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Thread: Assuming stack and reach are OK, is there any downside to a steep seat tube angle-74

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    Default Assuming stack and reach are OK, is there any downside to a steep seat tube angle-74

    I'm hoping for some advice here. I'm looking at a bike with a short-ish top tube, but it has a 74.5 degree seat tube angle which pushes the reach back out. Other dimensions of the frame are OK. I can't test ride because it's on ebay. Is there any downside to the 74.5 seat tube angle?
     

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by txsimple View Post
    I'm hoping for some advice here. I'm looking at a bike with a short-ish top tube, but it has a 74.5 degree seat tube angle which pushes the reach back out. Other dimensions of the frame are OK. I can't test ride because it's on ebay. Is there any downside to the 74.5 seat tube angle?
    Can you get the setback you need with a 74.5 sta, your favorite saddle, and a reasonably-available seatpost? If not, that's a hard stop unless you're planning to adjust your position.
    William M. deRosset
    Fort Collins, CO

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by txsimple View Post
    Is there any downside to the 74.5 seat tube angle?
    What angle normally puts the seatpost head centered on the saddle rails?
    Generally, a change of 1 degree in seat angle moves the position horizontally by about 1 cm.
    If that allows you to still repostion the saddle where you need it, there is no downside.
    The other option is to compensate for the delta in the angle with a different offset seatpost.

    Personally, I need about a 75 degree seat angle and a 25mm set back post to get where i need
    the saddle to be, but i also can also get to the same spot with a 74 degree angle combined with
    a 15mm set back post, or a 73 seat angle and a zero setback post.

    This chart shows the amount of set back for each frame size and various seat tube angles:

    -g

    seatpost setback chart.JPG
    Last edited by GrantM; 11-07-2014 at 04:55 PM.
    EPOst hoc ergo propter hoc

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    What Grant said.

    An example from my world. Using roundish numbers for easy math, I have one bike with a 74.3 STA and a 25mm setback "post" and another with a 73.3 STA and a 14mm post. They both fit perfectly and allow me to achieve the identical saddle setback (nose of saddle to center of bottom bracket) and saddle height that I need. The STA just dictates how much setback I need in my seatpost to get the saddle where I need it. That saddle position number I don't change, however. I suggest thinking of the STA as just one factor in putting your butt where you need it. And as the others said, if you can get your saddle in the right position with a reasonably available post, the 74.5 STA will be fine.
     

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    well, with the way you phrase the question, no. i.e. if that sta yields the right stack and reach, then no problem. to rephrase what the guys above said correctly already, here's the gist. you'll find a steeper sta on smallish bikes because those folks have short legs and have to be more forward to get the seat setback they like. likewise, taller people and those who like to have the seat way behind the bottom bracket will never get on with a steep seat tube because no seatpost and saddle combo will let them get far enough back. so no one can possibly tell you the answer of whether it will work for you without knowing your current set up. if you are smallish (ride a frame in the 50-53 size range) and have a setback under something like 5cm (saddle nose to bottom bracket horizontal difference using a plumb bob), then you might be ok. Be aware also that a short top tube will be even shorter with the steeper seat tube. Are you seeking a short top tube? what size do you ride?
     

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by giordana93 View Post
    you'll find a steeper sta on smallish bikes because those folks have short legs and have to be more forward to get the seat setback they like
    The real reason is that designers of production bikes need to make the size run look well differentiated on the geometry chart.

    Road bikes don't vary in reach more than a few cm, but the public is fixated on fit bullshit. A steep STA lets you sell someone a bike that will satisfy their tape measure without derping the whole front end.


    Quote Originally Posted by giordana93 View Post
    Be aware also that a short top tube will be even shorter with the steeper seat tube.
    It'll measure that way in isolation, but the bike will actually be longer!

    The OP already understands this in his question
    Fred Blasdel

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by GrantM View Post
    Personally, I need about a 75 degree seat angle and a 25mm set back post to get where i need
    the saddle to be, but i also can also get to the same spot with a 74 degree angle combined with
    a 15mm set back post, or a 73 seat angle and a zero setback post.
    Keep in mind that seatpost setback numbers are not at all comparable between products! A "16mm" bent Thomson has less actual offset than a "0mm" Ritchey 1-bolt.

    For some stupid reason the clamp offset is measured center-to-center from the post, even though clamps vary widely in width.

