Here's the basic frame geometry and how it's measured!
TT: Top tube length
Measured along the top tube from middle of head tube to middle of seat tube. Normally between 20-21.5''
This is often referred to as the size of the frame (even though we'll see later in this "guide" that there are
more variables to look for)! Normally a longer top tube is better for tall people and shorter for shorter people etc.
HT: Head tube angle
Measured as angle of the head tube compared to axle line. Normally between 74-75 degrees
The steeper the head tube angle the quicker the steering will feel.
SO: Stand over height
Most company's measure stand over height from the middle of the BB to middle of the top tube, but I've seen
measurement from axle line to top tube and from middle of BB to top of top tube.
A higher stand over makes for stronger triangles (front and rear) while a lower can be more out of the way for
BB: Bottom bracket height
Measured from axle line to middle of BB but often seen as bottom bracket height above ground (witch is confusing
due to variables like tire sizes forks and wheels).
Lower BB's makes for a more stable ride while a higher bottom bracket makes the bike feel quicker and easier to spin.
ST: Seat tube angle
Measured as angle of seat tube compared to axle line. Normally between 71.5-69 degrees
CS: Chain stay length
Most commonly measured from middle of BB to middle of dropout but can also be measured "slammed" (all the way
forward in the dropout). Normally around 13.25-14''
A shorter rear triangle makes for quicker steering and is easier to get into the balance point in manuals but harder to
balance while there. A longer chain stay length creates a more stable ride and are easier to balance in manuals but
harder to get into the balance point.
When looking at a new frame most people consider the top tube length to be the "size" of the frame, believing their new
21'' frame to have equal room in the front as their current 21'' one (for example). It's really not that simple and I'm gonna try
to explain why and what to look for!
Remember, the size that matters (when considering front end room for the rider, NOT how the frame handles) is the difference
between where your hands and feet's have contact with the bike!
Top tube slope:
A high or low top tube actually makes a different of how much room the front end has, a taller top tube height pushes the BB
forward compared to a frame with the exact same dimensions with a low stand over height making a tall top tubed frame have
less room for the rider. Note that it also shortens the wheel base slightly with makes it react and "feel" like a slightly shorter
frame as well.
Head tube angle:
The steeper the head tube angle the more room you'll have up front but it also shortens the wheel base and therefore
makes the frame react quicker.
Seat tube angle:
The seat tube angle also changes the front end room of you bike, the steeper the seat tube angle the farther back
the bottom bracket will be creating more room for the rider (while standing), although the seat will be further forward.
Bottom bracket height:
A lower bottom bracket makes the distance between your bottom bracket and handlebars longer creating more room
for the rider.
So how much different does it make?
Let's compare two frames both with the same top tube length, same handlebars (8'') and stem (0 rise 2'' long) and a 3rd frame
with a shorter top tube length.
|Bottom bracket height||11.5''||11.9''||11.5''|
|Stand over height||7.5''||9''||7.5''|
As you can see in the example picture Frame #1 have more then 1/2'' more room between the bottom bracket and grips
then frame #2, that's very noticeable, actually, as you can see in the example, Frame #2 (21'') is closer in size (front end room),
and actually has less room up front then Frame #3 (20.5'' top tube length)!
The big difference between Frame #2 and #3 is the handling. While both frames have around the same front end room for the rider
the handling will be completely different on the two frames. Because of the wheelbase and head tube angle Frame #3 (20.5'')
will be much more unstable and quicker to turn then the longer wheelbased Frame #2.
There you go, there's alot to think about when deciding on your next frame! Enjoy! .