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What problems, if any, should I expect from an undersized frame, and how small would the frame have to be before these issues occur? For the sake of the question please assume an older steel diamond-framed bike with a quill stem.

Context:

I am looking for a used bicycle for commuting. I'll be on city roads and highway shoulder, mostly flat with occasional short hills or sections of packed gravel, 12-15 miles (20-25km) each way. Since there is a very small selection in my area/budget, I am unlikely to find a frame that fits just right. Information I've found so far is that a too-large frame can have unfixable fit issues as well as safety hazards if the standover clearance is too small. However, I can't find much information about how to tell when a frame is too small or what the consequences of an undersize frame would be.

It seems like a small frame could be made to fit a larger rider by adjusting the seat post and stem height and stem/handlebar forward reach, but I'm not sure how this will affect handling or durability. I'm aware of increased potential for toe overlap and the need for minimum insertion length on stems and seat posts - are there any other problems I should be aware of?

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    I have found advice for when someone is between two sizes. In this case, I'm wondering about sizing down significantly - for example a 50cm frame when recommended is 54-56cm.
    – Loralrose
    Commented Feb 5, 2022 at 21:52
  • On modern frames I’d say it can work quite okay since you can replace a typical stock 90mm stem with anything up to 140mm or even 160mm. I think with quill stems this wide range is not available.
    – Michael
    Commented Feb 6, 2022 at 7:13

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In short - a small frame can be somewhat extended using longer seatposts and raising the stem.

However the leverage afforded by a long seatpost will put extra forces on the frame. I've personally cracked several frames in this area.

A steel bike also implies smaller tube diameters, which means a narrower seatpost. These are more likely to bend under sudden impact than the larger diameter modern seatposts. I've bent two seatposts, one was a very long one on my folding bike, the other was a 25.4mm on a too-small MTB frame. Both happened on sudden impact where I was on the saddle, one on the road and one offroad.

I suggest you get the frame you find most comfortable to ride.

The seatpost should be high enough to let your knee extend to almost-straight, but not so high your hips wiggle while pedalling. And you have to have at least the minimum seatpost inserted into the frame, preferably more.

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  • Thanks for the answer. When you say the seatpost may bend under sudden impact, does that mean a serious impact like a crash? Or is it likely to happen under normal riding conditions, such as hitting an unexpected pothole?
    – Loralrose
    Commented Feb 5, 2022 at 22:56
  • @Loralrose for me, first one was a bump in the road from road-works, perhaps 50mm high that I didn't see. Bike went up, I didn't, seatpost bent. However that had 500mm of stickout but was a ~28mm post. The Offroad one was a small jump that went badly - bike is a rigid so no suspension. That one had 350mm of stickout. By comparison, an `80s road bike might have had from 0 to 50mm of stickout.
    – Criggie ♦
    Commented Feb 6, 2022 at 1:57

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