Bike Fit

Discussion in 'Cycling Equipment' started by Scott Morrison, Mar 3, 2006.

  1. How do I determine the appropriate top-tube length for a road bike? I
    know that there are a variety of "rules of thumb" for chosing the
    "correct" seat tube length.

    Ideas?
    Scott

    If you are a pro please indicate how where your info has come from. Year
    of experience is a good source.

    Thanks for you ideas.
    Scott
     
    Tags:


  2. idea one-the seat post secured at the saddle rail's mid point should
    position the saddle so as your thighs will clear the seat when in a
    seated climbing power push forward with the legs riding mode.
    which is a bit further back with the knees than the drop a string/plumb
    bob from the knees and have the bob touch the - is it the pedal shaft ?
     
  3. Art Harris

    Art Harris Guest

    Scott Morrison wrote:

    > How do I determine the appropriate top-tube length for a road bike?


    There is no formula or exact method for determining top tube length.
    Fortunately, changing the handlebar stem extension can compensate for a
    less than ideal top tube length as long as you stay within reasonable
    limits.

    For a classic road frame with a level top tube, the top tube length
    will be roughly proportional to the seat tube length. So a bike with a
    60 cm seat tube will likely have a top tube of about 59 cm. For a given
    "frame size" (i.e., seat tube length), you probably won't find more
    than about 1 cm variation in top tube length from one model to another.
    Choose a slightly longer top tube if your arms/torso are propotionally
    longer than your legs, or if you like a stretched out position.

    The handlebar stem extension is also usually proportional to frame
    size. A 50 cm frame might have a 90 mm stem, and a 62 cm frame might
    have a 130 mm stem extension.

    When you get into "compact" frames with sloping top tubes, huge seat
    posts, and S, M, L sizing, the whole concept of top tube length gets
    really murky.

    Bottom line: First determine how stretched out you want to be on the
    bike. Then choose a top tube length that will get you in the ballpark.
    Finally, fine tune the reach by choosing the right stem.

    Test ride the bike before buying, and don't accept it until it feels
    right.

    Art Harris
     
  4. Mark Hickey

    Mark Hickey Guest

    Scott Morrison <[email protected]> wrote:

    >How do I determine the appropriate top-tube length for a road bike?


    The best way is to compare the bike to the one you currently ride. If
    your current bike fits properly, you'll just need to compute the stem
    you need to replicate the saddle to bar "cockpit length" once the
    saddle is in the same relative position over the cranks.

    To start from scratch with just body dimensions is a lot more
    complicated, and beyond the realm of generalizations.

    > I
    >know that there are a variety of "rules of thumb" for chosing the
    >"correct" seat tube length.


    The seat tube length is relatively unimportant. The only real "fit
    parameter" it affects is standover clearance. That is, if you have
    two bikes that put the bars, saddle and crank in the same relative
    position, they "fit" the same, regardless of the seat tube length.

    Also, beware of the fact that bikes are measured one of three ways -
    center to top (of top tube), which is the logical one, since it tells
    you where the point of human-bike contact will be. Center to center
    is nice for the builder, but is slightly less informatic. And some
    manufacturers/builders use center to top of seat tube, which never
    made any sense to me at all since a bike of the same "size" can vary
    by several cm's depending on the amount of seat tube extention the
    designer chooses.

    Mark Hickey
    Habanero Cycles
    http://www.habcycles.com
    Home of the $795 ti frame
     
  5. G.T.

    G.T. Guest

    Mark Hickey wrote:
    > Scott Morrison <[email protected]> wrote:
    >
    >
    >>How do I determine the appropriate top-tube length for a road bike?

    >
    >
    > The best way is to compare the bike to the one you currently ride. If
    > your current bike fits properly, you'll just need to compute the stem
    > you need to replicate the saddle to bar "cockpit length" once the
    > saddle is in the same relative position over the cranks.
    >
    > To start from scratch with just body dimensions is a lot more
    > complicated, and beyond the realm of generalizations.
    >
    >
    >> I
    >>know that there are a variety of "rules of thumb" for chosing the
    >>"correct" seat tube length.