    It should really be measured to the leading edge of the upper clamp.
    Fred Blasdel

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by blasdelf View Post
    The real reason is that designers of production bikes need to make the size run look well differentiated on the geometry chart.
    I highly doubt that Ernesto Colnago and Ugo DeRosa vary both the head and seat angles for marketing,
    since they have been doing this for fifty years, long before a consumer ever saw a geometry chart.

    For the bell curve of sizes, in order to keep the saddle centered on the rails over the range of frame sizes,
    you can't get there by keeping the seat angle constant from 48 to 68cm frames. You have to steepen and
    slacken it to keep the seat in the right range for traditional "road" positions as we recognize them... as in a
    "normal" road fit. Yes, I agree, the effect this has on changing the top tube length is largely misunderstood,
    but the fact remains, varying the seat angle across sizes is necessary for best results.

    -g
    EPOst hoc ergo propter hoc

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by blasdelf View Post
    Keep in mind that seatpost setback numbers are not at all comparable between products!
    Measure twice, cut once

    -Norm Abram
    EPOst hoc ergo propter hoc

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Thanks all! There's a couple of other questions I have asked seller about bike and will then make a decision.
     

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by blasdelf View Post
    Road bikes don't vary in reach more than a few cm, but the public is fixated on fit bull**t.
    As we've discussed before, the "reach" (distance from bb centerline along the top tube to the center of the head tube)
    is just one of multitude of factors that affects how far a rider has to reach their hands in the riding position:

    1) Saddle design: rail length, and where the center "sit" spot is
    2) Seatpost setback: the distance the head offsets from the shaft
    3) Seat Angle: Determines how much the centerline of the post is set behind the bb
    4) Top tube "reach": the forward distance of the frame ahead of the bb
    5) Stem length: the forward extension along the horizontal axis
    6) Handlebar forward reach: the amount the bend of the bar sweeps ahead of the stem
    7) Lever length: the length of the body of the shifter/lever
    8) Headset spacers: Adding height to the stem above the top tube moves the bars back towards the rider.

    This last point is often misunderstood, but remember that "stack" is measured along the top tube, and the
    head tube is not 90 degrees. So any spacers that raise the bars also reduce the reach. So for example,
    if you compare two frames with the same amount of "reach" but one has a 3 cm shorter head tube,
    you can't replicate the position by adding 3cm of spacers under the stem, and this will reduce the reach
    by almost 1cm.

    -g
    EPOst hoc ergo propter hoc

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by blasdelf View Post
    The real reason is that designers of production bikes need to make the size run look well differentiated on the geometry chart.

    Road bikes don't vary in reach more than a few cm, but the public is fixated on fit bullshit. A steep STA lets you sell someone a bike that will satisfy their tape measure without derping the whole front end.




    It'll measure that way in isolation, but the bike will actually be longer!

    The OP already understands this in his question
    Ahem. It's not bull shit and b) nothing is measured in isolation. Grant and
    I both are in the small end of the frame size spectrum. I can assure you that the difference between 73 and 75 is not some fickle designer whim. I cant ride a set back post on a 73 sta without cramming the saddle all the way forward, and so far I refuse to ride a zero set back post even though it is indicated for my fit on a 73 sta. period full stop no bs. Whereas 75 is too steep for general riding and with 90% of saddles out there. Not enough set back. And sure, maybe
    I could run a 9cm or a 14cm stem to account for the supposedly negligible differences in reach, I dont because it is stupid; and it was not at all clear that op understood this so a few of us agreed to chime in on this fairly neophyte and much discussed issue, because, yeah, for some of us, we are "fixated" on not buying bikes with fucked up fit. And if you think a bike with a steep seat tube will "actually be longer" (than what? a slacker bike with same top tube length?), then, well, I just dont know. Maybe someone has had the cervelo koolaid?
     

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by giordana93 View Post
    I both are in the small end of the frame size spectrum. I can assure you that the difference between 73 and 75 is not some fickle designer whim. I cant ride a set back post on a 73 sta without cramming the saddle all the way forward, and so far I refuse to ride a zero set back post even though it is indicated for my fit on a 73 sta. period full stop no bs. Whereas 75 is too steep for general riding and with 90% of saddles out there. Not enough set back.


    at a short person's saddle height the difference at the clamp between 73 and 75 is 2cm

    even the short rails on leather saddles have 9-10cm of adjustment room


    Quote Originally Posted by giordana93 View Post
    I could run a 9cm or a 14cm stem to account for the supposedly negligible differences in reach
    You're still overestimating the difference, on a size run with TT Lengths that vary by 8-10cm, the difference in reach is usually around 3cm (2cm if the headtubes are tall!)

