One question that I hear all the time is whether or not an athlete should run a rear disc wheel for a particular course and wind conditions. Assuming you want to smash PRs and crush dreams, the answer is almost always DISC! I started working on a physics analysis along the lines of the treadmill running analysis, but eventually decided that a simple flowchart was more approachable. Share it with your friends… or not… and read on for more info.
|The last question should be in pink font…|
Assuming discs are permitted at a race, the question comes down to three issues:
1. The trade-off between aerodynamics and weight. Disc wheels or disc covers are usually a little heavier than non-disc wheels, but the penalty for this extra weight is made up many times over by reduced drag on all but the most extreme courses. And I mean extreme… like climbing Alp d’Huez and not coming back down!
You can expect a disc wheel to save 10-200 grams of drag (100 gram of drag equates to roughly 1 second per kilometer), depending on whether it replaces a shallow box-section training wheel or a state-of-the-art deep aero wheel, and on the extent to which your bike frame and position shield your rear wheel. Savings also depend on the the specific yaw angle distribution you will encounter (discs perform relatively better a higher yaw angles).
2. Handling & safety in the wind. Many triathletes and some overzealous race officials are uncomfortable with disc wheels with the slightest breath of wind. In my experience, this concern is blown way out of proportion. It is true that disc wheels in the presence of any crosswind component increase the side force on a bike and induce a torque that tries to push the cyclist over. But most of the time this is easily and automatically compensated for and has little impact on handling. In fact, many people report that disc wheels have a stabilizing influence on handling under certain circumstances. The exact mechanism is the subject of debate, but probably arises from a complex interplay between the bike and front wheel’s steering axis, center of pressure and a phenomenon called countersteering.
Disc wheels can become a liability in high, gusting crosswinds, which is one reason they are not permitted at some races like the Hawaii Ironman World Championships where the ride follows the coastline. In any case, even a moderately deep front wheel (40-60 mm) typically has a much greater impact on handling and safety than a rear disc.
The only way to determine your comfort level with aero wheels in windy conditions is to experiment in training. Personally, I will run a disc and deep front wheel under pretty much any conditions and I’m not a big guy or a particularly skillful bike handler.
3. Comfort is another potential issue with discs. Some people find that some discs give a harsh ride compared to spoked wheels because they have less vertical compliance (i.e. they are stiffer in the vertical plane). This issue can be addressed in a few ways without abandoning discs. First, some disc wheels such as offerings from HED and FLO are actually spoked wheels with (mostly) non-structural carbon fairings. This design should preserve most of the characteristics of a normal wheel. The same is true of adding a disc cover to a normal wheel. Another option is to run a larger rear tire (e.g., 24+ mm). The combination of increased volume and contact area and lower pressure can improve comfort and handling with only a tiny aerodynamic penalty, especially when paired with the latest generation of wide rims.
For me, comfort is a non-issue with my structural carbon disc, but then again, I’ve never experienced rough roads at the end of an Ironman ride…
What about cost? Carbon disc wheels are generally pricey, but disc covers are an inexpensive option with comparable benefits. For more info on disc covers check out #AskTriNerd: Disc covers vs. real disc wheels