Stupid frame size question

frankspencer1979
frankspencer1979 Posts: 525
edited December 2012 in MTB buying advice
My current bike is a 20" frame with a 600mm top tube. If I can get an extra 2" out my seat tube (easy done) there's no reason why an 18" frame with 613mm top tube wouldn't fit me, is there? :? :?:

Comments

  • cooldad
    cooldad Posts: 32,599
    Depends. Different manufacturers measure in different ways. Even different frames within the same manufacturer can be measured differently.
    I don't do smileys.

    There is no secret ingredient - Kung Fu Panda

    London Calling on Facebook

    Parktools
  • The Rookie
    The Rookie Posts: 27,812
    In terms of the frame size, you need the effective top tube length (horizontal from top of headtube back to seat tube/post) different fixing points at each end can mean the same 'top tube' length can give very different length bikes.
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • frankspencer1979
    frankspencer1979 Posts: 525
    Things are never simple are they! I'm not too concerned with the top tube length really, I'm tall so I'm used to quite an upright position already. I just wonder about me leg length and the pedal stroke. It seems logical to me that pulling the seat tube out 2" more would put me in the same position on the smaller frame?
  • The Rookie
    The Rookie Posts: 27,812
    It would, yes.....which is why leg length is the least importnat measurement when it comes to sizing a frame, you have massive adjustment from the seatpost, far more than you can affect cockpit length by.
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • querhoch
    querhoch Posts: 111
    it might have the effect of sitting you at the same hieght, but if you have a shorter seat tube then you might have a shorter top tube and that will change your riding position. if youre a tall rider then you need the correct length top tube or you'll end up with back problems.
  • frankspencer1979
    frankspencer1979 Posts: 525
    Well depending on how the manufacturers measure, i think the top tube will actually be longer. It's the difference between a ragley and a vitus, which were designed by the same person so should use the same measurements.
  • querhoch
    querhoch Posts: 111
    why should they use the same measurements simply because they were designed by the same person? theyre different bikes and unless they have exactly the same geometry then top tube lengths will be different. it might be longer or it might be shorter, but changing the length of your seatpost wont automatically give you the same riding position.
  • cooldad
    cooldad Posts: 32,599
    The seat tube is angled, so how far the saddle will move backwards depends on the angle.
    I don't do smileys.

    There is no secret ingredient - Kung Fu Panda

    London Calling on Facebook

    Parktools
  • frankspencer1979
    frankspencer1979 Posts: 525
    The top tube measurements are both 'effective' so should be comparable. My primary concern is that the drop from 20" to 18" will be too cramped for my lanky legs, but as there's 4" of seat post between my current ride position to the minimum insert point, I think I should be ok.
  • passout
    passout Posts: 4,425
    It is complicated but remember that the most important measurement is 'effective' top tube length. Also try for a test ride if possible.
    'Happiness serves hardly any other purpose than to make unhappiness possible' Marcel Proust.
  • frankspencer1979
    frankspencer1979 Posts: 525
    Test ride not really an option due to the nature of the deal i'm looking at, it's a piglet frame for £190. I could wait for next years models, but I'm not fussed about new colours, and can survive without ISCG tabs. It seems to me the top tube on the piglet will be longer than my current one, and by poking more seat post out combined with the angle of the seat tube, that will create even more length allowing for quite a bit of adjustment. I'm already used to being quite uppright so this may potentially be more comfortable.

Categories