Frame Sizing again !

Discussion in 'Cycling Equipment' started by Graham, Feb 6, 2003.

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  1. Graham

    Graham Guest

    What is the difference between a bike with a 73.25 deg. seat tube and 54.5cm top tube, and a bike
    with a 74 deg. seat tube and a 54.3 cm top tube ? Which one has the longer top tube ? I am setting
    the saddle to pedal position on both bikes the same but the second bike works out longer to the
    levers than the first. Despite both having 11 cm stems ! Can anyone help.

    Graham
     
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  2. "Graham" <[email protected]> wrote in message news:[email protected]...
    > What is the difference between a bike with a 73.25 deg. seat tube and
    54.5cm
    > top tube, and a bike with a 74 deg. seat tube and a 54.3 cm top tube ?
    Which
    > one has the longer top tube ? I am setting the saddle to pedal position on both bikes the same but
    > the second bike works out longer to the levers
    than
    > the first. Despite both having 11 cm stems ! Can anyone help.
    >
    >
    > Graham
    >
    Well, the one with the longer top-tube has the longer top-tube. I think it depends what you're doing
    with the saddle. Assuming both frames have 53cm seatposts (you didn't specify) the top of the seat
    post on the
    55.25 degree frame will be 0.67cm further behind the bottom bracket than for the 74 degree frame.
    The difference at the saddle will be greater. If you're pushing the saddle on the 73.25 degree
    frame forward on its rails to correct for this difference you'll end up with a shorter distance
    between saddle and bars than on the 74 degree frame. Is this the effect you're seeing?

    Michael MacClancy
     
  3. Graham <[email protected]> wrote:

    > What is the difference between a bike with a 73.25 deg. seat tube and 54.5cm top tube, and a bike
    > with a 74 deg. seat tube and a 54.3 cm top tube ? Which one has the longer top tube ? I am setting
    > the saddle to pedal position on both bikes the same but the second bike works out longer to the
    > levers than the first. Despite both having 11 cm stems ! Can anyone help.

    According to my quick calculations, everything else being equal, on the second bike the distance
    from the saddle to the handlebar should be 0.5 centimetres longer. This is because the top tube,
    even if it's slightly shorter, connects to the seat tube closer to the bottom bracket horizontally.

    This is only true if the saddle is set fore-aft exactly the same amount behind the bottom bracket on
    both bikes. If you use the same seat post and saddle, on the bike with a 73.25-degree seat tube the
    saddle should be positioned more forward relative to the seat post clamp or it will be farther
    behind the bottom bracket, increasing the difference you are experiencing.

    Of course, these differences are really small. I don't think calculations like this are very
    meaningful because you can't really count on the accuracy of the manufacturer's frame geometry. If
    the difference really bugs you, just use a slightly shorther stem on the second bike.

    -as
     
  4. Harris

    Harris Guest

    Graham <[email protected]> wrote:
    > What is the difference between a bike with a 73.25 deg. seat tube and 54.5cm top tube, and a bike
    > with a 74 deg. seat tube and a 54.3 cm top tube ? Which one has the longer top tube ? I am setting
    > the saddle to pedal position on both bikes the same but the second bike works out longer to the
    > levers than the first. Despite both having 11 cm stems ! Can anyone help.

    These are very small differences. I'll assume you have the saddle and stem heights adjusted the same
    on both bikes and that the cranks are the same length. The distance to the levers could be due to
    the handlebar shape and/or where the levers are mounted on the bars.

    Making the KOPS measurement to within a couple of mm isn't easy; just moving slightly on the saddle
    can make a difference. I wouldn't worry as long as you're setup is within 1 cm on the two bikes. How
    much difference are you seeing?

    Art Harris
     
  5. To compare top tube lengths on frames with different seat tube angles, measure from the bottom
    bracket forward.

    Top_Tube_Length - (COS(Seat_Tube_Angle) * Seat_Tube_Length)
    --
    David Blackburn

    "Graham" <[email protected]> wrote in message news:[email protected]...
    > What is the difference between a bike with a 73.25 deg. seat tube and
    54.5cm
    > top tube, and a bike with a 74 deg. seat tube and a 54.3 cm top tube ?
    Which
    > one has the longer top tube ? I am setting the saddle to pedal position on both bikes the same but
    > the second bike works out longer to the levers
    than
    > the first. Despite both having 11 cm stems ! Can anyone help.
    >
    >
    > Graham
     
  6. graham-<< What is the difference between a bike with a 73.25 deg. seat tube and
    54.5cm top tube, and a bike with a 74 deg. seat tube and a 54.3 cm top tube ? << Which one has the
    longer top tube ?

