PSA Geometry adjustments-
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  1. #1
    mtbr member
    Join Date
    Aug 2007

    PSA Geometry adjustments

    Since it seems there is a lot of confusion about geometry adjustments I wrote up this overview.

    Let me give a summary of geometry adjustments here for future reference.

    Offset bushing (reducers):
    Do not affect amount of suspension travel. They will, on most bikes, affect the suspensions behavior, since you are on a different part of the leverage curve.

    They need sufficient clearance in behind the seat tube. If itís tight, going to a smaller rear tire will help (in standard orientation). If used in ďextendingĒ orientation you need clearance in front of the seat tube (with shock yoke or swing arm linkage).
    If used in the standard orientation(to shorten eye to eye distance of frame) they will:
    • Drop the bottom bracket
    • Slacken Headangle
    • Slacken seattube angles (you might be able to compensate for this by sliding saddle forward on the rails/ reversing the seatposts saddle clamps)
    • Increase stack
    • Decrease reach
    • Lengthen wheelbase
    • Increase front center/ rear center ratio

    Adjusting headsets:
    Need a non-integrated headset. If the head tube is on the smaller side, you will need to go to external cups. Usually this is at the bottom due to the larger dimension of the tapered steerer there.
    If you go from internal to external cups on the bottom, this will:
    • slacken head angle
    • Slacker seattube angle (you might be able to compensate for this by sliding saddle forward on the rails/ reversing the seatposts saddle clamps)
    • Raise the bottom bracket
    • Shorten the reach
    • Increase the stack
    • Lengthen the wheelbase

    If you switch from internal to external cups on the top, this will increase the stack, but you can compensate by removing spacers, going to a negative rise stem or a lower rise handle bar (unless you already were as low as possible before. Technically it will also reduce reach, but only by about 3mm.
    All this is in addition to any changes you are doing with the actual angle adjust!

    The first option for adjustment headset is straight offset. This will increase or reduce the reach, front center and wheelbase, in addition to any possible changes caused by switching to an external headseat cup.

    The other option is angled adjust headsets. Commonly these are used to make the head angle slacker, so thatís what I will describe. If you install them the other way around, all effects are reversed.
    In addition to any effects from switching to external cups an angled headset will:
    • Slacken the head angle
    • Slightly drop the bottom bracket
    • Slightly increase the reach
    • Slightly decrease the stack
    • Increase front center / rear center ratio

    Did I miss anything important?

    Letís not get into whether or not any of these changes are better or worse. That depends on geometry as a whole, rider size and riding style to such an extent that it will only lead to confusion and unnecessary arguing. letís leave it at the facts(geometry) and then everyone can decide for themselves whether or not the like those changes.

    I hope this helps anyone considering adjusting their bike!

  2. #2
    mtbr member
    Join Date
    Aug 2007

    Making your seat tube angle steeper

    If you need(want) a steeper seat angle, what we really care about is the position of the saddle, the tubes underneath donít matter at all.

    So, if your saddle is to far back (perhaps due to adjustments like the ones above), you can slide your saddle forward on the rails. There is a limit there though. If that is not sufficient you can try the following:

    On many seatposts, including the Reverb, the plates that clamp the saddle rails are offset, even though the head itself is not. You can simply flip these plates around to move the saddle forward quite a bit.

    If that isnít sufficient, there are several dropper posts available with offset heads, and many of them can be mounted backwards, so that the saddle is offset forwards. Before you commit to buying one of those for that purpose, make sure that there is enough rotation to get the saddle level when the post is facing backwards.
    Models with one bolt running side too side (like older Specialized Command Posts) are a safe bet.

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