I know that fluid isn’t compressible, I’ve just assumed that oil is forced through a valve on compression the same as it is on rebound, and the air gap allows for initial movement which is undamped. Gamphy suggested that oil weight doesn’t affect compression damping, however if there’s 2 valves (comp & rebound) it must. As I said, more research required on my part I think. I have had bikes which have had adjustable air pressure forks, which no doubt adds another layer of complication.
Your forks are nothing like that diagram, that's a cartridge fork set up. Your forks are similar to this set up. we just have a fork piston ( fork damper) with 2 big holes (one on each side) in the bottom and one tiny hole at the top.
Your spring compresses when you hit a bump, that compression action is slowed down by forcing the oil through the 2 big holes at the bottom, how fast it compresses is determined by the size of the big holes that allow the fluid to pass through, this is the damping action on compression. Air compression, the air gap at the top of the forks has a very small affect on compression, but is negligible with our fork set up, the air gap left at the top of the forks is to allow space for the oil to travel into when the forks are compressed and the whole volume is reduced - too little air gap and the forks cant compress to their full extent because there is no room for the oil to travel into, its all low pressure stuff,( under this low pressure, oil is not really compressible). Under fork compression the viscosity (thickness and fluidity) of the oil is not much of a factor because the holes are big enough to allow easy passage of the oil.
Once your forks have stopped compressing, the spring wants to push back and extend your forks, but the oil in the upper chamber is slowing down how fast the spring can push the forks open, because the oil can only pass through the tiny hole at the top of the damper rod, this damping action is your rebound damping. The viscosity of the fork oil very much affects how quickly the oil can pass through this tiny hole and therefore affects the rebound speed.
Our fork set up is a balance by how strong the spring is, how quickly the spring compresses, how easy it is for oil to flow up on compression and how easy it is for the oil to flow back down on rebound. Harris did a pretty good job finding the middle ground.
First thing , before anything else, buy a set of preload adjusters of Amazon/Ebay and set you fork sag for yourself with all your gear on to about 60mm. then give them a try, it might be all you needed to do!
Next point to start is with oil viscosity, Use an oil with a viscosity around 25, this will slightly quicken the rebound, making the forks feel at a bit more reactive, but use enough oil, too little and your forks will bottom out too easily, too much and your forks will stiffen up too much when you hit a bump. The magic number on our forks is 450 - 455ml (but make sure the forks are completely empty, by fully pumping them at least 10 times, because of the fork piston seals and the position of the bottom holes in the damper rod, they can retain up to 50ml of oil and you would never know).
Others have mentioned that you can experiment by adding more holes at the bottom of the damper rod to aid faster fluid flow under compression, but if you make it too easy for the oil to pass out of the bottom chamber, your spring may not be damped enough and it will compress your forks very quickly, your forks will feel very soft, but then you need to find the balance point for this on rebound.
If after sag setting and oil change you still feel you want your forks to be a bit more compliant, then add 2 more 3mm holes in the bottom of the piston rod about 15 mm above the two big ones. That will slightly quicken compression, but the 25cst oil will balance that by slightly quickening rebound. With stock springs thats about all you can do.
NEVER EVER add more holes to the top, or make the existing rebound hole any bigger (the tiny hole at the top). The only way to adjust rebound damping on our stock fork set up is by changing the viscosity of the oil, this determines how quickly the oil can pass back down to the bottom chamber, through this tiny hole, as the spring is extending , this is rebound damping, too quick and you have a pogo stick, too slow and the forks cant get back to the set length before the next bump (so don't use something like 20W fork oil, its simply not viscous enough for our basic damper tube set up.
If your still not happy, then go with somebody like Racetech for springs, emulators and oil, at this point you will be drilling bigger holes in the bottom of the damper rod because the emulator takes over oil flow duties for compression , but be honest about yourself and your usage when determining which spring and emulator set up, its a pricey set up but have heard a few really good reports, there are also reports of folks saying the set up was too stiff (I suspect they may not have been totally honest in their set up and usage , perhaps they though they were more MX than trail?, Or maybe the emulators are just not set right, dunno). There are other suppliers such as YSS but the springs are not bespoke to you and have very mixed reports. If your going pro set up and pro spend, RaceTech, the alternative could be the Ohlins cartridge set up, but dont know if its still available or at what costs? many swear that Ohlins is the Daddy.
For info, my set up is a drilled damper piston, different and longer springs, 25cst oil and preload adjuster, it (in my opinion) is significantly better than stock and or YSS emulator and stock spring set up, I have 2 sets of forks (did have 3) but sold the stock set up. I've had all 3 set up on my bike a week apart to help me determine which was best, current bespoke set up is best by far, but second was stock with preload adjuster and 25cst oil, very similar to the YSS emulator set up and a lot cheaper (my current set up is also cheaper than YSS emulators, in my region).
Read the suspension bible by race tech, I did a link to it on this forum, just can't remember which post, however it's easy to find online.
Hope that helps.
