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Which is faster up 14,000 feet? Light wheels or deep aero wheels?

We test 42mm wheels vs 65mm wheels up multiple laps of the 5% Lookout Mountain, keeping average power output the same.

As cyclists, we know that deep aero wheels are usually faster than lighter, shallower wheels when riding on flat roads. And we know that when climbing very steep roads, lighter wheels will be faster than deeper wheels. But what about conditions that are in between? At what point do aerodynamics, er, outweigh the benefits of slightly less weight?

In this episode of Project 14er, presented by Cadex, we set out to test our model-based theory that a gradient of 5% or less and a speed of 9mph or more is that tipping point.

To test our theory, we picked Lookout Mountain outside of Golden, Colorado, which averages 5% up 1,273 feet of elevation gain over 7.4km. Doing 11 laps up Lookout puts us at 14,000 feet.

Why 14,000 feet? Our colleague Betsy Welch cooked up Project 14er as a challenge earlier this year. It’s far more accessible than Everesting, and is an ode to all the 14,000ft mountains here in our great state of Colorado.

Lookout Mountain outside Golden, Colorado, averages 5% over 7.4km. Photo: Hannah DeWitt

The light wheels vs deep wheels testing protocol

Dan Cavallari and I alternated among riding three sets of wheels:

My plan was to standardize everything I realistically could: I used the same bike, the same new Cadex tires at the same pressure, the same position as much as possible, and one bottle per lap. I aimed to keep average power the same for each lap, at about 275 watts.

Since conditions would likely change throughout the day, we switched wheels every lap or every other lap.

We switched between a shallower set, a deeper set, and this mixed set throughout the day. Photo: Hannah DeWitt

Modeling and theoretical physics vs the real world

Best Bike Split, if you’re not familiar with it, is a sweet modeling software created by Ryan Cooper and now part of the TrainingPeaks family. It can do a bunch of different things, but I used it to model the difference that changing wheels could make up Lookout.

Best Bike Split allows you to enter in the specifics or at least approximate all the variables in the cycling equation: weight, aero drag, rolling resistance, speed, wind, and more. You can enter a course and even plug in the forecasted weather to get dynamic data into the model for wind direction and speed. It’s super cool.

I plugged in all my data for the two set-ups on the Lookout course. The predicted difference at an average 278 watts? One second advantage — to the deeper wheels.

Best Bike Split is an excellent modeling tool. I compared my bike with the Cadex 42s to the same bike with 65s over the Lookout course with the day’s predicted wind conditions included in the analysis.

Part of the reason we choose Lookout was based on the research of our colleague and all-around tech guru Lennard Zinn, who recently did a deep dive on this very topic in the July issue of VeloNews. In that piece, Lennard talks about the flywheel effect of wheels: The energy you put into getting a rim up to speed comes back to you as long as you don’t brake.

A lighter rim has a lower moment of inertia, so you can get it up to speed easier — something most riders can feel. A heavier rim has a higher moment of inertia, but you still get that same amount of energy returned to you — it’s just harder to feel.

Dan didn’t look at data – he just paid attention to how the various wheels felt going up and down Lookout. Photo: Hannah DeWitt

In any event, the real world proved to provide more obstacles than a theoretical model. While I rode the planned four laps on the deep wheels at pace, I was only able to complete three laps at pace on the shallower wheels, and only two on the mixed-depth set. First filming and then fatigue got in the way.

As the first lap of the day had the strongest headwinds, I threw out the first lap on the 65s.

The results

@278w Cadex 42 Cadex 65
Lap 1 26:08 26:03
Lap 2 25:50 24:34
Lap 3 24:56 24:44
Average 25:38 25:07

The handling and ride feel

While numbers on a spreadsheet are all fine and good, ride feel of course matters! Dan spent the day paying attention to feel, not numbers, and came away convinced that he would prefer to ride the shallower wheels, or perhaps the mixed-depth set, for his everyday hoops.

Dan and I both enjoyed the overall feel of the Cadex 42 wheels. Photo: Hannah DeWitt

Deep wheels have a two-way relationship with the wind: Yes, they help you move through it faster, but they also subject you to more side input from it. On gusty days deep wheels can be a little unnerving, especially if you’re flying downhill.

During our test, coming back down the serpentine Lookout Mountain road, the deeper wheels often required more attention and a tighter grip on the bars in windy sections.

Conclusion

Our two-man experiment wasn’t exactly laboratory science, but I was generally happy with the repeatability of the laps and the overall conditions. And I was also happy to find that the average of my paced laps generally agreed with our modeling in that the deeper wheels were slightly faster.

At gradients of less than 5%, deeper wheels are definitely faster. And at gradients of greater than 5% — at least at our amateur speeds — lighter wheels will be faster. For this average of 5% and an average speed of 10.5mph, the deeper wheels were slightly faster.

These statements are still generalizations, of course, based on our modeling and our experience. Your results may vary, as will of course your wheels, body position, power, wind conditions, and other factors.

One thing we didn’t examine in modeling was how deep wheels feel to ride, and in the real world we found what you may well have experienced yourself: Deeper wheels can absorb more side force from wind that you as a rider have to counteract.

Cycling is always a balancing act, and choosing wheels is no different. Photo: Hannah DeWitt