    The 54.5cm is longer.

    << I am setting the saddle to pedal position on both bikes the same but the second bike works out
    longer to the levers than the first.

    Saddle to pedal position isn't going to work unless you put your knee in the same place on both
    framesets. So, if you knee is proper on the 73.25 degree one, then it needs to slide back on the 74
    degree one... .75 of a cm, which increases top tube(effectively) of the second bike . Also, need to
    have the seat height the same and also have the headtube angle the same...

    Peter Chisholm Vecchio's Bicicletteria 1833 Pearl St. Boulder, CO, 80302
    (55)440-3535 http://www.vecchios.com "Ruote convenzionali costruite eccezionalmente bene"
     
  7. Graham

    Graham Guest

    "Qui si parla Campagnolo" <[email protected]> wrote in message
    news:[email protected]...
    > graham-<< What is the difference between a bike with a 73.25 deg. seat
    tube and
    > 54.5cm top tube, and a bike with a 74 deg. seat tube and a 54.3 cm top tube ? << Which one has the
    > longer top tube ?
    >
    > The 54.5cm is longer.
    >
    >
    > << I am setting the saddle to pedal position on both bikes the same but the second bike works out
    > longer to the levers
    than
    > the first.
    >
    > Saddle to pedal position isn't going to work unless you put your knee in
    the
    > same place on both framesets. So, if you knee is proper on the 73.25
    degree
    > one, then it needs to slide back on the 74 degree one... .75 of a cm,
    which
    > increases top tube(effectively) of the second bike . Also, need to have
    the
    > seat height the same and also have the headtube angle the same...
    >
    >
    > Peter Chisholm Vecchio's Bicicletteria 1833 Pearl St. Boulder, CO, 80302
    > (303)440-3535 http://www.vecchios.com "Ruote convenzionali costruite eccezionalmente bene"

    Thanks for all the replies so quickly, and very useful too ! I should also have said that on the
    first bike the head tube angle is 73 deg and on the second 71.7 deg. Both bikes use 3ttt bars
    with, according to the 3ttt web site, the same reach and drop. The bikes are both 53 cm c-c but
    the first uses a 3ttt Synthesis stem and the second an ITM millennium stem, both 11 cm. I am,
    through my own measurements, finding that the first bike is all other things being equal approx.
    1cm shorter saddle to levers. In line with what you all have been telling me. I hope you don't
    mind me not giving the brand of bikes, but this is to avoid any bias. I find that people have a
    lot of pre-conceived ideas when you give them a brand name. But it would be interesting if
    anyone could have a guess at the manufacturers of both bikes.

    Thanks again

    Graham
     
  8. "Graham" <[email protected]> wrote in message news:[email protected]...
    >
    > "Qui si parla Campagnolo" <[email protected]> wrote in message
    > news:[email protected]...
    > > graham-<< What is the difference between a bike with a 73.25 deg. seat
    > tube and
    > > 54.5cm top tube, and a bike with a 74 deg. seat tube and a 54.3 cm top tube ? << Which one has
    > > the longer top tube ?
    > >
    > > The 54.5cm is longer.
    > >
    > >
    > > << I am setting the saddle to pedal position on both bikes the same but the second bike works
    > > out longer to the levers
    > than
    > > the first.
    > >
    > > Saddle to pedal position isn't going to work unless you put your knee in
    > the
    > > same place on both framesets. So, if you knee is proper on the 73.25
    > degree
    > > one, then it needs to slide back on the 74 degree one... .75 of a cm,
    > which
    > > increases top tube(effectively) of the second bike . Also, need to have
    > the
    > > seat height the same and also have the headtube angle the same...
    > >
    > >
    > > Peter Chisholm Vecchio's Bicicletteria 1833 Pearl St. Boulder, CO, 80302
    > > (303)440-3535 http://www.vecchios.com "Ruote convenzionali costruite eccezionalmente bene"
    >
    >
    > Thanks for all the replies so quickly, and very useful too ! I should also have said that on
    > the first bike the head tube angle is 73 deg and on the second 71.7 deg. Both bikes use 3ttt
    > bars with, according to the 3ttt web site, the same reach and drop. The bikes are both 53 cm
    > c-c but the first uses a 3ttt Synthesis stem and the second an ITM millennium stem,
    both
    > 11 cm. I am, through my own measurements, finding that the first bike is all other things being
    > equal approx. 1cm shorter saddle to levers. In line
    with
    > what you all have been telling me. I hope you don't mind me not giving the brand of bikes, but
    > this is to avoid any bias. I find that people have a
    lot
    > of pre-conceived ideas when you give them a brand name. But it would be interesting if anyone
    > could have a guess at the manufacturers of both
    bikes.
    >
    > Thanks again
    >
    > Graham

