Those forks are not open bath (hydralic) so rebuilding them is no as straight forward as you might think.
the USD forks on the AGB29's are designed to run air pressure in the lower chamber, this is what resists the bottoming.
Basically in a hydraulic fork, the top section of the fork is full of air and as the fork compresses, it compresses that air which resists bottoming and makes the fork more progressive.
the AGB29 forks do not work like this,
the top of the fork is basically just open and has a plunger shaft running down the middle, with a couple of weak springs on them. the plunger runs down into the bottom of the leg, where a sealed up cartridge is enclosed, which has the oil in it, the stanchions are not lubicated by the damping oil (so you need to lubricate the stanchions regular) when the damping cartridge is full of oil, it does not assist with spring action as it does not contain air, underneath the cartridge is a floating base, which underneath that is a air chamber. the more air in that chamber, the more the oil is compressed and the harder it is for the fork to reach full compression, the air pressure also assists the spring rate, and sag slightly.
To adjust the air pressure, you WILL NEED A PROPER SHOCK PUMP!!
Downhill mountain bike shops are more likely to stock a shock pump, these are around $50.00 and can pump up to 300psi, Specifically though they pump VERY SMALL Amounts of air each pump. the chamber under the fork leg is TINY, DO NOT USE A AIR COMPRESSOR!
As for how much air is safe to use, the forks do not work properly at all without air in that lower chamber, they will just continuously bottom out and be too slow to operate. They DO REQUIRE AIR IN THOSE CHAMBERS!!
Suggest start out with around 30-40psi in each leg, I have no idea how much air pressure these forks can take without that section of the fork failing. (nor can anyone anywhere tell me how much air is safe to use) from memory my forks had around 30psi from the factory. You may wish to use nitrogen this will compress less, and be less susceptible to heat than air, but much harder to obtain and continuously tweek.
Suggest you move slowly upwards until you reach the desired compression characteristics.
You should bottom out your forks on occasion anyway, on larger or bader landings, but for most of the time not jumping you should use 70-80% of your travel, and landing normal jumps around 90-95% of your travel.
it is possible to change the oil in those forks, but it's a nightmare I found at least, compared to regular hydraulic forks. but unless your AGB29 forks are alot different than the ones on both bike i have, you'll be wanting to opt for lighter oil, not harder, these forks have way too much rebound damping from factory.
Juls
