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I have an opportunity to buy a barely-used Allez Elite for a very small amount of money (around $100). However, its frame size may be slightly different than the "ideal" size for me. If this difference in size will make the bike uncomfortable on extended rides, I'd rather purchase a different one that fits well, even for more money.

Based upon my height and leg length, I ought to be using a 55cm bike frame.

How much smaller/larger of a frame can I use before it will be uncomfortable? +/- 5cm? More? Less?

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Is it too big or too small? It's often easier to make a small bike bigger than to make a big bike smaller. – Kibbee May 9 '14 at 23:52
I had the chance to measure it today, and the frame is closer to 53cm in than 55cm. – CmdrMoozy May 10 '14 at 19:22
Presumably the true frame size was written somewhere on there. Still, for 100 dollars for a decent condition Allez Elite, I'd be inclined to buy it and sell it later if it didn't fit me - probably would turn a profit. – Batman May 10 '14 at 19:29
up vote 6 down vote accepted

First, note that frame sizes aren't standard - you may ride a 55cm in one model, but a 52cm in another model (say, comparing road racers to cross bikes). And, you do pick your model based on preference - I prefer a larger bike than most people of my height, since I have relatively long legs and long arms.

Most of the time, you can go about 1 frame size above or below the ideal frame size (thats around +/- 2 cm in model numbers, typically ) with part tweaking - the most important measure being top tube length, which can be drastically different between similarly labeled models (This can be offset with saddle and stem replacements to some extent, but the adjustment ability is relatively limited). This is why getting good bike fit is important (and why people have to try bikes before buying them) - the geometry chart isn't super useful unless you know how that chart fits you (and some relevant swappable parts like saddles come into play as well). Of course, you have to be careful with this, especially for bikes which will be used off road (you wouldn't want to hit the wedding jewels on the top tube for example).

This is a good article on the topic of frame sizing.

For what its worth, if I had to intuitively place a 55cm, 60cm, and 50 cm road bike, for males, I'd guess a 50cm being for people closer to 5'2" ish, 55cm for people closer to 5'6", and 60 cm for people who are probably 6'1" ish. But again, this is completely useless without trying the bike.

To add, most likely, a wrong frame size (or misadjusted frame) will lead to back pain or knee pain. Both are not fun to deal with once you're past being a teenager.

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This is a pretty good answer, although a 55/56cm frame for most people would fit closer to 5'11"-ish, and a 60cm frame closer 6'3"+, typically. Aside from that detail, which was already noted by @Batman as only an intuitive guess, +1 – zenbike May 10 '14 at 4:55
As a personal example, I am 5'11" and my road bike is a 58. But I guess I do have some odd proportions. – Batman May 10 '14 at 5:09
As you noted, that is why a proper bike fit with a trained technician is important. :) For example, with my body (also 5'11"), a 56cm Scott or Specialized or Trek works well, but a 55cm Storck, a 55cm Bianchi, and a 52s Colnago C59 all fit pretty close to the same for me. Partly, that is a difference in how the bikes are measured, and also with the varying geometries used in their creation. Someone who is 5'11" with a long torso and shorter legs, may fit a 58cm better than a 56cm. Not on the "average" person, though. Even your flexibility can mean a change in frame size. A more flexible... – zenbike May 11 '14 at 5:09
...rider will tend to naturally reach farther than a less flexible rider, and will thus be more comfortable on a larger frame. – zenbike May 11 '14 at 5:10

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