Size specific tubing is a technology that many of the modern bicycle manufacturers are using on their higher end bikes. Usually it is seen on carbon frames but can also be applied to steel or aluminum. Size specific tubing is exactly as the name sounds, the tubes that make up the bicycle frame are sized according to the frame size. For example a smaller frame like a size 48 will have tubes with smaller diameters then a size 60 frame.
This technology is cool because it simply makes sense. A larger frame has a longer top tube, seat tube, and down tube length so why not have tubing that is proportionate to the frame size. A bigger frame with longer tubes means that the tubes will have more flex to them and also the frame will typically have a larger heavier rider. For a smaller frame the opposite is true, shorter tubes don’t need to be gigantic and super stiff for smaller riders. Smaller frames with narrower tubes will be lighter weight and still give the performance the rider should expect from a nice frame.
The reason some manufacturers are not using this technology is that it is simply more expensive to create all of the different molds for each frame size. A non size specific frame will require different molds for all of the frame sizes but these molds can be more easily adapted from one mold to the next. Size specific molds require much more adaptation from mold to mold. Non size specific frames require the manufacturer to pick a happy medium in tube size. The middle frame sizes will be sized very appropriate while the larger frames will have slightly more flex to them and the smaller frames will be slightly overbuilt.
I have taken some photos of two Jamis Supernova Elites to demonstrate what size specific tubing looks like and how much of a difference in tube size there is. Keep in mind these measurements are not 100% accurate as these are just quick measurements for demonstration purposes.
This first bike is a size 51 Supernova frame.
The tubes look very proportionate to the frame size but I wanted to measure them to see exactly what difference we are looking at when we go to the next frame.
This frame has a diameter of 43mm for a top tube with a length of 515mm (51.5cm).
This Frame has a diameter of 47.3mm for a down tube with a length of 580mm (58cm)
This next bike is a size 58 Supernova (only 3 sizes larger).
This frame right away looks to have larger more stout tubes.
This frame has a diameter of 53.1mm for a top tube with a length of 570mm (57cm).
This Frame has a diameter of 57.3mm for a down tube with a length of 610cm (61cm).
So side by side the top tube diameters are 43mm (size 51) and 53.1mm (size 58). That is about 1 cm difference in the diameter of the top tubes.
The down tube diameters measure very similar 47.3mm (size 51) and 57.3mm (size 58) also about 1 cm difference.
So from all this data it is obvious that as the frame gets larger the tubes get larger in both length and width. We can take these numbers and see a ratio of tube length to diameter to see where these numbers come from. The size 51 frame has a top tube ratio of 11.97 to 1 meaning that for every 11.97 millimeters in length the tube gains 1 millimeter of thickness. The down tube of this frame has a ratio of 12.26 to 1. The larger frame has a top tube ratio of 10.73 to 1 and the down tube has a ratio of 10.64 to 1. This shows not only that the tubes are getting larger in diameter as the frame gets bigger but also that the larger frame is more overbuilt then the smaller frame. I’m guessing that the reason it has a higher ratio and a stiffer bigger tube is to compensate for a larger rider with more weight putting out higher watts.