BMC Kaius 01 One review

BMC brought along the Kaius to complement its more rugged and extreme-terrain focused URS and URS LT gravel bikes.

Instead of suspension and slackened off-road geometry, the Kaius is all about low weight, aerodynamics and straight-line speed.

Technically, it’s quite an achievement, and could set a new benchmark for racing gravel bikes.

However, it’s a bold move to make a bike so focused on pure gravel racing. How does the Kaius fare on more varied terrain, topography and trails?

BMC Kaius 01 One frame specifications

The Kaius was conceived to be a road-race-light gravel machine from the outset, with the frame and fork design sharing plenty of tube shapes with the road-race BMC Teammachine SLR.

The sculpted angular head tube, oversized down tube and tapering top tube all look to have come straight from the race bike playbook.

At the rear, a cutaway seat tube pulls the rear wheel inward to keep the back end tight for quick handling.

The thin seatstays are spaced broadly to enable tyres up to 44mm wide to be fitted. The fork has equally generous clearance.

The frame features a wide spectrum of carbon fibre specifications throughout.

BMC says it has opted to use more rigid high-modulus fibres concentrated in areas that require stiffness (namely, the head tube and bottom bracket area).

More compliant fibres have been used elsewhere to help reduce vibrations.

Despite its obvious performance gravel bike leaning, the Kaius frame has also been reinforced in areas such as the bottom bracket and underside of the down tube. In theory, this adds resilience against rock strikes and shocks over rougher surfaces.

Even with that reinforcement, BMC has kept the frame weight low. It carries a claimed weight of just 910g in a size 56cm, and the fork a further 400g.

Completing the frameset ‘module’ is a 160g carbon seatpost and an aero-optimised cockpit that adds another 315g.

The 58cm bike on test here tipped my scales at an impressively light 7.72kg.

BMC Kaius 01 One geometry

The Kaius is all about speed, so its ride position comes influenced by road race bikes rather than gravel bikes.

That means a long 410mm reach, which effectively shifts the front wheel forwards for improved stability and traction.

The ICS integrated cockpit brings an effective 100mm stem length to this 58cm test bike.

It’s a little shorter than you’d typically expect on a bike of this size, but compact enough that in gravel races it’ll help sharpen up the steering reactions.

Comparisons might also be made between the Kaius and BMC’s suspension-equipped URS LT. However, the Kaius has a much lower stack (595mm) and that longer reach.

The Kaius also steepens up the head angle to 72 degrees, compared to 74 degrees on the URS.

The wheelbase on the Kaius is also significantly shorter – a 58cm bike comes in at 1,039mm, compared to the Large URS LT’s 1,081mm. The Kaius has a road-endurance like 68mm trail, while the LT has a much longer 77mm.

In short, instead of being compared to a gravel bike such as the URS LT, the Kaius’ numbers wouldn’t look out of place on a pure road bike.

BMC Kaius 01 One build

On a bike with a price tag that’s into five figures, I don’t expect to make any complaints in the build department. For the most part, that’s true here.

The Zipp 303 Firecrest wheels are my current favourite do-anything wheelset.

The low weight, sublime ride quality and responsive nature make for a wheel that’s – in my opinion – unbeatable for the money you’d spend aftermarket.

The same goes for Fizik’s brilliant Argo saddle.

While choosing a bike saddle is highly subjective, the Argo seems to bring a great blend of supportive shape and generous padding (along with low weight in this Argo 00 form).

The drivetrain and brakes come from SRAM’s gravel flagship Red AXS XPLR, the 44-tooth chainring paired with a wide 12-speed 10-44t cassette.

That gives a top-end equivalent of a 52/12t on a road bike groupset. The bottom gear (which provides a 1:1 ratio) is more than light enough to keep you spinning on the steepest climbs, traction allowing.

While the bike is a challenging ride at times, shift speed is superb, and the chain management from the fluid-damper equipped XPLR derailleur keeps the chain taut no matter how rough the going gets.

