Seat tube angles - why so much variation?- Mtbr.com
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  1. #1
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    Seat tube angles - why so much variation?

    I try to setup all my bikes (city, road, mtb, fat) so that my saddle always has the same amount of setback relative to the bottom bracket. To far forward or backwards and my knees will start to hurt. I just don't get why there is so much variation in STA in the MTB world. I don't get the super steep angle trend of enduro bikes that put the rider over the BB, which combined with a zero-offset dropper pushes all your weight forward when seated and pedaling and messes up your knee angle.

    Please educate me!

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    Have you ridden one of the newer geo long travel 29ers? It seemed off the me as well, just considering the seat position relative the bb/pedals. While you are definitely more on top of the cranks, and it takes some getting used to, it works in tandem with the long front center and slack HA. And, does it ever improve climbing, especially if you’re taller and used to hanging out over rear axle on a slack STA. Also, on longer travel bikes the STA slackens more as shock moves into its travel.

    I coincidentally moved cleats all the way rearward on shoes. I tried it based on others experience with improved feel, power and control from middle of foot. This helped with being more forward and allowed me to lower seat and COG.

    This geo configuration with long travel is not as comfortable or as fast I believe on long flat smooth sections of trail, but it benefits the downhill, climbing especially technical climbs, high speed chunk, lower speed tech, berms and turns.

  3. #3
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    ^^What he said^^ Flatlanders are being left out of the equation, steep seats make steep climbs easier. Also keeps TT lengths in check as reach grows. Seems to be geared towards the enduro bikes for now but could creep into downcountry.

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    Quote Originally Posted by ToiletSiphon View Post
    I don't get the super steep angle trend of enduro bikes that put the rider over the BB, which combined with a zero-offset dropper pushes all your weight forward when seated and pedaling

    Please educate me!
    Go find some steeper hills to ride up.

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    Quote Originally Posted by Ratt View Post
    ^^What he said^^ Flatlanders are being left out of the equation, steep seats make steep climbs easier. Also keeps TT lengths in check as reach grows. Seems to be geared towards the enduro bikes for now but could creep into downcountry.
    Makes sense, as long as all of your riding is going up one side of the mountain the going down on the other side.

    As someone with abnormaly long femurs compared to my height, I already struggled to get proper setback on many road bikes, even if I moved my cleats back a lot. Anything with more than 74.5 sta is automatically out of the equation. Although I admit it climbs better, flat riding will kill my knees if I'm too forward.

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  6. #6
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    Have you tried a set back seatpost?

    Seat tube angles - why so much variation?-fa21dcec-5bf7-4859-8daf-82c8a97ad0ac_1.aad5678b709ae29296f7de48ab233b8f.jpeg
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    I'm 6'5" with a 36" inseam and never going back to slack seat tubes. With the steeper seat tube you have to raise the saddle to get the same extension. The only issue I noticed is if you don't run your bars high enough you'll experience more weight on your hands on flat ground.

  8. #8
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    Why are there so many variations?

    Because one manufacturer thinks their design is better than another manufacturer.


    Why do some bikes have a longer wheelbase than others?
    Chainstay length?


    The list goes on.

    I have 2 bikes, no idea if the seat is set up the same relative to the two bikes. I do know the seat is set up for what feels right for me based on the difference in length of front-center. The two bikes pedal differently for sure -I can easily feel a difference in how my feet contact the pedal.
    At the end of the day, I don't really care because as a whole, the bike feels natural.

  9. #9
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    Quote Originally Posted by Gasp4Air View Post
    Have you tried a set back seatpost?

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    I actually had my LBS change the stock Dropper on my Stumpjumper alloy to a Command Post, which has 20mm of setback - but I use it with a shim since there wasn't a dropper with setback available in 34mm diameter

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  10. #10
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    Seat tube angles - why so much variation?

    Quote Originally Posted by ToiletSiphon View Post
    Makes sense, as long as all of your riding is going up one side of the mountain the going down on the other side.

    As someone with abnormaly long femurs compared to my height, I already struggled to get proper setback on many road bikes, even if I moved my cleats back a lot. Anything with more than 74.5 sta is automatically out of the equation. Although I admit it climbs better, flat riding will kill my knees if I'm too forward.

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    I get that - 35in inseam- 6’2”-long femurs. My new bike has 76 STA and I had same concern. But I’m really liking it. Did take some adjustment and felt like used different muscles. Started out with dropper with 25mm setback because also concerned about haw STA shortens cockpit. But went to straight one after moving cleats. Interestingly my handle bars are about 32mm closer on this bike vs last that had a 73 STA. I’m liking that too.

