Questions, questions, so many questions! Lately people have been reaching out to me with all sorts of requests for advice. I’m flattered, but I’ve been struggling to keep up with the steady stream of questions. I’m not good at providing short answers, or answers without references, graphs and calculations. So I’m going to start tackling some of the more common and interesting questions in a new blog series: #AskTriNerd.

I’ll be the first to admit when I’m out of my depth. I don’t presume to call myself a coach or an expert in any field. But I’ll do my best to provide science-based answers and combat the spread of broscience.

#AskTriNerd: My friend Bruce asked: “Disc wheel cover vs real disc wheel. Give me the scoop.”

For those unfamiliar, disc covers are plastic or carbon fiber fairings that fasten to a normal rear wheel to convert in into a disc wheel. Options include  US-based Catalyst and the Wheelbuilder AeroJacket, UK-based Raltech, and Australia-based Dyma. Accomplished DIYers can even make their own cover for super cheap (instructions).

zipp-wheelbuilder-aerojacket-cover
Photo: Wheelbuilder.com

How does a disc cover compare to an actual disc wheel?

The short answer is that the minor disadvantages compared to actual disc wheels make it hard to justify the $500-2000+ price difference. With options starting under $100, a disc cover is one of the most cost effective upgrades that you can make to your tri bike.

I raced many times with a Wheelbuilder disc cover without issue before eventually “upgrading” to a carbon disc when I found a bargain. I am surprised by how few covers I see at local triathlons given their benefits.

Pros of disc covers

  • Similar aerodynamic performance to actual discs (further discussion below). Both will save you roughly 0.5-1 seconds per kilometer compared to a regular (non-aero) rear wheel depending on your specific setup and race conditions.
  • Cheaper than actual discs (save $500-2000+).
  • Same ride quality and handling characteristics as a regular, spoked wheel. Aluminum rims also offer better braking performance than carbon brake tracks found on many disc wheels.
  • Insignificant weight penalty: Disc covers add 200-400 grams to a wheel (less for a deeper rim). A light rear wheel with a cover can weigh even less than some disc wheels. In any case, aerodynamic gains almost always trump weight savings.
  • Flexibility: If you are traveling to races with one wheelset, a deep rear wheel with a cover gives you two wheel options. This is useful for potentially windy races.
  • Train and race with power: A disc cover on a PowerTap wheel is one of the more affordable ways to train and race with a power meter. Choose a sturdy wheel for training and slap cover on for race day.

Cons of disc covers

  • Annoying to install/remove on a regular basis (takes 10-20 minutes).
  • Not compatible with certain frames and wheels (details here).
  • Can rattle on rough roads, although this problem can be solved with tape.
  • Decidedly unsexy.
  • No awe-inspiring, soul-crushing “WHOMP! WHOMP! WHOMP!” sound of a disc wheel.

 

Jesse-Thomas-cover
Even pros use disc covers. This is Jesse Thomas’ bike from his 2012 Wildflower Triathlon win. Photo: Triathlete Magazine

Aerodynamic performance

There is little publicly available data comparing different types of disc wheels (e.g., 1,2,3,4). The most commonly cited data from Wheelbuilder (below) suggests that covers offer a similar aerodynamic advantage to actual disc wheels. Other data comparing different disc wheels with a full bike are rarely “apples-to-apples” comparisons (i.e., same bike, same front wheel, same tire, same rider, same conditions, etc.).

wheelbuilder_wind_tunnel_data_all
Wind tunnel test results comparing Wheelbuilder cover and two Zipp disc wheels (900 and Sub9). Copied from Wheelbuilder.com. Conclusions and details of the testing procedure can be found on their website. As always, it’s important to take test results with a grain of salt when the company has a financial interest in the outcome, but these results seem credible.

Wheels are often wind tunnel tested by themselves, not with a full bike. Data from such tests is somewhat deceptive, as it tends to overstate differences in drag among rear wheels. A rear wheel on a bike is shielded by the frame and rider, and encounters air that has already been disturbed (aka “dirty” or “turbulent” air). Furthermore, most wind tunnel tests are conducted at a speed of ~50 km/h (~30 mph), much faster than the average speed of a typical age group athlete. For these reasons, apparent differences in drag among rear wheels may appear to be significant in test results, but will be much smaller in reality. No data that I am aware of has conclusively shown major differences among disc wheels.

Disc wheels fit into three general categories: flat (straight walls), lenticular (lens-shaped) or other hybrid shapes. Disc covers on shallow rims are lenticular, while disc covers on deep rims have a hybrid shape. Non-flat discs often show superior results in wheel-only wind tunnel tests; however, a few aerodynamic gurus have hinted that flat discs tend to perform better in frames that shield the rear wheel with a cutout (a design popularized by the classic Cervelo P3 and now found on most modern “superbikes”). For a more conventional frame design, a non-flat disc may be a better choice, but this is really splitting hairs.

A note on test results

All aerodynamic test results are subject to the universal caveat, “but it depends”. You can never precisely translate test results directly into your own potential performance gains, since your individual equipment choices, body, position, riding style and course conditions all interact to markedly influence aerodynamic properties. However, in the case of disc wheels, there is overwhelming evidence that they are a good choice in almost all cases.

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