will drive rebound into an underdamped condition. If you are trying to create a stable downhill suspension platform underdamped mid-stroke is bad. But, if you are trying to bunny hop the bike underdamped is good.

So - the MegNeg chamber effect - it depends………

Frankly, stuff gets tuned the other way around.
  • Hack around on spring rate to get the bump compliance and bottoming resistance you want
  • Retune damping to get whatever "feel" you are after
 

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Thank you for your thorough reply. I'm already settled on my megneg setup. I was just wondering to what extent is your tune applicable in my case. I'm fairly light at 67kg (147lbs) ready to ride and my bike is quite linear with average rate of 2.43:1.
 

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Discussion Starter · #15 ·
I'm tuning for a 100 kg rider on a 3:1 link ratio.

Above, Jayem mentioned the 2.5:1 link ratio bikes with medium tune shocks he has been working with are overdamped. I believe him. With your weight and link ratio I suspect you would want lighter rebound than I am running.

Something like: (LR.1/LR.2)*(w.1/w.2)= 1.8 times softer rebound damping
 

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Discussion Starter · #16 ·
Clean up on aisle 7!
Five rebound clicks is too fast and 4 clicks is too slow. I want ½ click. Shim stack tuning would fix that, but air shocks give another option.

Instead of tuning damping to match spring rate, the shock spring rate (air pressure) can be tuned to match damping.

The Specialized rider app gives the recommended spring rate and rebound clicker setting for rider weight changes. That curve, and some spring-mass-damper theory manipulation, spec’s the shock pressure change needed to soften damping by one click when running the same rider weight with a spring rate change.

Increasing the shock pressure by 50/2= 25 psi reduces damping by ½ click. Starting at 30% sag, the increase in shock pressure reduces race sag to 27% and softens rebound damping by ½ click. For the RockShox Deluxe Select + changing race sag over the 30 to 25% range just happens to speed up rebound response by one click. So, twiddling shock pressure over the sag range fills in the gaps between clicker settings.

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On the fork, 23/2= 11.5 psi reduces damping by ½ click and decreases race from 20 to 18.1%. That gives the same confusing range of tuning options. Rebound damping can be reduced by opening the clickers or increasing the gas pressure.
 

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Hi. I have a slightly off topic question, but this seems like very much the right audience for it so I hope you don't mind:

I have a new frame that came without a shock, and I've purchased a Deluxe Select+ shock for it. The shock is new but it's OEM, and I don't know what tune the shock has currently.
I've found out the recommended tune from the frame manufacturer, so I'm going to open up the shock to determine the tune and change it if necessary.

However, my question is this: Since I am a larger rider (100kg kitted up) should I consider going for a different shock tune than the frame manufacturers recommended tune? e.g. would a firmer compression tune be better given my weight? Thanks, P.
 

· Elitest thrill junkie
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Hi. I have a slightly off topic question, but this seems like very much the right audience for it so I hope you don't mind:

I have a new frame that came without a shock, and I've purchased a Deluxe Select+ shock for it. The shock is new but it's OEM, and I don't know what tune the shock has currently.
I've found out the recommended tune from the frame manufacturer, so I'm going to open up the shock to determine the tune and change it if necessary.

However, my question is this: Since I am a larger rider (100kg kitted up) should I consider going for a different shock tune than the frame manufacturers recommended tune? e.g. would a firmer compression tune be better given my weight? Thanks, P.
Yes, that is common. Sometimes tunes are already on the heavy side, it depends on the leverage ratio of the bike and the tune of the shock, as a low-leverage bike with a medium tune is effectively pretty firm, vs. the opposite will be effectively pretty light. But it's also common to need a firmer tune for a heavier rider.
 

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Discussion Starter · #20 ·
Suspect shock damping curves
The damping curves from the Specialized rider app are suspect. Moving the clickers from one to two clicks out normally creates a large change in damping force, which would cover a large change in shock pressure. But, the Specialized rider app shows a small shock pressure change compared to the other end of the curve at 8 to 9 clicks out. Opening the clickers from 8 to 9 clicks produces a small change in damping force – but Specialize recommends a large change in shock pressure. Seems backwards.

I also tried the RockShox trailhead app. RockShox recommends more open clicker settings and the curve implies near linear damping over the clicker range. Clicker orifice damping produces progressively smaller changes as the clickers are opened up. Not linear as RockShox shows or progressively larger as the Specialized app implies.
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It looks like RockShox and Specialized did not spend much time thinking about the shock damping force curves when specifying the recommended clicker settings for the shock or the fork.
 
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