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  1. #1
    Cannondale Snob
    Reputation: RiskEverything's Avatar
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    Mar 2006
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    Single-Pivot rear suspension: where is the best place for the pivot?

    On most of the older designs (Super V, Jekyll, Raven) the rear pivot is above and behind the BB, the shock mount is on the down-tube, and the shock has a straight-line compression stroke.

    On the newer designs (Rush, Prophet) the rear pivot is above and ahead of the BB, the shock mount is on the top-tube, and the shock has an angle to it that changes during the compression stroke.

    Still other bikes have the rear pivot mounted ahead of and level with the BB on the underside of the down-tube.

    What changes with relation to the position of the rear pivot to the BB?
    What changes with relation to the position of the shock mount and angle of the shock?

    Thanks in advance for helping me understand my bike a little better
    '06 Cannondale Rush 1000 4" travel 27lbs
    '04 Cannondale F600 SOBE -STOLEN!
    '96 Cannondale Uber-V 6" travel 30lbs

  2. #2
    mnt bike laws of physics
    Reputation: yogiprophet's Avatar
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    Jan 2006
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    Rear suspension design is not an easy thing to understand because there is so much going on.

    The location of the pivot ideally would change for each individual and BB height. This is because every person needs a different amount of anti-squat(what some may call anti-bob). A tall person has a higher center of gravity and there for tends to squat the srear suspension under acceleration more than a person with a lower center of gravity. So, a bike that suits one will not suit all people and that is one why one may like and the other not like.

    A bike with a higher BB height already has more anti-squat because the swingarm has more upward angle to it. With each pedal stroke there is more tendancy for the suspension to extend simply because the rear axle is lower than the pivot. So, all else being equal, it will need a slightly lower pivot location to get the same anti-squat.

    A rearward pivot location is better in all respects(besides having to deal with the front derailleur) because it is shorter it can be made stiffer with less mass, in the 2 smaller chainrings the chainline stays closer to the pivot point which keeps the suspension more activein those gears, and it has slightly less pedal kickback(too complicated to explain why here).

    The shock rate may be more of a preference and depend on the type of riding one does. I prefer a linear rate. The Rush/Prophet have falling rates, the Rize has a rising rate. The Super V is a rising rate. With a linkage, the designer can tune the rate to some degree depending on the design. A falling rate tends to blow through its travel more easily. A rising rate stiffens up more rapidly and will not bottom out. Also the shock ratio plays a role. The propblem with the Super V is that its shock ratio was too large. In other words, you had to put a lot of air in the shock because the it has about a 4:1 ratio(this is a guess) that combined with a rising rate meant that you will never get full travel and the suspension is harsh.

    OK, I am tired now. Hope you enjoy this stuff. I do.

  3. #3
    mnt bike laws of physics
    Reputation: yogiprophet's Avatar
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    OK, I'm rested again.
    I want to add that where a pivot is located in the fore/aft position makes very little difference. What makes much more of a difference is the fit of the bike, the geometry(which include the dimensions) and the shock rate.
    For instance, I like the design of the Rize, but the geometry is too slack for my preference so it is a no go for me. Best to stick with a bike that fits well and suits your riding style etc.

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