Reba tips (update from "how long are my rebas")

ddraver

Yes i ve changed the name, trying to keep these together as an experiment


What is the standard length that Rebas come in, I thought it was 100mm but written on the side is 115 and the length of the exposed slider is 115mm, but then i know air forks rarely use full travel so I thought this was the "extra bit",

plus the spacers seem to thick to drop it to 85, which is a little less than i'd hoped

Nicklouse you found a detailed spec drawing that gave 115,100 and 85mm too

Any Ideas

Sorry for the vagueness

Dave

nicklouse

the drawing on the RS webby give the axel to crown heights but if you are measureing 115 then thats the travel.

ddraver

so having had a look at the tech details....I nned to put one travel spacer in to drop it tio 100mm, and 2 to drop it to 85mm yes?!

Although i think this may be a shop job............

ddoogie

The U-turn models can be adjusted between to anywhere 85-115mm of travel. The "standard" Reba is 100mm but, as you have found out, it can be internally adjusted, via spacers, to either 85 or 115mm.

And if you are measuring 115mm of travel on the stanchions then that is the travel you are getting.

ddraver

so merlin have removed a spacer, or do they all come in 115mm...?!

and how difficult is it to put them in, i don't have, many tools at the mo, unfortunatly

nicklouse

depends on what you order. (or what they ordered).

they can be supplied is all 3 lengths.

ddraver

THE UPDATE!!!!!!!

I'm loosong air (i think) from the negative chamber, after i remove the pump it seems to let about 30-40 psi out, I think this is the pump but isn't this a hell of a lot?! makes accurate adjustment almost impossible!!!

plus would you counteract bob using the air pressure chamers or the compression setting

PPS - i noticed whilst doing some sprits that even when locked out the fork was compressing alot, is this because of the pop-loc setting?!

nicklouse

re the presure. set it. remove pump. reattach pump make note of new reading, reset pressure, reattach pump make note of new pressure reset...... see a pattern?

low volume high pressure settings see a big pressure change when the pump is attached.

set the flood gate to where you want it to let the fork work.

also check the valve core is tight or try changing it. same as car valve cores.

ddoogie

Does the fork definately lose 40psi of pressure when you remove the pump? Or is it the air that is trapped in the pump being released from the tubes? It shouldn't allow any air to be released from the fork when you start undoing the pump from the valve.

As for the dive and bob, you should top up the negative air pressure. This will firm up the suspension, making it less sensitive to small bumps but it will also stop excess movement.

ddraver

well that makes sense to me two ddoogie but the manual says to INCREASE negative air pressure to get better small bump response, and decrease it to get less bob....

that doesnt seem to be logical to me cos surely increasing negative air pressure would push you back up faster?!

or am i mixing up how these things work (my old one was an simple air pressure in both legs only fork so all this compression/rebound/damping are all a bit new

and nick louse...is 40psi lost just into the pump, it doesn't loose that much with the +ve chamber, only10 in fact, so thats why i ask, i reckoned the valve-pump seal is not so good but i'm no expert, i can trey another pump tonigh though and see if that seals it any better

nicklouse

ddraver as i said at the end of the post check the valve core.

and with the comment that you are only losing 10 on the +ve side it does seem more than likely.

ddraver

how would i know if theres a problem there....never changed them before

nicklouse

pop into halfrauds or some other place and ask for a valve core tool and some cores ( dont confuse them with the fact it is a MTB fork) for a car valve. insert tool into valve and unscrew, put in new and pump up.

ddraver

ok then i ll have a go...any idea on cost?!

Thanks very much for all your help nick...much appreciated

nicklouse

cores not much, tool a little. at a guess less than a fiver total. but if you go to a tyre place you might gets some cores for nothing.

ddoogie

ddraver wrote:

well that makes sense to me two ddoogie but the manual says to INCREASE negative air pressure to get better small bump response, and decrease it to get less bob....

that doesnt seem to be logical to me cos surely increasing negative air pressure would push you back up faster?!

or am i mixing up how these things work (my old one was an simple air pressure in both legs only fork so all this compression/rebound/damping are all a bit new

My mistake.

S_J_P

Negative-pressure equates to the initial resistance to the movement, i.e. the higher the negative pressure the more sag, and easier the fork is to start movement. The positive pressure equates to the amount of movement you get for a given load/bump.

If you ran 0 negative, and 100 PSI positive pressure, then you'd need 100 PSI (over a 38mm piston that's equivilent to around 25kg) to start movement. The negative chamber reduces this initial movement mass requirement. If you run similar positive/negative pressures, then initial small-bump response is improved and sag increased.

As for your excessive movement when locked-out, the Reba usually moves up to 40mm when locked-out, and the amount of movement is depenant on the positive pressure. However, if the floodgate is set too lax, then you'll get full piston movement even when locked-out. Make sure you hold the lever in the locked position, and adjust the gate as detailed in the manual.