    >
    >
    > The seat tube length is relatively unimportant. The only real "fit
    > parameter" it affects is standover clearance. That is, if you have
    > two bikes that put the bars, saddle and crank in the same relative
    > position, they "fit" the same, regardless of the seat tube length.
    >
    > Also, beware of the fact that bikes are measured one of three ways -
    > center to top (of top tube), which is the logical one, since it tells
    > you where the point of human-bike contact will be. Center to center
    > is nice for the builder, but is slightly less informatic. And some
    > manufacturers/builders use center to top of seat tube, which never
    > made any sense to me at all since a bike of the same "size" can vary
    > by several cm's depending on the amount of seat tube extention the
    > designer chooses.


    Yeah, I got hit by that on my latest bike. I pay attention to
    center-to-top so I know how much seatpost I'll have extended. I figure
    the less seapost extension the less chance I'll break a seatpost.
    Seatpost's are one bike part that I have broken on more than one
    occasion. But on my Soma there is very little seat tube extending above
    the top tube. The bike fits perfectly so I'm not that concerned, I
    have plenty of top tube clearance but if I want to race 'cross I don't
    have a lot of clearance for a foot forward dismount.

    Greg


    --
    "All my time I spent in heaven
    Revelries of dance and wine
    Waking to the sound of laughter
    Up I'd rise and kiss the sky" - The Mekons
     
  6. Scott Morrison wrote:
    > How do I determine the appropriate top-tube length for a road bike? I
    > know that there are a variety of "rules of thumb" for chosing the
    > "correct" seat tube length.


    Scott,

    Lay your forearm along the top tube
    so your elbow is backed up against
    the seat post, and extend your fingers.
    The distance between your longest
    finger and the head tube should be
    about 3-3.5 inches.

    > If you are a pro please indicate how where your info has come from. Year
    > of experience is a good source.


    That's what I was told when I was
    a young grommet, and it definitely
    works out for me and for my particular
    forearm.

    Robert
     
  7. Ron Ruff

    Ron Ruff Guest

    To fit a bike properly you are really just locating your seat relative
    to the bottom bracket and your bars relative to the seat... and there
    are a wide range of options that can achieve that quite well. Other
    dimensions like top tube length and seat angle are secondary... ie they
    are usually important only if they *prevent* locating the bars and seat
    in the right place.

    There are several fit calculators online that take into account
    detailed body measurements, and allow you to estimate the proper
    dimensions... it is a good place to start.

    I was thinking about the murky world of seat-tube angles, seat-tube
    lengths, top-tube lengths and angles, etc... and it occurred to me that
    there are two frame dimensions that seem most important for sizing a
    frame.

    The first is a "reach" dimension that can be universally applied...
    unlike the top-tube length that varies with seat-tube angle, and
    top-tube angle as well if the frame is "compact". I propose that a
    plumb line be projected upward from the center of the bottom bracket,
    and the horizontal distance from there to the top-center of the head
    tube would be the "reach" of the frame. This a dimension that would
    make it easy to compare one frame to another... provided of course,
    that you knew what the value was!

    The other is a "height" dimension that can be universally applied... as
    in the height of the center of the steerer tube just above the headset
    (relative to the BB), which directly effects the height of the bars
    and/or what you will need to do in order to get the bars at the proper
    height relative to the seat.

    Now, you just need to determine the proper seat height and setback,
    figure the reach and height that would work best for you and you'd be
    able to select the equivalent size for any model of frame.
     
