It’s been more than a year since the UCI scrapped its limitation on frame design under the so-called “3:1 rule.” The code required the length and width of cycling equipment to not exceed a three-to-one ratio. In practical terms, it meant that extreme aerodynamic tube shapes were illegal in bike frames, handlebars, and other components.
Now that the rule is gone, bike manufacturers have had time to work with this new freedom. So what has changed with aero bicycle technology? Well, not a whole lot. First of all, the rule still applies to components, including seat posts and handlebars. And frame tubes must still be able to fit within a rectangular 8-centimeter-square box as they did before.
Manufacturers still cannot fill in the triangles of a bike frame like a sail. Nor can they create a fairing from the down tube to the front wheel. In essence, the maximum depth of frame tubes will not increase.
“If the UCI box limitation is not changed, the design of aero bikes will remain almost the same,” says Kepa Otxoa, an engineer at Orbea.
Coincidentally, Orbea’s Orca Aero was one of the first bikes to come to market after the UCI scrapped the 3:1 rule. Otxoa and his team made only minor tweaks that were previously illegal. They extended the fork blades, for example. The changes are so subtle, in fact, that you wouldn’t notice the fork is outside of that 3:1 ratio unless you had a pair of digital calipers.
The primary tubes of a frame — the down tube, top tube, and seat tube — all still have a minimum dimension of 25 millimeters. Under the old rule, that meant they could only be at most 75 millimeters deep, to comply with the 3:1 limit. The rule changes allow them to become more slender, but not much: 25 by 80 millimeters. Likewise, other elements such as the fork and seat stays can now use slightly deeper profiles, as Orbea has chosen to do.
“Some further aerodynamic optimization is possible,” says Nathan Barry, an aerodynamicist with Cannondale. “However, we are unlikely to see forks or seat stays getting significantly deeper or approaching that 80-millimeter depth now, even though that is permitted. This would likely have a negative impact on weight, stiffness, and handling without marked improvement in aerodynamic performance.”
As with any bike design, there are compromises to be made. Making an aero bike better isn’t just about aerodynamics.
“A frame that is not stiff enough flexes, and this means an aero disadvantage,” Otxoa says.
So what comes next? Both Otxoa and Barry believe the future lies in integration. Working within the constraints of the box rule for a given area of the frame, each agrees that we’ll start to see the fork, seat stays, and chain stays better hide frame features like bosses, axles, and disc brake calipers.
So, if you’re waiting for bikes to look like they’re from the future, keep your fingers crossed for further rule changes.