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A question for the more knowledgeable out there: why does seat tube angle matter? It seems that this is a number that gets tossed around with near religious zealous, with some people stating that more relaxed gives more power or more upright promotes better spin.

But with several centimeters of seat rail adjustability and another couple available between zero-setback and extreme setback (like some Easton's) posts, I would think the STA is nearly irrelevant -- just put your hips where they need go.

So please enlighten me, what am I missing?

Thanks!
 

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Ideally, your bike fitting should start with the position of your knees relative to the pedals or crankset. You should pedal most efficiently with a certain KOP (knee over pedal) position.

Assume that a bike with a 73 seat tube angle is standard. So with a frame with a steep STA (eg, 74), you generally have to move your saddle rearward to achieve the ideal KOP. That effectively lengthens the seat tube, and it's why bikes with steep STAs often have shorter top tubes.

Conversely, on a bike with a slack STA (eg, 72), you generally have to move your saddle forward to achieve the ideal KOP. This effectively shortens the top tube and explains why you can get away with a longer top tube length on frames with slack STAs.

Here's a real world example. I have a De Bernardi with a 56 cm top tube and 74 STA. It fits nearly the same as my Merckx with a 56.8 top tube and 72.5 STA, and my Bob Jackson with a 57 top tube and 72 STA. I use a 10 cm stems on all of these frames and my reach is nearly identical.
 

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Well, it matters because it is connected to the rest of the geometry. If two bikes have the same TT length and ST length but one has a steeper STA it will have a longer reach for a given saddle position (i.e., same real angle, height, distance behind the bottom bracket, etc. on both bikes).
 

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some people stating that more relaxed gives more power or more upright promotes better spin.


It depends on the person.
With a 74 sta, it would be pretty impossible for me to get seated correctly, even with an Easton post. A 73 sta, and an Easton post puts me in just the right spot. If I had a bike custom made with a 72 sta, I could use a "normal" set back post, and if I went with a 71 sta, I could use a straight post.
I have no trouble spinning with my position.
 

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Besides the obvious stuff everyone has already mentioned, certainly for shorter folks like myself, STA can help get you on a frame with enough clearance for standard 700c wheels without any funky cutouts or the like. Maybe minor for most folks.
 

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duh...
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cuz you don't want to have to get a custom seatpost... doesn't have much to do w/ power or spin, it's about getting your saddle in the proper position relative to the bb
 

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You are right in the broadest sense - you can get yourself positioned pretty well with most any near-normal angle. Unless you are on one tail of the bell curve, and the bike is on the other.

One thing that is often missed is that various sizes of people aren't directly proportional. Generally speaking, taller folks need more setback than smaller. So it makes more sense to slacken the larger sizes, so that the user can use the variability in rails and post setbacks to accomodate themselves rather than the bike.
 

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bigreen505 said:
A question for the more knowledgeable out there: why does seat tube angle matter? It seems that this is a number that gets tossed around with near religious zealous, with some people stating that more relaxed gives more power or more upright promotes better spin.

But with several centimeters of seat rail adjustability and another couple available between zero-setback and extreme setback (like some Easton's) posts, I would think the STA is nearly irrelevant -- just put your hips where they need go.

So please enlighten me, what am I missing?

Thanks!
It ALL depends on what theory on positioning you adhere to.

If your theory excludes certain seat angles or seatposts because they won't enable you to position yourself properly then yes, seat tube angle matters.

Why it matters to you is, if you know your seat tube angle and your position theory (KOPS, for instance) precludes you from obtaining that position, then you'll have to change it to achieve the position you want.

Studies HAVE shown that positioning your hips more over the bottom bracket foster/facilitate faster pedaling while positioning your hips farther behind the BB allows you to apply more power, albeit at a slower cadence.

I would like to point out that zero setback posts came out and really screwed up the theories. They kind of threw all the old seat tube angles out the window, but the manufacturers never changed their geometries to accommodate them.
 

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What about those that choose a bike for crits that is a size too small in order to get a smaller wheelbase for tight cornering.

This might require a seat post that is angled.
 

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Assume that a bike with a 73 seat tube angle is standard. So with a frame with a steep STA (eg, 74), you generally have to move your saddle rearward to achieve the ideal KOP. That effectively lengthens the seat tube, and it's why bikes with steep STAs often have shorter top tubes.

tarwheel2 ^^^^^^ You ment "effectively lengthens top tube". Right? ^^^^^^
 

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???

krisdrum said:
Besides the obvious stuff everyone has already mentioned, certainly for shorter folks like myself, STA can help get you on a frame with enough clearance for standard 700c wheels without any funky cutouts or the like. Maybe minor for most folks.
While the steeper STA does increase tire clearance, it has nothing to do with frame size or rider size. Large frames often have the same chainstay length as the small ones, these days. Most brands now use 73 degrees as the minimum angle on their road frames. That's steep enough as long at the chainstay length is at least 405mm.
 

