Donald in South Africa has a frame dimension question. He wants to know if it is better to adjust stem length as opposed to top tube or head tube lengths? This is a frame dimension and position problem, and this is what we think.
Is it better to adjust stem length versus top tube length and head tube length?
Yes, the stem length is the easiest way to make a bike fit your body. This is quite normal, the same as saddle height and stem height. The frame designer/builder will design and build the frame so that it will perform well when climbing, descending, sprinting, etc. The performance of the frame is all dependent on the frame angles, tube length, and dimensions and has to be within specific parameters, or the bike will not ride well.
If you are having a frame built to your measurements, you still need to make adjustments to your position with stem length/height and saddle position, to make everything perfect. This is the same as buying a stock frame off the peg that is designed to function properly and fit a few different body sizes.
Many factory made frames are now made in small, medium, and large. They fit many different body sizes and shapes. Previously frames were sold with 1 centimeter or half-inch size differences measured seat tubes with top tube lengths to match. This gave you a small frame with about the same seat tube as a top tube (19-inch seat tube with a 19-inch top tube). As the seat tube gets longer, the top tube will shorten to match (26-inch seat tube with a 24,5-inch top tube).
Now with the new frame designers, instead of having nearly 20 different frame sizes, a shop only has to stock three main sizes and at the most five different frame sizes. With the stem length and the saddle adjustments and sometimes crank length, you can fit anybody to a perfectly proportioned and performing frame.
Stems come in many lengths. From 8 centimeters to 14 centimeters in 5-millimeter increments, and with different angles. If the stem is an Ahead stem, then it can be turned over the other way. This gives you two angles with every stem, level or with a rise.
remember to set-up the bike so that your legs are positioned properly. Your knee over the pedal when the cranks are horizontal. With a slight flex in the knee when the pedal is at the bottom of its stroke, the position is correct. From then, you can work out your stem length for your best comfort and power output. Being well measured for a bike is an excellent idea. Usually, the cost is refundable when you buy the bike.