I’m just trying to figure out the theory behind my forks operation, that’s it, I’m not looking for stack recommendations or anything. It’s driving me crazy, It’s like a song you can’t get out of your head. Any questions, comments, funny stories would be appreciated.
Anyway, while dissecting my 01 WP forks I made the following observations:
1. “Mid-valve” or active valving
Mid-Valve 24 mm OD * 8mm ID * .10mm (4X)
Very light mid-valve spring
Shims open approximately 5 mm
Correct me if I’m wrong, but it seems to me that the “mid-valve” or active piston has nothing or little to do with how much oil is pushed into the base valve ports. It would seem that would be a function of the ratio between the rod area vs. cartridge area.
So, as the rod moves its causes a differential pressure between the top of the active piston and the bottom of the active piston.
Example:
Rod OD= 14mm Area =154 mm2
Cartridge ID= 28 mm Area =616 mm2
So the area above the piston would be 462 mm2 vs. the area below the piston 616 mm2
So it would seem that any change in the rod dia. would have a substantial effect on the resulting pressure downstream with the same force. (Assuming the cartridge ID stays the same)
So I would guess your base valve stack would have to be based on the ratio between the rod and cartridge diameter.
Again, correct me if I’m wrong. A base valve compression stack that works so good on your friends Yamaha might not work so well on your KTM or Honda unless the rod/cartridge ratio is the same.
So this brings me to the question. Why the need for a mid-valve? The area of the active valve ports is almost as big as the area of the cartridge itself (I would guess 75%) and the shims are so easily pushed of the face of the piston (light spring) that it seems like there is no way there could be any kind of flow restriction (or very little) except at very, very, very slow rod speeds.
Would it not be more beneficial to use a substantially higher rate mid valve spring?
Wouldn’t this give a more progressive damping curve during very slow rod speeds?
It seems that if you could “shift” more of the dampening to the mid-valve for low speed stuff (landing jumps, G-outs) you could then substantially reduce the “stiffness” of your base valve stack, theoretically reducing harshness on roots, rocks, breaking bumps, etc.
OK, I know I’m starting to ramble and I’m probably way off in left field, but I’m almost done, I think.
2.“Base Valve” or passive valving
Everything I’ve read has said your valving is speed sensitive and not position sensitive, after looking at the stack I believe this to be true. Pressure can be traded off against velocity, by placing a different effective area at each side of the piston (the top shim / piston interface). The same pressure on a smaller area will move the piston at a higher speed but lower force for a given rate of fluid delivery. Translation: The more shim deflection the less resistance on the cartridge rod, thus less deflection at higher shaft speeds. So maybe this explains why if your high-speed stack is to “stiff” your forks will feel harsh and deflective in the choppy stuff.
Again correct me if I’m wrong. Now I think I understand why the race tech gold valves I used on my last bike did not work so well. All the dampening was placed on the base valve compression shim stack. The forks worked very well on the low speed stuff (jump landings, G-outs) but on the fast choppy stuff (roots, rocks, etc.) it would beat my arms to a pulp. When I reduced the HS stack so it was acceptable in the choppy stuff, I would loose low speed control. I could never find the right combination that gave me the best of both worlds.
OK, I think I’m done for now, my head is starting to hurt.
Anyway, while dissecting my 01 WP forks I made the following observations:
1. “Mid-valve” or active valving
Mid-Valve 24 mm OD * 8mm ID * .10mm (4X)
Very light mid-valve spring
Shims open approximately 5 mm
Correct me if I’m wrong, but it seems to me that the “mid-valve” or active piston has nothing or little to do with how much oil is pushed into the base valve ports. It would seem that would be a function of the ratio between the rod area vs. cartridge area.
So, as the rod moves its causes a differential pressure between the top of the active piston and the bottom of the active piston.
Example:
Rod OD= 14mm Area =154 mm2
Cartridge ID= 28 mm Area =616 mm2
So the area above the piston would be 462 mm2 vs. the area below the piston 616 mm2
So it would seem that any change in the rod dia. would have a substantial effect on the resulting pressure downstream with the same force. (Assuming the cartridge ID stays the same)
So I would guess your base valve stack would have to be based on the ratio between the rod and cartridge diameter.
Again, correct me if I’m wrong. A base valve compression stack that works so good on your friends Yamaha might not work so well on your KTM or Honda unless the rod/cartridge ratio is the same.
So this brings me to the question. Why the need for a mid-valve? The area of the active valve ports is almost as big as the area of the cartridge itself (I would guess 75%) and the shims are so easily pushed of the face of the piston (light spring) that it seems like there is no way there could be any kind of flow restriction (or very little) except at very, very, very slow rod speeds.
Would it not be more beneficial to use a substantially higher rate mid valve spring?
Wouldn’t this give a more progressive damping curve during very slow rod speeds?
It seems that if you could “shift” more of the dampening to the mid-valve for low speed stuff (landing jumps, G-outs) you could then substantially reduce the “stiffness” of your base valve stack, theoretically reducing harshness on roots, rocks, breaking bumps, etc.
OK, I know I’m starting to ramble and I’m probably way off in left field, but I’m almost done, I think.
2.“Base Valve” or passive valving
Everything I’ve read has said your valving is speed sensitive and not position sensitive, after looking at the stack I believe this to be true. Pressure can be traded off against velocity, by placing a different effective area at each side of the piston (the top shim / piston interface). The same pressure on a smaller area will move the piston at a higher speed but lower force for a given rate of fluid delivery. Translation: The more shim deflection the less resistance on the cartridge rod, thus less deflection at higher shaft speeds. So maybe this explains why if your high-speed stack is to “stiff” your forks will feel harsh and deflective in the choppy stuff.
Again correct me if I’m wrong. Now I think I understand why the race tech gold valves I used on my last bike did not work so well. All the dampening was placed on the base valve compression shim stack. The forks worked very well on the low speed stuff (jump landings, G-outs) but on the fast choppy stuff (roots, rocks, etc.) it would beat my arms to a pulp. When I reduced the HS stack so it was acceptable in the choppy stuff, I would loose low speed control. I could never find the right combination that gave me the best of both worlds.
OK, I think I’m done for now, my head is starting to hurt.