Why do women's bikes have a diagonal bar in the frame instead of a horizontal frame? Is this the only difference?
Sorry if this has been asked (but I cannot find it by searching) and I also just cannot think of a reason why...
Way back when safety bike were becoming popular women wore skirts. Skirt lengths were to the ankle. The dropped top bar made getting on possible while maintaining resectability. Women were considered to fragile to risk hitting the top tube hence the slanted bar. The trend continued even long after women stopped riding in long skirts. Modern WSD (women specific design) geometry differs from conventional bike geometry. The difference goes beyond just size. Whether it is needed depends on who you ask. While some women may be smaller than the average male many are as tall, height is not the only issue. Many women have a different leg to torso ratio than the male average and the frames are built to fit that difference. This is not to say that a women needs a female specific frame or that a male wouldn't fit on a WSD frame. It all depends on the individual physical traits of the rider.
For MTB I have observed that many bikes marketed as woman specific are almost a scam, they are only the same frame with different colors and lower grade transmission parts (I remember one where the male model was 9 spd while the female was 8 spd, but the frame had pink patches... )
Before any feminist starts pointing out that I'm a male rider, let me state I have built a couple of bike for female riders, and also have shared the trails with many females that got their bikes on their own...
I have studied many models of "women's bikes", and even from reputable brands I often find that the only meaningful diference is in the saddle.
Another difference that can really mean something is that some makers size their frames "Small", "Medium", "Large" and "Extra Large" rather than giving a size in inches, so it may happen that for some brand "Medium" for male means 17" and for female means 15". Outside that, the rest of specifications in the geometry for the same model of frame are usually the same, or small enough that are negligible.
Traditionally, a woman's bike has a very sloped top tube. This applies mostly to classic and urban bikes. This feature was initially created to let women ride while wearing thir dresses. It also allows for easy mounting and dismounting without requiring to raise any leg prominently. However, this design is a bit weaker compared to the diamond frame design, so it is less used in mountain bikes.
At least in my country, the people has too many different sizes (because we have such a big mix of races in our ancestors) and while it's true that women have different proportions than men, it is also true that there can be huge differencies among males as can be among females. For example, given two people of the same gender and height, one of them can have longer legs than the other, this can hold true both for women or men. These two people clearly can not ride the same bike without some sort of adjustment.
Frame size and geometry is only one step in bike fit, you still have seatpost length, stem length, stem angle, steerer tube length and the corresponding steem spacers, handlebar style and size and saddle fore/aft position and saddle angle. As long as my experience recalls, adjusting these variables is enough to achieve a proper fit (as long as your frame is not too far from the ideal size) regardless of your gender.
However, there are a few tips that usually can narrow your search for components:
Saddle: As I said before, this is the only area where I have seen that there are real diferencies. First, while it is true thar female anatomy is diferent from male anatomy, it is also true that there can be huge differencies from one person of one gender to another of the same, so not every woman will be O.K. just with a woman's saddle.
The female riders I know are usually fine with wider saddles than a man of similar size, remember women ussually have wider hips. When the saddle has a cut groove in the center, females usually have a saddle that has a wider groove towars the front, while males are fine with a saddle that has a groove of uniform width. Also, when using a saddle that does not have any grooves, women tend to "nose down" the saddle angle.
This remembers me that the most reasonable "woman's saddle" design I have ever seen, rather than having a longitudinal groove, had a curved depression that begins roughly and inch behind the tip of the nose and ended there the "wings" begin to spread.
Grips: Women tend to have smaller hands, so a thinner grip (resulting in smaller overall diameter) can be more suitable. Also adjust brake lever reach so the lever is closer to the grip.
Handlebar: Its width is related to the rider shoulder to shoulder width, but then you have to account for riding style and technique, so I dont take the bait on women's handlebars. For any rider, if a narrower-that-can-be-bought MTB handlebar is needed, it is fine to simply cut it off (just cut the same length from both sides). I have also seen womens handlebar marketed as such because they have smaller diameter at the ends, but, unless you buy the corresponding grip, you wont be able to use it. That's why I rather recommend to buy thinner grips (or thicker if that's the case) that provide the same final result. And unless there is color and decorative design involved, you don't necesarily need grips to be designed for women to be thinner (smaller outer diameter), there are smaller grips at te same price of regular ones, you just have to find them.
