Geometry Help

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  • Geometry Help
  • soobalias
    Member

    you need to define what “more difficult” relates to.

    but that change, from 150 to 120 will only really steepen the HA by a degree – so shouldnt make the bike unrideable.

    roughly 1″ extra = 1 degree slacker

    what was the original forks A2C, compared with the new forks? how does shortening the forks change the sag?

    not the head angle thats the problem, the seat tube angle of 76 is really steep in the first place (if it really is that steep) so increasing that to possibly 78 when forks dropped could mean your knees are way past the pedal axle, knee should always be behind pedal axle by 5mm minimum.

    philfive
    Member

    that could be the issue VC, it feels like you are suddenly rising through clay, its very strange and yes it supposed to be that steep according to RM site.

    The change in geometry could also cause increased pedalling losses through the rear suspension.

    philfive
    Member

    it’s bloody confusing

    philfive
    Member

    Hi all

    i have a 2009 Rocky Mountain Altitude, which comes specced with 140mm, a 69 degree head angle and a 76 degree seat tube.

    i have recently replaced the forks with some 150mm dual position revs and apparently the increase should slaken the head by 1/2 degree.

    question is when i use the dual position setting which drops it to 120mm it is noticable more difficult when peddaling, is this because the head angle becomes to steep?

    Premier Icon brant
    Subscriber

    knee should always be behind pedal axle by 5mm minimum.

    #facepalm

    http://sheldonbrown.com/kops.html

    philfive
    Member

    lol brant 😀

    is it still relevant now though, it is 14 yr old?

    Premier Icon brant
    Subscriber

    Why would it be less relavent now?
    The “5mm behind pedal spindle” is nonsense.
    Triathlon bike have 78deg seat angles and they seem to go quite fast.

    I pedal best when I’m standing up and my knees are way ahead of the pedal axles when doing so.

    specialised bike fit still fits to have knee behind axle, had one done in Feb

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