Mountain Bike Reviews Forum banner
1 - 10 of 10 Posts

·
notabouttoseeyourlight
Joined
·
2,664 Posts
Discussion Starter · #1 ·
I know a lot of you on this forum have quite an extensive knowledge of the internal workings of various forks on the market and I thought that you might be able to help me with this question.

I know that companies use different methods to adjust travel and that some are safe for extreme riding at lower settings and some are really only suitable for climbing. I want to put a Pike with U-Turn on my hardtail and ride it at 110mm for street and dirt jumping. Would this be a bad idea?

Thanks.
 

·
carpe mañana
Joined
·
7,308 Posts
coma13 said:
I want to put a Pike with U-Turn on my hardtail and ride it at 110mm for street and dirt jumping. Would this be a bad idea?
Not at all. PIKE's spring is in the same state at each travel setting. With Manitou's IT for example, the fork is prevented from extending by a mechanical lock, which prevents the spring from full extending, putting force on that lock, which could eventually break, especially if it is perpetually travel limiting the fork. PIKE's U-Turn is a sleeve which the spring threads into. When you set the PIKE to full travel setting you simply thread out a maximum amount of spring allowed by the system. When you reduce the travel, you just thread the spring into the sleeve. Therefore at full extension of a given travel setting, the spring is fully extended, so there is no force acting on the topout limiter.

_MK
 

·
notabouttoseeyourlight
Joined
·
2,664 Posts
Discussion Starter · #3 ·
MK_ said:
Not at all. PIKE's spring is in the same state at each travel setting. With Manitou's IT for example, the fork is prevented from extending by a mechanical lock, which prevents the spring from full extending, putting force on that lock, which could eventually break, especially if it is perpetually travel limiting the fork. PIKE's U-Turn is a sleeve which the spring threads into. When you set the PIKE to full travel setting you simply thread out a maximum amount of spring allowed by the system. When you reduce the travel, you just thread the spring into the sleeve. Therefore at full extension of a given travel setting, the spring is fully extended, so there is no force acting on the topout limiter.

_MK
BRILLIANT!

Thanks a lot. That's exactly what I needed to know.
 

·
Save Jesus
Joined
·
2,947 Posts
MK,

Do you know how the U-Turn system works in the Reba air forks? I was wondering if there was a way to ghetto-rig the non-U-Turn forks to also give them some kind of external height adjustment. I know that the negative chamber pressure is a form of preload, so maybe that could be a way to adjust the ride height?
 

·
Registered
Joined
·
60 Posts
beanbag said:
MK,

Do you know how the U-Turn system works in the Reba air forks? I was wondering if there was a way to ghetto-rig the non-U-Turn forks to also give them some kind of external height adjustment. I know that the negative chamber pressure is a form of preload, so maybe that could be a way to adjust the ride height?
You can try...it's cheap :)
don't know if you'll obtain a decent compression curve...let us know.
(fyi, uturn air works differently...)
 

·
carpe mañana
Joined
·
7,308 Posts
beanbag said:
MK,

Do you know how the U-Turn system works in the Reba air forks? I was wondering if there was a way to ghetto-rig the non-U-Turn forks to also give them some kind of external height adjustment. I know that the negative chamber pressure is a form of preload, so maybe that could be a way to adjust the ride height?
I don't know for sure, but from the drawings on RockShox website it appears that when you reduce travel on the U-Turn Air you decrease the size of the positive and negative air chambers at the same time. Decrease in the volume of the air chambers, increases the pressure, which in turn increases the spring rate. Since negative chamber decreases with the positive, you dont' lose small bump compliance as the pressures are roughly equal at the beginning of the stroke.

In order to externally reduce the extension of the fork, try simply pumping up the negative chamber until your fork sinks to a given height. You shouldn't lose much in terms of ride quality.

_MK
 

·
Dude...
Joined
·
1,309 Posts
Air U-turn seems to decrease the size of at least the positive air chamber without changing the pressure. My guess is it moves a piston in the air chamber, decreasing the volume, but somehow closes the chamber back up after moving it. The air not being used would need some sort of one way piston to ensure that it doesn't effect the spring rate itself... Essentially the same thing as spring U-Turn. My pike is not any stiffer at 110mm, but definetely more progressive, reinforcing this hypothesis.

I'd like to know how RS does this if anyone knows.... Or fox for that matter.
 

·
carpe mañana
Joined
·
7,308 Posts
Jessep said:
Air U-turn seems to decrease the size of at least the positive air chamber without changing the pressure. My guess is it moves a piston in the air chamber, decreasing the volume, but somehow closes the chamber back up after moving it. The air not being used would need some sort of one way piston to ensure that it doesn't effect the spring rate itself... Essentially the same thing as spring U-Turn. My pike is not any stiffer at 110mm, but definetely more progressive, reinforcing this hypothesis.

I'd like to know how RS does this if anyone knows.... Or fox for that matter.
Fox and RS do it very differently. The way RS does it is much simpler. At least based on my understanding of the inner workings, which could be off. In any case, look at this diagram posted by RS and the one below:




In red, I circled air volumes subtracted from total volumes when travel has been reduced. It does not appear that the air has been allowed passed the piston, the two chambers have simply been decreased. The reason you don't see a difference in spring rate is because the increase in positive pressure is offset by increase in negative pressure (have you actually connected you air pump to the air spring when the travel has been reduced to verify that, naturally, you would have to account for air loss due to engagement, so maybe pumping the pressure in the hose up to what you had at full extension would do, prior to engaging the valve?). At bottomout the air volume in the positive is identical in both instances so you wouln't observe any difference there. The negative chamber volume increases as the fork compresses so it has progressively less effect on the positive spring, therefore you observe the fork as a more progressive.

_MK
 

·
Dude...
Joined
·
1,309 Posts
the plot thickens...

Just attached an air pump the the positive spring with the fork at 140mm, pushed it down to 110 and watched the pressure increase abut 30 psi. Then turned the u-turn down to 110 and saw no pressure drop.

Uncompressed there is blue above the adjustable piston, but compressed, there is not. I would assume that this means the air above the piston is still at the uncompressed pressure, and the blue shows the air being used as a spring?

or something like that...
 
1 - 10 of 10 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top