29-inch wheels, a 67.5 degree slack head angle, short 429 chainstays and a 455 millimetre reach in size L. What the Swiss bike manufacturer BMC has concocted sounds like a fine trail bike that you can both bolt and play with. But far from it. With the Fourstroke for 2019, BMC presents the latest iteration of its most thoroughbred race fullys. While the 10-kilo race file is also nice and light, it's also one of the most forgiving and extreme race bikes ever to dig its lugs into a World Cup track. And BMC has a few more special features in store. In this article, you can find out how this bike came about and how it proved itself in an initial practical test.
BMC developer: "We wanted to provoke."
Flashback: World Cup in Lenzerheide more than two years ago. Nino Schurter takes victory for Scott on a fresh 29-inch Scott Spark and goes on to win Olympic gold in Rio that year. The strongest BMC rider is record world champion Julien Absalon in second place, of course on the Fourstroke, which is now several years old. But it could be even better, BMC thought, and the very next day they put a prototype in the team's garage that could hardly be more extreme. A 2.5 degree slacker head angle, one centimetre more reach, a two degree steeper seat angle and a few millimetres shorter chainstays. An angry Fourstroke with a geo that the developers stole from the trail bike drawer with the "Aggressive" label. "We actually wanted to provoke the guys with it," we were told later, but the expected reaction failed to materialise.
Titouan Carod actually wants the bike just like this, with even shorter chainstays and perhaps a 60 mm stem, and record champion Julien Absalon is also visibly impressed. And from then on, the story takes its course. The chainstays become even shorter, as does the stem, and soon a vario support is also an integral part of Absalon's race bike and thus an integral part of the concept. And the development goal is clear: as light as the old Fourstroke, but with aggressive geometry and an integrated dropper post. And, of course, the whole thing has to be rock-solid so that it sprints forwards properly when the chain is pulled, but that kind of goes without saying for a World Cup bike.
As light as the old Fourstroke: stiffer, more modern and with Vario support
There's just one problem: stiffness, aggressive geometry, Vario support, it all weighs quite a bit. So a radical diet is definitely needed in the right places. Firstly, the dropper post. The developers said that everything on the market was too heavy, so they put all their eggs in one basket and dared to come up with their own design. The result was a mechanically articulated support with a spring and a closed, internal cartridge with a pressure of just two bars. This minimises the operating forces, an important factor on the world's racetracks. For weight reasons, the post runs in plain bearings directly in the frame, saving the outer tube of the Vario post compared to conventional systems. At the same time, the oval diameter of the post ensures smooth running and protection against twisting. In addition, the design is said to be lighter than a classic round post, which usually has to be secured against twisting with brass pins running in grooves.
The dropper post called "RAD" offers 80 millimetres of travel. Definitely sufficient for XC races and a value that was allegedly also the clear wish of the team riders. According to the motto "keep it simple", there are only two states: extended or retracted. The "RAD" cannot be fixed in intermediate positions. Some people will now grumble, but we didn't actually find this to be a hindrance in an initial practical test. And the weight of the "RAD" is really impressive. The telescopic seatpost weighs around 370 grams, BMC says it weighs a good 140 grams more than conventional, non-retractable seatposts. We would rather say a good 160 grams lighter than good conventional dropper posts.
Radical diet for the Fourstroke, investment in rigidity
The slimming cure goes into the nitty gritty, because as we all know, small things make big things: thru axles have been tapered, shock bolts are made of aluminium instead of steel and the rocker is now also made of carbon fibre instead of aluminium as before. These small details save just over 100 grams, which can then be invested in stiffness in the frame. The bottom bracket area has been made significantly stiffer and the headset is also stiffer. A separate, particularly wide 60 millimetre stem transfers the stiffness of the frame to the handlebars - important in tough terrain and when pedalling hard. According to the manufacturer, the overall stiffness of the frame has been increased by 15 per cent compared to the already very stiff predecessor, and the new Fourstroke now even beats the BMC Teamelite hardtail. And of course the rear triangle is not least to blame for this.
Wide bearings can be seen here at the rear, carbon everywhere, only the connection between the rear triangle and the main frame is made of aluminium. This makes the bike narrower and enables the short chainstays. And speaking of chainstays: Unlike in many race fullys these days, those of the BMC do not flex. Another conscious decision by the BMC developers, as flexing stays are supposed to produce a fluctuating anti-squat value in the rear triangle. Kinematics Latin? Yes, but with real consequences: namely noticeable pedal kickback when pedalling over bumpy terrain. And that's exactly what the new Fourstroke isn't supposed to have.
First contact: The Fourstroke on the World Championship course in Lenzerheide
To summarise: Aggressive geometry, lightweight dropper post with 80 millimetres of travel but only two saddle positions, stiff frame, total weight of the top model Fourstroke 01 One of 10.4 kilograms in size M with tubes without pedals, just over 10 kilos in tubeless setup. And how does the whole thing perform in practice? We have already been able to ride the new BMC Fourstroke on the World Championship course in Lenzerheide and got a first impression of the new bike there. On the only long climb on the course, right at the start, it is noticeable that not too much has been promised in terms of stiffness. Even with the shock open, the bike only bobs minimally. If you close the damping with the remote control, the bike shoots up the climbs and feels as stiff as a mid-range road bike. Nothing twists, nothing wobbles, not even under the test rider's almost 80 kilograms. First test passed.
When it's time for the descent, you click the suspension elements open again with a quick push of the thumb, retract the actually quite smooth-running vario support and shoot towards the first root field. The moment of truth, but no nasty surprises. The suspension is plush, but still remains high in its travel, even a botched line choice or a large drop onto the flat won't make the Fourstroke break a sweat. The tyres also work well right from the factory, the Swiss didn't install too-light tyres here, but decent "ready-to-race" goods. Even on flat, bumpy sections and with traction on the chain, the rear suspension remains as smooth as promised and generates a lot of traction. We didn't have any problems with the front wheels rising on difficult climbs. The first impression is therefore positive. Perhaps the only catch: the price. Starting at a good 6000 euros, the top model BMC Fourstroke 01 One costs almost 10,000 euros.
Adrian Kaether
Editor
