Seat Tube Angle -- Clear Up Some Confusion...- Mtbr.com
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  1. #1
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    Seat Tube Angle -- Clear Up Some Confusion...

    I was reading up on seat-tube angles, particulary steepish seat tube angles, and it made mention that steep seat-tube angles lead to cramped cock pit conditions (Pinkbike article). https://www.pinkbike.com/news/opinio...be-angles.html

    This cramped condition confuses me. Let me explain the way I see it. If the seat angle is steepened, and the ett and HA stays the same, everything just moves forward (headtube, fork). If the rider stands up on the pedals, there is more of the "bike" in front of him.

    I know I am a novice, but what am I not seeing that leads to this "cramped cockpit"?
    You cannot go against nature, because when you do, its part of nature too.

  2. #2
    pvd
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    Ignore anything you read about bicycles technology on Pinkbike. They are a marketing company.

    The position of the saddle relative to the wheels affects the balance of the bike in pitch. The position of the saddle relative to the bottom bracket affects the weight born by the arms when seated.

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    Quote Originally Posted by Ottoreni View Post
    If the seat angle is steepened, and the ett and HA stays the same, everything just moves forward (headtube, fork).
    The assumption is that the ETT does not stay the same, and the fixed value is the frame reach. ETT is pretty meaningless, especially on modern mountain bike designs that don't have straight seat tubes.

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    Yes, Now that I am going down this rabbit hole, I am beginning to see that all these bike media companies seem to be preaching more or less the same ice cream.
    You cannot go against nature, because when you do, its part of nature too.

  5. #5
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    Quote Originally Posted by dr.welby View Post
    The assumption is that the ETT does not stay the same, and the fixed value is the frame reach. ETT is pretty meaningless, especially on modern mountain bike designs that don't have straight seat tubes.
    I ride mostly flat terrain with steep drops. Some I jump, others that are too big to jump I roll down. My thinking is that with a steeper seat tube, the reach will increase, so when I stand up on the pedals I will have more of the bike in front of me when rolling down or rolling over some steep chunk. Seat Tube Angle -- Clear Up Some Confusion...-seat-tube-angle.jpg

    This sketch from this website illustrates what I am thinking. All values are the same, except STA.
    You cannot go against nature, because when you do, its part of nature too.

  6. #6
    pvd
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    You should not use that website for reference. The geometry calculation is not being done properly.
    Make your own drawing. Learn the difference between driving and driven parameters.

  7. #7
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    Quote Originally Posted by Ottoreni View Post
    My thinking is that with a steeper seat tube, the reach will increase
    Only if you keep the ETT the same. But you don't. It only describes an imaginary horizontal distance between two centerlines and tells you nothing about where the bars are going to end up unless you know several other numbers.

    If you want a bike with a long reach, you pick a bike with a long reach. That's the main number you need.

  8. #8
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    Quote Originally Posted by pvd View Post
    Ignore anything you read about bicycles technology on Pinkbike. They are a marketing company
    They're marketing-backed entertainment. I'm there for the pun threads.

    But yah even their veteran writers spout a lot of silliness.

    Quote Originally Posted by Ottoreni View Post
    I was reading up on seat-tube angles, particulary steepish seat tube angles, and it made mention that steep seat-tube angles lead to cramped cock pit conditions (Pinkbike article). https://www.pinkbike.com/news/opinio...be-angles.html

    This cramped condition confuses me. Let me explain the way I see it. If the seat angle is steepened, and the ett and HA stays the same, everything just moves forward (headtube, fork). If the rider stands up on the pedals, there is more of the "bike" in front of him.

    I know I am a novice, but what am I not seeing that leads to this "cramped cockpit"?
    Yes, that's how i look at it too.

    The dimension you want is front-center. Steepening the seat angle affords you more of it (while preserving cockpit length), as does a slacker head angle, or a longer reach. All of those means of lengthening front-center involve some sort of compromise, and the fun in frame design is managing the compromises so you hit nirvana.

    The other two measures are problematic. Reach is half a measurement floating in space, once you're designing the frame, and head angle is 1 of like 5 dimensions that impact steering behavior.



