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I am going on a two week bike trip with a friend and I can only afford a second hand bike. My concern is that I will wind up buying a bike that I later discover was the wrong size for me.

I know that in an ideal situation I would try out a bunch of bikes at a shop and pick the one that fits the best, but if I am buying one second hand it's going to be more of a, 'ride it around the block and then make a decision' scenario.

Given that, are there some tips / things to keep in mind that would help to lower my chances of buying an ill-fitting bike?

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If a bike shop fits you to a 55 then a 56 may or may not work but you can eliminate a 60 or a 52. Not really fair to the bike shop to have them fit you with no intent of buying a bike from them but you could get a feel for what a proper fit feels like. –  Blam Jun 18 at 20:03
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See this question: How much does non-ideal frame size matter? It's not exactly the question you're asking but will give you a good idea of the "tolerances" involved –  PeteH Jun 18 at 20:48
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For me there are two factors. One is inseam; I want to be able to stand on the ground with both feet while straddling the bike. The other, related to that, is leg extension when pedaling; to use the full leg, I generally want the seat high enough that my leg is not-quite-fully-extended at bottom of stroke (so I can tilt the foot and pull back across the bottom of the cycle if I need to). For my 29" inseam, that seems to mean a 19" or 20" frame to keep the seat post from being set unreasonably high. Your mileage will vary. –  keshlam Jun 19 at 2:14
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If you're actually buying a touring bike, you could likely get fit on something pretty similar (Trek 520 or something - That thing hasn't changed since the stone age) at your LBS for a modest fee. Then, look for second hand touring bikes with similar sizing. –  Batman Jun 19 at 4:29
    
Do note that unfortunately there is no single standard for frame size -- two identical frames can have a "size" difference of about 3cm, depending on how measured. "Standover height", OTOH, is a fairly precise number. –  Daniel R Hicks Jun 20 at 21:57

1 Answer 1

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How:

The low tech way:

  1. Measuring tape Simply take it with you and measure five bikes that are comfy, and five that are not.
  2. Digital camera Ask someone take a picture of you sitting on the bike, then review the picture to analyse yourself the sitting positions, both those that feel comfortable, and those that feel uncomfortable.

Geometry:

  1. Chainstay. You need long enough chainstay for panniers. My recommendation is 45 cm. Below 42 cm your feet can hit the panniers even with small feet.
  2. Comfort. You can select the ideal frame size based on different criteria: aerodynamics, efficiency, comfort, resulting speed, etc. For touring, the comfort is 10x more important then speed or anything else. This is because you sit 3-6h a day, for many days. On a tour, you get further by pedaling more and less by pedaling faster. Example: you can be 30-50% faster in an uncomfortable, but speedy position, but you can maintain that only for 1-2h. However, in a comfortable position, you can stay even 6-7h in the saddle.

I'd rather select a slightly smaller bike vs. a slightly bigger bike.

By buying a second hand bike, what you risk is hidden failures vs. the gain of lower cost.

  • Steel frames: check for "bents" and corrosion
  • Alu frames: check for cracks.

Bents usually come from accidents (falling with the bike), and cannot be corrected. You feel it only in the first moments of the riding, after a couple of minutes the feeling can go away. This cannot be repaired. I do have a steel bike that is bent.

Cracks come from big impacts and overload. This can be corrected by welding the alu frame, you need to find an alu-welder. I do have an alu bike with a crack (high quality brand, I didn't take yet the time to find a welder and get it repaired.

For more details, see my response here: How do you distinguish a touring bike...

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