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I found a used 2016 Trek 7.4 FX bike for sale and I tried it today and I liked it very much, but its size is large and my appropriate size is medium according to Trek’s classification. (I'm 170 cm tall and 79 cm inseam) I can stand over the top tube with flat feet but without any more space between me and the tube. Do you think it is a bad idea to own this bike despite the size?

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    I don't find bikes that don't fit me to be fun to ride for any significant amount of time. Top tube clearance is only one aspect of fit, however. What matters most is your comfort on the bike.
    – Paul H
    Commented May 1 at 19:50
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    Actually, almost more important than the stand-over height is the reach to the bars. A bike that's too large will also be too long, and may force you to reach too far or bend over too much for comfort.
    – DavidW
    Commented May 1 at 20:51

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Length (reach) is usually the most important aspect in bike sizing.

Once you have a frame in a certain size you can only adjust reach by changing the stem to a shorter or longer one (disregarding going for a different handlebar shape). You can usually go about 2cm to 3cm shorter or longer. This is pretty much the same difference you get with one frame size up or down. So by buying one frame size too small or too large you’ve potentially exhausted all your reach adjustment “budget”.

Standover-clearance is usually not a problem with modern frames. Seatposts can usually be raised and dropped quite a lot (like +-5cm from a “normal” position). Saddle fore-aft position can easily be adjusted in a wide range by going for seatposts with more or less setback.

So it’s really reach which has the least possibility for adjustment and is therefore the most critical measurement. If you feel quite stretched on the bike even if it already has a short-ish stem, absolutely don’t buy it. If you feel comfortable and it has a normal length stem (i.e. default length it came with) it’s probably fine and if it turns out you need slightly more or less reach you can easily go 1cm up or down.

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  • I agree on the importance of reach. I was messing about on a smaller framed bike we have in the shed - I adjusted the Seat height so my legs were at almost full extension when pedalling, while this was comfortable, the handle bars were now uncomfortably far away. The stem was already on about 40 mm of extension rings, but it was nowhere near close to high enough. This has been my issue with bike fit my whole life.
    – RominRonin
    Commented May 19 at 12:40
  • @RominRonin: If your handlebar is too low your stack is too low, not reach. Has never been an issue for me since I can tolerate the saddle quite a bit higher than the handlebar (like 15cm).
    – Michael
    Commented May 19 at 15:46
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If the price is right, it can be worth it as long as the size is close. You can get a short stem to reduce the reach if needed, and as long as you can pedal comfortably, the size might be close enough.

That said, there are a lot of bikes out there, so if you're on the edge, don't force it. Another bike will be available in the right size before too long.

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    On the other hand, if OP is excessively small/tall/etc then those rare times a "close enough" comes through it can be worth getting just in case. I bought a 61 cm frame with no real need for it, then 2 weeks later my main road bike cracked.
    – Criggie
    Commented May 2 at 3:58

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