BMX Frame Geometry information
What size BMX frame should I ride?
We recommend that riders from 160 - 180cm tall should choose a top tube length between 20.25 and 21 inch.
Riders 180cm and taller should choose 21 inch top tube frames and up. A shorter size frame could be chosen for street riding or competition park style riding, while a longer frame could be chosen for bowl riding or trail riding.
What is a '21"tt'?
BMX frames are typically sized in inches, with the top tube, or 'tt', being the determining factor for suiting riders of different heights.
So in this example, a '21"tt" refers to a 21 inch top tube length frame.
What type of BMX frame geometry is best?
The short answer is, there's no 'best frame geometry'. Frame geometry differs to suit different styles of riding.
Generally the shorter the chainstay and top tube, the more suited to quicker spins and movements, while a longer chainstay and top tube provides a more stable ride.
BMX frame geometry terms
Top tube length: The distance measured from
the center of the head tube, to the center of the seat tube.
The top tube length is the only factor that comes into play when looking for
a BMX frame size that suits your height, as it provides more room
between your seat and handlebars.
Head tube angle: The angle of the head tube from the ground.
Generally anywhere from 74° to 76°. The bigger the
number, the steeper the head tube angle, which translates to faster
steering.
You will find steeper head tube angles on BMX frames that are
catering towards street riding and quick movements, while mellower head tube angles allow for a more stable bike at higher speeds.
Stand over height: The length of the seat tube, measured from the center of the bottom bracket, to the center of
the top tube.
The stand over height gives a BMX frame more stability by
having more frame under your legs! A taller stand over can also help
with tricks like barspins, as your seat is higher. On the contrary, a
low standover will help move your legs over the frame for tricks like
tailwhips.
Chainstay length: The length of the
chainstay (bottom rear triangle tube), measured from the center of the
bottom bracket to the dropout. Generally distances between 12.5 to 14.8
inches. This measurement affects being able to lift the front wheel off
the ground, as well as ability to spin a BMX bike quickly, due to change
of distance between the wheels. A shorter chainstay length comes at a
cost of stability at faster speeds.
Bottom bracket height: The height of the
center of the bottom bracket shell from the floor, measured based off a
20 inch wheel in the dropouts. Generally ranging from 11.3 - 11.8
inches. The bottom bracket height determines your center of gravity on
the bike. A higher bottom bracket creates a lively feel BMX frame, while
a lower bottom bracket makes it more stable.
Seat tube angle: The angle of the seat tube
from the ground. Generally anywhere from 69°to 72°. The smaller the
number, the further back the seat would be, giving you the feel of
a larger BMX frame without having to make any other measurement larger.