  8. Ron Ruff wrote:
    > ... and it occurred to me that there are two frame dimensions that
    > seem most important for sizing a frame.
    >
    > The first is a "reach" dimension that can be universally applied...
    > unlike the top-tube length that varies with seat-tube angle, and
    > top-tube angle as well if the frame is "compact". I propose that a
    > plumb line be projected upward from the center of the bottom bracket,
    > and the horizontal distance from there to the top-center of the head
    > tube would be the "reach" of the frame. This a dimension that would
    > make it easy to compare one frame to another... provided of course,
    > that you knew what the value was!
    >
    > The other is a "height" dimension that can be universally applied... as
    > in the height of the center of the steerer tube just above the headset
    > (relative to the BB), which directly effects the height of the bars
    > and/or what you will need to do in order to get the bars at the proper
    > height relative to the seat.
    >
    > Now, you just need to determine the proper seat height and setback,
    > figure the reach and height that would work best for you and you'd be
    > able to select the equivalent size for any model of frame.


    Great minds think alike! Dan Empfield has come to exactly the same
    conslusion, only you wrote it in far far fewer words. He calls these
    two frame dimensions "stack" and "reach". See his thoughts at:
    http://www.slowtwitch.com/mainheadings/techctr/stack.html
     
  9. Ron Ruff

    Ron Ruff Guest

    [email protected] wrote:
    > Great minds think alike! Dan Empfield has come to exactly the same
    > conslusion, only you wrote it in far far fewer words.


    But he was much more eloquent...

    I thought of this when I was researching my position, and noticed that
    it was common for road bikes to have slacker seat tubes as you went up
    to bigger sizes... but my seat belongs in a particular spot... I'm not
    going to vary it just because the seat tube angle changed. After I did
    some calculations, I realized that sometimes the *smaller* frame would
    have the longer "reach"... in other words they increased the top tube
    length on the bigger size, but not as much as slackening the seat angle
    increased it!

    Another thing... why do road bikes slacken the seat angles and steepen
    the head angles as you move up in size, but MTBs are all the same
    (usually 73/71)? OK... I can understand some reason for the seat tube
    angle changing since KOPS is normally used, and crank length varies
    little. But why should a 50cm frame have a 72 deg head angle, while a
    60cm frame is 74? That is a fairly big difference in handling. I
    realize that toe overlap is a problem on smaller frames, but wouldn't
    it make more sense to use a longer TT and a short stem to solve that?

    If it was up to me I think I'd cover the range of sizes with maybe 4
    evenly spaced "reach" dimensions, and 2 "height" (head tube) choices
    for each one. Seat tube angles can vary as appropriate, but I think I'd
    leave the head angle the same.
     
  10. In article
    <[email protected]>,
    "Ron Ruff" <[email protected]> wrote:

    > [email protected] wrote:
    > > Great minds think alike! Dan Empfield has come to exactly the same
    > > conslusion, only you wrote it in far far fewer words.

    >
    > But he was much more eloquent...
    >
    > I thought of this when I was researching my position, and noticed that
    > it was common for road bikes to have slacker seat tubes as you went up
    > to bigger sizes... but my seat belongs in a particular spot... I'm not
    > going to vary it just because the seat tube angle changed. After I did
    > some calculations, I realized that sometimes the *smaller* frame would
    > have the longer "reach"... in other words they increased the top tube
    > length on the bigger size, but not as much as slackening the seat angle
    > increased it!
    >
    > Another thing... why do road bikes slacken the seat angles and steepen
    > the head angles as you move up in size, but MTBs are all the same
    > (usually 73/71)?


    Because the wheels do not get larger for larger frames. As
    the top tube and seat tube get longer to accommodate a
    tall rider the center of mass moves forward relative to
    the wheel base axis; and this is a bad thing: too much
    weight over the front wheel. Moving the seat tube angle
    away from the vertical shifts the center of mass back
    again. Also chain stay length increases for larger frames.

    I do not know why head tube angles are more vertical for
    larger frames. Maybe because they can. A longer top tube
    allows for more toe-wheel clearance, so they can build a
    quicker handling turning geometry. Increasing the head
    tube angle also increases the cockpit length.

