Luft Fusion Tuning-Luftkammer

[RGTec]

Kann man das LF Kit mit der AWK kombinieren?
Der "Patient" ist eine RS Zeb mit 180mm.

No problem.
The piston of the LF is 2-3 cm lower than stock.
Ratio of high/low pressure chambers needs to change. AWK is about 2:1 I think... with LF should be ~1.4:1

 

[jatschek]

Hätte eine Luftfusion 180mm für eine Fox 38 abzugeben.

Bei Interesse bitte melden.

 

[Moehrenprincess]

Worauf wechselst du?

 

[jatschek]

Intend upsidedown

 

[javisst44]

Anyone who tried both Luft Fusion and Truetune and can compare them? for me in a Zeb 170 with debonair+, thinking about Luft Fusion 180.

 

[scylla]

Anyone who tried both Luft Fusion and Truetune and can compare them? for me in a Zeb 170 with debonair+, thinking about Luft Fusion 180.

I never tried Trutune myself. However, to my technical understanding, Truetune does the absolute opposite to LF in the last third of the travel, while Truetune doesn't do a whole lot in the middle of the travel, where the LF gives more support.
The LF spring essentially enlarges the negative chamber compared to the stock spring. The negative chamber "helps" the fork into its travel, thus increasing sensitivity at the beginning of the stroke, and it allows to use more pressure, which "lifts up" the spring curve in the middle of the stroke, and also makes these systems quite progressive.
Whereas Truetune token is supposed to remove progressivity by increasing adsorption of air molecules throughout the travel, thus reduction of pressure .
So you have to ask yourself, what is the problem you want to address:
Is your fork too progressive and you are unable to use as much travel as you'd like to? Then the Truetune token could be your way to go.
Does your fork dive too much in the middle of the travel, and is it too harsh at the beginning, but you don't mind a good bit of progression? Then Luft Fusion is for you.

Another interesting option derived from another air chamber tuning system: Everflow seems (seemed) to think that it is a good idea to combine their "Airtank" system (which basically follows a similar concept to the LF: enhancing negative volume) with the Truetune. This combination is supposed to lead to a quite linear spring rate, with the large negative volume lifting up the spring curve in the middle, and the Truetune lessening the progressive ramp-up at the end.
Read here: https://www.instagram.com/p/CoAekpnMBLf/
For a while you could buy a set containing both Airtank and Truetune from Everflow. Not any more it seems, at least I can't find it anymore. Probably, because Truetune now sells its own negative chamber system "HIT" to be combined with the Truetune "Flow" token.

In theory, it should be possible to do the same combination with the LF. As @RGTec wrote a few posts above, the LF piston sits even a bit lower than stock, so there should be more than enough space for the Truetune token.

 

[javisst44]

I never tried Trutune myself. However, to my technical understanding, Truetune does the absolute opposite to LF in the last third of the travel, while Truetune doesn't do a whole lot in the middle of the travel, where the LF gives more support.
The LF spring essentially enlarges the negative chamber compared to the stock spring. The negative chamber "helps" the fork into its travel, thus increasing sensitivity at the beginning of the stroke, and it allows to use more pressure, which "lifts up" the spring curve in the middle of the stroke, and also makes these systems quite progressive.
Whereas Truetune token is supposed to remove progressivity by increasing adsorption of air molecules throughout the travel, thus reduction of pressure .
So you have to ask yourself, what is the problem you want to address:
Is your fork too progressive and you are unable to use as much travel as you'd like to? Then the Truetune token could be your way to go.
Does your fork dive too much in the middle of the travel, and is it too harsh at the beginning, but you don't mind a good bit of progression? Then Luft Fusion is for you. ...

Well since truetune lets you use more pressure most reviews states that the fork rides higher with more support while still being supple. Higher pressure in the + is also higher pressure in the negative. I'm on a Zeb Debonair+ which is like a luftkappe (big negative chamber).
So while I understand you theory that doesn't seem to be how it feels and higher pressures moves the spring curve.

"The suppleness of the fork’s initial stroke was still very much present but the fork stood up far more in its travel, which results in better support generally, but even more so when tackling steeper tracks."

