steep STA's, pedaling dynamics and knee health?- Mtbr.com
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  1. #1
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    steep STA's, pedaling dynamics and knee health?

    TLDR: With so many bikes going to a super-steep seat tube angle (75-77), aren't people going to start wrecking their knees?

    I've been professionally fitted on a bike a couple times and I've had some knee/hip pain issues at other times when I tried to move my saddle too far forward (on my road bike) to a more "aggressive" and "powerful" forward pedaling position. KOPS = Owch.

    When fitted, my knee is about 3cm behind the pedal spindle when the cranks are at 90. That's on the road bike, on the MTB I used to move it back about 5cm and lower the saddle a bit. But now that we have dropper posts, I can set my saddle up to exactly the position on my road bike and then drop it as needed for cornering/descending. Cool.

    However, now it seems new bikes are coming with steeper and steeper STAs. I have short legs and a long torso, so I need a slacker STA for good fit. My current bike has a 73 STA, and this is borderline too far forward, I need a setback seat clamp to fit it to me right with my preferred saddle. Many new bikes with "Modern Geometry" are going as far forward as 77! No setback post (remaining) in the world is going to compensate for that.

    So am I missing something here. I get that the steeper STA is meant to put more weight on the front wheel, but can that really be a good trade off if it shoves ones knees so far forward over the pedals and risks injury?

    This is not a frame building question per-se, but I figured the folks here would have the most understanding of this issue and the best advice.
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  2. #2
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    The key piece of information here is that mountain bikes are typically angled back when you're pedaling under load. Example:

    If you have a 76 degree STA and you're pedaling up a 7% grade, the bike will be tilted, on average, 4 degrees backwards from flat. That makes your effective pedaling STA 72 degrees.

    Some people ride up hills on their road bikes. Some do not. But generally, most people are riding up hills on their mountain bikes, so they can ride down.

    STA is one of the more subjective geometry numbers on a mountain bike. I consider this decision to be very much based on where and how you ride. If you're riding long, steep grades for long periods like we are in Colorado or like the fire road climbs of the Pacific Northwest, the steep seat tube angle is a godsend. In the foothills where climbs tend to be made at a easier angle, and sometimes with more ups and down, the benefit can be diminished.

    They should do mountain bike fits with a couple 2x4s under the front of the fit cycle
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  3. #3
    pvd
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    Seat angles are restricted by arm strength and terrain. If you live in a flatter area and have low arm strength, go with slacker. If you live in very steep ares (PNW) and have great arm strength, go with steeper.

  4. #4
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    Tell me about how arm strength factors in?


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  5. #5
    pvd
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    The more forward your saddle on the bike, the more your arms and Core have to support your body on near-flat sections of the ride. Thus, your arms and Core could fatigue out before the ride is over. This is an important consideration for the modern rider as dead arms mean no shred.

  6. #6
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    ^^^this is why I love slack seat angles...I dont have to work on my core.

  7. #7
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    Quote Originally Posted by FishMan473 View Post
    TLDR: With so many bikes going to a super-steep seat tube angle (75-77), aren't people going to start wrecking their knees?
    No.
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  8. #8
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    There really isn't a lot of useful information in your post OP. Not trying to be snarky...but from a fitment standpoint "I have short legs" doesn't really mean anything. One, a road bike calls for a relatively static position vs. riding off-road, especially trail riding. As someone who has prior experience in designing and building custom bikes, seat angle is not just based off your inseam in comparison to your torso length. In modern day terms, your torso/arm length would more dictate the "reach" of a bike. Seat angle (for road bike purposes) would start with (I say "start with" as other issues are taken into account where applicable) a ratio of upper (femur) to lower (Tib/fib) leg lengths. For example, someone with a 1.2:1 upper/lower ratio would have a slacker seat angle than a person with a 1.1:1 ratio. This is not generally a consideration in mountain bikes. You simply move around too much. Constantly sliding backwards and forwards, etc... Current seat angles are a function of performance design vs. strictly fitment. Also, KOPS is highly controversial in the fitting world. It was also pretty much thrown to the wind by most mountain bike designers at an early stage. Now, if you have existing knee issues that are exacerbated by your current riding position, that is something you will have to deal with. You may need professional help with that, you may be able to figure it out yourself...dunno. It may dictate your choices in frames and components to get a fit you're comfortable with.
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  9. #9
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    Joe, I'm not speaking for myself, just mountain bikers in general adapting this new steep seat angle paradigm. Your note about KOPS being "thrown to the wind" is exactly my point. It is my understanding that with KOPS many people were aiming to have their knees right over the pedal spindle and developed knee issues as a result. Further back is easier on the knees.

