Seat Angle Question??

Discussion in 'Cycling Equipment' started by FasTrack, Jan 7, 2005.

  1. FasTrack

    FasTrack Guest

    What is the therory why smaller road bikes have steep seat angles?

    Most smaller bikes have 74-75 degreee seat angles...

    What would a 72 degree seat angle do to a say 50cm or smaller road
    frame?

    Thanks,

    Dave
     
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  2. FasTrack wrote:

    > What is the therory why smaller road bikes have steep seat angles?
    >
    > Most smaller bikes have 74-75 degreee seat angles...


    Small frames with big wheels often have to resort to various levels of
    bogosity in frame design.

    > What would a 72 degree seat angle do to a say 50cm or smaller road
    > frame?


    If you kept the short top tube that should be on such a small bike,
    there'd be excessive toe overlap of the front wheel.

    As a work-around to this problem, some manufacturers make the head angle
    shallower, some make the seat angle steeper, some a little of both.

    None of these changes improve the ride or handling of the bike.

    The steeper seat angle leads to the rider putting more weight on the
    handlebars.

    Shallower head angle could be OK if the fork rake were also increased to
    provide a desirable amount of trail. Unfortunately this is not always
    done. The guys who design such bikes never ride the smaller sizes.

    The real answer is smaller wheels, or at least a smaller front wheel.

    Sheldon "Georgena Was Right" Brown
    +------------------------------------------------------+
    | If I have been able to see farther than others, |
    | it was because I stood on the shoulders of giants. |
    | -- Sir Isaac Newton |
    +------------------------------------------------------+
    Harris Cyclery, West Newton, Massachusetts
    Phone 617-244-9772 FAX 617-244-1041
    http://harriscyclery.com
    Hard-to-find parts shipped Worldwide
    http://captainbike.com http://sheldonbrown.com
     
  3. A Muzi

    A Muzi Guest

    FasTrack wrote:
    > What is the therory why smaller road bikes have steep seat angles?
    > Most smaller bikes have 74-75 degreee seat angles...
    > What would a 72 degree seat angle do to a say 50cm or smaller road
    > frame?


    It would make a small 50cm-sized rider (with maybe a shorter
    femur than a big bike rider) slide her seat forward on the
    post to get back to where she would have been (in relation
    to the pedals) with a 73 or 74 degree seat angle.

    Or not.

    Depends on the rider's femur length.


    --
    Andrew Muzi
    www.yellowjersey.org
    Open every day since 1 April, 1971
     
  4. David

    David Guest

    "A Muzi" <[email protected]> wrote in message news:[email protected]
    > FasTrack wrote:
    > > What is the therory why smaller road bikes have steep seat angles?
    > > Most smaller bikes have 74-75 degreee seat angles...
    > > What would a 72 degree seat angle do to a say 50cm or smaller road
    > > frame?

    >
    > It would make a small 50cm-sized rider (with maybe a shorter
    > femur than a big bike rider) slide her seat forward on the
    > post to get back to where she would have been (in relation
    > to the pedals) with a 73 or 74 degree seat angle.


    Yup. All seat-tube angle really does is position the saddle with respect to the bottom bracket.
    If your legs are short, you'll want a steep seat-tube, else not (I suppose--I only have experience
    with the former).
     
  5. AB/9000

    AB/9000 Guest

    A Muzi wrote:

    > It would make a small 50cm-sized rider (with maybe a shorter femur than
    > a big bike rider) slide her seat forward on the post to get back to
    > where she would have been (in relation to the pedals) with a 73 or 74
    > degree seat angle.
    >
    > Or not.
    >
    > Depends on the rider's femur length.


    If you belive in KOPS - Knee Over Pedal Spindle - and if shorter riders
    in general have shorter femurs. They don't. The average femur/leg
    ratio is constant.

    ---
    AndersB
     
  6. Fastak writes-<< What is the therory why smaller road bikes have steep seat
    angles?
    > Most smaller bikes have 74-75 degreee seat angles...
    > What would a 72 degree seat angle do to a say 50cm or smaller road
    > frame? >><BR><BR>


    It's because most smaller riders have shorter femurs, meaning a steeper seat
    tube angle to get the knee over the pedal spindle.

    Peter Chisholm
    Vecchio's Bicicletteria
    1833 Pearl St.
    Boulder, CO, 80302
    (303)440-3535
    http://www.vecchios.com
    "Ruote convenzionali costruite eccezionalmente bene"
     
  7. AB writes-<< If you belive in KOPS - Knee Over Pedal Spindle - and if shorter
    riders
    in general have shorter femurs. They don't >><BR><BR>

    Ahhh but they do have shorter femurs. kops is a starting place generally, the
    only thing in bike fit that is sort of a constant, but not that 'firm.