    Larger road bikes get reachier almost entirely by using longer stems and more drop


    Quote Originally Posted by giordana93 View Post
    And if you think a bike with a steep seat tube will "actually be longer" (than what? a slacker bike with same top tube length?)
    Yes, it absolutely is.

    And it hits short people the most, since they're the ones walking into shops saying that they "need a 50cm top tube for their fit", which is meaningless in today's market where people are perfectly willing to sell you a bike with a 76 STA to satisfy that 'need'
     

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by blasdelf View Post

    And it hits short people the most, since they're the ones walking into shops saying that they "need a 50cm top tube for their fit", which is meaningless in today's market where people are perfectly willing to sell you a bike with a 76 STA to satisfy that 'need'
    Would not a steeper SA push out the front center given the same top tube, I imagine that changes design that would have to be addressed somehow for desired handling and toe clearance on a small frame?
     

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by Loki View Post
    Would not a steeper SA push out the front center given the same top tube, I imagine that changes design that would have to be addressed somehow for desired handling and toe clearance on a small frame?
    It does. It would atmo.

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by Loki View Post
    Would not a steeper SA push out the front center given the same top tube, I imagine that changes design that would have to be addressed somehow for desired handling and toe clearance on a small frame?
    I think the point Fred makes in these discussions is a valid one, we should stop thinking about
    top tube length as one number.

    Think of it this way: the top tube doesn't "push" it's the result that happens to connect the seat tube
    and the head tube. For the purposes of design, think of the top tube as an elastic. Change the seat angle
    so the saddle is positioned correctly, preferably centering the rails with an appropriate seatpost.
    Position the head tube so that the front centre puts the front wheel in the right spot, and works with
    a proportional stem length appropriate for the size of the bike. The top tube merely connects the dots.
    It's total length is not important, what is essential is that the two segments (setback and reach) are
    BOTH the right numbers. The total number may be the same on two entirely different geometry designs.

    -g
    EPOst hoc ergo propter hoc

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by GrantM View Post
    I think the point Fred makes in these discussions is a valid one, we should stop thinking about
    top tube length as one number.

    Think of it this way: the top tube doesn't "push" it's the result that happens to connect the seat tube
    and the head tube. For the purposes of design, think of the top tube as an elastic. Change the seat angle
    so the saddle is positioned correctly, preferably centering the rails with an appropriate seatpost.
    Position the head tube so that the front centre puts the front wheel in the right spot, and works with
    a proportional stem length appropriate for the size of the bike. The top tube merely connects the dots.
    It's total length is not important, what is essential is that the two segments (setback and reach) are
    BOTH the right numbers. The total number may be the same on two entirely different geometry designs.

    -g

    This ^ would all make more sense if bicycle designs were expressed as contact points above and between the wheels. But as long as one is quoting a spec sheet or the closest metric rule that's measuring a frame, we'll always be speaking two different languages here. And add a third one if you consider that some of us work in virtual center lines that exist in 2D only atmo.

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    Quote Originally Posted by giordana93 View Post
    Ahem. It's not bull shit and b) nothing is measured in isolation. Grant and
    I both are in the small end of the frame size spectrum. I can assure you that the difference between 73 and 75 is not some fickle designer whim. I cant ride a set back post on a 73 sta without cramming the saddle all the way forward, and so far I refuse to ride a zero set back post even though it is indicated for my fit on a 73 sta. period full stop no bs. Whereas 75 is too steep for general riding and with 90% of saddles out there. Not enough set back.
    Well, we don't scale crankarm length beyond what usually is a 5mm range. So if you're going for anything vaguely KOPS, smaller feet will pull the rest of the body forwards.

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    Default Re: Assuming stack and reach are OK, is there any downside to a steep seat tube angle

    I wish Grant Petersen's Z-factor had caught on and became a standard (i.e. oft published) dimension. It would certainly have helped un-muddle the issues of setback / top tube length / reach / STA.

    Z-Factor (or Reach) = (Virtual Horizontal C-C Top Tube Length) - [(C-C Virtual Seat Tube Length) * cos (STA)]

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