New BMC Kaius 01 gravel race bike closes the gap between aero and all-terrain

We went to the rugged backroads of Sardinia to test the Swiss brand's claim that the new Kaius 01, which now has clearance for 52mm tyres, is a 'no compromise' race bike

Photos: BMC Words: Simon Smythe

A “no compromise gravel race bike that’s faster, lighter, and more capable than before.” That’s how BMC introduces the new Kaius 01. The Swiss brand is claiming improvements in aerodynamics, stiffness and weight, while tyre clearance expands to a prodigious 52mm – up from the 44mm of the outgoing model. BMC has also worked on what it calls “race ready compliance” for more extreme terrain, and equipped the frame with “smart” cargo options. Like the previous Kaius, the new bike borrows heavily – or perhaps lightly – from BMC’s Teammachine SLR 01, its flagship lightweight road bike.

Gravel racing has come a long way in not so many years, and expectations of the bikes have grown exponentially. Average speeds have dramatically increased, meaning that aerodynamics play a much bigger role. Terrain can be wildly unpredictable: traction and control over rugged ground at higher speeds necessitates higher-volume tyres. And thirdly, as with road racing, we’re seeing a return to prioritising low weight – or at placing it back on near equal terms with aerodynamics. That said, with steeper off-road gradients gravel racers are subjected to, lightness is arguably even more important than it is for the road race bike. This is BMC's response.

Proof of concept

Thanks to the indomitable Pauline Ferrand-Prévot winning the gravel world title on the first generation of the Kaius in 2022, BMC was able to demonstrate that the DNA of the Teammachine SLR 01 worked off-road too. Rouleur’s Rachel Jary rode Dirty Reiver in 2023 on the original Kaius and said: “The Kaius 01’s closeness to a road bike made it one of my favourite gravel bikes I’ve ever ridden.” The new Kaius builds on that same thinking, with reshaped aerodynamics borrowed from the latest Teammachine SLR 01, launched at the end of last year, as well as the Teammachine R aero bike, as raced by the Tudor pro road team.

BMC says its engineers took all they learned from the first generation of Kaius, combined it with the lessons from the Teammachine series of bikes and applied them to this newest generation: “From the dramatically re-engineered aerodynamic headtube profile to the addition of the F1 inspired Halo fork, and the significantly increased tire clearance, the latest generation of Kaius has been reinvisioned to meet the demands of high performance gravel racing.” It has added aero, tyre clearance, cargo capability, preserved stiffness while claiming to lose 63 grams from the frameset.

Why lightweight gets harder

What were the learnings exactly? At the new Kaius’s presentation to the media in Sardinia last month, BMC’s head of R&D, Stefan Christ, told us: “In between the Kaius generations we had two major projects that were pushing the weight boundaries. One was the Teammachine R, but the really important one was the last generation of the Teammachine SLR. We completely revised our material choice, our thinking about how to build a bike with a certain stiffness that can be super light."

He continues: “From the Teammachine SLR, from one to the other generation, we saved 16 per cent or 220 grams, which was huge. This new way of thinking carbon layups helped us to make this bike a little lighter than the previous generation but with things that normally push weight up: better aerodynamics means more surface area; so does more tyre clearance and maintaining the stiffness. So working on those three things that go against weight, we were still able to save four per cent.”

A sub-kilo gravel bike frame is challenging to create, especially with aerodynamic elements, says Christ. The claimed frame weight is 998 grams in a size 54 (with black paint) with a full build with the top SRAM Red XPLR AXS spec weighing a claimed 7.1kg.

Fork and seatpost

One of the crucial pieces of the puzzle is the new Halo fork, which weighs a claimed 400 grams. “You think ‘Oh, it could be lighter’ but we’ve done our homework on the frame and fork to make sure the rider experiences the best overall feel,” says Christ. “The tyre clearance on the fork is huge, and that means you have to go a longer way between the force introduction points, which typically does not add stiffness. So maintaining the lateral stiffness, which is super important for keeping your line but also so that you can steer around obstacles that you see at the last moment. But at the same time the fork has to be compliant to absorb the small vibrations, and this is not easy. Additionally, we clearly designed it as an off-road bike, so we built in a little bit more mechanical strength. I want certain wall thicknesses for robustness of the frame as well. So the fork was one of the big parts of the development.”

