Ovalizing and Tapering Tubes

An oval tube is stiffer in its major axis and more flexible in its minor axis. Although ovalizing is often touted as a major contributor to stiffness, it is actually more useful as a means to improve flexibility. Ovalizing does add some bending stiffness in the major axis, but at the same time it reduces torsional stiffness. Since most frame tubes see both bending and torsion, ovalizing is not a panacea.

Also, tubes see bending stress along their entire length. Ovalizing a tube over a very short section-for example, ovalizing a seat tube at the bottom bracket shell-results in marginal bending stiffness improvements along the tube+s major axis while making it more flexible through its minor axis. And of course torsional rigidity suffers as well.

Tapering

Tapering was first used on steel bikes to help soften the ride over the poor road conditions at the turn of the century. At that time, virtually all bicycles had tubing with relatively thick walls, primarily because the cost of more accurately drawn thinwall tubing was prohibitive. Tapering was a less expensive way to bring resiliency to the frame, since a tube becomes more flexible as it tapers (that is, its moment of inertia drops). Tradition and cosmetics have continued this practice in modern bicycles, but tapering serves little purpose in improving the ride of any high-quality frame, whether steel or titanium.

Perhaps the easiest way to see why tapering is not generally meaningful is to imagine a hypothetical standard steel frame which has enough stiffness to ensure good ride characteristics. The only way to remove weight from this frame without altering the ride (ignoring fatigue issues for the moment) is to juggle tube diameters and wall thicknesses along the entire length of each tube; otherwise, the torsional and bending stiffnesses will change, spoiling the ride.

Tapering a tube can give the illusion of greater overall stiffness, but it really depends upon which end of the tube you view. From the small end it appears as if you have increased the stiffness. From the large end it appears that you have created a more flexible tube.

Certainly, a tapered down tube that is larger at the bottom bracket shell will be stiffer in that area than the same tube with no taper. But it must also be thicker, and therefore heavier, to avoid upsetting the tube's diameter-to-wall ratio; otherwise, the tube will collapse.

Thus, the most weight-efficient way to limit flex is with a tube of constant diameter and wall thickness. For example, say you want to increase the stiffness of a 24-inch down tube by 50%. One approach would be to taper half of the tube until the 50% stiffness increase (viewed from the larger end) was met. This method would also increase weight by roughly 25%. A second approach would be to increase the overall diameter of the entire tube, which would raise weight by 20%. In the end, both tubes would display the same deflection under a given load, but the untapered tube would be lighter.

When resiliency is the goal, a better approach is to start with a smaller diameter tube with a thinner wall. This can give the same flexibility over the length of the tube while saving weight.

Tapering in titanium also creates problems with grain orientation in the metal (see -3-2.5 Tubing Processing Variables). Tapering forces the molecules to align with the longitudinal axis of the tube, rather than to maintain their optimum radial orientation. This has a detrimental effect on fatigue life.

There are some good uses for tapering, however, particularly in the seatstays. The main function of seatstays in a rigid frame is to provide a place to put the brakes. A tube that is very rigid in bending and torsion at the brake mounts is useful, but the rest of the tube does not contribute much to the ride of the bike. A tapered seatstay could cut weight slightly without harming performance. Any weight savings would have to be carefully balanced against losses in fatigue endurance, however.

The evolution of suspension frames may trigger more applications for tapered tubing. Clearance issues arising from ergonomics and standardized components may require some interesting tube configurations.

Finally, it is worth noting the one drawback of a straight-gauge tube is that, compared to the large end of a tapered tube that is equally stiff in bending, the straight-gauge tube will have higher stress at the joint. This concentration can be resolved through butting.