The 2009/2010/2011 Rockshox SID Race forks have the following adjustments:
- positive air spring;
- negative air spring;
- compression; and
Positive air spring
The positive air spring provides the pressure in the fork that holds you up.
This is the first thing you want to tweak. The fork has a rough air pressure guide on a sticker on the fork leg. Use this pressure as a starting point.
Check that you've got the pressure right by measuring the amount of sag. To measure the sag you need to be balanced on your bike in the attack position. It helps if you've got someone to help you measure the sag. The SID's have sag % markings on the fork legs. Aim for between 20 and 30% sag. Once your fork is fully dialed in you may want to revisit these sag settings and tweak them further within the 20-30% range to get things just perfect.
Negative air spring
Negative air pressure influences how easily the fork begins to compress. More negative air pressure makes the fork "plusher" in its initial travel. Rockshox recommend matching the negative air pressure to the positive air pressure. If you've got the time, I'd go for a quick ride with no pressure in the negative air spring and then another quick ride with the same pressure as in the positive air spring. You'll get a really good idea of what the negative air spring does this way.
You might be wondering why the fork doesn't collapse with equal positive and negative air pressures? Well, if you take a look at the air spring assembly you will see that the positive air pressure acts on the entire surface area of the air spring head. The negative air pressure acts on a much smaller surface area because of a smaller diameter air head and the presence of the air spring rod through the middle of the negative air spring reservoir.
Here's what Rockshox has to say about the negative air spring .
Notes on setting air spring pressures
You only get an accurate reading on the air spring pressures when you are pumping the air spring up. If you connect your pump to check the air spring pressure you will end up with a lower value because you have had to let air out of the air spring to fill up the pump hose and guage. This is particularly so with the negative air spring that has a very small volume of air, so letting even the smallest amount out will change the pressure drastically.
You also need to take care when connecting your pump to the air valve. Tighten the air pump no more than a 1/4 turn once the seal makes contact. If you tighten it any further, the pin in the pump fitting may hold push on the schrader valve pin and let lots of air out when you disconnect your pump.
The foolproof way to get your baseline air-spring pressures
- deflate both air pistons;
- inflate the pos to something much less than what you will need, say 50psi;
- leave the shock pump attached to the pos piston and get on the bike, balanced in the attack position;
- get your mate to pump up the pos until you reach your desired sag amount - the shock should extend as your mate pumps it up;
- this will give you the correct positive pressure - note this pressure;
- stay on the bike as your mate takes the pump off the pos valve and puts it onto the negative valve;
- get your mate to pump up the neg piston until the shock just starts to get sucked down (sag just starts to increase) - now you know what neg pressure you need to balance the pos at the particular amount of sag you are running - write this pressure down as well.
There is always some stiction in the fork, so the neg pressure you get maybe a few psi too high. Disconnecting the pump will dump a little psi out of the neg anyway.
Test ride and tweak your preferred pressures around the values you got using this method.
Low speed / high speed compression
When suspension guys talk about low and high speed compression damping its not about how fast you are going when you hit that bump. Low and high speed refer to the rate of travel of the fork through its stroke. Landing a massive G-out actually relies more on your low speed compression damping. On the other hand, small square edge bumps on the trail realy should be controlled by your high speed damping.
The floodgate adjuster on the SIDs operates as a compression circuit bypass when pressure against the motion control damper "spring" exceeds a certain amount. The position of the compression adjuster determines how much pressure the motion control damper will see from the fork compressing. With the compression adjuster in the maximum/locked position, the motion control damper will experience the maximum amount of travel induced pressure.
Motion control damper and floodgate
Have a look here for a pretty good explaination of the motion control damper. More interesting views on the motion control damper and floodgate here as well.
What are the differences between the motion control damper and the blackbox motion control damper?
Have a look at this MTBR forum thread for some info.
How to set your floodgate?
Start with the fork positive and negative air spring pressures properly adjusted so that you have the "right" amount of sag (there's no real right and wrong amount of sag on a bicycle fork like the SID - anything from 15-30% is OK, what you like is up to you):
- Find a gentle up-hill that you can get out of the saddle and crank hard on without accelerating too much or running out of room;
- Set the compression adjuster to the locked (MAX) setting and set the flood gate full open (fully counter clockwise);
- Do a test run up the hill, with the flood gate full open the forks should bob up and down even in the fully locked position;
- Keep going up and down the hill, each time you go up add a clockwise click to the floodgate until aggressive cranking out of the saddle does not compress the fork;
At this point you should have the minimum amount of "platform" via the floodgate that you need to stop fork bob when climbing out of the saddle. You don't want to run more "platform" than you absolutely have to, it'll just make your suspension harsh without any performance gain.
The floodgate setting also seems to affect the high speed compression even when you have the low speed compression fully open. So even if you never run the fork locked out, you can adjust the floodgate to get the sort of HSC you like. If the fork feels harsh on square edge bumps even though you are running the LSC wide open, then try reducing the floodgate setting.