ARC8 Evolve FS is a lightweight, progressive XC MTB with flex stays and slider suspension

The ARC8 Evolve FS is the latest addition to the brand’s lineup of lightweight cross-country mountain bikes, entering the market at the very competitive weight of 1.78 kg (frame and shock), with the top-end XX1 build weighing a claimed 9.5 kg in size medium. For an XC bike, the geometry of the Evolve is very aggressive; the head angle comes in at 65° in the slackest configuration, a relaxed setup more commonly reserved for much longer travel bikes belonging to trail or even enduro categories.

ARC8 tell us the Evolve FS is the XC Bike they had wanted to make 4 years ago, as it is the kind of bike that co-founder Jonas enjoys riding himself. They’ve held off until now as they were convinced the market simply wasn’t ready for such a progressive geometry back then. With the passing of time, a new category has emerged; Downcountry. Love it or hate it, it’s here to stay, but there’s no denying that the lines between the Downcountry and Cross-Country categories are blurring at a rate of knots. ARC8 see the Evolve FS as the embodiment of that. 

In its most race-ready configuration, the Evolve FS pairs a 100mm fork with 100mm of rear wheel travel, while the less expensive builds get slightly more suspension travel; 110mm rear with a 120mm fork. On both, the rear wheel travel is delivered via a new suspension platform dependent on flexible seatstays with the shock driven via a slider mounted to the top tube.

We had the pleasure of meeting ARC8 co-founder, Jonas Mueller, at the Winter Bike Connection Event in Italy, where he talked us through everything one might need to know about the Evolve FS, along with a bit of backstory on how ARC8 themselves came into being.

ARC8 Evolve FS

• Bike: ARC8 Evolve FS
• Intention: XC/Marathon MTB Racing
• Frame Material: Carbon
• Fork Travel: 100-120mm
• Rear Wheel Travel: 100-110mm
• Wheel Size: 29″
• Weight: 9.5 kg (claimed) XX1 build in Medium
• Frame Price: 3,299 €

Behind the ARC8 branding sits Jonas Mueller, an engineer who previously spent 7 years at BMC developing their carbon mountain bike frames. More recently than that, Jonas was a frame designer for Bicycle.Engineering, where he was head of development of open mold bicycle frames that were licensed out to be sold by numerous brands. In fact, the very first ARC8 bicycle, the Essential, is one of those aforementioned open mold frames designed by Jonas. The Evolve Hardtail and Extra Enduro frames followed shortly afterward. 

In the four years that have passed since the creation of ARC8 as a standalone bicycle brand, Jonas has been hard at work designing frames (and components for sister brand Faserwek) that aren’t open mold; the Evolve FS you see above is the very first frame that is specific to the ARC8 brand.

Development of the Evolve FS came not from a desire to make the Evolve Hardtail a full suspension mountain bike; it is, in fact, more an evolution of the Arc8 Essential, their 4-bar linkage 120mm travel trail bike. Though that bike was never really meant to be a fully-fledged XC race bike, customers, especially those in Switzerland, were using it as such, adapting it for the purpose with 110mm of rear wheel travel paired with a 120mm fork. Indeed, Nicola Rohrbach spent two years racing on the original ARC8 Essential frame, winning the Swiss Epic on it in 2021. In that marathon/cross-country configuration, complete builds as light as 10.4 kg were possible.

Weight was never a top priority during the development of the original Essential, but the suspension layout did lend itself well to a fairly lightweight design. Naturally, Jonas became curious to see just how lightweight a full suspension XC frame they could build if they set their mind to it. And so they did.

In their quest to shave grams off the already lightweight Essential frame, ARC8 were not prepared to compromise on wall thickness. To reduce material, they instead opted to make the tubes themselves smaller in diameter. Before they even turned their attention to the suspension platform, they were able to save over 250g simply by reducing the frame’s surface area.