The braking is full of progressive control and I like that BMC has opted for large 160mm rotors. It gives the Kaius such fine braking control, which is essential if you want to ride quickly in all conditions.

The Pirelli Cinturato tyres are brilliant on tarmac and hard-packed dirt roads, though they aren’t equipped well for anything approaching mud. If you have that sort of terrain in mind, a switch to a better suited set of gravel tyres will be a good idea.

The Kaius has a big emphasis on aero, evidenced by BMC’s Aerocore bottle cages – like those found on the Teammachine – which integrate with the frame to smooth airflow.

The one-piece ICS Carbon Aero cockpit follows the same design cues as the BMC’s road-going ICS systems.

Here, though, the bar shape is very different.

The top section of the bar is just 360mm wide from hood to hood, in keeping with the current racing trends towards narrow widths designed to benefit aero performance.

The bar drops then flare out at 12.5 degrees, widening to a more familiar 420mm at the drops.

The trend of super-narrow bars for road racing has some traction, because making your frontal area as compact as possible saves watts. I’m not convinced the upsides are compelling off-road though.

Instead, the Kaius 01 Two model, which comes with an ICS2 stem and a wider RCB01 gravel bar (as found on the URS), could offer similar lightness and speed, but with a steering feel that suits more varied terrain.

BMC Kaius 01 One ride impressions

The Kaius is a pure race machine, even more than it is a gravel bike. As soon as I took it out on its maiden ride, it felt every inch a rapid bike.

On tarmac, it rides much like the brilliant Teammachine SLR, albeit running 700 x 40c tyres with a tightly packed knobbly tread.

The ride position is long and low, and the inherent stiffness through the cranks is more noticeable than almost any other gravel bike I’ve ridden.

Throughout the many hours of testing I’ve taken the Kaius on, and over a multitude of surfaces, it’s arguably the most rapid gravel bike in contention for Bike of the Year 2023, providing conditions and terrain are in its favour.

On broken roads, wide forest fire roads and unmetalled surfaces, the Kaius is without peer. Hunker down into the slim drops and you can really motor it along at road bike speeds with ease.

The generous 40c Pirelli Cinturato Gravel tyres quash vibrations incredibly well – I really thought I’d found an epic all-day ride partner.

However, when the surfaces turn choppy, rocky, rutted or rooty, the Kaius becomes quite brutal.

The stiffness and efficiency so endearing on smoother dirt make way for bone-shaking, finger-numbing harshness up-front, compounded by the cockpit’s compact dimensions and stiffness.

Only dropping tyre pressures down below recommended levels began to cure this.

On rough roads and trails, line choice is paramount. But the amount of effort required to keep the Kaius on line was very high.

When things were more technical and tricky, I felt the road-derived geometry and the narrow bars hampered me at every stage.

When I hit my favourite singletrack sections in the woods, or pointed the Kaius downhill on double-track and singletrack trails, the front end always felt as if I was fighting with it.

It meant I spent much more time down in the drops rather than the hoods. However, even the 42cm outside-to-outside measurement at the bottom of the drops is arguably too narrow.

There’s a good (handling) reason you don’t see mountain bikes with super-narrow bars, so considering BMC makes some of the best cross-country mountain bikes around, the bars on the Kaius seem an odd choice.

BMC Kaius 01 One bottom line

On rolling, wide unmetalled roads, and forest fire roads (and even on tarmac, for that matter), the Kaius 01 One is brutally, intoxicatingly fast.

If that’s your idea of gravel-riding heaven, the Kaius is very hard to beat.

However, by its very nature, gravel isn’t always like that, and the Kaius can quickly become a real handful when you approach more technical or twisty terrain.

The racy geometry and narrow focus of the Kaius play a part, but the very narrow ICS cockpit spoils much of the good work the frame can do.

In this build, the Kaius One is an impressively speedy weapon aimed squarely at racers, but it doesn’t leave much for anyone else to enjoy.