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I've been riding my bike for a while it's a 55cm square frame. I recently used the online calculator on competitive cyclist. It told me I'd need a seat tube of 53 and top tube of 57cm.

So I'm riding a bike essentially too small for me...

It's not a massive amount out, so I'm sure I can tweak things for the correct position. Layback seatpost, slamming the stem etc.

--But what is the natural position on the bike to ensure no stresses on the back or wrists?

--What are the signs that something is too small?

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There ain't nothin' "natural" about riding a road bike. It's a question of how much stress your body can stand, and how comfortable you want to be vs being "competitive". A shorter "reach" is generally more comfortable but less "competitive". You can adjust the "reach" within a few cm by changing stems, though doing so also changes stability. –  Daniel R Hicks Sep 3 '12 at 12:28
    
Yeah that's why I was considering the layback seatpost. Since my stem is already inline with my front hub. I've heard that having the seat behind the BB can affect performance. –  will Sep 3 '12 at 15:25
    
(And it's generally better to have a bike a bit too small rather than a bit too large, so long as your back isn't being forced into a hump, and you can raise the seat high enough.) –  Daniel R Hicks Sep 3 '12 at 17:54
    
You can find answers on Selle Royal site about how to choose a saddle. They describe four riding positions: sport, athletic, moderate and relaxed. –  mouviciel Sep 20 '12 at 19:01
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3 Answers

The "natural" riding position would be one with the following criteria:

  1. Your knee never fully extends (locks out) when the crank is at "dead bottom center"; basically in line with the seat tube

  2. You can sit mostly upright when on the flat part of bars. You have a comfortable slight bend in your elbow when on the hoods. You can reach the drops without your knees hitting your chest/stomach/elbows.

  3. When in either the hoods or the drops there is little upward or downward bend in your back. Its just in a neutral, comfortable, position.

  4. You feel no uncomfortable pressure on your perineum

  5. Your knees are (relatively close to) over the pedal spindle with the crank arm level.

Obviously there are millions of ways to get this position depending on frame size, geometry, seat post height, stem length, handle bar drop, etc. Not to mention your personal flexibility and level of fitness.

To answer your second question, you generally know a fram is too small if you must extend your seatpost beyond the manufacturers limits or you must use a stem that is longer than about 130mm or with a rise of more than about 12 degrees.

Any of these things should signal that you are trying to make the bike "bigger" by adding length to parts.

edit: In regards to your comment about a "set back" seatpost, I would add this to the list of "ways you know a bike isn't the right size". You are effectively trying to lengthen the seat tube. Unfortunately, this will put your weight heavily on the back wheel and will effect steering/control.

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This video may help you -

Certainly made me more comfortable on my road bike.

Summary:

  1. Comfort is rule #1
  2. Adjust the seat height so that the knee makes a 10 or 20 degree angle at maximum extension
  3. Move the seat forward and back so that, when your foot is forward, your knee is above the pedal
  4. Adjust the handle bars so that your back is at 45 degrees, and the bars obscure your view of the front axle (or higher for more comfort, or lower for better aerodynamics)
  5. Handle bars as wide as the shoulders (or wider for a more open diaphragm, or narrower for better aerodynamics)
  6. Adjust cleats so they're under the ball of the foot
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Welcome to Bicycles! While this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the video for reference. –  freiheit Sep 3 '12 at 20:59
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Two quick and easy points to remember as a rough rule of thumb which 80% of the people you see out there ride a bicycle don't seem to realise:

a) put your heel square on the middle of the pedal, your leg should be straight and only just able to touch the pedal. Of course you don't cycle on your heel of your foot! But the ball of it, which means a greater length and thus your knee would never actually lock in actual cycling unlike in this quick and easy little "test".

b) while actually cycling don't lock your elbows! Transmits too much road shock straight up your arms, leading to soreness.

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