[Jrsandiego]

http://m.youtube.com/watch?v=J_Kpg6oUSFc

http://m.youtube.com/watch?v=ZoOSKyE74TM

I wasn't happy with the performance of the shock on my Norco Sight 7.2 so I decided to attempt to tune it... So happy that I did... I have since re-tuned it with multiple shim stack configuration and found a set-up that is really dialed for the way I ride..

 

[Gman086]

Kewl vid.

A couple things to understand tho...
The shim stacks control HIGH SPEED compression and rebound only. Low speed are controlled by bypass ports and the shock knobs adjust how much oil can flow thru those ports. Changing the oil weight does zero for high speed (viscosity has no effect on shims) but will speed up low speed damping thru the bleed ports. So... you effectively reduced the overall tune of your shock which may be perfectly fine for trail applications, especially if you wanted a plusher shock over smaller bumps. For larger bumps at high speeds you may find you don't have enough damping now (I'm not convinced that removing a shim from a "low" tune HSC stack is a good idea). For FR/DH riding you'd actually add shims (or use thicker shims). Here's some good info:
RockShox Rear Shock Tuning Experience - Pinkbike

Cheers,

G MAN

 

[PUSHIND]

The shim stacks control HIGH SPEED compression and rebound only. Low speed are controlled by bypass ports and the shock knobs adjust how much oil can flow thru those ports.

This is incorrect. In this application, and most MTB applications, the shims have a large effect on the entire damping force curve. In the particular shock the adjustable rebound port also has a secondary shim stack which specifically effects low speed.

Changing the oil weight does zero for high speed (viscosity has no effect on shims) but will speed up low speed damping thru the bleed ports

This is incorrect as well. Because of the relatively low displacement and port volume increasing oil viscosity will change the entire damping force range.

Darren

 

[Gman086]

Well that's odd Darren because you were the one who told me that viscosity barely affects shim damping during a phone conversation where I had asked about using a higher visc oil in my Monarch RT-AM I'd purchased from you (you talked me out of that idea)!!! Maybe you were referencing MX applications?!

Have FUN!

G "moar shimz" MAN

PS - Still bottoming out badly with your "custom tune" and all the volume spacer bands (I think there's 6 in there now) inside the high volume air can at 20% sag no less!

 

[PUSHIND]

Well that's odd Darren because you were the one who told me that viscosity barely affects shim damping during a phone conversation where I had asked about using a higher visc oil in my Monarch RT-AM I'd purchased from you (you talked me out of that idea)!!! Maybe you were referencing MX applications?!

No it effects it for sure whether MTB or MX. I would've talked you out of it because we want to use the lightest viscosity fluids in air shock dampers where we see very high temperatures. The greater the viscosity, the greater the percentage of change you get during heat cycles.

PS - Still bottoming out badly with your "custom tune" and all the volume spacer bands (I think there's 6 in there now) inside the high volume air can at 20% sag no less!

Sounds like it's time to switch to a standard volume air can to get the spring curve more progressive.

Darren

 

[Gman086]

Good info on the heat situation Darren! That makes sense.
Is there anything I can do with the IFP pressure or is the standard can my best bet? (sorry for thread derail OP but hopefully this info is beneficial to everyone's greater understanding of shock tuning). The shock damping is quite good, just bottoms on bigger hits. I think we'd all like to get a better understanding of how IFP pressure affects the tune.

Thanks,

G

 

[Jrsandiego]

Darren, I really appreciate you chiming in... Really solidified my theories on tuning the suspension..

Gman, My understanding of the shim stack came from DVO suspensions website..
Piston & Shim Technology ? DVO Suspension

My experience from understanding suspension came from racing radio controlled off-road vehicles... When we raced on tracks that were rough and our suspension tended to "pack up" we would switch to lower viscosity oil or use pistons/dampeners with bigger ports.. And when the track was smoother we would go with thicker oil or piston/ dampeners with smaller ports.. The shim stack in MTB suspension adds another dimension which I was unsure of... Some of the things I have learned through this experience is that the shim closest to the piston has a huge affect on how the suspension functions.. Going smaller gets you coil like performance but no dampening because the oil just flows thru the ports on the piston... And going to a size that just covers the ports will give coil like performance but with a functioning rebound adjustment that is within a usable range.. And I think it caused by the slight resistance that you get with the shim that forces oil thru the adjustment rod in the rebound circuit..

 

[ktm520]

In the particular shock the adjustable rebound port also has a secondary shim stack which specifically effects low speed.
Darren

Hmmm, I have always assumed that the secondary stack, either on the reb or comp side, was purely a check valve to prevent cross flow. It's such a light stack with high port face area.