PFP Adjustment and Bottoming Control

[singletracker]

I would like to take a course on suspension. I understand the math (I teach some math courses), but since I've never torn a fork a part and looked at in depth I don't have a good comprehension of how it works.

 

[singletracker]

I called EE today and asked Dave about the best way to improve the bottoming resistance. Of course he recommended the same thing as you guys. The first suggestion was to increase the fluid in the outer chamber. When I asked him directly about the PFP he said increasing the preload increases the pressure on the inner chamber and will effect the action throughout the travel including increasing bottoming resistance and decreasing plushness on trail junk. He said that I may want to increase the preload for HS and decrease it for enduros.

I noticed that when I increased the PFP preload by a half turn that it seemed to handle high speed rutted corners a little better. BTW, I'm happy with my suspension, but I'm trying to get it set up the best I can.

 

[CDBiker220]

So I think I understand the cause and effects of adjusting the PFP adjuster. My question is how is this much different from adjusting the compression clicker? What differentiates adjusting one over the other? (currently have a 1.46kg/mm pfp spring compared to stock 2kg/mm)

 

[GMP]

It has nothing directly to do with base valve compression valving. I agree with Steve that the PFP preload will affect the overall force at which the fork starts to move, while the PFP spring rate itself will be additive to the main spring rate throught the range.

Its all about reducing cavitation on the midvalve side of the rebound piston. The stiffer the midvalve, the greater the pressure drop or differential and the lower absolute pressure. Bubbles form when this pressure drops below the vapor pressure of the fluid. If the total cartridge pressure is kept high, the differential across the midvalve will not drop the pressure low enough to cause this cavitation effect.

Just like a shock, but shocks have been pressurized for a long time as the single piston design with high differentials requires it.

 

[CDBiker220]

Ok I understand everything your saying, and understand how it works. I am an engineer by trade and have worked with springs, I get the how the preload and spring rate add to the main spring by a 9/1 ratio due to the area of the piston. I understand that its purpose is to keep the cartridge fluid pressurized. What I'm asking though is not how it works but how its adjustment affects real world performance. According to what your saying adding preload to the pfp will only make fork movement harder to initiate, therefore just adding harshness. So that doesnt sound productive. I guess im wondering what the changes in the feel of the fork are when adjusting the pfp in compared to adjusting compression dampening in.

 

[Vindicator]

From my experience the more you preload PFP the firmer the suspension feels. Not necessarily harsh feeling, just firmer . It's a simple and easy way to adapt the suspension for your trail at the trail . For sand and MX I would go with more preload, for rocks less preload. Depends a bit on your personal taste .I don't think PFP has a huge effect on the fork sag.

 

[STM Suspension]

It's just acting, basically, to increase the total spring preload in the system.

Does adding preload on the spring act to increase overall resistance to movement? Yeah, because it increases static spring force.

I think of the PFP as a FINE TUNING tool. If your main springs are wrong, the front end will never work as good as it could.

And damn! Glenn. Have you been studying suspension a LOT or do you have an ME degree as well?

 

[GMP]

Dave,
I make it a point on understanding what I work on. Educated in electrical/electronics, make my living in process control software design, but I've done my share of mech stuff as well. I think a guy with a basic understanding of physics can learn a lot from the
Restackor software.


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[nick H]

How much adjustment is there supposed to be on the pfp valve? I get ten turns and I have spoke to others and they only have 3. All bikes are 13s and not race models. I guess my question is there something wrong with my forks?
Forgot to mention my suspension was redone professionally and they did not do anything to pfp

 

[GMP]

Ten turns is correct, what is available, but god would that be stiff! I do not think that part of the fork has changed since its introduction.

I just installed .46 springs with only 1mm preload, made custom shims. Just enough to hold the spring. Now the PFP is a lot more useful, you can really feel the difference in preload as you go in on the adjuster. First ride with it Sunday so we will see how it goes.


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[nick H]

I rode with them at 8 in the rocks and it was a touch stiff ha ha I messed up on adjusting them thought it was 2 turns from all the way in not from all the way out
Wonder why everyone else I spoke to only came up with 3 turns of adjustment

 

[GMP]

So your setting of 8 turns out was actually 2 turns in then, which is the stock setting. For rocks I like a softer initial travel (less preload) but not a soft sloppy fork either. I think that the stock main spring preload plus minimum PFP is just a bit too much total. Why jack the main springs up 5mm and only work the first turn or two of a ten turn PFP adjustment? Less main preload should allow more useful range of PFP adjustment. The math and the bounce test in the shop agree, we'll see if the trail does too.


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[nick H]

no i was all the way in(clockwise) and went 2 turns out this is the wrong way to set it your supposed to be 2 turns in from all the way out so basically it was as stiff as possible.
at least this is how my tuner told me how to do it