    And though mountain biking is generally more dynamic than road cycling, you still spend a lot of time pedaling in the saddle seated, at least I do. I'm pretty sure I spend more time sitting and pedaling on relatively flat terrain than I do in any other position. Even when I lived in Flagstaff I spent most of my time grinding it out on trails or roads that were probably less than a 3% grade most of the time, nothing that made me feel the need to scoot forward on the saddle.

    I may be misunderstanding this, but it seems like steep seat angles and modern geometry are about climbing and descending only, when most of us spend most of our time traversing.
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  10. #10
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    Quote Originally Posted by FishMan473 View Post
    ...I may be misunderstanding this, but it seems like steep seat angles and modern geometry are about climbing and descending only, when most of us spend most of our time traversing.
    I think the steep SA is definitely advantageous for FS bikes if your mainly doing ups and downs and not riding too many flats. I believe part of the idea is to counter rear sus squat when riding up steeps since more weight is transferred to the rear which slackens out the seat angle as the rear shock compresses and the fork extends more than when pedaling on flats. Depending on rear sus design, bikes may need need a steeper or slacker static SA to achieve a similar sa degree when climbing as some sus designs compress more when pedaling and some extend more.

    Another thing is that bikes with super slack head angles may try to get the rider more forward with a longer reach to help keep the front wheel weighted. If the reach is increased a lot and the seat angle is kept slackish, then the ETT gets really long making pedaling while seated very stretched out. Steepening the SA brings the ETT back to a more reasonable number making it so your not so stretched out when seated and pedaling on long reach bikes.

    Now, for hard-tails, steep seat angles do not make as much sense. First of all, the SA steepens by about a degree just by sitting on the bike. Second, there is no rear sus that compresses when climbing so no need to counter that.

    Also, I see it was mentioned that shorter riders would need a slacker sa. This seems to be the opposite of what is needed as there are bike companies slacken the SA by a by a half degree or so to account for the longer femurs of taller riders. I have never seen a bike that has a slacker sa for the smaller sizes than the larger sizes because of this. I have seen many bikes and have bought bikes with slacker sa for the larger sizes however.


    Here is some more insight from a review of the new privateer 161 that has a 80* seat angle...

    Is an 80 seat tube angle too steep?
    This is the big question and one that will divide opinion. Over the last few years, disruptive brands like Pole and Geometron have made seat tubes increasingly steeper and have arguably started dragging the entire industry with them. However, is 80 a step too far? A quick calculation shows that given the same seat height and reach, a bike with an 80 seat tube will position the seat around 5 cm closer to the bars than a bike with a 76 seat tube angle. Geometry trends often swing like a pendulum and while radicals push the extremes what is often best for most riders is a compromise in the middle. On steep climbs the extreme steep tube angle of the Privateer 161 is advantageous, allowing you to sit tall and relaxed, conserving energy. However, on flatter trails and mellow climbs, the extreme angle and moderate reach transfer a lot of weight on to your palms, which can become tiring. If your normal rides resemble a Toblerone in profile, with steep ups and downs, you will love the effortless winching of the Privateer 161. However, if your rides are more mellow we would say 80 is a bit too far and there are more versatile bikes. Funnily enough, weve heard a rumour that Privateer have a shorter travel 141 on the way, with a slightly slacker seat tube angle for increased versatility.

    https://enduro-mtb.com/en/privateer-161-2020-review-2/
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  11. #11
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    Quote Originally Posted by FishMan473 View Post
    I may be misunderstanding this, but it seems like steep seat angles and modern geometry are about climbing and descending only, when most of us spend most of our time traversing.
    I think right here is the key to why you don't get the steep seat angle thing.

