A tubeless or tubed carbon wheelset with high-end DT Swiss internals and a huge amount of engineering homework invested in optimizing weight and aerodynamics.
Fast-feeling in virtually all conditions: uphill, downhill, accelerating, in the wind; plays nice with clincher and tubeless tires
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Keith Bontrager is often quoted for saying, “Light, strong, cheap — pick two.” The new Bontrager Aeolus RSL 37 wheels are not cheap. At a feathery 1,370 grams, the wheels feel great climbing and accelerating, and they boast some impressive aerodynamics with their 37mm-tall rims. As for strength, after a month or so of use and abuse, they are still perfectly true and seemingly on par with other top-shelf options.
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Light weight or aerodynamics or both?
Bontrager — the brand founded by the man himself and owned by Trek since 1995 — claims these Aeolus RSL 37 wheels to be “lighter than shallower wheels, faster than deeper wheels.” That’s a bold statement, and of course, one that must be quickly qualified.
The weight part is relatively easy to assess. This set is claimed to weigh 1,325g. My Feedback Alpine scale found them to weigh 1,370g without tape or valve stems. Compare that to Zipp’s 202 NSW wheels that weigh a claimed 1,506g with 32mm rims (and a $3,200 price tag) or Specialized’s Roval CLX 32 wheels that weigh a claimed 1,350g at with 32mm rims. Both of these are disc sets. You can certainly find lighter wheels at the 32mm depth in a clincher design.
Comparing the aerodynamic qualities of this wheelset versus others is impossible without a wind tunnel, and frankly, even with a wind tunnel, there is a huge amount of room for interpretation about what ‘the best’ or ‘the fastest’ actually means.
So let’s start simpler. Bontrager claims this wheelset generates less side force in crosswinds than deeper wheels, which only makes sense. I can take this claim at face value.
As for this wheelset being ‘faster’ than other wheels of similar or even slightly deeper, that’s more complicated. For its purposes, Bontrager measured drag from -20 to 20 degrees every 5 degrees, plus at -12.5, -7.5, 12.5, 7.5 degrees, and calculated for a rider averaging 28mph with a 7mph average ambient wind. The company used modeling for a weighted mean to compare this wheelset to others, giving more emphasis to what the company’s engineers believe are the angles more commonly encountered in normal riding.
The fun but infuriating thing about aero testing is the seemingly infinite amount of variables. Trying to answer a simple question — which wheelset is faster? — quickly unspools into a plethora of more questions: Faster for whom? On what bike? At what speed(s)? At what wind angle(s)? Paired with what width tires? You can get situations with two similar wheels where one is faster at one configuration of yaw and wind speed, and the other is faster at another configuration. Which is better? Not easy, right?
For me, I leave the science to the scientists, understanding that wheels will always be a balance of factors, and go ride.
Riding the Bontrager Aeolus RSL 37 wheels
My first ride on these wheels was for the debut of Project 14er, our VeloNews project that comprises climbing 14,000 feet in one ride. We chose Mt. Evans in Colorado, because it’s a Fourteener, and because the top 14 miles of the paved climb — the highest paved road in North America — are closed to cars. Cool, right? It seemed like a great way to test lightweight aero wheels, going up and down a massive mountain twice.
The one bummer about Mt. Evans is that the frost/thaw cycle chews up the road, effectively pulling the pavement apart every 40 feet or so and leaving deep gaps that can really rattle you on the descent.
Bontrager sent me 32mm tubeless tires and 25 and 28mm standard clinchers. I stared at the tires for a while. Did I want a plusher ride for the descent or a lighter ride for the ascent? The 28s seemed like the Goldilocks option.
Switching from alloy wheels to lightweight carbon hoops with fast tires is a delicious experience. The lower rotational weight and resistance is noticeable from the first few pedal strokes, and noticeable once riding in every switchback and other acceleration. Everything about the wheels feel like a dedicated climbing setup.
Once at the top of Mt. Evans, we donned all the clothing we had in our handlebar bags and bombed back down the rough pavement. While the deep cracks didn’t exactly disappear, the wheels were clearly more forgiving than deep aero wheels. (Skinny steel spokes will always have more give than thick carbon rims. The higher the rim-depth-to-spoke-length ratio, the stiffer the ride, right?)
The wind whipped us around a little bit, but the wheels never seemed to catch or pull in crosswinds as we ripped down the completely exposed corners above treeline.
Snowdrifts blocked the road in a few spots, forcing a little walking and a bit of deep-gravel riding. A few rocks dinged off the rims, but didn’t cause any damage.
Part of the weight savings seems to come from the small-diameter hubs. The flanges extend out towards the rims, but the central portion of the hubs are noticeably narrower than other wheels. This design doesn’t seem to negatively affect lateral stiffness.
While virtually all my rides since mid-March have been solo or with a coworker, I did jump into a fast group ride this week where the wheels felt like the perfect tools for the job.
Bontrager Aeolus RSL 37 construction details
The centerlock wheels are built with 24 DT Aerolite spokes front and rear on Ratchet EXP hubs with DT Swiss 240s internals. The rear hub has DT’s pleasant mechanical purr when coasting, and a quick engagement when you pedal.
The tubeless-ready wheels ship with both Bontrager’s plastic TLR rims strips and rim tape. The rim tape is light and reusable. The TLR strips are single-use as they snap tidily into place, but lose their shape when you remove them. That is to say, you can’t switch back and forth between tubes and tubeless without getting more TLR strips. On the plus side, the TLR strips are pretty close to foolproof for the initial tubeless setup.
The warranty situation is excellent. Trek offers a lifetime warranty to the original owner, and its Carbon Care program offers free replacement for any riding damage in the first two years of ownership. All carbon wheels should come with such a policy.
Notably, there is no weight limit.
Other Bontrager Aeolus options
The $2,400 RSL wheels are wonderful, but they are $2,400. Bontrager also has Aelous Pro 37 ($1,300) wheels that weigh a claimed 1,505g. Both the RSL and Pro are 37mm tall, with 21mm internal rim and 28mm external rim widths. The Pros just use DT 350 hubs and different material in the rim.
There are also new Aeolus Elite 35 and 50 wheels, both $900 per pair, with 19mm internal rim widths. The 35s weigh a claimed 1,665g, and the 50s a claimed 1,730g.
Verdict: Exceptional all-around road race wheels
Aside from the not-insubstantial price, it is hard to find fault with the Bontrager Aeolus RSL 37 wheels. For a high-end disc race set-up, the RSL 37s are lively and quick to accelerate, easy to control in the wind, and, if you believe Bontragers’ engineers — which I do — aerodynamically efficient.