Frame Sizes - Compact and Traditional

starless

Hi There,

Although I've been road biking for a few years, I have only ever had the one bike which was measured for me by my LBS. Now I've started looking for another bike, but I'm quite confused about the measurements of frames. :?

For example, if a frame is said to be 56cms, what is that measurement? Also, it's obvious that a compact bike with a sloping top tube will have a smaller frame size for the same rider. But how can you tell which are equivalent?

Now I know that there are other factors to consider, such as the bike's geometry, but can anyone give me a relatively simple explanation?

The reason this has become important to me now is that I'm after a new bike, or a used one, more likely. My current bike is a 7 or 8 year old Principia RS6, which has a small compact frame. I am about 5' 9" tall.

Thanks for any help on this matter.

thebikeliker

According to:

http://www.principiabikes.com/pdf/Principia_2002_back.pdf

"S6 and off-road frame top tube lengths are measured horizontally, from the headtube to an
imaginary, extended seattube."

So, if I understand that correctly, the fact that it is a compact frame doesn't matter as the size given is adjusted for this.

If you look at page 8 of the link above you'll see that the frame size in cm is equal to the seat tube height minus 30mm. I don't know why 30mm difference but I'm sure someone here will explain.

Garybee

If only it were so simple. Sadly there is no convention, manufacturers all do it their own way.

The measurement is almost always the length ot the seat tube, but even with a horizontal top tube manufacturers do it different ways. Some measure from the BB to the centre of the top tube, some measure to the top, some measure to the top of the seat tube.

With sloping top tubes some manufacturers still measure it the same way while others give the equivalent measurement (as if it had a horizontal top tube).

Basically it's just a giant mess. In my opinion the most important measurement is the length of the top tube (taken as if it was horizontal).

Rich Hcp

You need to try some, it's the only way, I'm afraid

starless

Oh bugger! :roll: I thought it was a bit easier than that. But then, why should it be? Why should the manufacturers come to an agreement that would help their customers, after all?

So, the bottom line seems to be, if you want a bike that definitely fits properly, go to a LBS and make sure.

If, however, you want to buy a bike at a knockdown price on the internet or from eBay, then you need to know the manufacturer's method of measurement and you should have either tried the type of bike physically, or you are taking a calculated risk.

Happy days! :shock: