Bontrager announces next generation of Aeolus RSL superwide aero road wheels

Published April 15, 2021 01:00PM

Bontrager has today released its latest range of Aeolus carbon fiber aero road wheels. Offered in 51, 62, and 75 mm depths, most of the claims and features are about what you’d expect. They’re more aerodynamically efficient, more stable in crosswinds, and they’re wider than what came before. However, all three of the new wheels are not only disc-only, but also clincher-only, including for Trek- and Bontrager-sponsored pro teams, who have supposedly been using them since early this season.

The latest frontier in aero road wheel performance

The progression of aero road wheels over the years can be summed up in several key stages. First, designers were primarily concerned with shapes that were fast in a straight line, leading to all sorts of nominally V-shaped rims. Then crosswind speed became a bigger deal, which blunted all of those shapes, making the cross-sections look more like a U than a V. When engineers then put greater emphasis on crosswind stability, those U shapes got rounder still, and also fatter in general.

Tire beds have continued to grow in size — although continually smaller relative to the outer rim width — as designers now look more at rolling efficiency as a way to eke out some more speed.

Bontrager’s latest rim shapes were developed with the help of fancy new computers at Trek HQ, which not only run computational fluid dynamics simulations in 3D, but also actively help the engineers tune the shapes to get the desired characteristics. So yes, Bontrager is actually saying that while meaningful drag improvements are getting increasingly hard to eke out of modern aero road wheels, they’re still some juice left in there.

Aero matters aside, though, what will perhaps be more tangibly noticeable to everyday riders is the broader inner rim width. All three of Bontrager’s latest Aeolus aero road wheels have very generous 31 mm outer widths matched with 23 mm inner ones, both of which have supposedly been optimized for 25 mm-wide tires (printed width, which would plump up to about 27 mm on that wide a tire bed). In that configuration, Bontrager is claiming some “off the charts” gains in terms of both efficiency and stability.

Things get more interesting when you factor rolling resistance into the equation. Wider rim beds have long been seen as a way to improve tire efficiency given the broader shape, increased air volume, and lower operating pressures, but it’s only recently that wheel brands have been so deliberately incorporating that aspect into their design toolboxes.

When taking both aerodynamics and rolling resistance into consideration (including the switch from tubulars to tubeless clinchers), the simulated numbers get pretty interesting. Bontrager says a rider at the front of the peloton traveling at about 45 km/h (28 mph) will save 6.3 watts of effort when using a tubeless-equipped Aeolus RSL 51 instead of a tubular-equipped Aeolus XXX 4 (disc-brake for both). And in a 1,500-watt sprint — where aerodynamic gains would be magnified — Bontrager is claiming a whopping 34 W of total savings when using the new Aeolus RSL 62 with tubeless tires instead of the Aeolus XXX 6 with tubulars.

Speaking of racing, Bontrager says the Trek-Segafredo riders have been using the new wheels this season, including with tubeless tires from team sponsor Pirelli. In fact, Bontrager says that Jasper Stuyven started on an Aeolus RSL 75 rear and Aeolus RSL 62 front wheel in his win at Milan-San Remo (only switching to an older XXX front tubular after a puncture due to supposedly limited supply of the newer wheels at the time).

Tubulars and rim-brake wheels aren’t completely dead at Bontrager, by the way, though they’re clearly on life support.

“We will continue to have rim-brake options in carry-forward wheels, and actually, the Pro 5 rim-brake is one of our best selling wheels,” said Bontrager marketing manager Alex Applegate. “As for tubular, we will continue to have the Aeolus RSL 37 offered in a tubular. The 37 has a narrower profile, is optimized more for lighter-weight climbers and ‘cross, and makes more sense for tubular applications. That wheel will stay in the line-up.”

Model details, pricing, and availability

In addition to inner and outer rim widths, the three Aeolus RSL TLR Disc wheel models — the RSL 51, RSL 62, and RSL 75 — also share the same DT Swiss Aerolite bladed stainless steel spokes (24-hole front and rear), DT Swiss Pro Lock externally located nipples, and low-profile Bontrager cartridge bearing Center Lock disc-brake hubs, which combine features from DT Swiss 240s and 180 hubs and use that company’s latest Ratchet EXP driver mechanism.

Claimed weights for the three depths are 1,410 g, 1,520 g, and 1,645 g per set, all with a retail price of US$2,400 / AU$3,800 / £2,000 / €2,400. Wheels will be sold individually in case riders want to put together a staggered setup, and none of the wheels have any sort of rider weight limit.

For riders on more reasonable budgets, Bontrager is also offering the Aeolus Pro 51 TLR Disc, which uses the same rim shape as the Aeolus RSL 51, but with a less expensive grade of carbon fiber, the same spoke and nipples, and similar Bontrager hubs based on DT Swiss’s mid-range 350 model. Retail price is US$1,300 / AU$2,200 / £1,300 / €1,300, and claimed weight is 1,590 g per set.

One thing to keep in mind regarding the tire compatibility and claimed weights is that Bontrager quotes those figures in tubeless guise, including the molded semi-rigid plastic rim strips that provide the inner seal and beadlock capabilities needed to keep the whole system airtight. Each of those rim strips weighs 60 g, but since they’re not needed if you’re going to run tubes, the real-world weight is actually a bit lighter. Bontrager also offers a tape-based tubeless system at the ready, too, which still keeps things airtight while saving about 100 g.

I’ve got a set off Aeolus RSL 51s here for review, and the actual weight is just 650 g for the front wheel and 770 g for the rear, for a grand total of 1,420 g without rim strips at all — a truly impressive figure for disc-compatible carbon clinchers this depth from a mainstream brand.

We’re now apparently in “fourth winter” here in Colorado, but hopefully this is the final round before spring arrives in earnest so I can finally get out on these things. Stay tuned for a more in-depth review soon.