PS. Someone (it may even have been Supersonic!) advised me to run the pressures of both chambers around 20% lower than the "book" values, this improved ride immensely.

ddraver

so SJP - what do you use the compression dial for then is this only for use when the fork is locked out ?!

what does it affect - speed of movement, initial force to cause movement... ...


many thanks

Dave

supersonic

The compression damper is a low speed compression adjustment, which will slow the fork down over low speed hits. At maximum it 'locks' the fork out, and the threshold of this lock its adjusted via the floodgate.


Just to clarify, increasing the negative pressure opposes the main spring, and tries to suck the fork down. This makes it MORE sensitive at the beginning of the stroke. It is best to inflate the positive chamber first.

S_J_P

The compression dial locks the fork, or sets (very coarsely!) the amount of movement.

Unlocked = full movement.
Locked = 40mm movement
In-between = between 100 and 40mm movement.

Mine is really loose though, and in practice it's impossible to set it usefully except locked, and unlocked.

Broadly:

Positive Air - How far fork compresses (High pressure = less movement)
Negative Air - Initial resistance to movement (High pressure = easier movement)
Gate - How much force is required to "blow-off" the lock (low-floodgate = easily blown off)
Compression Dial - Fork lock/unlock
Rebound - rate at which the fork re-extends after compressing

ddraver

many Thanks

ddraver

so an update nick.....the other shock pump does nt loose air....dodgy shock pump?!

nicklouse

could be the End being a tad different.

but you are sorted?

did you check the valve was tight?

i have had similar issues but only with the lower valve. but just replaced the cores.

odd.

ddraver

i wanted to try the other pump...havent tried yet

you think its more likely to be the valve than the pump?!

nicklouse

I know that some pumps can be funny on RS's.

ddraver

strangely is the RS pump thats playing silly buggers....I ll have a play around tonight with the pair and see whats going on

then i ll try a new valve

pots

ddraver wrote:

THE UPDATE!!!!!!!

I'm loosong air (i think) from the negative chamber, after i remove the pump it seems to let about 30-40 psi out, I think this is the pump but isn't this a hell of a lot?! makes accurate adjustment almost impossible!!!

i had the exact same problem with my rebas using a rs pump that came with my old sid xc.
bought a topeak pump (pocket shock dxg) & this solved the leakage when disengaging

MarvintheAndroid

ddraver wrote:

well that makes sense to me two ddoogie but the manual says to INCREASE negative air pressure to get better small bump response, and decrease it to get less bob....

that doesnt seem to be logical to me cos surely increasing negative air pressure would push you back up faster?!

or am i mixing up how these things work (my old one was an simple air pressure in both legs only fork so all this compression/rebound/damping are all a bit new

Adding air to the +ve increases the effective spring rate and makes the fork ride harder and more resistant to bobbing.

Adding air to the -ve will work against the +ve for part of the travel, making the fork bob a little more and increasing bump sensitivity. It will also tend to reduce the available travel.

In effect, more +ve pushes you up, more -ve pulls you down.

However the -ve doesn't completely counteract the +ve, just part of the stroke. So increasing both +ve and -ve by a similar amount will give similar sag but greater ramp-up as you compress the fork.

The reba's motion control can be used to selectively engage the lockout / increase compresion damping to iron out bob when climbing out of the sddle. I get approx 15-20mm of travel (on an 80mm fork) when locked out and climbing hard, as Nicklouse said this is set up according to rider weight using the floodgate adjustment.

Marv.

DomA

I have exactly the same problem with my Revelations. The negative looses about 30-40psi when diconnecting the pump, Topeak DXG.

The core seems tight enough, can I swap it with one from an inner tube? or must it be car tyre spec.

Dom

dvatc_mark

I thik you are all forgetting one inportant factor here.....

When you connect the shock pump to the fork you are increasing the volume of the forks air chamber (this applies to pos and neg air) , this increase in size may only be small but it will have an effect on the fork pressure as when you increase the volume of the chamber without putting anymore air into the fork the pressure will decrease. If anybody remember can remmeber back to science at school, it's boyles law.

If you don't belive me try leaving the shock pump connected to the fork and compress the fork, then do the same with the shock pump disconnected and you should feel a difference in the forks responce becasue with the shokc pump connected the volume of the air chamber is bigger which will effect the spring rate of the fork.

As Rebas only have small air chambers the effect of connecting the shock pump on fork pressure is more obvious than a fork with a large air volume like a Marzocchi.

Hope this helps.

nicklouse

dvatc_mark wrote:

I thik you are all forgetting one inportant factor here.....

When you connect the shock pump to the fork you are increasing the volume of the forks air chamber (this applies to pos and neg air) , this increase in size may only be small but it will have an effect on the fork pressure as when you increase the volume of the chamber without putting anymore air into the fork the pressure will decrease. If anybody remember can remmeber back to science at school, it's boyles law.

If you don't belive me try leaving the shock pump connected to the fork and compress the fork, then do the same with the shock pump disconnected and you should feel a difference in the forks responce becasue with the shokc pump connected the volume of the air chamber is bigger which will effect the spring rate of the fork.

As Rebas only have small air chambers the effect of connecting the shock pump on fork pressure is more obvious than a fork with a large air volume like a Marzocchi.

Hope this helps.

allready covered.