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Teach (___) How to Choose the Right Size Stem

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Teach (___) How to Choose the Right Size Stem

Old 03-16-21, 04:32 PM
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Moisture
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Teach (___) How to Choose the Right Size Stem

I know this is a very difficult topic with many differing opinions. Let's keep this concise and try not to coincide with each other opinions too much.

The top tube length of most bikes is usually designed around the use of a 100mm stem in mind. This means that anything more or less than whatever length stem the bike was designed around would be for a differently proportioned rider or use case than what the manufacturer initially intended.

like aniki said, saddle position is always going to be a starting point for frame fit. There is no "right" or "wrong" saddle position. Lets say you found a position that more of less works from the perspective of comfort and power transfer. Other than crank arm length which is a whole other topic of discussion, the next thing we consider is the stem length, right?

i learned that you should have roughly a 90 degree bend in your elbows when riding your bike. As such, wouldn't this be a general guideline for choosing an ideal stem length? Example. I have never seen, myself included, anyone have a comfortable 90 degree bend at the elbow whilst using a 100mm stem. Unless you already have achieved this right angle in your elbow, wouldn't you want to use whatever length stem it takes to achieve this?

Whether i use a 100mm, or 40mm length stem, I will be equally low on the bike, except I will be a hell of a lot closer to comfortably maintaining a right angle at the elbows with the shorter stem. Is this a good starting point?
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Old 03-17-21, 05:55 AM
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Originally Posted by Moisture View Post
like aniki said, saddle position is always going to be a starting point for frame fit.
I didn't say that; stop misquoting me. I said that fit ALWAYS starts with the feet; nothing to do with saddle position. FOOT ON PEDAL is first!
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Old 03-17-21, 07:02 AM
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Originally Posted by Moisture View Post
The top tube length of most bikes is usually designed around the use of a 100mm stem in mind.
You lost me right there.
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Old 03-17-21, 07:19 AM
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Originally Posted by Moisture View Post
I have never seen, myself included, anyone have a comfortable 90 degree bend at the elbow whilst using a 100mm stem.
I think the angle will change for most folks- hoods, flats, drops. You may be unique, however....
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Old 03-17-21, 09:19 AM
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Old 03-17-21, 10:16 AM
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Originally Posted by Moisture View Post
i learned that you should have roughly a 90 degree bend in your elbows when riding your bike. As such, wouldn't this be a general guideline for choosing an ideal stem length? Example. I have never seen, myself included, anyone have a comfortable 90 degree bend at the elbow whilst using a 100mm stem. Unless you already have achieved this right angle in your elbow, wouldn't you want to use whatever length stem it takes to achieve this?
None of this makes any sense at all. Elbow bend, as long as there's some, is quite uninteresting. And that said, while you are completely wrong about all this, the one fact which stands out is that you haven't looked at cyclists to see what elbow bends are used. Try this:


There you have elbow bends of 90°, also greater than and less than. What's immediately obvious in any case, is that stem length has nothing to do with elbow bend, which has every thing to do with hip angle. Yeah, the more you bend over at the hips, the more you have to bend your elbows in order to keep your hands on the bars. Who knew?

Actually, I think you made an error in your OP and didn't mean to talk about elbow bend at all. I'll give you that, OK?
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Old 03-17-21, 04:04 PM
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IIRC, there is some research that seems to say the shoulder angle should be 90 degrees, but I don't buy it. I think I'd need a much longer stem to reach that angle. More important, it doesn't seem to take into account the variation in bone length. JMO, of course.
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Old 03-17-21, 05:25 PM
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Old 03-22-21, 07:11 PM
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First get the proper height and get balanced over the saddle. Then choose a stem that gives you an 80-90 degree angle between your humerus and back, with hands on the hoods. Then readjust your saddle for balance.
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Old 03-23-21, 03:52 AM
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Moisture : I applaud your passion for bike fit and your dilligent pursuit of info. However, it seems that you often clutter your own path to progress with flawed assumptions.

In other words, the goal is good (i.e. how do I determine the best stem size for "me"), but you then put the goal out of reach by building your formula upon an unstable foundation. I have no idea why you believe that all bikes are designed around a ~100mm top tube, or that there is no "wrong saddle position". You are sabotashing your own path to success with certainty.

It is very difficult to be both curious and certain when searching for answers.
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Old 03-23-21, 08:40 AM
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Originally Posted by oldbobcat View Post
First get the proper height and get balanced over the saddle. Then choose a stem that gives you an 80-90 degree angle between your humerus and back, with hands on the hoods. Then readjust your saddle for balance.
I find that the 90 degree angle is very accurate, and something I strive to subconsciously achieve with my bike fit. Thanks for sharing.

It doesn't necessarily promote me to get very aero though, at least for me. I'm not sure why, because my legs are not significantly longer in proportion to my torso when compared to the average.

Originally Posted by Cycletography View Post
Moisture : I applaud your passion for bike fit and your dilligent pursuit of info. However, it seems that you often clutter your own path to progress with flawed assumptions.

