Equilibrium, Fork breathing

The importance of fork oil change's.
Ok, you have filled your forks with fresh oil,and thats it for many years to come?. well many of us have pulled apart a set of forks to find the internal's red rusty and parts pitted... said oil as emulsified, its become a water based sludge.
Bmwbob [forum member] and Lansdowne fork kit user asked me what differance 20cc of oil less would make to the kit?
So i connected a pressure gauge to a fork assembly to observe the pressure readings.
The first "pump" increased the air pressure to 30 psi which then dropped just as quickly back to zero when the stanchion returned..But! and here is the interesting part of this test;
Repeated pumps decrease the readings! Yes you read this right...pumping the fork's decrease's the pressure.
here is why this happens;
The effect ; You are riding down the road and brake firmly,your forks compress the air in the leg,simply because the slider take's up the extra space.thus decreasing the air gap....and yes most people know that :shock:
But here is the BIG BUT :!: when this happens the compressed air escapes past the seal slightly reducing the pressure,so when the fork extends back we now have a slight neg pressure, as this repeats several times the fork starts to equalize [negitive-possitive] a single lipped seal will suck air easly,because the lip faces towards the oil.
When the bike comes to a stop the fork is under a slight negitive pressure condition...But this slight vacuum is short lived .and air [fresh and moisture laiden ]is "sucked" back in via the seal.
This constant refreshing emulsifie's your oil to produce a sludgy rust promoting goo, that many have see dribbling from the drain hole!!
So put a fork oil change on the "the winter lay up" tasks...

I would guess new double lipped leak proof seals will help. But again its the air getting in...not the oil getting out thats important for good operation,and long life.
Best wish's John Lansdowne Eng

Very useful objective report in fork air pressure John,
Exactly the opposite findings that MX-dirt bikers experience,
fork action pumps up their air pressure so factory and after market
supplies air nipple on forks to bleed off pressure periodically.
Something to consider-monitor on a double lip seal experiment.

This also contradicts various folks telling me that tight strapping
down Roadholder forks to transport are hard on seals leaking after
arrival unloading. Does it do seals a bad deal?

hobot

It must be the seal shape in early design, none double lip mayby? I have done the test again and the results are the same, first pump will see 30-40 psi. constant pumps show a steady fall in pressure.
Being as the seal lip face's inwards the lip is forced against the tube on the compression stroke, then the seal allows air to enter because the lip is faceing away from the outside.
I will connect my vacuum/pressure gauge and the depression/pressure can be measured,
Results comming soon.

Ugh John,
You just threw me a reality concept recoil.
I thought the compression pressure was venting to the atmosphere,
but you state it must be more into the lower fork volume. hmm.

Where does escaped air go? What do it do there?
Would there be some advantage to reduce the air space suction?
Would a tad less pre-sag spring tension nullify the suction drop
on road and brake pumping?

Fascinating data to ponder its influence on Roadholding.
With the Fauth-hobot kit + brace I have not been able to create
or encounter any conditions of fork limiting Peel's wild behavior.
On Peel's prior and my 2nd factory Roadholders, they can be a loud
damaging terror.
NOT clamming its the best upgrade possible and I just gotta try
your neat cartridge install, but my benchmark is going to
be hard to top. I expect a combo of them may thrill me most.

The rebound dampening is yet a mystery to me, but its
got to be important, as hi powered turns tend to lift
front out of traction, regardless of centrifugal - apparent forces.
There is no real force pushing bike outward, that's just
inertial resistance to forces pushing it inward.

hobot

It would be interesting to repeat the test with more modern double acting seals, maybe a set of Leak Proof's would fit the bill, I don't know of any other easily available aftermarket products.

Hi rick, I will undertake a test with new leakproof seals,the double lip should resist the incoming air due to the top outmost lip facing outwards, I know the lansdowne Kit as seen oil pressure readings over 350 psi,so a air pressure reading of 30-40 will have little effect on damping. Regards John Lansdowne Eng

john robert bould said:

Hi rick, I will undertake a test with new leakproof seals,the double lip should resist the incoming air due to the top outmost lip facing outwards, I know the lansdowne Kit as seen oil pressure readings over 350 psi,so a air pressure reading of 30-40 will have little effect on damping. Regards John Lansdowne Eng

Click to expand...

No, it won't do much to the damping but if we get air tight forks then the oil level controls the air volume and the air starts to act as a secondary spring so we have another variable to play with ...this is usually the start of trouble.

From what I've learned so far playing with cartridge emulators the standard spring rate is already too high, adding an air effect may not be a good thing.

john robert bould said:

when this happens the compressed air escapes past the seal slightly reducing the pressure,so when the fork extends back we now have a slight neg pressure, as this repeats several times the fork starts to equalize [negitive-possitive] a single lipped seal will suck air easly,because the lip faces towards the oil.

Click to expand...

That "compressed" air could also be leaking past the fork top nut threads?

Rich_j said:

john robert bould said:

Hi rick, I will undertake a test with new leakproof seals,the double lip should resist the incoming air due to the top outmost lip facing outwards, I know the lansdowne Kit as seen oil pressure readings over 350 psi,so a air pressure reading of 30-40 will have little effect on damping. Regards John Lansdowne Eng

Click to expand...

No, it won't do much to the damping but if we get air tight forks then the oil level controls the air volume and the air starts to act as a secondary spring so we have another variable to play with ...this is usually the start of trouble.

From what I've learned so far playing with cartridge emulators the standard spring rate is already too high, adding an air effect may not be a good thing.

Click to expand...

Well ..30 plus p.s.i is approx the norm for 180cc of oil...adding more oil increase's it .

Interesting to hairy handling features appear when filling to hydro lock
level,
that does not quite show up in static testing till road lumps
must pump down what little air space remaining.
Pogo's crazy side ways off small road imperfections for one thing.
Be Aware.

Intriguing to me that this also reverses fork input results,
as it does when slowed up for parking lot off the hwy or
excessive turn speeds one end or the other can't take any
more lean.
This is also zone just prior to untamed isolastic Commando
hinged antics onset.

hobot