 

[RGTec]

@scylla If I remember correctly you had the first version which was based on a modified fox 38 air spring (EDIT: and you had the debonair C1 before)
The current version increases significantly also the positive chamber (actually the first version did too but much less)
The increase in positive volume is more than enough to compensate for the higher pressure. In the case of the Debonair+ models with the "luftkappe" piston, the difference is much greater and the final result is a decrease of progression about equivalent to 1 token. In addition, the progression from the lower leg "chamber"is also halved.

 

[scylla]

Thanks for the correction. Yes your're right, I had the old version based on a Fox38 spring. I wasn't aware of the amount you managed to increase positive volume with the new version, so sorry for causing confusion with my outdated statement about progressivity.

In addition, the progression from the lower leg "chamber"is also halved.

Compared to Buttercup springs?

 

[javisst44]

Still the same progression in the other leg as well I guess.

 

[SovietElmo]

Since the bottom of the stanchion is sealed shut by the foot stud, the air between the airshafts foot stud and the bottom of the lowers acts as a secondary air spring. Given the increased shaft diameter of the new Luftfusion, there simply is less air in the lowers and hence less progression.

The buttercups take up a slight bit of air volume, too. But it is margins less than with the Luftfusion.

By the way: Because the dampers aren't open bath, those stanchions are not sealed off. Thats why connecting the pressure relief valves on Zebs does a lot.

 

[scylla]

Well since truetune lets you use more pressure most reviews states that the fork rides higher with more support while still being supple. Higher pressure in the + is also higher pressure in the negative. I'm on a Zeb Debonair+ which is like a luftkappe (big negative chamber).
So while I understand you theory that doesn't seem to be how it feels and higher pressures moves the spring curve.

"The suppleness of the fork’s initial stroke was still very much present but the fork stood up far more in its travel, which results in better support generally, but even more so when tackling steeper tracks."

As well as I'm aware, that some kind of blackbox magic concerning initial suppleness at higher pressure is attributed to the Truetune in some reviews, I don't see a feasible explanation, how it should achieve this feat? Without changing volume or pressure ratios (apart from adding a piece of plastic in the positive), there is no reason at all, why the system should not respond very classically to increasing pressure in the static equilibrium state, which is a fork at full lenght/beginning of stroke.

They measured a bit, and the curves don't show any magic in the initial stroke. All that's happening is a flattened out spring curve further down the travel.
https://www.pinkbike.com/news/review-trutune-suspension-inserts-unlock-more-travel.html

 

[scylla]

Since the bottom of the stanchion is sealed shut by the foot stud, the air between the airshafts foot stud and the bottom of the lowers acts as a secondary air spring. Given the increased shaft diameter of the new Luftfusion, there simply is less air in the lowers and hence less progression.

The buttercups take up a slight bit of air volume, too. But it is margins less than with the Luftfusion.

By the way: Because the dampers aren't open bath, those stanchions are not sealed off. Thats why connecting the pressure relief valves on Zebs does a lot.

More is less, not less is less.

 

[SovietElmo]

That was a terribly helpful full quote. What do you mean?

 

[scylla]

there simply is less air in the lowers and hence less progression.

here you go

 

[Maffin_]

here you go

The question is: How big is the change in casting volume when the fork compresses.

If the Casing Volume is smaller, but the amount of absolute volume reduction during compression stays the same the fork gets more Progressive since the relative change in volume is bigger.


For example: (numbers are completly made up I have no idea how big the volumes are)
Original Casting volume is 200 ccm
Reduction during compression is 50 ccm
Volume left when the fork is fully compressed is 150ccm
The Volume gets 25% smaller.
In this case the casting pressure would rise form 1 bar to 1,333 bar

Tuning Casting volume is 150ccm
Reduction stays 50ccm
Volume left when fork is compressed is 100ccm
the volume gets 33% smaller
In this case the casting pressure would rise form 1 bar to 1,5 bar

the smaller base volume is more progressive.

 

[RedSKull]

The question is: How big is the change in casting volume when the fork compresses.

If the Casing Volume is smaller, but the amount of absolute volume reduction during compression stays the same the fork gets more Progressive since the relative change in volume is bigger.


For example: (numbers are completly made up I have no idea how big the volumes are)
Original Casting volume is 200 ccm
Reduction during compression is 50 ccm
Volume left when the fork is fully compressed is 150ccm
The Volume gets 25% smaller.
In this case the casting pressure would rise form 1 bar to 1,333 bar

Tuning Casting volume is 150ccm
Reduction stays 50ccm
Volume left when fork is compressed is 100ccm
the volume gets 33% smaller
In this case the casting pressure would rise form 1 bar to 1,5 bar

the smaller base volume is more progressive.