    You next mention that femur/leg ratio is constant and that is true, shorter
    femur and shorter lower leg,

    Peter Chisholm
    Vecchio's Bicicletteria
    1833 Pearl St.
    Boulder, CO, 80302
    (303)440-3535
    http://www.vecchios.com
    "Ruote convenzionali costruite eccezionalmente bene"
     
  8. AB/9000

    AB/9000 Guest

    Qui si parla Campagnolo wrote:
    > AB writes-<< If you belive in KOPS - Knee Over Pedal Spindle - and if shorter
    > riders
    > in general have shorter femurs. They don't >><BR><BR>
    >
    > Ahhh but they do have shorter femurs. kops is a starting place generally, the
    > only thing in bike fit that is sort of a constant, but not that 'firm.


    Doh, I missed the word "relatively".

    ---
    AndersB
     
  9. AB/9000

    AB/9000 Guest

    Qui si parla Campagnolo wrote:

    > It's because most smaller riders have shorter femurs, meaning a steeper seat
    > tube angle to get the knee over the pedal spindle.


    Not true!

    Smaller riders have shorter femurs and shorter lower legs = same
    femur/lower leg ratio as larger riders = same optimum seat angle - KOPS
    or not.

    Steeper seat angles on smaller frame sizes has to do with what Sheldon
    says. It's a precaution against toe-overlap and not based on fit and
    position.

    In the larger sizes you see the opposite: steeper head angles and
    shallower seat angles to avoid overly long wheelbases (= slow handling
    bikes), but at the same time chain stays are often longer to obtain a
    more even weight distribution. This is especially the case with Italian
    frame builders

    If frame manufacturers should offer proportionally fit frames to riders
    of all sizes, it would require proportionally scaled wheels.
    And ideally proportionally scaled courses with proportionally scaled
    turns (corners!) ......

    Take a look at:
    http://bikefitting.com/English/Theory/SeatAngle.aspx

    Hope you understand. English is not my first language.

    ---
    AndersB
    Denmark
     
  10. fastrac-<< What is the therory why smaller road bikes have steep seat angles?
    >
    > Most smaller bikes have 74-75 degreee seat angles.. >><BR><BR>


    Because most smaller riders also have shorter femurs, so to get KOPS, a steeper
    seat tube angle is required. If it were 72 degrees, and to get KOPS, a reverse
    setback seatpost would often be required.

    Peter Chisholm
    Vecchio's Bicicletteria
    1833 Pearl St.
    Boulder, CO, 80302
    (303)440-3535
    http://www.vecchios.com
    "Ruote convenzionali costruite eccezionalmente bene"
     
  11. > Steeper seat angles on smaller frame sizes has to do with what
    Sheldon
    > says. It's a precaution against toe-overlap and not based on fit and
    > position.


    A shallow head tube angle will move the front wheel forward to mitigate
    toe overlap, but the seat tube angle doesn't matter. The bottom bracket
    is still in the same place if the seat tube angle was 72 deg. or 76
    deg. A steep sat tube angle will move the riders arse and knee further
    forward, but their feet down on the pedals don't move forward at all.
    --Jim
     
  12. An anynymous poster wrote:

    > A shallow head tube angle will move the front wheel forward to mitigate
    > toe overlap, but the seat tube angle doesn't matter. The bottom bracket
    > is still in the same place if the seat tube angle was 72 deg. or 76
    > deg. A steep sat tube angle will move the riders arse and knee further
    > forward, but their feet down on the pedals don't move forward at all.


    Depends how you look at it. If you keep the top tube length constant, a
    steeper seat tube angle will move the bottom bracket back with respect
    to the front wheel, the handlebars and the rider's butt.

    Sheldon "Relativity" Brown
    +-----------------------------------------------+
    | Who has deceived thee as often as thyself? |
    | -- Benjamin Franklin |
    +-----------------------------------------------+
    Harris Cyclery, West Newton, Massachusetts
    Phone 617-244-9772 FAX 617-244-1041
    http://harriscyclery.com
    Hard-to-find parts shipped Worldwide
    http://captainbike.com http://sheldonbrown.com
     
  13. Sure, but when you fit someone up, don't you get their saddle, knee,
    KOPS stuff delt with first, then move forward and work on tt lenght?

    It's a strange concept, we're presuming tt has to be constant yet we're
    ok with changing the seat tube angle. My feeling is one really can't be
    tinkered with too much before the other has to be adjusted also.

    I definetly don't envy shorter riders, as their bikes always seem to
    have a compromise somewhere, but I also think way to much is made of
    toe overlap. I once had a 58cm Italian frame with overlap, and although
    it was a very quick handeling bike, the overlap was never an issue.
    --Jim
     
  14. Sure, but when you fit someone up, don't you get their saddle, knee,
    KOPS stuff delt with first, then move forward and work on tt lenght?

    It's a strange concept, we're presuming tt has to be constant yet we're
    ok with changing the seat tube angle. My feeling is one really can't be
    tinkered with too much before the other has to be adjusted also.

