Stefan Christ and his team of engineers in Grenchen, Switzerland have a happy knack of simplifying complexity. It’s a pleasing trait whose advantages are not lost on, say, BMC Racing’s Rohan Dennis.
The few, subtle modifications made by Christ and his colleagues to their already impressive BMC trackmachine TR01 were enough to provide the Australian with a machine on which he could travel 52.491km in an hour, sufficient at the time to break the Hour Record.
It says much for Christ’s design that such a specific application was far from his thoughts when he was tasked by the Swiss federation with designing a single bike for use in all disciplines in the omnium. That the same machine proved suitable, with only the smallest adaptation, for a task as specific as the Hour is a testament to its ingenuity.
Christ is as rational as one would imagine a Swiss engineer to be. Putting Dennis on BMC’s trackmachine TR01 and not its road-going sister, the timemachine TR01, was, he shrugs, a straight forward decision; never mind that none of the recent slew of Hour Record holders before or since have ridden a track bike (all rode modified road time trial bikes).
“The decision was pretty easy for us,” he says, gesturing at Dennis’ machine, brought into the quiet anteroom of BMC’s Grenchen HQ in which our interview takes place, and propped against a wall. “From a pure front-end, aero performance point of view, we were convinced that the trackmachine was the better choice.”
That suitability, specifically, lies in the tube shapes. Those of the trackmachine TR01 have a profile designed to perform at “zero degree yaw angle,” Christ explains. Put simply, there are no crosswinds in a velodrome, and the tube profiles have been optimised accordingly.
There’s more. Look closely at the tubes and you’ll notice a low ridge. This is a “tripwire”, moulded to disturb airflow. If this seems counterintuitive (“aerodynamic” designs are smooth skinned, right? Surely air should be allowed to flow unimpeded where at all possible?) it is because engineers have learned that by initiating turbulence they are able to exhibit some level of control upon it.
“Instead of letting the profile by its shape define the point where the flow goes from laminar to turbulent, you can create a small turbulence boundary layer. By that you can delay where the air really starts to separate,” Christ explains.
Of the two people involved in this conversation, it’s fair to say that I am the more impressed. “It’s pretty old now,” Christ shrugs. Tripwire technology was first used on gliders, he continues, whose comparatively low speed offered an obvious application to bicycles. Now the concept is widely used, including by Mavic on the tyres supplied with their CXR wheels.
I look closer. The tripwires on the front of the fork blades are easy to spot, now I know what I am looking for. I fail to notice them on the chainstays, and for good reason: the concept works only on tubes that “see the wind”; that is to say, the vertical, rather than the horizontal.