    > OK... I can understand some reason for the seat tube
    > angle changing since KOPS is normally used, and crank length varies
    > little. But why should a 50cm frame have a 72 deg head angle, while a
    > 60cm frame is 74? That is a fairly big difference in handling. I
    > realize that toe overlap is a problem on smaller frames, but wouldn't
    > it make more sense to use a longer TT and a short stem to solve that?
    >
    > If it was up to me I think I'd cover the range of sizes with maybe 4
    > evenly spaced "reach" dimensions, and 2 "height" (head tube) choices
    > for each one. Seat tube angles can vary as appropriate, but I think I'd
    > leave the head angle the same.


    --
    Michael Press
     
  11. Ron Ruff

    Ron Ruff Guest

    Michael Press wrote:
    > Because the wheels do not get larger for larger frames. As
    > the top tube and seat tube get longer to accommodate a
    > tall rider the center of mass moves forward relative to
    > the wheel base axis; and this is a bad thing: too much
    > weight over the front wheel.


    Then it would certainly make sense for MTBs to do it... they are much
    more susceptible to too-forward weight distribution (technical
    descents).

    Ideally on a road bike you'd like to have a 50/50 weight distibution...
    and I don't think any exceed that on the front except for some TT
    positions.

    > Also chain stay length increases for larger frames.


    Only because they need to for seat tube clearance, since the seat angle
    is slacker.

    > A longer top tube
    > allows for more toe-wheel clearance, so they can build a
    > quicker handling turning geometry.


    True... but if a steeper angle is good on the larger frames, wouldn't
    it also be good on the small ones?

    > Increasing the head
    > tube angle also increases the cockpit length.


    How so? Assuming that other dimensions are the same, that doesn't
    follow.
     
  12. In article
    <[email protected]>,
    "Ron Ruff" <[email protected]> wrote:

    > Michael Press wrote:
    > > Because the wheels do not get larger for larger frames. As
    > > the top tube and seat tube get longer to accommodate a
    > > tall rider the center of mass moves forward relative to
    > > the wheel base axis; and this is a bad thing: too much
    > > weight over the front wheel.

    >
    > Then it would certainly make sense for MTBs to do it... they are much
    > more susceptible to too-forward weight distribution (technical
    > descents).


    You are the expert on mountain bicycles. Perhaps the
    manufacturers just don't care.

    >
    > Ideally on a road bike you'd like to have a 50/50 weight distibution...
    > and I don't think any exceed that on the front except for some TT
    > positions.
    >
    > > Also chain stay length increases for larger frames.

    >
    > Only because they need to for seat tube clearance, since the seat angle
    > is slacker.
    >
    > > A longer top tube
    > > allows for more toe-wheel clearance, so they can build a
    > > quicker handling turning geometry.

    >
    > True... but if a steeper angle is good on the larger frames, wouldn't
    > it also be good on the small ones?


    Toe-wheel clearance.

    >
    > > Increasing the head
    > > tube angle also increases the cockpit length.

    >
    > How so? Assuming that other dimensions are the same, that doesn't
    > follow.


    Moving the head tube angle toward the vertical means the
    steering tube moves forward.

    --
    Michael Press

    `It was a very ordinary filling station,
    in front of a house with a bicycle shop below.'
    -- Georges Simenon, L'Auberge aux Noyés.
     
  13. Ron Ruff

    Ron Ruff Guest

    Michael Press wrote:
    > > True... but if a steeper angle is good on the larger frames, wouldn't
    > > it also be good on the small ones?

    >
    > Toe-wheel clearance.


    That can be achieved by making the TT (reach) longer, and fitting a
    shorter stem. Shorter cranks and/or smaller would probably be a better
    solution for shorter people.

    > Moving the head tube angle toward the vertical means the
    > steering tube moves forward.


