freedomrider1

I ride a 56 road bike now, iam looking at new bikes and the one i was interested in comes in 55 and 58. So my question, is there much difference in 55 compared to a 56.

pacificaslim

What you need to look at is the top tube length and head tube height. If the top tube length is similar, you can adjust to be the same as your current bike by changing stem size (range of 100mm to 120mm is usual, meaning you can compensate for a 2cm difference in top tube length). You need to look at head tube height to see if the position of the bars is going to be about where you want it without using a stem with a crazy angle or a bunch of spacers.

DaveSSS 

There's little more to it than just TT length and head tube length. Differences in the seat tube angle can make the reach longer or shorter, just like changes in the TT length. The effect is about 1cm per degree. A 55cm TT with a 74 degree STA would be about the same as a 56cm TT with a 73 degree STA and no, I did not get that backwards. Steeper STAs increase the reach.

Also, when comparing the head tube length, the headset stack height needs to be included. Conventional headsets have 25-35mm of stack height and integrated models are usually in the 8-20mm range.

I pay no attention at all to frame size numbers and never compare them between brands.

deadprez012

Is seat tube angle determined relative to the chain stay? Or do you use the seat post and top tube as reference vectors? Or some imaginary plane parallel to the floor?

If you're considering buying a frame, what measurements are necessary to make fit similar or equivalent to a current bike? Seat tube angle, top tube length? Does chain stay angle (compared to top tube) matter, since that would affect BB height?

Pb_Okole

Seat tube angle is measured against a line parallel to the floor or running from front axle to rear axe if you prefer to think of it like that.

freedomrider1

thanks for the help, everyone