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C-40 said:
While the steeper STA does increase tire clearance, it has nothing to do with frame size or rider size. Large frames often have the same chainstay length as the small ones, these days. Most brands now use 73 degrees as the minimum angle on their road frames. That's steep enough as long at the chainstay length is at least 405mm.
Ok, I guess I was speaking out of school. I was under the impression most framebuilders go with a steeper STA on smaller frames was to accommodate a 700c rim and tire. I guess the 75 degree STA recommended during my fitting is based on other factors. It seems to work pretty well for me, so I can't really complain. Guess I am just a bit confused now.

Think I figured it out. My bike has 403 chainstays. I guess that is one factor that was taken into account.
 

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Peter P. said:
Studies HAVE shown that positioning your hips more over the bottom bracket foster/facilitate faster pedaling while positioning your hips farther behind the BB allows you to apply more power, albeit at a slower cadence.
Got links to those studies? I run a lot of setback for a few reasons, to support lots of saddle/bar drop and because i have really long femurs (they're longer than my upper body) at 183cm and 92cm of that being inseam. I have no problem pedaling at 120rpm in this position. I suspect people sometimes talk themselves into certain situations like "these 175mm cranks reduce my cadence".
 

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Assume there is an ideal size and shape for each rider made by points at the top of the seat over the seatpost, the center of the handlebars, and the center of the bottom bracket. Change the the angle of the seat tube without changing dimensions of that triangle and you move the rider's weight forward or backward on the bike. I find that change in balance one of the few changes I actually notice.
 

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C-40 said:
While the steeper STA does increase tire clearance, it has nothing to do with frame size or rider size. Large frames often have the same chainstay length as the small ones, these days. Most brands now use 73 degrees as the minimum angle on their road frames. That's steep enough as long at the chainstay length is at least 405mm.
I know nothing other than I'm tiny enough to have 700cc issues if I'm not careful. I think getting the front tire on there without a ridiculous headtube angle or unsafe toe overlap is also a consideration that a steeper STA would help with. I always assumed it was this, rather than rear wheel clearance, that was the issue. Maybe I'm incorrect as well :confused:
 

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info...

krisdrum said:
Ok, I guess I was speaking out of school. I was under the impression most framebuilders go with a steeper STA on smaller frames was to accommodate a 700c rim and tire. I guess the 75 degree STA recommended during my fitting is based on other factors. It seems to work pretty well for me, so I can't really complain. Guess I am just a bit confused now.

Think I figured it out. My bike has 403 chainstays. I guess that is one factor that was taken into account.

It is common for the STA to increase on the smaller sizes. As leg length decreases, so does the femur length. Most riders with shorter legs need to be further forward to achieve KOP, which is a common measure of a "proper" fit. Personally, I like the saddle further back, so I end up using 32mm setback post with most saddles, since my 51cm frame has a 74.5 degree STA.

The current LOOK 585, as an example, has the same 405mm chainstay length in all sizes, but the STA varies from 73 to 74.5.

Older LOOK models like the KG481 used a 72.5 degree STA in all sizes with the same 405mm chainstay length. There was very little tire clearance on these frames.
 

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They matter for a number of reasons...already mentioned.

However, many are stating that STA should be more relaxed than they currently are. Well, that's great for those with long legs or long femurs...however those of us with short legs and or femurs need steeper STA's to get a proper pedaling position on the bike.

I'm 5'11" or so (depends on the time of day you measure me :) ) but only have a 32.5" inseam with short femurs. The ideal STA for me is 74 degrees...and that's with a 0 degree post, but need a top tube length of 56.5cm - 57cm to get the best feel with a normal length stem. However, it's quite rare to find that in a mass produced bike.

So for bikes, I now downsize to get the steeper STA and compensate with a longer stem for the TT length.

The first thing I look at with a bike is the STA, then the length of the head tube, then the length of the TT. I need a shorter head tube than most due to the fact that I have short legs which means my seat post/seat isn't as high as others which drops me farther down on the bike. This makes normal length head tubes too tall for me....but then I like a lot of saddle to bar drop....currently running 11cm and could go deeper as I still spend the majority of my time in the drops.
 

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C-40 said:
It is common for the STA to increase on the smaller sizes. As leg length decreases, so does the femur length. Most riders with shorter legs need to be further forward to achieve KOP, which is a common measure of a "proper" fit. Personally, I like the saddle further back, so I end up using 32mm setback post with most saddles, since my 51cm frame has a 74.5 degree STA.

The current LOOK 585, as an example, has the same 405mm chainstay length in all sizes, but the STA varies from 73 to 74.5.

Older LOOK models like the KG481 used a 72.5 degree STA in all sizes with the same 405mm chainstay length. There was very little tire clearance on these frames.
Got it, thanks. I guess I wasn't so off base, just wasn't totally aware of all the nuts and bolts that went into the recommendation. Short femur = increased STA to get close to KOPS makes sense to me. Slowly but surely I am figuring all this stuff out.
 
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