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Suspension Fork Question
#1
This has troubled me for months, everytime I search the intrawebs I never find an answer.

I have a Rock Shox Reba SL 29. You can pump air into one side of the shock. My question is: Does air stay on the one side? does is travel to the other side? Or is there some other area to add air. I have yet to break the bike down into parts. It's still brand new.
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#2
Hi

First question:
Is that a Solo air or Dual air? There is an easy way to tell: Solo air can only have air pumped in from a valve on the crown (the top bit the steerer is connected to. This is very straightforward, air stays inside the stanchion (bronzy anodized bit) and acts as a spring. The more air inside the harder the fork will be (set PSI according to your weight and personal preference).

The lowers should only contain a bit of lubrication oil for the seals and play no part in the damping. If you take them off then the fork will still work properly. Everything is contained within the stanchions. The crown that connects the stanchions is a solid bit of metal, air cannot escape from one leg into another.

Dual Air is another kettle of fish altogether.
It has the conventional air spring like the solo air, but at the bottom of the same leg (on the lowers where the QR goes) there is another air valve. This becomes a bit of a dark art but the best way to set these up is to inflate the upper valve as per the solo air, then pump air into the bottom to change how it feels (up to about 5-10 PSI below the upper valve pressure)

Both these chambers are separate and if air is moving between them you have a faulty seal. The lower air chamber is set just below the main air piston and dictates how sensitive the fork is. With a significantly lower pressure the fork only compresses in extreme movements (i.e. off drops, etc.), which makes it good for pedaling but rubbish at soaking trail chatter. Conversely a high pressure forces the fork to move more, so it is better at soaking chatter, but less efficient at pedaling unless you whack the lockout on (blue lever). However if you overdo the lower chamber or do not set it up specifically in this order then it will reduce the travel of the fork. If you want to adjust the upper chamber, completely empty the lower and then adjust it before resetting the lower to where you want it.

Hope this makes sense.
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#3
Wow, Thanks. I'm not sure if it's dual air or not. I don't know what the red knob on the bottom is for yet. I have only used the lock our once. I know the black knob on the top comes off and air can be pumped in there.
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#4
Interesting info JonB.

I have an older Rock Shock which is air over oil, and spring for rebound and compression.

Shocks either have spring or a polymyler inset to compress and spring back. The variable psi air pocket provides slower and additional compression and the oil slows the rebound.

Some shocks use one leg for compression and one for rebound. BMW motorcycle pioneered this idea around 1990.
Never Give Up!!!
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#5
Not sure which model it is (might be an outdated one) but one of them takes a crazy looking injection needle with a gauge on it. I'll try and see where I saw it.
Good maintenance to your Bike, can make it like the wheels are, true and smooth!
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#6
(08-07-2010, 11:43 PM)Jordan300 Wrote:  Wow, Thanks. I'm not sure if it's dual air or not. I don't know what the red knob on the bottom is for yet. I have only used the lock our once. I know the black knob on the top comes off and air can be pumped in there.

Here you go: http://www.sram.com/en/service/rockshox/view.php?catID=1&subcatID=8

The red knob is the rebound adjuster.

If the forks are dual air their will be a schrader air valve at the top and bottom of the left leg.

There is air only in the left leg. The left leg is an air spring and the right leg contains the damper.
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#7
(08-08-2010, 05:18 AM)GeorgeET Wrote:  Interesting info JonB.

I have an older Rock Shock which is air over oil, and spring for rebound and compression...compression and one for rebound. BMW motorcycle pioneered this idea around 1990.

Yeah, my Downhill fork (04 Rockshox Boxxer) is very similar. It doesn't use compressed air at all, but uses one leg to control compression and another for rebound. I really like this because it gives much better independent adjustment and has very little to go wrong. I love it. I didn't realize this technology was that young, I know DHers borrow a lot of technology from motorbikes but I hadn't realized they'd only come up with this in the 90s. It feels very outdated in comparison to modern Downhill forks.
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#8
Well suspension and shocks have been around for a long time. My first motorcycle was a 1968 250cc Ducati . I can attest that the suspension systems available today are vastly superior and very expensive. They have remote oil reservoirs, adjustable springs, dampening , air psi and enough adjustments to screw them up real bad, unless you are a pro. :-)

As per bicycle suspension, as far as I remember MB came into being in mid 80's and I did not see a lot of shocks till a few years later.So as far as MB's are concerned 1990 design was contemporary.

The BMW K75S forks still work good . Of course the high end Ohlins are amazing and more than most need.
Never Give Up!!!
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