    The steep seat angles just aren't as beneficial if you're riding flatter terrain. If I'm on my full suspension (with a 76 degree STA) I'm basically never riding flat terrain unless I'm riding a road to get to a trail, and even then it's less than %5 of my ride. I'm either on a steep climb or a steep descent, and that's where this geometry shines.

    It's hard to know what the majority of mountain bikers spend the majority of their time riding, but at least around here, it's pretty difficult to go on a mountain bike ride and not have at least one sustained steep climb. In other parts of the country? probably a different story. Which brings it all back to what I said in my earlier post about this being subjective to what kind of terrain you ride your bike in.
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  12. #12
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    up for debate: many bikes are designed with actual mountain terrain in mind but ridden on trails with relatively little elevation change. I would hesitate to buy most modern mountain bikes for this reason. it feel like buying a chainsaw when I need a pen knife.

  13. #13
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    Quote Originally Posted by mack_turtle View Post
    up for debate: many bikes are designed with actual mountain terrain in mind but ridden on trails with relatively little elevation change. I would hesitate to buy most modern mountain bikes for this reason. it feel like buying a chainsaw when I need a pen knife.
    It all depends on where you live. For the trails where I live modern geometry is a god send. There is nothing flat, its all up or down. If I drive an hour to ride with my brother on trails around where he lives in the outskirts of Sacramento I am miserable. His local spot is very flat with lots of very short climbs and descents. Its all about picking the right tool for the job.


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    Quote Originally Posted by Little_twin View Post
    It all depends on where you live. For the trails where I live modern geometry is a god send. There is nothing flat, its all up or down. If I drive an hour to ride with my brother on trails around where he lives in the outskirts of Sacramento I am miserable. His local spot is very flat with lots of very short climbs and descents. Its all about picking the right tool for the job.
    exactly.

  15. #15
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    I had some knee issues moving to a steeper STA until I realized my saddle height was way to high. Old bike was 71.5cm, new bike I had to lower to 70cm for some reason but it feels great. My knee issues were also over exertion but the lower saddle was near instant relief before I changed the way I ride.

    Did you give your body time to adapt to a forward saddle position? Your muscles are used to firing in time with your saddle further back. You need to let your body adjust to the new position and pay attention to a smooth pedal stroke.

    I don't understand how a forward saddle position could screw up your knees more than any other position. You're still exerting the same force on the pedals. If your timing pedal strokes wrong and pushing down closer to the 12 o'clock position (relative to your hips) it's like trying to push against an immovable object over and over again. I can definitely see that messing up your knees.

