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I have a lady friend who's interested in getting into road cycling. The smallest frame sizes I've seen so far in the market are 48 and, in one instance, a 47. Is this about the most common and/or smallest starting point for frames for these riders? She's average weight (around 100lbs). Any recommendations on frame size would be appreciated.

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I suggest you look toward large kids bikes to see if they fit the bill? Downside, they're more likely to be BSOs; good ones will be as pricey as an adult road bike. – Criggie Mar 13 at 18:26
    
Depending on budget, she could also consider a custom frame. That's probably at least $3,000, but the result can be very nice. – Reid Mar 13 at 23:28
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Probably not the best thing to do for your first bike. – Batman Mar 13 at 23:33
up vote 9 down vote accepted

My wife is 5' 2" and raced elite road for many years. For her proportions 48-49 cm frames seemed to work well. Sometime she had mentioned interest in a smaller frame, but when I looked at how she fit on the bike I think it was due to flexibility issues (at that time) rather than frame sizing issues.

Of course this is one person's experience and therefore not a canonical answer. There are a lot of factors, other than frame size, that govern fit, but 48/49cm is likely a good starting point for someone 5'2".

NOTE: different brands will have slightly different fit dimensions (stack and reach) for the same quoted frame size. Frame sizing is not standardized.

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This is correct. A persons proportions are everything when sizing. Most women tend to have longer legs than men. I have have a particularly long torso and have met many women who are 4+ inches shorter than I am, but ride the same sized frames. "Women's" frames are generally designed with a shorter top tube (among other changes) to accommodate this generalization of proportions. See also bicycles.stackexchange.com/questions/13826/… – Suspended User Mar 13 at 18:17
    
@SuspendedUser the notion that on average women have longer legs than men is actually a myth. Empirical data says otherwise. – Rider_X Mar 13 at 18:34

You have to try the different frame sizes. I'd suggest looking at 650b or 26" wheel sizes if possible though, or possibly a kids frame -- I would not recommend using 700c wheels, as is standard on road bikes, for someone under maybe 5'5". Some of the women's specific marked bikes go a bit lower in size than the mens bikes and most major manufacturers (certainly Trek/Specialized/Giant) produce them.

Different frame sizes work for different people of the same height (depends on leg length, torso, arm length, etc.) and frame sizes aren't consistent between models of bike. So she has to try a bunch of bikes.

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650b is a nice option for smaller riders, but it is not a common size in road cycling. At races or charity rides neutral wheel support will only have 700c. On club rides no one will have 650b tubes. Unless you are only riding on your own, nonstandard wheel sizes can be a hassle. – Rider_X Mar 13 at 18:37
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It's becoming more common. Plus, would you rather have a bike that fits well, or one that works well for races? And you should always carry your own tubes. – Batman Mar 13 at 21:07
    
@Rider_X Come on. So carry two tubes because of a non-standard size. I will take that for fit every time. On a club ride no one will have 650b? – Paparazzi Mar 13 at 21:16
    
@Frisbee and you have never had two flats in one ride? Plus you don't have anything to lend others. 700c on the right frame can work fine for smaller riders. 650b has some advantages plus a lot of bike geek buzz. Most non-geeks would benefit from cross comparability over niche bike-geek cool factor. – Rider_X Mar 13 at 21:22
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@Rider_X What part of "So carry TWO tubes because of a non-standard size" is not clear? Yes I will take fit every time. And 650b is not that uncommon. – Paparazzi Mar 13 at 21:26

Pay attention to the effective tube length (ETT, VTT and whatever it might be called) which is the horizontal length from the handlebar to the saddle tube. While saddle height ('conventional' size) can be changed easily, ETT is hard to adjust.

The easiest way to determine the right ETT is a calculator.

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