    I’m much more balanced on this bike. Riding position more comfortable. When I stand to pedal, I’m in roughly same position and therefore weight doesn’t dramatically shift forward as it did on last bike. Shoulders feel better. Climbs much, much better - especially short steep tech. Descends better. Better on tech and turns in flats. Only things less efficient is flat, straight and smooth.

  11. #11
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    Anyone ever consider that any real mtb trail, steep enough to require a super steep STA, will require you to be out the of saddle navigating tech anyway ???

    Just kidding, sort of.


    It's because pushing the limits of STAs is the latest and greatest thing, and at least to some degree driven by marketing. Mostly a bandaid for riders on "over-biked" rigs that sag too much into their travel as they sit and spin up their buffed out, manicured, man-made, hokey, bike paths.

    Oooops. There I go again.

    I do agree that less than 74 degrees on a FS mtb is too slack - even a shorter travel rig.

  12. #12
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    We've figured out that we can use seat tube angle as a handling tuning tool just like head angle. A designer can use a steep seat angle to get an adequately long front-center w/o having to slow down the steering or dork up the weight distribution. It also facilitates the transition from seated to standing, which is helpful on a mtb.

    I'm not sold on 77* for everyone, but i do think it's folly to write off a bike because the seat angle doesn't fit your preconceptions. I think there's room in the market for 71-79*, depending on application. Same as road bikes.

    I spent 5-6 years rehabbing a knee that couldn't tolerate a steep seat angle. Worth it.
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  13. #13
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    Quote Originally Posted by ToiletSiphon View Post
    I try to setup all my bikes (city, road, mtb, fat) so that my saddle always has the same amount of setback relative to the bottom bracket. To far forward or backwards and my knees will start to hurt. I just don't get why there is so much variation in STA in the MTB world. I don't get the super steep angle trend of enduro bikes that put the rider over the BB, which combined with a zero-offset dropper pushes all your weight forward when seated and pedaling and messes up your knee angle.

    Please educate me!

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    Quote Originally Posted by David R View Post
    Go find some steeper hills to ride up.
    With allowances for tongue-in-cheek, this is the answer.

    Bike geometry should vary with intended use. If the intent is steep climbs and descents, then steep STAs make a lot of sense. Especially when combined with a dynamic, out-of-the-saddle riding position.

  14. #14
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    As a single element, Seat Tube angle relative to the knee is actually meaningless.
    Why? Well, as you sit on your saddle and your foot is fixed to your pedal at the correct height/extention, nothing changes. You can change the seat tube to any angle and the fixed position of the leg does not change. The knee will not be effected by a change in its free movement. Crank length can alter knee angle within a given fixed length, but not seat tube angle as a design element.

    What does change is the relationship Pelvic tilt has to the body as it is altered to reach the handlbars which is the only other connection point on the bike. I use the steep seat tube angle to relieve stress on the lower spine, develop more effective power to the pedal and place the COG more into the middle of the 2 axles. There are other reasons to go Geo-forward, but these are my own. I design and build my own bikes and have been doing my bikes this way well before the so-called Geo-forward phrase was coined.

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  15. #15
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    Quote Originally Posted by Eric Malcolm View Post
    ...I use the steep seat tube angle to relieve stress on the lower spine, develop more effective power to the pedal and place the COG more into the middle of the 2 axles. There are other reasons to go Geo-forward, but these are my own. I design and build my own bikes and have been doing my bikes this way well before the so-called Geo-forward phrase was coined.

    Eric

    Interesting.

    When moving back and forth between my bikes with relatively slack STA's, like 72.5, and less slack, like 74.5, I can feel a definite difference with being able to lay down the power over longer, pedals distances.

    Sure, no doubt, on super steep, seated climbs, a very steep STA I'm sure would be a benefit, but it has its drawbacks. First off, its not often I'm climbing steep tech in the saddle. Too rocky, rough. The farther forward I am the more quads come into play and let off the posterior chain, including the hamstrings. When laying down the power on rolling terrain, over the long haul I've noticed a significant difference in the amount of power I'm able to generate - more with a slacker STA. The posterior chain in inherently much stronger in most people. Also, the slacker STA keeps weight off my hands, and lets me pedal a lot longer.

    This is not against steeper STAs. But, I don't agree with the blanket trend to the extent it is being taken.