Sources: The wife, the sister, a couple of female rider friends and 3 racer ladies.
**Update: ** A few months after I wrote this answer I saw a trend in certain brand where the frames designed for women are a bit smaller but also have a shorter effective top tube, that is, they have less distance between the seat tube and the head tube. This makes sense for women that have proportionally longer legs and smaller torso + shorter arms, since this design would allow for a handlebar that is "closer to the rider" without using a super-short stem (which can cause a funky steering feeling at first). Funny enough, this difference is visually noticeable only in their 29ers. Side note: This difference also makes for a shorter wheelbase bike (the wheel axles are closer) which makes the bike feel more agile and easy to maneuver for a smaller person whereas a larger person would feel the same bike as "twitcy".
Except for differences in size (because women tend to be shorter than men), women's frames are no different than men's frames, really.
Although "step-through" frames have typically been marketed as women's frames in the USA, they're ridden by both men and women elsewhere. This type of frame is very convenient for utility and commuter bikes in urban areas where one may want to mount/dismount without swinging a leg over the back of the bike.
In other words, the step-through is not really a women's frame. It is just a commuter/city-bike design that is not as sporty looking as a proper diamond frame, but if you're wearing regular trousers or a skirt they're easier to deal with.
Women have different leg/arm/torso ratios, so if a frame truly has a women specific geometry, you're going to typically see a shorter effective toptube and longer seat tube & headtube. For really small women's frames (and men's for that matter), the geometry starts to get really wonky because the wheel size and BB height typically don't change. I won't elaborate any further because I think you're asking more about step-through frames than you are women-specific frames.
There aren't more than two things going on here really. One, women are generally shorter. Two, someone probably thought the quickest way to be able to differentiate the women's models was to slant the top bar a little, and it caught on. It's not by any means a standard, and most well built bicycles don't have much difference in shape between the women's and men's beyond size.
Oy...How about this. Yes, geometries are different in marketed WSD frames vs "unisex frames", BUT...what if they weren't ever called WSD? What if they were just proportionally smaller bikes? Orbea has done that this year -- TT length/headtube height are proportional to the frames. It's not about whether a bike is men's or women's...it's "was it designed for a human"?
There are many, many men who have proportions that are "seen as" women's proportions. And many women who are porportioned like men. I wish companies never would've never made WSD frames...it's misleading and leads women (especially new riders) to believe that that is what they need. When in fact, many many women can fit a 50 or 52 cm frame so long as touch points are modified (narrower handlebars, for example). But, truth be told, men modify these touch points also -- shouldn't really be any different for any human.
(been in the industry for 20 years, raced for 25, female)
For some types the answer appears to be "just marketing". I've got a 58cm road bike with 700x23 tires, and 18" wide handlebars. My wife's old bike (which she no longer uses and is now the one that I can beat around in town with when I don't feel like wearing cycling shoes since it has platforms on 1 side of the pedals) is a 53cm frame, 700x23 tires, and 16" wide handlebars. Crank lever length is 7" for both of them. Dimensions on her "women's" bike match smaller "men's" bikes... And yes, it's a little small for me for a road bike, but actually very closely matches the dimensions on my MTB which I was using to beat around town on... :-)
The difference in height between a 5'10" (178 cm) man and a 5'5" (165 cm) woman is 8%.
So if men and women have the same shape, and if their bikes have the same shape, and if the man's bike has 28-inch (700 cm) wheels, then the woman's bike should have 26-inch wheels.
However I've found that small-framed road bikes or hybrid bikes, claimed to be suitable for women, still have 28-inch wheels: which changes their shape, and how well they fit the rider.
26-inch wheels exist but seem to be more standard for "mountain bikes" than for road bikes.
A couple of inches can make a real difference to how well a bike fits the rider.
Instead of or as well as asking the difference between a "mens" and a "womens" bike, it might be accurate/important to ask the difference between bikes for taller and shorter riders.
Apart from wearing a skirt, another reason seems to be so that her feet can touch the ground when she hops off the saddle at a stop, because of sitting too high to begin with, perhaps because of the wheels are bit too large.