And the final piece of the frameset’s weight-saving, drag-reducing and compliance-adding jigsaw is the AS10 seatpost. Christ says: “This is one of the specific elements we borrowed from the Teammachine SLR to complete the frameset. It is the latest of 10 different generations of seatposts that have been created over the decades. It is extremely light at 134 grams with hardware.”

Aero gains

BMC’s AeroSynthesis is a design philosophy and technology approach that the brand says is focused on harmonising aerodynamic performance and system integration in bicycle development. At the presentation, Christ explained what’s behind the name: “It exists first of all on the new ICS aero cockpit of the Kaius, but we don’t design only one component: we design a whole system – a frameset with everything including groupset and wheels. Making a bike faster is way more challenging than making a frameset faster. We’re looking at the impact of the rider on the frameset, of the components on the frameset. So that’s why we always develop all of our bikes aerodynamically with a rider, with equipment including helmets and clothing. We have 3D scans of real athletes, and we make sure that whatever we come up with in terms of aero solutions, we validate for different athlete profiles.”

That said, zooming in on the frameset only, BMC claims a 12 per cent reduction in aerodynamic drag over the previous Kaius at 40kph with 45mm tyres. The standout aero features are the Halo fork, which has a wide crown designed to push airflow away from the wheel and tyre as well as creating big tyre clearance. It’s a shape also used in BMC’s aero platforms such as the Teammachine R and even the Timemachine Mpc, the all-new TT bike. And of course it’s a fork shape used universally and more frequently now, beginning with the Hope-Lotus track bike and more recently deployed by Factor in its ONE aero road bike.

The AS10 seatpost, borrowed from the Teammachine SLR, is also faster than the previous version; the down tube is sculpted so that the bike is more aerodynamic with bottles and a deeper head tube improves airflow at the front.

Turning up the tyre volume

BMC points out, perhaps needlessly, that to maintain an aerodynamic position on a gravel bike, you need to be able to trust its handling and traction, and the number-one factor in a bike’s handling and traction is tyre width. “When developing the latest generation of Kaius, the BMC engineers knew that in order to futureproof the bike, they would need to increase the tyre width clearance,” it says. For this reason, the new Kaius is compatible with 52mm tyres while maintaining the ISO 6mm clearance. The Kaius’s geometry – mostly down to bottom bracket height – allows tyres all the way down to 32mm to be ridden without compromising handling.

1x only

As part of the futureproofing, BMC developed the new Kaius frame around a 1x groupset with the ability to fit gravel or MTB gearing. That means – if it wasn’t obvious already – that it’s not compatible with 2x groupsets. It’s not compatible with Shimano 12-speed Di2 groupsets that use a wired battery either, including RX810, RX825 or RX820. Only the RX820 in 1x configuration and RX710 are compatible since they feature a wireless rear derailleur with an integrated battery.

The frame has a mounting point for an integrated lightweight chain guide and can accept chainrings from 38t to 50t, or 32t to 50t without the chain guide.

The pressfit BB has been abandoned: now it's a threaded T47x85.5 system for SRAM Dub with bigger bearings.

Five storage cargo points

There are more points on the frame to stash essentials – five in total including a bottle cage mount under the down tube, as well as a new top tube cargo plate mount that can be used to stash an extra inner tube, a CO2 cartridge or an electronic pump. The idea is for racers not to have to fill their pockets or fumble with zips on bags.

Signature ride feel

BMC says there are multiple contributors to its signature ride feel. With the new Kaius, its Tuned Compliance Concept (TCC) is engineered with the aim of keeping a rider fresher, faster, and in control over rough terrain. This is done through the carbon layup, and then with a number of new technologies developed for Kaius, such as the seatpost, seat tube and seat stays. The AS10 aero seatpost, as found on the latest Teammachine SLR 01 helps it to supply what BMC claims is the same compliance as the previous generation Kaius. The new compliant Arete seat tube, in combination with the AS10 aero seatpost, suspends the rider, offering a few centimetres of vertical deflection (obviously dependent on rider weight).

