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  #1  
Old 01-28-2018, 06:35 PM
moonhoo moonhoo is offline
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Does a steep seat tube angle make the effective size of a big larger?

I read somewhere — and I'm paraphrasing — that a 56cm bike with a steep seat tube angle like 74 or 75d would "feel more like a 58cm".

This was in reference to a bike that had a 56.5cm effective top tube, but it had a 74d seat tube. Some people were calling it a 56, some people were call it a 58.

I know size by numbers is a bit arbitrary. However I am curious about how the seat tube angle changes the effective top tube and size of the frame.

If I'm not mistaken, a big part of Rivendell's sizing is that their bikes often have seat tubes in the 72-73d range, which I think contributes to the idea that one should always "size up" when buying a bike from them.

Am I conflating two different things? Anyone care to school me on this?
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Old 01-28-2018, 07:08 PM
Kontact Kontact is offline
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Quote:
Originally Posted by moonhoo View Post
I read somewhere — and I'm paraphrasing — that a 56cm bike with a steep seat tube angle like 74 or 75d would "feel more like a 58cm".

This was in reference to a bike that had a 56.5cm effective top tube, but it had a 74d seat tube. Some people were calling it a 56, some people were call it a 58.

I know size by numbers is a bit arbitrary. However I am curious about how the seat tube angle changes the effective top tube and size of the frame.

If I'm not mistaken, a big part of Rivendell's sizing is that their bikes often have seat tubes in the 72-73d range, which I think contributes to the idea that one should always "size up" when buying a bike from them.

Am I conflating two different things? Anyone care to school me on this?
If the seat tube angle is steeper than 73°, it is wise to add 1cm for every 1° over 73. And that's because most people are going to end up correcting their saddle back that distance to get a reasonable set back from the cranks. (Reasonable set back from a fitting or KOPS, for example.) Realistically, racing bikes should all just have 73° seat tube angles and some brands have done this. Since I ride smaller frames that often come with 74 or 74.5° seat tube angles I end up with my saddle scooted pretty far back on the post.

With Riv they are suggesting a different kind of fit, where the rider sits further back from the cranks than they would on a racing bike. It is a different position and makes you sit further back on your pelvis and more upright. In this case the TT length is just as useful as on a racing bike. But if you want to have racing bike set back on a Riv you need to subtract that extra angle from the TT length.
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  #3  
Old 01-28-2018, 10:21 PM
moonhoo moonhoo is offline
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I think I see.

Are you saying effectively that if you fit a 56cm frame with a 73d ST angle, that you’d likely fit a 57cm frame with a 74d ST angle?
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Old 01-28-2018, 10:23 PM
Peter P. Peter P. is offline
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All else being equal, as you steepen the seat angle you'll want to slide your seat back on the rails to maintain the same relationship of your knees over the pedals (I'm not talking KOPS; I'm talking WHATEVER your current position is.)

Sliding your saddle back in such a case will lengthen your effective top tube length.

Rivendell's sizing is not just top tube; they advocate an overall more upright riding position. If I were buying a frame/bike from them, I'd buy based on a top tube length I'm comfortable with, and ignore their sizing recommendations.

You could also try matching up whatever your current reach measurement is, if you know that, and compare it to a Rivendell, but I don't know if Rivendell even publishes such numbers.
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Old 01-28-2018, 10:33 PM
jtakeda jtakeda is offline
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Quote:
Originally Posted by moonhoo View Post
I think I see.

Are you saying effectively that if you fit a 56cm frame with a 73d ST angle, that you’d likely fit a 57cm frame with a 74d ST angle?
If we’re going by the math above it would be a 57cm with a 73* is the same as 56 with a 74* seat tube. All else equal
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Old 01-28-2018, 11:17 PM
Kontact Kontact is offline
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Quote:
Originally Posted by Peter P. View Post
Rivendell's sizing is not just top tube; they advocate an overall more upright riding position. If I were buying a frame/bike from them, I'd buy based on a top tube length I'm comfortable with, and ignore their sizing recommendations.
So when you do that, do you then convert the TT length by subtracting for the slack STA?

Quote:
Originally Posted by jtakeda
If we’re going by the math above it would be a 57cm with a 73* is the same as 56 with a 74* seat tube. All else equal
Exactly.
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Old 01-29-2018, 11:29 AM
moonhoo moonhoo is offline
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Ok, thanks. That makes sense.

I've ridden lost of compact frames. But I start riding a square frame and am noticing how remarkably different the sizing is. Still wrapping my head around it a bit, hence the question.
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Old 01-29-2018, 03:13 PM
Kontact Kontact is offline
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Originally Posted by moonhoo View Post
Ok, thanks. That makes sense.

I've ridden lost of compact frames. But I start riding a square frame and am noticing how remarkably different the sizing is. Still wrapping my head around it a bit, hence the question.
It really shouldn't be. A true compact has all the same important angles, just with less seat tube.
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  #9  
Old 01-30-2018, 08:42 AM
moonhoo moonhoo is offline
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Quote:
Originally Posted by Kontact View Post
It really shouldn't be. A true compact has all the same important angles, just with less seat tube.
The tricky variable is that I’d be on a 74 or 74.5 STA on a custom frame. In a square-ish frame that looks to my eye at least a bit strange, not to mention that my stand over height would be borderline uncomfortable if I didn’t want to have 3” of stem showing. Fro a POV if looks, I’m finding the “best” square frame a bit hard to find
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  #10  
Old 01-30-2018, 12:57 PM
Kontact Kontact is offline
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Quote:
Originally Posted by moonhoo View Post
The tricky variable is that I’d be on a 74 or 74.5 STA on a custom frame. In a square-ish frame that looks to my eye at least a bit strange, not to mention that my stand over height would be borderline uncomfortable if I didn’t want to have 3” of stem showing. Fro a POV if looks, I’m finding the “best” square frame a bit hard to find
I guess I don't know what a "square" frame is.
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  #11  
Old 01-30-2018, 01:05 PM
jtakeda jtakeda is offline
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Quote:
Originally Posted by moonhoo View Post
The tricky variable is that I’d be on a 74 or 74.5 STA on a custom frame. In a square-ish frame that looks to my eye at least a bit strange, not to mention that my stand over height would be borderline uncomfortable if I didn’t want to have 3” of stem showing. Fro a POV if looks, I’m finding the “best” square frame a bit hard to find
I’m not sure I understand this.

Having stem showing is a function of head tube length and not necessarily the angle of the head tube.

I have a square frame with a big head tube and it helps to keep my bars higher with less stem exposed.
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