Not my writing but it may help
Rockshox Lyrik Coil U-Turn Guide for Dummies
First off, I ride a Transition Covert. I'm 6'3" and weigh 205lbs geared up. I don't do any big drops but do ride rocky, southern California, technical terrain. I'm basically writing this so other people won't have to search through thread after thread like I did to find the information they need.
When the fork arrived, I noticed that the 160mm black mark wasn't lined up with the wiper seal. This is normal. I figured that when Rockshox made the marks, they didn't take into account the wiper seal which is a couple millimeters wide. You will still get the full 160mm of travel.
According to the spring guide from Rockshox, I needed a firm spring. My sag with the stock (medium) spring and both high speed and low speed compression fully counterclockwise is approximately 34mm. This is 22% of the full travel. The spring guide must have recommended one spring higher than normal since I'm at the top end of the Firm spring limit.
This fork is a blast to ride! I took it to my local "aggressive XC" trail and coming from a Fox Float 130 RL, this fork feels way more stiff and plush. It felt like it just rolled over everything! I could take drops of about 3.5ft without coming close to bottoming. Small bump sensitivity was excellent as well.
Reducing the travel for climbing is easy, just twist the U-turn knob. You might want to set it the way you want well ahead of a long climb because it does take a bit of effort to twist the knob. I found that I like to keep it around 140mm for just general riding, dropping it to 120 for the climb and topping it out at 160 for descents. According to the manual, spring rate is supposed to increase when you shorten the travel. If it does, I can barely tell.
Fork tuning is something I found that you can't learn by reading about it. I found the best way to find which settings you need is to set the rebound slow enough that the fork won't kick back, then ride with the high speed and low speed compression knobs fully counterclockwise for a little bit, then ride again with them fully clockwise. This way you can tell what each one actually does. Once you do that, then you're ready to tune.
High speed controls bottom out and square edged hits, however, you will have to compromise if you go either way. Increasing high speed compression will prevent bottoming out, but the ride will be harsh. Reducing high speed compression will make the fork feel more plush on the square edged hits, but will cause the fork to bottom out. The idea is to tweak to fit your riding style and terrain. To tune high speed compression, start with the knob in the middle, ride, and adjust as necessary. If your bottoming out your fork with the high speed compression knob fully clockwise you need a stiffer spring. Keep in mind that this fork isn't very progressive so you must compromise.
Low speed compression is just as easy to tune as high speed. Higher compression means less pedal feedback (bob) and brake dive. Lower compression means better small bump sensitivity. Find a good medium that will both reduce user feedback while still maintaining small bump sensitivity.
Changing springs is very easy once you've figured out how to do it. Place the bike so the fork is horizontal. You don't want any oil to leak out the bottom. Remove the U-turn knob being careful as there are 3 bb size bearings and 3 small springs inside that will pop out. I like to do this on my back porch with towels on the ground so the springs and bearings don't go bouncing everywhere. Now don't just undo the bottom screw and top cap and start yanking on the spring like I first tried to do and wonder why it won't come out. You have to loosen the screw at the bottom of the fork about 3/4 of the way out. Then undo the top cap. Be careful as the black paint is easily scratched. Once the top cap is undone and it pops up, cover the bottom screw with a towel or something soft and hit it lightly until it loosens. Then take the screw all the way out and the spring should slide out. Lube up the new spring with grease and insert. Some twisting is required to get it to go in. Then reverse the process. Also, make sure that the gasket in the top cap doesn't get mashed. Surprisingly, little torque is required on the top cap. You're ready to ride.
I installed a firm spring in my fork just to see what Rockshox was talking about when they said I needed a firm spring. My sag was about 15% at 25mm. With a firm spring installed, the ride was harsh even with compression all the way soft. It did not contour the terrain very well. Also, the U-turn knob was noticibly harder to twist and after double checking my spring installation and found everything to be fine, figured it was due to the stiffer spring. I rode the heck out of it anyway for a while to see if it needed breaking in. It didn't get any better. If you're into big drops and such and don't care about a plush ride, then the firm spring may be for you. Since the highest drop I may encounter is going off a 4 or 5 foot staircase made of railroad ties and dirt, I decided to keep the medium spring.