  16. #16
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    I wonder why people weren't running layback posts backwards (forwards?) or slamming their saddles forward to get the same effect years ago? Do dropper post make the forward position more enticing?

  17. #17
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    Quote Originally Posted by Eric Malcolm View Post
    and place the COG more into the middle of the 2 axles. There are other reasons to go Geo-forward, but these are my own. I design and build my own bikes and have been doing my bikes this way well before the so-called Geo-forward phrase was coined.

    Eric
    This isn't much of a factor if you don't care about short chainstays. Not all of us do.

    What do you think about bar position driving seat tube angle?

  18. #18
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    Quote Originally Posted by knutso View Post
    I wonder why people weren't running layback posts backwards (forwards?) or slamming their saddles forward to get the same effect years ago? Do dropper post make the forward position more enticing?
    I like the squirrel. Cool pic.

  19. #19
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    I think bike companies have jumped the shark when it comes to trail bike geo. The bikes we pull out for 90% of the riding we do. The one bike you'd take on a road trip. I've been playing with low, long and slack geo since 2005. I've invested thousands of dollars into custom frames playing with numbers. I've been laughed at on the starting line with that goofy combo of steep HA and slack SA, and a long TT. IMO, a 74* SA and 66.5* HA is perfect. I'm not claiming to have invented anything. There are many others, but it was born out of necessity from the early days of Super D where you were hauling A down, and up. The trend of slacker frames is messing with wheelbase and the flickability of trailbikes.
    Ripping trails and tipping ales

  20. #20
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    Quote Originally Posted by knutso View Post
    I wonder why people weren't running layback posts backwards (forwards?) or slamming their saddles forward to get the same effect years ago? Do dropper post make the forward position more enticing?
    Shorter reach and longer stems.

  21. #21
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    Quote Originally Posted by jeremy3220 View Post
    Shorter reach and longer stems.
    If the hands are in the same position relative to the BB, the saddle probably should be in the same place, no?


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  22. #22
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    Quote Originally Posted by Le Duke View Post
    If the hands are in the same position relative to the BB, the saddle probably should be in the same place, no?


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    No, weight on the front wheel changes

  23. #23
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    Quote Originally Posted by ToiletSiphon View Post
    I try to setup all my bikes (city, road, mtb, fat) so that my saddle always has the same amount of setback relative to the bottom bracket. To far forward or backwards and my knees will start to hurt. I just don't get why there is so much variation in STA in the MTB world. I don't get the super steep angle trend of enduro bikes that put the rider over the BB, which combined with a zero-offset dropper pushes all your weight forward when seated and pedaling and messes up your knee angle.

    Please educate me!

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    Dang TS, we're on the same page. I missed your post and started my own.

    I don't get it, either. Steep STA might be helpful to reduce the lateral change in position when you use a dropper, maybe. Otherwise, messing with the saddle fore-aft relative to the bottom bracket will just optimize for climbing (only) or descending (only).


  24. #24
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    I don’t understand why one would want a road bike, a city bike, and a mountain bike to all have the same cockpit dimensions. You ride them in different settings, for different purposes, and in different ways. Why should they fit the same? (My fatbike is more or less the same as my MTB, with allowances for a slightly smaller fit for soft/challenging conditions.)

  25. #25
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    Quote Originally Posted by evasive View Post
    I don’t understand why one would want a road bike, a city bike, and a mountain bike to all have the same cockpit dimensions. You ride them in different settings, for different purposes, and in different ways. Why should they fit the same? (My fatbike is more or less the same as my MTB, with allowances for a slightly smaller fit for soft/challenging conditions.)
    Cockpit should not be the same, but the saddle fore-aft position relative to the bottom bracket should be more or less the same, reflective of your femur length. Max saddle height might be reduced 1-2cm on MTB. Obviously your MTB will have shorter stem, wider bars, higher bars, etc., than road bike.




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  26. #26
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    Quote Originally Posted by knutso View Post
    I wonder why people weren't running layback posts backwards (forwards?) or slamming their saddles forward to get the same effect years ago? Do dropper post make the forward position more enticing?
    Absolutely.

  27. #27
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    Quote Originally Posted by ToiletSiphon View Post
    As someone with abnormally long femurs compared to my height, I already struggled to get proper setback on many road bikes, even if I moved my cleats back a lot. Anything with more than 74.5 sta is automatically out of the equation. Although I admit it climbs better, flat riding will kill my knees if I'm too forward.
    Same experience here right down to the fully rearward cleats. Big time spinner too (you?). Kept trying to nudge the saddle forward (to help with steep climbs and put more weight on the front wheel) but my knees HATED it. So I spent 10 years mainly riding SS before buying a 429 Trail in 2016 (slack STA, long stays, short reach). It's been OK and I never considered swapping it out because I figured it was the best I could do (fit wise).

