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My girlfriend wants to buy a used road bike as well. She is pretty small 5'2'' with shorter legs so it has been tricky finding anything that would fit her.

She suggested that we should look at something with a lower top tube. Ie something like this:

enter image description here

(Not sure why but the images don't appear to be working. Here is a link: http://massachusetts.inetgiant.com/Boston/AdDetails/COUGAR-10-speed-women-s-road-bike-NH-MA-border/8075274)

I don't know why but this does not seem like a serious bike to me. We want to do some faster riding but I don't know how to justify why this isn't a "fast bike". (Or is it a fast bike?)

I know some benefits of having a regular top tube are:

  • the bike has less flex and will lose less power on each pedal stroke. (Is this significant?)
  • The geometry is right for the person riding the bike. If you buy a bike with a lower down tube you are not really solving the problem. With a solution like this you will not be able to get your handle bars at the right height (ie: at the same height or lower than your seat post).

Question: Will a bike with a regular top tube be faster than a bike with a lower slanted top tube? Why?

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2 Answers 2

up vote 4 down vote accepted

Frame flex. The step-through frame means that the seat tube has a bending force applied to the middle of it, so it has to be stronger and heavier than the same tube in a diamond frame. Usually the down tube is also beefed up. Even so, there will be more flex both laterally and longitudinally/torsionally. I've also not seen a decent lightweight frame in that geometry. If you want light in a step-through design perhaps look at a mixte frame instead.

You can get some seriously tiny road bikes these days (google). The real choice (IMO) is between 650c and 700c wheels. 650c wheels look better, make for a lighter bike and generally have fewer compromises. They are more of a scaled down full size bike. 700c wheels mean the bike looks stretched a little with the rider low down between the wheels. The advantage is that tyres are easier to find and the manufacturer already has jigs etc to build bikes around 700c wheels. Tyres are only slightly easier, these days Schwalbe and others make a decent range of 650c tyres (they're not just a track & time trial size) and shopping online means you can definitely buy them (your LBS should be able to order them in too).

To find one of those second hand I suggest checking out the second hand ads at your local racing clubs. Finding one in the giant mess of second hand bike ads has proved tricky for us in the past, mostly due to the overwhelming number of cheap crappy childrens bikes that pollute the search results. However if you want to sell one just approach short adults... you'll sell it very easily.

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Yes. "Women's" bikes were intended primarily for one purpose; so that women could wear skirts while riding.
They are inherently weaker than a "diamond" frame and as a result must be made heavier.

Other solutions are the "mixte" frame, with dual top tubes running directly from headstock to rear dropout.

Modern "compact" frames are a sort of modified diamond, likely taken from mountain-bike design. For extremely short riders, there's always the possibility of using smaller wheels.. 650mm rims. The problem is ground clearance for the cranks.

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why is ground clearance for the cranks a problem - with slightly smaller wheels it makes sense to use slightly shorter cranks, leaving ground clearance unchanged. Stock cranks go down to 150mm, it's only shorter than that where there'd be a problem with supply. And the difference in diameter between 700c (622mm bsd) and 650c (571mm bsd) is only 25mm, so dropping from 165mm cranks to 150mm cranks would actually increase ground clearance for the same effective frame geometry. –  Мסž Apr 11 '11 at 22:06
    
Well, I meant modifying an existing bike with smaller wheels... Normally not done due to brake placement problems. If the thing is built properly with a smaller wheelset, then shorter cranks would be standard. –  M. Werner Apr 12 '11 at 2:24
    
even just swapping wheels on an existing frame wouldn't cause ground clearance problems as long as you added new cranks to the list of parts. As you say, that swap already means changing wheels, brakes and quite possibly gearing. A much more common swap is 27" to 700c, because long arm caliper brakes will cover the change in wheel size. I can't imagine that finding brakes to jump an extra 12.5mm would be easy. –  Мסž Apr 12 '11 at 3:08
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