In other words, the goal is good (i.e. how do I determine the best stem size for "me"), but you then put the goal out of reach by building your formula upon an unstable foundation. I have no idea why you believe that all bikes are designed around a ~100mm top tube, or that there is no "wrong saddle position". You are sabotashing your own path to success with certainty.

It is very difficult to be both curious and certain when searching for answers.
Sorry, someone told me this about the 100mm stems. I realized this doesn't make any sense. Each bike is designed around it own specific stem length according to the manufacturers intended application as well as the proportions of the riders they envision will be using their creation
.

Last edited by Moisture; 03-23-21 at 08:44 AM.
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Old 03-23-21, 08:46 AM
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Cycletography , as I continue to learn, you may have noticed that I quickly shifted my focus away from what simply works for me.. its not a useful way to learn by generalizing one type of rider proportion.
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Old 03-23-21, 04:11 PM
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I don't get the 90 degree shoulder-torso angle recommendation, given the wide variation in bone sizes, conditioning, flexibility, and desires from person to person.

The position of the hands on the 'bars comes from the combination of, at least, top tube length, torso size, humerus size, forearm size, shoulder width, riding style (bent back vs. bending at the pelvis), degree of elbow bend, etc., etc., etc. Add to that the way a rider wants to ride (tops, ramps, hoods, drops, upright vs aero) and the handlebars themselves (width, reach, and drop). Then there's the question of stem height, especially for those of us with quill stems which can vary from essentially slammed to Technomic 9" stems. Then, those of us who use biking to get into shape (whatever shape one wants to get into) are likely to get more flexible as we cycle more, and that can affect stem length.

I know there's at least one study that shows maximum power output when a rider's shoulders form a 90 degree angle with his body, but IIRC (can't find it now) the sample size was minuscule and averaged. Meanwhile, one of the 1st photos a web search turned up was of a woman in cycling kit, 90 degree angle at shoulders ... and straight elbows, a sure recipe for pain on roads around here.

I'd say choosing a stem that obscures the front hub when on the drops (which was the recommendation when I set my bike up 39 years ago) can work - except that I understand the current standard reco is obscuring the hub when on the hoods, and that probably has some basis in evidence.

My bet is that there's no formula for stem size that doesn't cause pain for a LOT of people.

*****

My bet is that stem lengths are chosen by manufacturers based on looks, based on the proposition that smaller stems look better on smaller bikes and bigger stems look better on bigger bikes.
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Old 03-23-21, 05:12 PM
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There's nothing empirical about that 90-degree angle. It just seems to be a decent compromise for controlling the bike, opening the chest and diaphragm, and getting power from the glutes and back muscles. And let me qualify this. It's for performance riding, road or MTB. For tootling on the bike path on a cruiser or comfort bike, that angle will be closed up as the rider sits more upright and power is less of a concern.

As for getting aerodynamic, that's all in your back angle and it's your choice to go as low or high as you want. It's a matter of simply changing the handlebar height and modifying the stem length slightly, to maintain that angle.

And stock stem lengths aren't selected on looks. They're based on assumptions on the average proportions of the average rider with average posture for a given frame size. Larger frames come with longer stems because taller riders tend to need more reach relative to the cockpit length. They also tend to use more saddle setback and slacker seat tube angles to balance the weight of their larger torsos.
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Old 03-23-21, 05:25 PM
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Originally Posted by philbob57 View Post
I don't get the 90 degree shoulder-torso angle recommendation, given the wide variation in bone sizes, conditioning, flexibility, and desires from person to person.

The position of the hands on the 'bars comes from the combination of, at least, top tube length, torso size, humerus size, forearm size, shoulder width, riding style (bent back vs. bending at the pelvis), degree of elbow bend, etc., etc., etc. Add to that the way a rider wants to ride (tops, ramps, hoods, drops, upright vs aero) and the handlebars themselves (width, reach, and drop). Then there's the question of stem height, especially for those of us with quill stems which can vary from essentially slammed to Technomic 9" stems. Then, those of us who use biking to get into shape (whatever shape one wants to get into) are likely to get more flexible as we cycle more, and that can affect stem length.

I know there's at least one study that shows maximum power output when a rider's shoulders form a 90 degree angle with his body, but IIRC (can't find it now) the sample size was minuscule and averaged. Meanwhile, one of the 1st photos a web search turned up was of a woman in cycling kit, 90 degree angle at shoulders ... and straight elbows, a sure recipe for pain on roads around here.

I'd say choosing a stem that obscures the front hub when on the drops (which was the recommendation when I set my bike up 39 years ago) can work - except that I understand the current standard reco is obscuring the hub when on the hoods, and that probably has some basis in evidence.

My bet is that there's no formula for stem size that doesn't cause pain for a LOT of people.

*****

My bet is that stem lengths are chosen by manufacturers based on looks, based on the proposition that smaller stems look better on smaller bikes and bigger stems look better on bigger bikes.
I think that 90 degrees is a good starting point for balancing performance with optimal muscle activation and comfort.
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