Seat angle versus set offset- Mtbr.com
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  1. #1
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    Seat angle versus set offset

    Moving the seat forward or backwards has the same effect as steepening or slackening your seat angle right? Are they really the same net effect?

  2. #2
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    Same exact effect. With a dropper it end up in slightly different location when dropped. Seat tube angle only matters if you run out of adjustable range. It's not an important measurement like HTA or BB height..... Reach is the most important measurement in fitting a bike.

  3. #3
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    Reach is the measurement from bb to stem. The SA will dictate how far back from the BB. The actual seated reach should take into account the SA. Reach alone is a good metric for how much room while out of the saddle. Isnt this right?

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  4. #4
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    Yes. Seat location is determined by your leg position over the BB. STA is one of the only measurements that effectively changes when you slide your seat forward or back. Reach is for out of the saddle and determines the length of stem you run. Short reach = long stem. The reason long frames are the rage right now is for short stems.

  5. #5
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    Thx. My arms a little shorter than avg plus the inflexibility makes me prefer a more uprights seated position but at times i feel like im more over the front. Currently running a 40mm stem. Trying to see if there is a really precise way of comparing bikes via geometry to predict fit. There's a window and its good to understand the limits. Many ppl are in between sizes and always seems like everything is a compromise. Kind of frustrating. I seem to think downsizing is better because it might be easier for me to control the bike. No one can agree on the optimum position for pedaling which kops tried to address. I know demoing is the way to go but there are so many brands which isnt accessible via a lbs.

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  6. #6
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    Seat position and seat tube angle are related, in that seat tube angle along with seatpost setback will determine a narrow range of seat positions (at a given seat height) that can be achieved. Seat tube angle can, however, become a factor in minimum chainstay length, and if you're playing in the narrow-margin areas, you may need a higher setback seatpost to achieve a desired seat position.

    It's all related.

  7. #7
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    Quote Originally Posted by alexbn921 View Post
    Yes. Seat location is determined by your leg position over the BB. STA is one of the only measurements that effectively changes when you slide your seat forward or back. Reach is for out of the saddle and determines the length of stem you run. Short reach = long stem. The reason long frames are the rage right now is for short stems.
    Last sentences are wrong. Long front is not just to be able to run short stems. Long front shifts weigh back making the bike more stable at speed and reducing OTB issues.

    Short stem/wide bars is for control and again, keeping weight back to reduce OTB and maintain more stability at speed and descending.

    As for on topic:

    It's all related. People say their "between" sizes when actually their not. Seat height, forward/rearward position, stem length, bar width etc can all be adjusted to make a bike handle how the rider wants it to.

    If "between sizing" was a problem that means that there would be no adjusting fit of a bike. It is physically impossible to make one bike fit everyone. That's why so many adjustments and a proper bike fit is done.

    Same goes for "optimal pedalling position". No such thing exists because every human is different. Each rider technically needs to be fitted like the pros are to get things "optimal". But unless your a pro race, it really doesn't matter.

    Worry less, fit the bike to be comfortable and handle as you want and ride. KOPS etc is industries way of trying to give the answers but truly only a starting point, nothing more. As are many other "universal" fitting methods. Only you and a physical therapist that specializes in cycling can determine how each component should be positioned.

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  8. #8
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    I have an older blur with a 460mm reach and real 73 STA. I don't want to run longer that a 90mm stem so it forces me to move my seat back more than I like.
    My new Tallboy has a 505mm reach with a 50mm stem and a fake 73 STA. Real STA is closer to 70. I still have enough rail to slide the seat forward to the spot I like. The wheelbase is much longer but my position on the bike is very similar relative to the BB.


  9. #9
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    What about fitting a rider based on their relative balance position between the WB depending on discipline. XC means closer to the BB(65% back ), trail (70% back), DH (75% back) with the latter closer to the rear. If the front center is too long, wouldn't that put you in a washout position?

    There is no standard it seems and no one agrees on anything. There should be a formula based on your own measurements. Ive done a fit on those fancy bikes at trek where the bike changes shapes but all the fitter did was ask how it felt. Kinda like an optometrist asking which is more clear. There has to be a more precise way of doing. Maybe start off with a discipline to get a ratio of fore and aft and work from there. I dont know. Im sure I'm beating the same horse that everyone has beat on in the last century. Poor horsey

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  10. #10
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    That's because how YOU fit on a bike is the basis for your comfort, how the bike handles is related to how the bike is designed around that fit. Going backwards will only result in compromises.

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