PFP Adjustment and Bottoming Control

[singletracker]

I'm still not 100% sure what the PFP effects. Does it have a noticable influence on bottoming resistance? If one wants to increase the bottoming resistance without negatively impacting the plushness over trail junk would he be better off adjusting the low speed compression or the PFP adjuster? I'm thinking the PFP, but I'm not for sure.

 

[GMP]

It has more of an effect on low speed stuff, as it also increases overall fork preload. Too much and the small stuff gets more harsh. I actually dropped my main spring preload down to 2mm to make better use of the PFP adjustment and gain a plusher ride on more technical trail. The biggest difference between this and a KYB/Showa is that the cartridge pressure range is higher. The Jap forks start out at essentially zero ICS preload while the Zoke is at 7mm on a 20Nm spring, with PFP full out. This should allow more aggressive valving without cavitation, but the stock valving is relatvely light. I'm still wrapping my head around the overall hydraulic effects, but my feeling is that with light valving this is minimal. You can learn a lot from the Shim Restackor software.

 

[singletracker]

Thanks GMP.

I should add that I had EE revalve my suspension this winter since I had good luck with them on KTM's. I told them that I was pretty happy with the fork, so I wasn't looking for any big changes there. One thing I had them do is put in a lighter PFP spring, 15Nm? I've been riding with the revalved suspension for about 2 months, and I'm happy so far.

I use more of the travel now, which is good, but I bottomed out hard two rides ago. Picture about a 20' tall 60d incline into a V-bottom (2' wide bottom) and then 50d up the other side. Add in a soft bottom, and too much speed and that equals a loud clank sound and hard impact. This is on my own property, so I've been through there many times and have a good gauge for how fast I can hit it, but the bottom is getting a bit soft.

I'm thinking I will continue to increase the PFP until the trail junk becomes harsh and then back it off a bit. I increased it by 1/2 turn before my last ride and it seemed like I had more control hitting high speed (3rd/4th gear) rutted turns. I'll probably keep increasing it until it gets too harsh on the trail junk, and then back it off.

 

[twowheels]

Thanks GMP.

I should add that I had EE revalve my suspension this winter since I had good luck with them on KTM's. I told them that I was pretty happy with the fork, so I wasn't looking for any big changes there. One thing I had them do is put in a lighter PFP spring, 15Nm? I've been riding with the revalved suspension for about 2 months, and I'm happy so far.

I use more of the travel now, which is good, but I bottomed out hard two rides ago. Picture about a 20' tall 60d incline into a V-bottom (2' wide bottom) and then 50d up the other side. Add in a soft bottom, and too much speed and that equals a loud clank sound and hard impact. This is on my own property, so I've been through there many times and have a good gauge for how fast I can hit it, but the bottom is getting a bit soft.

I'm thinking I will continue to increase the PFP until the trail junk becomes harsh and then back it off a bit. I increased it by 1/2 turn before my last ride and it seemed like I had more control hitting high speed (3rd/4th gear) rutted turns. I'll probably keep increasing it until it gets too harsh on the trail junk, and then back it off.

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.

 

[singletracker]

Is that as simple as removing the top cap and adding oil? I've changed fork oil in OC forks, but I haven't touched CC forks. It is currently set at 325mm.

 

[GMP]

You mean 325cc, you can't measure level in CC forks. The stock valving on the '12s is quite sloppy. Lots of bleed, which is why it works decent in the rocks and trail trash. I stiffened mine up and its still good in the sharp edge stuff, firm but no deflection. If your terrain is less sharp edge and more higher speed bumps maybe you would be better off with some stiffer valving. Just a thought. Are you running stock springs? I can use all the fork in whoops easily, 176 lbs, 205 geared up. Trying stiffer springs next.

 

[singletracker]

You're right, it's cc's. I'm not for sure what EE did as far as valving goes, but they made some adjustments. They told me what they were going to do, but I lost my notes. I think they adjusted the midvalve stack as well as changing the PFP. I still have to run the rebound in pretty far though, so I don't think they changed the rebound much.

Where I've been practicing the soil is pretty loamy without many sharp edged bumps, but it will be a bit different when I start racing. That's why I'm a little unsure about my settings. I plan to race every weekend in April, so I'll have a better feel for it soon.

I'm still on stock springs, and I weigh 167lb first thing in the morning, and about 10 pounds less than you geared up.

 

[twowheels]

You're right, it's cc's. I'm not for sure what EE did as far as valving goes, but they made some adjustments. They told me what they were going to do, but I lost my notes. I think they adjusted the midvalve stack as well as changing the PFP. I still have to run the rebound in pretty far though, so I don't think they changed the rebound much.

Where I've been practicing the soil is pretty loamy without many sharp edged bumps, but it will be a bit different when I start racing. That's why I'm a little unsure about my settings. I plan to race every weekend in April, so I'll have a better feel for it soon.

I'm still on stock springs, and I weigh 167lb first thing in the morning, and about 10 pounds less than you geared up.

Stock springs should be fine at that weight.

 

[singletracker]

Thanks twowheels!

 

[Front Royalty]

I am under the impression that adding pfp in conjunction with outer chamber oil level will turn this fork from a rock eating pillow top ride to more of an mx track setup. At least im hoping so, anyway!

 

[STM Suspension]

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]

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?