Reference the following for more details: Bicycles & Tricycles: Numerous Chapters Mototrcycle Chassis Design Racer’s Encyclopedia of Metals, Fibers & materials
We are going to put it out there right now. The simplest, lightest and usually strongest bicycle frame design is a double triangle.
Simple engineering tells us that using this design creates the strongest frame possible for the least weight, as a fully triangulated structure is superbly efficient. But, as with many things, when you start adding elements into a simple structure that it was not designed for, it means that the structure becomes compromised, hence is the case with a full suspension mountain bike frame.
Where once the bicycle’s frame sole purpose was to connect the head tube, bottom bracket and rear wheel in the stiffest possible manner (and keep the rider’s arse off the ground!), the frame now has to also allow for rear suspension, requiring pivot points as well as mountings for a shock absorber, linkages etc. etc; the simple clean double triangle no longer was going to cut it, at least not without some serious rethinking. Materials and process’ too have changed and while a simple round tube is still the the strongest structural element, we are now able to engineer shapes that can have specific characteristics that achieve more specific functions, ideal for full suspension designs that place unusual demands on what has to be a very light, strong and cost effective structure.
Also playing a part in all of this, is that the bicycle world, like the automotive, is engaging in ‘aesthetic’ designs. Simple round tubes no longer are enough to woo the customer, who is looking for swoops and curves, as they do when they buy a new car or motorcycle. As with everything else, these new and previously un-addressed design needs have placed more demands on the once simple bicycle frame. To help you understand the various methods talkedup these days, here is a brief run down…
Traditional round tube
As mentioned, a traditional round tube is the strongest and lightest of all structures. This is because it resists bending loads in all directions equally. This attribute also allows round tubing to be made lighter as by increasing the diameter of the tube, it becomes stiffer – ideal when using lighter weight materials. While for many designs, a round tube is the best option, their use is limited when it comes to achieving more complex solutions that require minimum joining, where any join is a weak point. Also, ultra light tubes with thin walls do not lend themselves well to even minor forming (bending etc.), meaning that from a design standpoint, the use of traditional tubes strictly limits design flexibility.
While round tubes deliver equal strength in all axis, basic shaped tubes ie. tubes with a constant cross section, can offer less or more strength for a given axis. The simplest way to think about this would be to think of a rectangular, or ‘box’ section. A box section can be employed to provide a high level stiffness in a single axis of bend via the deep section, while on the narrow section the stiffness can be significantly less. It is not unusual to see shaped and regular tubes used together to achieve a highly efficient design. Another form of shaped tube is where a basic round tube is subjected to a secondary process, such as swaging. In this example, swaging may be employed to modify a round tube where it is desired to create a area of increased stiffness. quite often you will see a round down tube swaged at the bottom bracket to create not only a more complete join but also to increase the stiffness in this specific area. Secondary forming can also be used to give a round tube an element of ‘cost effective’ form to create tubes with a similar, though simpler, to hydroforming.