Reach is the key measurement for mountain bikes as the majority of your time on the bike will be in a standing position, with your saddle well behind your body, making the length from the head tube to the seat tube somewhat irrelevant - because you’re not actually sitting on the seat.įor road riders, reach tells us exactly how far you have to ‘reach’ to grab the handlebars and is not subject to wonky measuring and sizing practices between brands. If you were to draw a line from the centre of the bottom bracket straight up through the frame and one from the centre of the head tube back 'reach' is the horizontal distance between the head tube and the vertical line. Reach is a slightly more difficult measurement to get your head around and refers to the horizontal distance from your bottom bracket to the top of the head tube. Realistically there is about 20mm of leeway shorter or longer than a stock road bike stem, any longer or shorter, and you will drastically change the handling characteristics of your bike. If you have a bike with a top tube that is too short or too long, small deficits can be remedied with a new stem of different length. The top tube relates directly to your arm and torso length, and it is a figure that doesn’t leave a ton of leeway to be fixed. This can increase the length of the physical section of tubing but does not increase the actual distance between the head- and seat tubes. Old school road bikes see a completely horizontal top tube, while the top tube on most modern bikes slopes downward. The top tube and effective top tube length can differ because of the modern compact or semi-compact geometry bikes. Effective top tube is the important one (Image credit: Colin Levitch) Effective top tube and top tube length can differ slightly.