Your calculation is wrong. Reduction isn't the same, the "volume left" is the same.
The stanchions with the sealed lower end always leave approx the same room, when the fork is compressed.
So you are going from 200 to let's say 50 in the first case and 150 to 50 in the second case.
That's 4:1 in one case and 3:1 in the other.

 

[Maffin_]

Your calculation is wrong. Reduction isn't the same, the "volume left" is the same.
The stanchions with the sealed lower end always leave approx the same room, when the fork is compressed.
So you are going from 200 to let's say 50 in the first case and 150 to 50 in the second case.
That's 4:1 in one case and 3:1 in the other.

Thats why I asked the initial question of "how big is the change in volume in each case."
The Calculation was based on a case in witch the smaller volume turns the fork more progressive.

But if the volume change is as you describe then the smaller volume makes the fork less progressive indeed.

 

[SovietElmo]

here you go

The air enclosed between the air shaft foot stud and the bottom of the lowers acts as an airspring. Opposing your compression. Lowering the air volume there gives you a weaker opposing spring. Where did I go wrong?

 

[Maffin_]

The air enclosed between the air shaft foot stud and the bottom of the lowers acts as an airspring. Opposing your compression. Lowering the air volume there gives you a weaker opposing spring. Where did I go wrong?

The stanchions with the sealed lower end always leave approx the same room, when the fork is compressed.
So you are going from 200 to let's say 50 in the first case and 150 to 50 in the second case.
That's 4:1 in one case and 3:1 in the other.

 

[SovietElmo]

@Maffin_ Should I now also quote my original post that was quoted twice, because it already contains the claim? What is this?

 

[Maffin_]

@Maffin_ Should I now also quote my original post that was quoted twice, because it already contains the claim? What is this?

Youre right.
@scylla is wrong in this case. so was my asumption that the total change in Air volume is constant not the Volume left.


As long as the volume left is roughly constant the smaller casting volume is less progressive.

 

[scylla]

As long as the volume left is roughly constant the smaller casting volume is less progressive.

Interesting assumption, which is right for comparing different travel length (one of the reasons why long-travel forks are said to be overly progressive), but not so for comparing different spring systems with different shaft diameters and shapes, and differently shaped base plates, at the same travel.

Comparing residual casting volumes with the LF spring and a Debonair+ with Buttercups is not straight-forward, given the complex shapes of the spring shafts (one with the oversized shaft and a smaller footstud below, the other with the Buttercups). Without litering out the volumes, or calculating with exact geometry, we will never know. @RGTec could shed some light...

The air enclosed between the air shaft foot stud and the bottom of the lowers acts as an airspring. Opposing your compression. Lowering the air volume there gives you a weaker opposing spring. Where did I go wrong?

The casting volume basically acts as positive chamber, and we are talking about progression, which means not spring rate, but change in spring rate.
Like @Maffin_ explained the relative change of the residual casting volume throughout the travel is the source of "casting progression".
In a simple system with straight cylindrical shapes and seal against the outer wall, you typically want to increase the base volume in order to decrease progression. Given that the volume you add to the fully extended state is similarly added to the compressed state, and not somehow consumed throughout the travel. Same thing you do with removing tokens in your airspring. In the lower leg, a simple way to increase both extended and compressed volume, hence reducing the relative volume change in the lower leg, would be to make a hollow baseplate for your stanchion seal.

Typically, the airspring side will be the main source of "casting progression", with a low volume in the lower leg, which changes a lot. Connection of both legs via the pressure relief valves, like you mentioned, will help the progression taking place in the left (spring side) leg, via releasing pressure build-up towards the cartridge-damper side.

 

[Maffin_]

Sorry @scylla I thought @RedSKull knew that the compressed volumes are the same and accepted it for this case as a Fact.

Since we dont have exact information about the actual change in uncompressed und compressed Volume for both systems we cant be sure wich version is more progressive.

Did I get it correct now?

 

[scylla]

That's why I initially asked RGTec, what he means with "half the casting progression" ... compared to what spring exactly, since Debonair+ can be with or without Buttercups.
The rest of the baseline discussion was not my intention, just trying to clarify a potentially misleading statement.