    Unless you are changing the TT length, the bottom of the steerer will
    move back. It sounds like you are fixing the front wheel position and
    pivoting everything around that.
     
  14. In article
    <[email protected]>,
    "Ron Ruff" <[email protected]> wrote:

    > Michael Press wrote:
    > > > True... but if a steeper angle is good on the larger frames, wouldn't
    > > > it also be good on the small ones?

    > >
    > > Toe-wheel clearance.

    >
    > That can be achieved by making the TT (reach) longer, and fitting a
    > shorter stem. Shorter cranks and/or smaller would probably be a better
    > solution for shorter people.
    >
    > > Moving the head tube angle toward the vertical means the
    > > steering tube moves forward.

    >
    > Unless you are changing the TT length, the bottom of the steerer will
    > move back. It sounds like you are fixing the front wheel position and
    > pivoting everything around that.


    OK, you are the expert. Explain the geometry changes for
    larger racing frames.

    --
    Michael Press

    `It was a very ordinary filling station,
    in front of a house with a bicycle shop below.'
    -- Georges Simenon, L'Auberge aux Noyés.
     
  15. Peter Cole

    Peter Cole Guest

    Michael Press wrote:
    > In article
    > <[email protected]>,
    > "Ron Ruff" <[email protected]> wrote:


    >>Another thing... why do road bikes slacken the seat angles and steepen
    >>the head angles as you move up in size, but MTBs are all the same
    >>(usually 73/71)?

    >
    >
    > Because the wheels do not get larger for larger frames. As
    > the top tube and seat tube get longer to accommodate a
    > tall rider the center of mass moves forward relative to
    > the wheel base axis; and this is a bad thing: too much
    > weight over the front wheel. Moving the seat tube angle
    > away from the vertical shifts the center of mass back
    > again.


    I think the reason is that the effective seat tube angle is actually
    that between the end of the seat post and the pedal axle at the "3
    o'clock" position. Since crank lengths vary by only 10% but seat tubes
    vary by 30+%, the effective seat tube angle gets steeper in large
    frames, so the actual seat tube angle is made shallower to compensate.


    > Also chain stay length increases for larger frames.


    Unfortunately, it usually doesn't. It should, to match the greater
    front-center distance created by longer top tubes. Wheelbase should be
    increased proportionally front and back. Failure to do this makes a good
    wheelie bike, and a lot of tall frames fall into this category.


    > I do not know why head tube angles are more vertical for
    > larger frames. Maybe because they can. A longer top tube
    > allows for more toe-wheel clearance, so they can build a
    > quicker handling turning geometry.


    Small frames have toe overlap problems. Slacking the head angle helps,
    it also allows you to put more offset in the fork, which also helps.

    > Increasing the head
    > tube angle also increases the cockpit length.


    By the tiniest amount.
     
  16. SYJ

    SYJ Guest

    Art Harris wrote:
    ---snip---
    > When you get into "compact" frames with sloping top tubes, huge seat
    > posts, and S, M, L sizing, the whole concept of top tube length gets
    > really murky.


    Not necessarily...Calculate your 'traditional' size, then look to find
    out what the 'effective TT length' is on the compact bike you're
    considering. This is the length the TT would have were the bike a
    'traditional' frame. Thus, as a starting point, this should more or
    less correspond with the 'rule of thumb' size for a given rider.
    Granted, with so many manufacturers switching to compact sizing to
    avoid a full run of sizes (S,M,L), finding the right size *may* be more
    of a challenge

    ---snip---

    > Test ride the bike before buying, and don't accept it until it feels
    > right.


    --/snip---

    Excellent advice.

    As the resident goofy morphology torch holder for compact bikes (short
    legs, long torso), I would be remiss if I didn't urge you to keep them
    in mind if you cannot find a traditional frame that fits you
    comfortably (as long as you can stomach the aesthetics).

    SYJ
     
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