Geometry

The reach, stack and bottom bracket height have been tweaked in order to deliver a bike that BMC says allows for a more aggressive riding position, but still has stable and confidence-inspiring handling.

Taking feedback from its pro gravel riders, BMC created a geometry with a 5mm shorter reach for an aggressive position and to shift more weight above the front wheel. There’s an 11mm higher stack aimed at better control on rough ground. There’s the same BB drop resulting in a 10mm higher ground clearance thanks to bigger tyres; 4mm longer trail to improve front end traction, and chainstays that are 5mm longer than before, for improved rear-end traction. The head tube angle remains as before.

First ride impressions

I hope this doesn’t come across as unoriginal, but I’m going to echo the comment Rachel Jary made after she rode the original Kaius. This felt more like riding a road bike than any gravel bike I’ve ridden so far – and I mean riding it over some very rugged terrain in the hills of Sardinia and not just small stones on a Forestry Commission fireroad.

Let’s be honest, many of us – perhaps even most of us – come from a road rather than a mountain bike background, and that means we absolutely need a gravel bike to be kind to us when we lack the skills or expertise to ride bikes as fast as we’d like to on technical, steep singletrack. My riding in Sardinia with the Kaius was the first time I’ve not once wished I was on a mountain bike when it gets a bit tricky.

As the BMC engineers themselves had told us, a lot of if comes down to the tyres. Our Kaius bikes were fitted with Continental Dubnitals in the new 50mm size that was launched in mid-April. This was an XC tyre that was downsized for gravel, so is very grippy indeed on steep, sandy climbs but, as we found, also surprisingly fast-rolling on asphalt. On the twisty road descent to our lunch stop, we hit 78kph behind BMC gravel pro Roman Holzer – and it wasn’t in the slightest bit worrying.

So the traction was fabulous and the rolling was fast, but the responsiveness of the Kaius to a change of line mid corner to hop to the other side of a dangerous-looking rut was also seriously impressive. We attacked every off-road section, only slowing down and bunching up again to skirt around the edge of the large puddles in order to keep socks dry and shoes white. Then the group would explode again, with the youngest and strongest of us – like Holzer, who was of course still riding easily for him – sprinting away down the dirt tracks or trails off into the mountains.

On long off-road climbs – and there were a couple of big ones in the 80-kilometre ride on day one – the Kaius felt light and efficient. It was easy to settle into climbing mode at FTP without getting distracted by obstacles or the unevenness of the trail. The geometry felt familiar, only almost imperceptibly higher at the front and shorter than that of a road bike, but exactly right for extra control when needed, or for when rider weight needed to be shifted rearwards on a steep descent. After a final stretch along the beach, where Stefan Christ took a tumble into the sea when he tried to ride the Kaius through a deep channel, I finished feeling much fresher than I expected, and no more beaten up than on a four-hour road ride. The comfort wasn’t something I had really noticed until we finished – it was perfectly judged.

For ride day two, a local loop closer to our base at San Teodoro in the north of the island, we were offered narrower tyres to test the difference. I was so taken with the Kaius’s performance with the 50mm Dubnitals that I chose to stick with them. For me, this was a sweetspot that I wanted to enjoy a little longer: a light, stiff bike with tyres that are wide but not so wide that they feel heavy or cumbersome, with enough grip and volume to get me up or down anything – that’s what it felt like. On this shorter loop there was the techiest climb of the two days, involving winching the bike in a rocky rut up a sandy hill between the bushes very slowly in bottom gear, almost track-standing at one point to change direction. I did it three times because I enjoyed the fact that I could clean it so easily.

It got me thinking about the Kaius’s designation as a ‘race bike’. Yes, it's the answer to the gravel racer's prayers, but was I misusing it because I wasn’t racing it? My conclusion is that for gravel-curious roadies who want a bike that doesn’t feel noticeably different from their regular ride, the Kaius is ideal. It is so capable that it literally turns off-road into road. He’s probably heard the same joke many times, and I fully expect a weary, exaggerated eye-roll, but although Christ couldn’t ride it on water, he and his team have nevertheless performed a miracle with the Kaius.