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  1. #1
    mtbr member
    Reputation: Bemiller's Avatar
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    Jun 2013

    Help with tuning rear shock - First full suspension

    So I just got my first full suspension bike, and really this is my first year riding aggressively... so it works out. I just built and got my bike tuned at the local shop but they did not touch the suspension (which is fine). However, I'm a bit confused about the terminology for some of the components and am wondering if someone could simplify this for me. So, for starters here is my new (used) bike (2009 Cannondale Carbon Moto 2).

    Help with tuning rear shock - First full suspension-moto-2.jpg

    The Fox 36 Talas front fork is easy enough to adjust because it seems generally weight sensitive and then there's the rebound nob which seems pretty intuitive.

    Just for the record I'm 5'7" and 150 lbs.

    It's the Fox DHX Air 4.0 rear shock that is a bit complicated for me. I set the main chamber at 140 lbs because it seemed pretty much in the middle range but I'm not really sure how to determine if there is too much sag or too little. Someone in a local bike shop informed me that they usually start w/ this pressure then adjust accordingly.

    I mostly understand rebound, I just haven't figured out yet what I'm looking for in my rebound adjustment. I might just set it in the middle and see if that's comfortable.

    I understand propedal which is fine, but I don't really get the "Boost Valve" which is a secondary chamber w/ a minimum of 125 and a max of 200 lbs. I set this one at 150. It says it is for compression damping but I don't really understand what that is and how to feel for it, and how it differs from rebound.

    So pretty much anything that you guys could contribute to my understanding, preferably accompanied by layman's terms, would be greatly appreciated!

  2. #2
    mtbr member
    Join Date
    Jun 2006
    Dig around in the shock/suspension forum here - there are a number of threads that should help you with setup.

    A lot of this tuning is going to be based on experimentation. Do some research to find a decent starting point and get an idea of exactly what each adjustment is supposed to change. Ride for awhile, change one thing, ride again. Get a seat of the pants feel for what happens when you tweak this setting or that.

    There are some good Youtube vids on suspension set-up basics.

    Compression damping does the same thing as rebound damping, but slows the shock on the way IN, rather than the way out. I think the Boost valve is more like an initial 'platform' setting that determines how hard a hit initiates suspension movement. Helpful to keep the bike from being too active under pedalling forces, but too much and you lose sensitivity on small bumps. Mess around until you find the balance that works for you.

    Helps to write down settings as you go through this process so when you find what works for you, you know how to get back to it if you change or rebuild your shock, or change things up for different terrain.

  3. #3
    Fat-tired Roadie
    Reputation: AndrwSwitch's Avatar
    Join Date
    Nov 2007
    I think it's about spring rate, then rebound damping, then compression damping, in that order.

    Spring rate's really pretty easy. Set the rebound and compression dampers fully open. Putting the minimum pressure in the boost chamber would be roughly equivalent. Then adjust preload/air pressure for about 30% sag and go riding.

    Your fork and shock probably both have O rings on them. Those let you know how much travel you're using. Put them all the way at the bottom. By the time you finish a ride on terrain representative of what you ride, they should be very close to the top. Most suspension components have a little unused stanchion/inner sleeve, so they won't top out all the way. If you're not using all your travel, try less pressure. You paid for however many millimeters... Modern forks and shocks don't have such a distinct bottom-out, so you may need to find out what too much pressure is to find out what's right. 10 psi increments generally work okay for me for high-pressure forks and shocks.

    If the fork or shock is rebounding too quickly, you can actually feel the bike bouncing under you, especially after rolling off a curb or a ledge. Dial up the dampers until that goes away.

    IME, compression dampers are pretty optional on a non-race bike. If your bike moves around too much under you on a climb, try more damping. If they're hard to tune on the fly, tune them for uphill singletrack - fire roads and pavement aren't really what your bike is about, and it would be silly to mess up its downhill performance to climb an immeaurably small amount faster.
    "Don't buy upgrades; ride up grades." -Eddy Merckx

  4. #4
    mtbr member
    Reputation: Bemiller's Avatar
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    Jun 2013
    Thank you for these thoughtful responses. I will take them into consideration as I further tune out my rear shock.

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