  16. #16
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    Quote Originally Posted by FishMan473 View Post
    Joe, I'm not speaking for myself, just mountain bikers in general adapting this new steep seat angle paradigm. Your note about KOPS being "thrown to the wind" is exactly my point. It is my understanding that with KOPS many people were aiming to have their knees right over the pedal spindle and developed knee issues as a result. Further back is easier on the knees.
    So, I see what you're getting at as far as where and the terrain you're riding. And yes, a slacker STA may be a better fit for you. However it would be better because you are not utilizing the steep STA for the intended design purpose, which is generally lot's of up/down trails with varied terrain such as chunk, flow, etc... I'll only add that to my knowledge, there are no definitive studies that show any correlation to knee issues as a result of STA (I mean, nobody would be a triathlete for very long if that were the case). Knee issues nearly always come back to one (or a combination) of a few things. 1) Bad fit e.g. saddle to high or too low. 2) Physiological issues such as overuse injury or preexisting injury or 3) a combination of injury and overuse. I'll give you an example of a bike we built for a customer. The man was older, I believe he may have been in his 70's (this was a while ago, so I don't exactly recall) and he had Polio as a child. As a result of that, he had significant loss of flexibility in his body. His hamstrings especially. He was virtually unable to have an angle more acute than 90 degrees between his torso and femur. Challenge accepted. We designed and built a frame for him with a nearly 90 degree seat tube angle. Of course, we had to radically design around this so the bike (this was a road bike BTW) so it handled/rode as normally as possible. The point being, none of the bike's design would harm his knees.
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  17. #17
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    Moving your feet forward on the pedal, or moving the cleat rearward, gets you a slacker STA technically. 1 degree for every 12mm or so. Adopt a mid-foot pedaling stance and that steep STA should be ergonomically the same as a slacker STA if you believe in KOPS.

    If the ball of your foot is 2" in front of the pedal spindle, but your butt is in the same place on the saddle, it's like your STA were over 4 degrees slacker, compared to if you had the ball of your foot over the pedal spindle.

    The benefit in general weight balance shouldn't be underestimated. When sitting on the saddle, over 70% of your weight is on the rear tire (and under 30% on the front; can measure with scales under each wheel). When hovering your butt over the saddle, it's immediately reduced to 60% or less. This should be readily apparent, when you roll over a curb with the rear wheel and judge by how much the rear compresses in each position. This also affects suspension feel--your rear shock sags more when sitting than when standing. Do you optimize it for standing and compromise for sitting, if you cared about susp tuning? With a steeper STA, this difference is lessened.

    Also, F kinked seat tubes. There are many new bike designs that are hard pass to me, since these designers seemingly wanted the seat post to be more parallel with the fork for aesthetic purposes. I blame this trend for bikes with misleading "effective" seat angles, including the Ibis Ripmo, which aren't as steep as they suggest at normal riding saddle height. People who compensate their fit based on the charts are left wondering why there's pressure on their palms. It's cause their heavy upper body is being propped up by their arms, as it's in a state of falling with it leaning forward too much for their core to hold up. Should let the upper body fall forward even more (so that the legs/feet can support the weight through the stretching of the back and leg muscles) or get more upright (so extension of the legs can support the weight). Might need to get a comfier saddle and shorter cranks for the latter position, as it don't work too well for spinners.
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  18. #18
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    Quote Originally Posted by Varaxis View Post
    Also, F kinked seat tubes. There are many new bike designs that are hard pass to me, since these designers seemingly wanted the seat post to be more parallel with the fork for aesthetic purposes.
    Huh? I've been designing with bent seat tubes for the past 7 years and have not once heard that as a reason for doing it. In fact, it has little to do with aesthetics. It's a pain in the a$$ but it's what is needed.

  19. #19
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    Quote Originally Posted by Varaxis View Post
    .

    Also, F kinked seat tubes. There are many new bike designs that are hard pass to me, since these designers seemingly wanted the seat post to be more parallel with the fork for aesthetic purposes.

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    Now that we have that out of the way, I thought bent seat tubes were a way of gaining tire clearance for shorter chainstays.



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  20. #20
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    Quote Originally Posted by Varaxis View Post
    The benefit in general weight balance shouldn't be underestimated. When sitting on the saddle, over 70% of your weight is on the rear tire (and under 30% on the front; can measure with scales under each wheel). When hovering your butt over the saddle, it's immediately reduced to 60% or less.
    (thread hijack) on that note, I often read about designing a bike to "balance the rider between the wheels." what exactly does that mean? is it measurable, like could you put a scale under each tire and find the balance point and have that mean something? or is it more of a subjective thing, the meaning of which varies considerably from one rider/ bike to the next?

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