    I definetly don't envy shorter riders, as their bikes always seem to
    have a compromise somewhere, but I also think way to much is made of
    toe overlap. I once had a 58cm Italian frame with overlap, and although
    it was a very quick handeling bike, the overlap was never an issue.
    --Jim
     
  15. Anynymous? Anonymous. No I ain't, I signed it "Jim" What else you need,
    my ssn?
     
  16. "Jim" wrote:

    > Anynymous? Anonymous. No I ain't, I signed it "Jim" What else you need,
    > my ssn?


    We have three "Jims" here at Harris Cyclery. Are you Jim Ammirato, Jim
    DaSilva, or Jim Wirtanen? Or perhaps a different Jim altogether...?

    Sheldon "Mystery" Brown
    +------------------------------------------------+
    | Too often we... enjoy the comfort of opinion |
    | without the discomfort of thought. |
    | -- John F. Kennedy |
    +------------------------------------------------+
    Harris Cyclery, West Newton, Massachusetts
    Phone 617-244-9772 FAX 617-244-1041
    http://harriscyclery.com
    Hard-to-find parts shipped Worldwide
    http://captainbike.com http://sheldonbrown.com
     
  17. On Tue, 25 Jan 2005 14:02:58 -0500, Sheldon Brown
    <[email protected]> wrote:

    >"Jim" wrote:
    >
    >> Anynymous? Anonymous. No I ain't, I signed it "Jim" What else you need,
    >> my ssn?

    >
    >We have three "Jims" here at Harris Cyclery. Are you Jim Ammirato, Jim
    >DaSilva, or Jim Wirtanen? Or perhaps a different Jim altogether...?
    >
    >Sheldon "Mystery" Brown
    >+------------------------------------------------+
    >| Too often we... enjoy the comfort of opinion |
    >| without the discomfort of thought. |
    >| -- John F. Kennedy |
    >+------------------------------------------------+
    > Harris Cyclery, West Newton, Massachusetts
    > Phone 617-244-9772 FAX 617-244-1041
    > http://harriscyclery.com
    > Hard-to-find parts shipped Worldwide
    >http://captainbike.com http://sheldonbrown.com


    Dear Sheldon,

    Mysteriously, there seem to be no Daves in your wanted
    posters, despite their teeming numbers here on
    rec.bicycles.tech:

    http://sheldonbrown.com/harris/index.html#articles

    Does Harris Cyclery discriminate on a first-name basis?

    The closest Shakespeare came to a Dave is a trivial servant
    named Davy in the second part of Henry IV.

    Carl Fogel
     
  18. Sheldon Brown wrote:
    > "Jim" wrote:
    >
    > > Anynymous? Anonymous. No I ain't, I signed it "Jim" What else you

    need,
    > > my ssn?

    >
    > We have three "Jims" here at Harris Cyclery. Are you Jim Ammirato,

    Jim
    > DaSilva, or Jim Wirtanen? Or perhaps a different Jim altogether...?

    A different Jim altogrther is correct.

    Jim "Jim" Potter
     
  19. Carl Fogel wrote:
    > Dear Sheldon,
    >
    > Mysteriously, there seem to be no Daves in your wanted
    > posters, despite their teeming numbers here on
    > rec.bicycles.tech:


    "Dave's not here!"

    Sheldon "C&C" Brown
    +--------------------------------------------------+
    | Some of my brother's paintings may be seen at: |
    | http://junila.com |
    +--------------------------------------------------+
    Harris Cyclery, West Newton, Massachusetts
    Phone 617-244-9772 FAX 617-244-1041
    http://harriscyclery.com
    Hard-to-find parts shipped Worldwide
    http://captainbike.com http://sheldonbrown.com
     
  20. ander-<< Smaller riders have shorter femurs and shorter lower legs = same
    femur/lower leg ratio as larger riders = same optimum seat angle - KOPS
    or not. >><BR><BR>

    Ya can argue this but if you believe KOPS is a place to start, not a hard
    constant(as most do, including the likes of DR Andy Pruitt), it is unlikely
    that a short person can get his KOPS with a 72 degree seattube angle even with
    a no set back seatpost.

    ander-<< Steeper seat angles on smaller frame sizes has to do with what Sheldon

    says. It's a precaution against toe-overlap and not based on fit and
    position. >><BR><BR>

    Sorry, don't see how not do I agree.
    << Take a look at:
    http://bikefitting.com/English/Theory/SeatAngle.aspx

    Hope you understand. English is not my first language. >><BR><BR>


    Theory is right....

    Peter Chisholm
    Vecchio's Bicicletteria
    1833 Pearl St.
    Boulder, CO, 80302
    (303)440-3535
    http://www.vecchios.com
    "Ruote convenzionali costruite eccezionalmente bene"
     
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