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How does the seat tube angle affect riding?

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How does the seat tube angle affect riding?

Old 06-03-14, 08:52 AM
  #26  
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Originally Posted by Kopsis View Post
Not to knock frame builders, but very few have a mechanical engineering background. They may have years of experience with what works and what doesn't, but if they're telling you that the main triangle flexes more with a slack STA, then they don't really understand the physics involved. If the main structure really did flex that much, it would feel like a wet noodle when putting down power.

On the custom frame I ride, the builder picked the STA so that with the seatpost I like and the saddle I ride, it put my sit bones exactly the right distance behind the bottom bracket to get the fit I want. The overall geometry was all about fit and handling. Ride quality was dialed in through the chainstay, seatstay and fork tubing selection and design.
Interesting. I understand that triangles are theoretically not distortable simply by the nature of their shape, and that's why we see them as common building blocks in architecture. There is no where for them to distort. Maybe "flex" is the wrong word. Would it be more accurate to say that a more vertical post transfers shock more directly and efficiently, and that a slack angled post might difuse or dilute the shock?

How did your builder acheive your desired level of compliance in the rear triangle, if triangles don't flex? That's a sincere question, not a sarcastic jab.
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Old 06-03-14, 09:04 AM
  #27  
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Originally Posted by SquidPuppet View Post
Interesting. I understand that triangles are theoretically not distortable simply by the nature of their shape, and that's why we see them as common building blocks in architecture. There is no where for them to distort. Maybe "flex" is the wrong word. Would it be more accurate to say that a more vertical post transfers shock more directly and efficiently, and that a slack angled post might difuse or dilute the shock?

How did your builder acheive your desired level of compliance in the rear triangle, if triangles don't flex? That's a sincere question, not a sarcastic jab.
I'm not a mechanical engineer (but am an electrical engineer). But the idea that triangles don't flex or distort is just wrong unless there are constraints on 'what is flexxing' that I don't understand.

dave
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Old 06-03-14, 09:04 AM
  #28  
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i think it's intuitive that the more disparate the angles in a parallelogram the more easily it can be distorted.

and although the bike's "main triangle" isn't a parallelogram, it sure isn't a triangle, it's got four sides and two are roughly parallel.
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Old 06-03-14, 09:22 AM
  #29  
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Originally Posted by hueyhoolihan View Post
i think it's intuitive that the more disparate the angles in a parallelogram the more easily it can be distorted.

and although the bike's "main triangle" isn't a parallelogram, it sure isn't a triangle, it's got four sides and two are roughly parallel.
I would judge that it depends on where you push on the triangle or parallelogram. Stretch out a parallelogram and push on the 'long axis' and it will be pretty stiff. Press on the short axis - very different.

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Old 06-03-14, 09:25 AM
  #30  
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Originally Posted by SquidPuppet View Post
How did your builder acheive your desired level of compliance in the rear triangle, if triangles don't flex? That's a sincere question, not a sarcastic jab.
That's a fair question. It's not that triangles don't deform, it's that given the forces and the size/stiffness of tubes used in the main triangle, there's very little deflection under any normal riding circumstances and certainly not enough that a few degrees of STA would have any effect. The rear triangle, on the other hand, is made from far more flexible tubes on two of the three sides. This does allow the triangle to deform (both the chainstay and seatstay have to flex to do so). By tuning the tube shapes and sizes in the rear triangle, a certain amount of compliance can be obtained.
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Old 06-03-14, 09:46 AM
  #31  
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I think that the idea behind the flex of a triangle is within the plane of the triangle, or quadrilateral as the case may be. The triangle seems intuitively more rigid since aside from twisting it to distort all three junctures, how do you flex it without bending one of the sides? With more sides and corners, you could flex it by just distorting angles.

It's also easier to bend a rod if you're at more of an angle to it, less in line with the rod. So a more slack seat tube angle does seem to me to be more flexible, since your weight is more in line with the tube with the steep one. It also seems to me that a smaller triangle will be more rigid overall, and a steeper tube makes the triangle smaller. The difference is usually just a few degrees though so I don't know if it really makes much difference in the grand scheme of things.

The answer here that I like best is the longer chain stay and wheel base of the slacker seat tube angle.
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Old 06-07-14, 05:50 PM
  #32  
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So, Booger1 and other pros who ride several different bikes: do you tend to choose bikes with the same or very similar seat tube angles because you are used to it, or is there anyone who deliberately rides bikes with quite different seat tube angles in order to strengthen different muscles?

After reading Booger1's reply, I made more comparisons between my two (hybrid) bikes, both by measuring various lengths/distances and by sitting on/riding them.

Bike 1:
74 seat tube angle, 70 head tube angle;
shorter chain stay, top tube horizontal, seat tube;
longer front center,
lower standover;

Bike 2 :
70 seat tube angle, 69 head tube angle;
longer chain stay length, top tube horizontal, seat tube;
shorter front center;
higher standover.

Overall, Bike 1 is more "compact" and Bike 2 more "spread out", so to speak (though Bike 1's wheelbase is 5mm longer). Bike 1 fits me better because I have a short torso relative to legs. I feel more natural and comfortable sitting on Bike 1 with hands on handlebar and pedaling. However, for some reason Bike 2 rides more smooth and less sluggish than Bike 1 , maybe due to certain bike parts and the tires (Bike 2 is brand new).

Thanks for the "tutorial". Now I know what to look for when I buy the next bike.
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Old 06-07-14, 06:08 PM
  #33  
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Originally Posted by fietsbob View Post
Its more how it makes You fit on the bike .. steep is on a race bike, down and on all fours , shallow on a Utility-Cruiser sit up bike.

and all the other parts picks are in service of that function-style ..
It also has to do with body shape - more upper body weight, you want more saddle setback to have your center of gravity balanced over the BB. This reduces hand pressure for a more comfortable ride. It may or may not have a penalty on power output.

Ride quality is affected: with my weight centered over my feet, I can just lift myself off the saddle when a big bump is coming. That's harder if I'm too far forward.
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Old 06-08-14, 08:14 AM
  #34  
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divergent ways small Bikes for women have been done .. typical steepen the seat tube shorten forward reach

Terry for her designs made the font wheel smaller so seaat tube could be used for body mass balance

and head tube was not lowered angle , since the smaller wheel resolved the foot/ wheel contact issues ..



rider posture in the bike in Vol's 2 .. 1) low sporty jump out of the saddle for sprint starts, bars a bit stretched out,

2) upright, near reach bars, cruiser, touring, seated starts..

Last edited by fietsbob; 06-08-14 at 08:17 AM.
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