PFP Adjustment and Bottoming Control

[STM Suspension]

Originally Posted by twowheels

You might try adding outer chamber fork oil, 5cc's at a time. That should leave almost all of the action as is while bolstering bottoming resistance.

Spot on.

Adding fluid to the outer chamber affects only the last third or so of the stroke and leaves the rest alone.

[GMP]

Maybe Dave could fill in the gaps in the PFP explanation if he has time.

[STM Suspension]

Originally Posted by GMP

Maybe Dave could fill in the gaps in the PFP explanation if he has time.

I've actually ordered in several different rate PFP/IC springs to be able to qualify what one feels besides just understanding what increasing the pressure on the fluid is doing.

My understanding of the Preloadable Free Piston is that it increases or decreases the total spring rate- the more you screw it in, the more spring rate you get overall. The same way that increasing the nitrogen pressure in your shock acts as more spring.

More later- I've got to head to the shop. You guys have me seriously busy this spring!

[GMP]

It adds to the total rate, but the main effect of winding in the adjuster is adding total preload, which is the devil for a good ride in the trash or steering, providing all is well before adjusting. I dropped my main spring preload to allow more PFP adjustment scope. Its the hydraulic effects of various pressure that I'm ALMOST clear on. Cavitation I understand, eliminating/reducing this this was the primary reason for all CC designs correct? Cavitation effects are also valving dependent. So, considering the stock or slightly modded valving is quite tame IMO, I would say it primarily functions as preload adjuster. This is just my logic, Dave is the suspension engineer.

[STM Suspension]

Ok, so let's think about this.

If we add preload to the spring pushing on the floating piston, it would increase the fluid pressure (literally the spring applies a force, spring rate times displacement, which is applied on the piston, which has some surface area- and pressure is psi, so pounds per square inch).

Except that as we increase the pressure the fork extends more- and goes back to some equilibrium, just higher in the stroke. [The pressure difference acts on the rebound piston- it extends the fork because the rebound side of the piston has more area than the midvalve side- the midvalve side has the damping rod, so no pressure on that area.]

So, yeah. It sure seems that the PFP acts as a preload adjuster in that it moves the front of the bike up and down.

Having said that, the new IC springs showed up today. My fork (not a 48 Marzocchi) originally came with 2.6 kg/mm IC spring. It currently has a 2.0. The new ones are 1.4 and 1.0 (may as well make a change to feel the difference), and I got a progressive rate guy to try as well.

[singletracker]

The new ones in my bike are the 1.4kg, but since the valving is different I can't really say exactly what it effected. I'm assuming it should be a little more compliant over the trail trash.

BTW, thanks for your input Dave. Your expertise is much appreciated.

[twowheels]

I once had an old suspension tuner tell me the best way to figure things out quickly was to go to the extremes.

What if the PFP spring was infinitely stiff? The fork couldn't compress, because the fluid that wants to be displaced by the rebound rod as it enters the cartridge couldn't go anywhere, and being incompressible would either bend the rod or split the cartridge.

And what if it was infinitely soft? You'd essentially have an open chamber design, where the fluid that is displaced by the rebound rod works on the compression stack and then enters an unpressurized chamber, frothing all the way.

As it is, the PFP spring adds additional stiffness to the fork because in order for the rebound rod to enter the chamber, the fluid displaced must move the floating piston and compress the PFP spring. On rebound, the energy stored in the PFP spring is returned to the fluid and acts to prevent cavitation on the backside of the rebound stack.

Lowering the PFP spring rate will lessen the overall spring rate of the fork. Lowering PFP preload will reduce the force needed to move the fork in the first place, but not change overall stiffness.

[STM Suspension]

That's one of the big things I learned in school- think of the extreme cases so you can understand what's going on.

But then we ought to ask other questions as well. Like, is the oil really frothing? What kind of difference do we need between the midvalve and main compression stack to maintain high enough pressure behind the rebound piston on high speed movement but get the plushness we're looking for? This is the same thought behind tuning the 'foot valve' (compression adjuster) on the shock- you need to maintain enough pressure above the foot valve so you don't have problems with the main shock.

And this is one reason why it's good to find someone local to work with for your suspension- Steve and I were talking and what they ride out there is nothing like what we ride out here- and could use a very different set up.

I know of one AMA road race team that ended up with so much midvalve damping that they actually pumped the open chamber cartridge dry...

KTM's WP suspension still uses an open chamber fork in a lot of models.

Me personally, I usually try to think about the big things that govern the situation and try to deal with those first. Fresh fluid, good seals, good grease, fresh bushings and being very close on the spring rate solves so many problems.

So, IC springs. I'm betting I'll feel a difference and I'm betting they can help your bike (which is why I got a bunch in to test.) But I bet they wouldn't make any difference if the fork was all bound up with worn out bushings, dead fluid and fork springs that were way off.

[GMP]

So you guys are in aggrement and makes perfect sense. The added spring rate and preload effect is merely a "side effect" of the system that is neccessary to stop cavitation under certain conditions with certain valving setups. If a change other than spring function was in effect with different adjustments or PFP spring rates, it would mean that cavitation is changing(getting better or worse), meaning that the valving requires more cartridge pressure to avoid it.

If I'm correct, the PFP piston bore/damper rod AREAS are a 9:1 ratio(36mm ID/12mm OD). So, total additive rate from the PFP spring should be PFP rate/9. This is why CC forks feel stiffer than OC forkes with the same rate main springs.

[STM Suspension]

Pressure = Force/Area

So, IC spring rate * spring displacement gives Force. Area is based on diameter of dividing/floating piston that the spring is pushing on.

So you now have pounds force/in^2 (psi) or N/mm^2

There is fluid force acting on the rebound shim side of the rebound piston (whole thing) and fluid force acting on the midvalve side of the rebound piston (whole thing minus area of damping rod.) That gives actual force difference seen by rebound piston- which acts to extend the cartridge.

And it increases as the IC spring compresses... Progressive linear spring. Huh. Whatdya' know?