Numbers are always good to illustrate a point. I took a shallow dive compared to how far down I
could have gone but bellow is a pretty good representation of top tube lengths vs seat tube lengths over time. I included the smallest sizes of each bike I looked at as well as ones that people suggest to small people.
As you can see, the earliest mountain bike has a top tube length of 55cm for the smallest frame they made…an 18”. A road bike of the same era with the same seat tube length has a top tube length that is 2cm shorter. Someone riding an 18” mountain bike should be riding a 54cm road bike which has the same top tube length as the mountain bike.
In 1994, the top tube did lengthen in the 18” size. The 15” size got the shorter top tube (but still long) of the mountain bike 10 years earlier. A 50cm Trek road bike similar to the 1200 kept the same 53cm top tube length. The 15” mountain bike is closer to what someone riding a 50 cm road bike would be riding but still too large for someone who should be riding a 43cm (17”) road bike
If you move on another 10 years, the top tubes lengthen a bit more. The 18” Specialized Hard Rock is longer than the 1994 Trek mountain bike but they were making smaller bikes by that time. The 13” mountain bike is closer to what someone who is riding a 42cm bike should be riding (3 to 4” smaller than road).
The real interesting stuff starts to happen in the late 2000s with the introduction of women specific designs. The bikes started to come in smaller sizes and the top tube length shortened significantly. Look at the length of the top tube of the 1989 50cm Trek which is what any women smaller than 5’4” got stuck on. It’s 53cm. All of the bikes after the 2007 Trek have top tubes that are between 1 and 5cm shorter than that road bike. They are 5 to a whopping 11cm shorter than the mountain bikes.
Saving the part that makes most people mad for the last, standover height
has to be considered as well. People who don’t have to ride bikes that are too large for them
always say that standover is unimportant. But if you have to spend your life trying to dodge a top tube with the sensitive bits, it becomes something that is very important. An 18” mountain bike has a standover of 76cm. The Terry in the above chart has a standover of 67cm. Gaining 9 cm of clearance is not insignificant and can make a huge difference if the rider just wants to comfortably dismount the bike…never mind if they have to do it in a hurry.
Bottom line: Women don’t have to settle for the wrong size anymore. Just as no women should have to buy a pair of size 12 men’s shoes and then stuff them with socks to make them fit, no woman should have to bike a bike that is far too large and stuff the bike with the bicycle equivalent of socks.