    Then in January, in a thread similar to this one, someone suggested an oversized flat pedal (Catalyst). It puts the arches over the pedal spindle (about 1" forward). Having been a geo nerd for a decade (looking for the fix!), these pedals intrigued me because it seemed they might attenuate my rearward bias (in an era of rider-forward geo). Bought the pedals, then lowered the saddle and moved it forward, then lowered my bar. REVELATION!!! I'm lower, more centered, more weight on the pedals, and I can get power down through my quads. I'm no longer fussing with micro adjustments of tires, suspension, bar height, saddle fore-aft, etc. (looking for the fix!). My saddle is now fully forward (straight post) which shrinks my cockpit, so I'm running a 800mm bar and 70mm stem, hunched over the front wheel and trying to maintain rear traction. Hardly ideal but still way better than before. And now that my riding position has caught up to these modern bikes, I need a new frame with longer reach, shorter stays, steeper STA.

    So for me, it took a change to these big flat pedals to make use of the new geo, partially because I have long feet, super high arches, spindly ankles, and long femurs - just a very unstable, inefficient lower body. I had been planning on going back to SS full-time but now haven't ridden SS in months and am about to buy a new FS frame.

    All that said, steep STA does dampen power output on flatter terrain for many, but few care because of the improved ascending and descending, plus the power you do generate is more efficiently transferred than with a slack STA, per at least one study (and IME). And with the big flat pedals, I don't get any knee pain with the steeper STA (as I did w/SPD) - super comfy.
    Last edited by Ryder1; 2 Weeks Ago at 03:46 PM.

  28. #28
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    Quote Originally Posted by BmoreKen View Post
    Cockpit should not be the same, but the saddle fore-aft position relative to the bottom bracket should be more or less the same, reflective of your femur length. Max saddle height might be reduced 1-2cm on MTB. Obviously your MTB will have shorter stem, wider bars, higher bars, etc., than road bike.




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    Sure, if you’re describing the radius. Most people have an ideal angle for efficiency in a seated position, but that doesn’t mean all bikes have to be designed or fitted around it. Road, time trial, and tri bikes are all intended for riding fast on pavement, but nonetheless they have different dimensions for accommodate different body positions. And at the far end of the spectrum are unicycles which have to prioritize balance. But people have ridden the Great Divide Route on them.

  29. #29
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    Can someone confirm what I thought I read once:

    "Slide back on the saddle to [use different muscles to] make power. Slide forward on the saddle to spin."

    If the saddle is out of whack to the bottom bracket, your feet are in a different place relative to your pelvis. Is that just a matter of re-training your muscles, or are some bikes too extreme?

    My fore/aft saddle adjustment is probably not critical - until it's past a certain point.

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  30. #30
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    Quote Originally Posted by Miker J View Post
    Interesting.

    When moving back and forth between my bikes with relatively slack STA's, like 72.5, and less slack, like 74.5, I can feel a definite difference with being able to lay down the power over longer, pedals distances.

    Sure, no doubt, on super steep, seated climbs, a very steep STA I'm sure would be a benefit, but it has its drawbacks. First off, its not often I'm climbing steep tech in the saddle. Too rocky, rough. The farther forward I am the more quads come into play and let off the posterior chain, including the hamstrings. When laying down the power on rolling terrain, over the long haul I've noticed a significant difference in the amount of power I'm able to generate - more with a slacker STA. The posterior chain in inherently much stronger in most people. Also, the slacker STA keeps weight off my hands, and lets me pedal a lot longer.

    This is not against steeper STAs. But, I don't agree with the blanket trend to the extent it is being taken.
    I'm with you on this one. For me it's about balancing the input of my quad's with my gluts/hamstrings. This is done with saddle position fore-aft. On terrain that isn't really steep I need a slack STA to do this.

  31. #31
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    Quote Originally Posted by BmoreKen View Post
    Cockpit should not be the same, but the saddle fore-aft position relative to the bottom bracket should be more or less the same, reflective of your femur length. Max saddle height might be reduced 1-2cm on MTB. Obviously your MTB will have shorter stem, wider bars, higher bars, etc., than road bike.

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