    Don't know about the first, but the second's a 55cm Colnago, eh? ;-)

    General rule of thumb is that for mid-sized bikes, a one degree change in seat tube angle will move
    the seat post forward or back about 1cm. As you've noted, a change in headtube angle will also
    affect the reach, but the end result will be affected by your bar height relative to your saddle and
    the stem's quill angle as well. A slacker head angle than seat angle will reduce the reach as your
    bar height increases, and vice-versa. For a given bar height and stem length, reach will increase as
    stem quill angle approaches 90 degrees.

    We all paid attention in Trig class didn't we? ;-)

    SB
     
  9. Graham

    Graham Guest

    "Steve Blankenship" <[email protected]> wrote in message
    news:p[email protected]...
    > "Graham" <[email protected]> wrote in message news:[email protected]...
    > >
    > > "Qui si parla Campagnolo" <[email protected]> wrote in message
    > > news:[email protected]...
    > > > graham-<< What is the difference between a bike with a 73.25 deg. seat
    > > tube and
    > > > 54.5cm top tube, and a bike with a 74 deg. seat tube and a 54.3 cm top tube ? << Which one has
    > > > the longer top tube ?
    > > >
    > > > The 54.5cm is longer.
    > > >
    > > >
    > > > << I am setting the saddle to pedal position on both bikes the same but the second bike works
    > > > out longer to the levers
    > > than
    > > > the first.
    > > >
    > > > Saddle to pedal position isn't going to work unless you put your knee
    in
    > > the
    > > > same place on both framesets. So, if you knee is proper on the 73.25
    > > degree
    > > > one, then it needs to slide back on the 74 degree one... .75 of a cm,
    > > which
    > > > increases top tube(effectively) of the second bike . Also, need to
    have
    > > the
    > > > seat height the same and also have the headtube angle the same...
    > > >
    > > >
    > > > Peter Chisholm Vecchio's Bicicletteria 1833 Pearl St. Boulder, CO, 80302
    > > > (303)440-3535 http://www.vecchios.com "Ruote convenzionali costruite eccezionalmente bene"
    > >
    > >
    > > Thanks for all the replies so quickly, and very useful too ! I
    should
    > > also have said that on the first bike the head tube angle is 73 deg and
    on
    > > the second 71.7 deg. Both bikes use 3ttt bars with, according to the
    3ttt
    > > web site, the same reach and drop. The bikes are both 53 cm c-c but the first uses a 3ttt
    > > Synthesis stem and the second an ITM millennium stem,
    > both
    > > 11 cm. I am, through my own measurements, finding that the first bike is
    all
    > > other things being equal approx. 1cm shorter saddle to levers. In line
    > with
    > > what you all have been telling me. I hope you don't mind me not giving
    the
    > > brand of bikes, but this is to avoid any bias. I find that people have a
    > lot
    > > of pre-conceived ideas when you give them a brand name. But it would be interesting if anyone
    > > could have a guess at the manufacturers of both
    > bikes.
    > >
    > > Thanks again
    > >
    > > Graham
    >
    > Don't know about the first, but the second's a 55cm Colnago, eh? ;-)
    >
    > General rule of thumb is that for mid-sized bikes, a one degree change in seat tube angle will
    > move the seat post forward or back about 1cm. As you've noted, a change in headtube angle will
    > also affect the reach, but
    the
    > end result will be affected by your bar height relative to your saddle and the stem's quill angle
    > as well. A slacker head angle than seat angle will reduce the reach as your bar height increases,
    > and vice-versa. For a
    given
    > bar height and stem length, reach will increase as stem quill angle approaches 90 degrees.
    >
    > We all paid attention in Trig class didn't we? ;-)
    >
    > SB
    >
    > Well done SB, any takers for the other.

    Graham
     
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