    Steep seat angles are suddenly of interest because they weren't previously exploited as a means to increase front-center. And because we've been using roadbike ~73* which isn't ideal for offroad seated climbing. Just as we've seen head angles coalesce around 64-67* over the last 10 years, seat angles will eventually land in a similar range. Steeper than what has been traditional, but probably not outside what we see in other cycling disciplines.

    ...actually, we may see mtb frames with >80* seat angles in the future; no other cycling sport pits our power output against the slope of the climb.
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

  9. #9
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    Quote Originally Posted by Ottoreni View Post
    I was reading up on seat-tube angles, particulary steepish seat tube angles, and it made mention that steep seat-tube angles lead to cramped cock pit conditions (Pinkbike article). https://www.pinkbike.com/news/opinio...be-angles.html

    This cramped condition confuses me.
    I'd say that if a steep (76-77) STA feels cramped then your bike is too small (for that STA)... and if you think a bike that would be long enough not to feel cramped is too long then I'd say you probably don't need a seat tube angle that steep. This is simply because if you're starting to feel cramped then you probably already have plenty of weight on the bars already and moving the seat forward isn't the change you need. This also has to do with how STA should be based on application. A hardtail and a 170mm enduro bike both with 77 seat tube angles will have a very different 'ride angle' once you're climbing a hill. The hardtail steepens under sag and the enduro bike slackens.

    My enduro bike has a 76 STA and feels very comfortable (not cramped at all). My hardtail isn't cramped but I wouldn't want my saddle any further forward. It has a 74.5 STA. The hardtail is a smaller bike and as I said before the STA steepens when you're actually on the bike. The hardtail STA feels plenty steep (steeper than the enduro bike) and I even run my saddle toward the middle of the seat clamp. I wouldn't want a 76 STA on that bike; not because 76 is objectively too steep and cramped but because 76 is too steep for that bike.

  10. #10
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    I just switched from a large Ibis HD4 to a large Transition Sentinel. My HD4 had a 74* STA, 455 mm reach, and a ETT of 628. I am 177 cm tall and I have always been right between a medium and large on just about everyone's sizing charts. The large Sentinel has a 76* STA, 475 mm reach, and a ETT of 622. The long reach on the Sentinel had me a little concerned at first but because of the steeper STA, the ETT is a touch shorter and what ultimately made me pick the large. Couldn't be happier with my choice and I am happier with the overall riding position of the Sentinel.

    I guess if you just steepened the STA on my old HD4 and didn't increase the reach to match, it would end up being a cramped cock pit. All the modern geo bikes that I have seen steeper STAs all have the reach adjusted to match.

  11. #11
    pvd
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    IMPORTANT:

    The usable seat angle is very different between a fully suspended bike used in the Pacific Northwest and New England (or wherever). If you do a lot of traversing, a very steep seat angle is going to murder you.

    Also, The usable seat angle of a well designed full suspended bike and a hardtail are very different. While I may be able to just get away with a 76.5 degree seat angle on a full boat enduro FS bike, I max out at about 74 degrees on a enduro hardtail.

    Thus, making overly generalized and untested comparisons that don't take into account your own arm/core strength, riding area, and riding style is going to result in a terrible bike, regardless of how the trendy kids are doing it on PinkBike.

    A race winning setup is not a comfortable or fun setup and will only work for the duration of an event.

  12. #12
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    Quote Originally Posted by pvd View Post
    IMPORTANT:

    The usable seat angle is very different between a fully suspended bike used in the Pacific Northwest and New England (or wherever). If you do a lot of traversing, a very steep seat angle is going to murder you.

    Also, The usable seat angle of a well designed full suspended bike and a hardtail are very different. While I may be able to just get away with a 76.5 degree seat angle on a full boat enduro FS bike, I max out at about 74 degrees on a enduro hardtail.

    Thus, making overly generalized and untested comparisons that don't take into account your own arm/core strength, riding area, and riding style is going to result in a terrible bike, regardless of how the trendy kids are doing it on PinkBike.

    A race winning setup is not a comfortable or fun setup and will only work for the duration of an event.
    As you say, this is an individual thing. I find 76* angle on a hardtail to be perfectly comfortable, and could go steeper if so inclined (get it?!). Just gotta figure out what works for you.

    In this case i think it's good that PB is pushing people to try it. Ultimately it will lead to better production frame geometry. Far better this than boost spacing and 15mm axles.
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

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