Reach & Top tube length
The top tube is the classic dimension for determining the length of a frame and is measured horizontally from the centre of the upper edge of the head tube to the central axis of the seat post or the saddle support. A second dimension for specifying the length of a bike has established itself, namely reach. This dimension is measured horizontally from the centre of the upper edge of the head tube to the centre of the bottom bracket.
This enables the geometry to be considered from two viewpoints. The classical top tube length is the longitudinal size of the frame while the rider is pedalling in the saddle. In contrast to this, the reach is the longitudinal dimension of the frame while the rider is standing on the bike, and is therefore in the descent position.
The reach in turn crucially affects the dynamic handling of the bike during a descent. As riders merely stand on the pedals in this situation and have no contact with the saddle, they have to be positioned as centrally as possible within the frame. If the reach is too short, this very quickly leads to the familiar sensation of flipping over the handlebars.
B: Top tube length
The top tube length always has to be viewed in combination with the cockpit (length/angle of the stem/bend and height of the handlebars) and essentially determines whether the rider is “compact” when sitting on the bike or more “stretched” – it should be regarded more as a static component. At the same time, the overall horizontal top tube length affects the climbing capability on uphill rides. If it is longer (“more stretched”) the front wheel rises only slightly when riding over obstacles and constantly maintains contact with the ground (= more traction).