Moving away from the four-bar linkage of the Essential, with its ten frame bearings and all of the hardware that came along with those pivots, ARC8 were able to save a further 118g. Jonas and the engineering team were delighted to have saved over 368g without having compromised stiffness or strength, and without making the frame too fragile. 

The Evolve FS carries over the reverse main pivot seen on the original Essential, wherein the stays sit inside the seat tube and the bearings are housed in the stay itself. That recessed portion of the seat tube, blocked from view by the stays, is in fact open, whereas it is closed on the Essential frame. Now, there is no risk of debris getting trapped and potentially causing damage. That debris can of course enter the lumen of the front triangle, but there is a hole on the underside of the downtube that will allow it to eventually escape.

Another place ARC8 were able to save on weight is at the brake mount, switching from a post-mount on the seatstay to a flat-mount on the chainstay. This was also necessary as, with the new suspension design, the seatstays flex as the bike is pushed through its rear wheel travel. 

That brings us to that new suspension design, which stands out thanks to its use of a slider.

The four-bar linkage of the Essential is replaced by a suspension platform dependent on flex seatstays and a slider mounted to the top tube that guides those flexing stays to drive the rear shock. Over a link, this setup offers opportunity to create a leverage curve that is much straighter throughout the bike’s entire rear wheel travel, lacking the classical Sine shape curve that four-bar linkage designs traditionally give rise to.

Jonas is able to tune progression by changing the angle of the rails that the shock slides along. Similarly, moving the rails up or down increases or decreases the rear wheel travel. Moving the main pivot changes the level of anti-squat in the system. The amount of friction the slider adds to the entire system is said to be minimal; Jonas says, if you were to remove the slider and hold the rods in your hand, the shock mount tab that slides along those rods would move down along them under its own weight.

In the bike’s unloaded state, the seatstays are themselves deformed. It is somewhere in the middle of the bike’s 110mm of rear wheel travel where the flex stays are unloaded, before they begin to flex in the opposite direction throughout the latter half of the travel. 

Evolve FS Geometry

The ARC8 Evolve FS can be built up as an all-out XC racing bike, with 100mm rear wheel travel and a 110mm fork, or as a 110mm travel bike with a 120mm fork. In its short travel configuration the head angle sits at 66.1°, slackening off to 65° in the longer travel setup. The BB drop comes in at 35mm and 43mm, respectively, with chainstays at 430mm on both frame sizes.

Somewhat surprisingly, it is only available in sizes Medium and Large, accommodating riders from 160-190 cm tall. Jonas tells us they had originally planned to bring a small frame to market also, but found that riders as short as 160 cm felt the 465mm reach of the medium frame delivered a very comfortable fit. After consulting with Andreas Gösele, doctor of the Swiss Olympic team and a well known bike fitting expert, the ARC8 team concluded that a size small was simply not needed.

That 465mm reach pertains to the medium frame with a 120mm fork; that increases to 477mm on the more race-driven shorter-travel setup. The large frame reach comes in at 495mm with the 120mm fork, increasing to 507mm with the 100mm fork.

Evolve FS Frame Details

Another place where ARC8 could have saved additional weight is at the bottom bracket; a PressFit BB would have enabled them to reduce the amount of material here, but Jonas chose not to go down this route, opting for the reliability and improved durability that comes with the use of a threaded bottom bracket instead.

The Evolve FS was designed with internal cable routing in mind, and it seems a clean-cut aesthetic was important too. ARC8 worked with Acros on a custom upper headset cap that tucks cables and hoses into the headtube.

Though it’s not super clear in any of the photos herein (largely thanks to that Matte paint job), the underside of the downtube is actually slightly aerodynamic, presenting little surface area in the direction of travel. More important than  aerodynamics, Jonas tells us this feature also helps prevent the downtube from holding onto too much trail debris when the bike is raced in very muddy conditions, again helping to keep weight off the bike.

The Evolve FS has boost (148mm x 12mm) rear end spacing, resulting in sufficient clearance for a 29″ x 2.4″ Wolfpack Speed tire. At the dropout, a SRAM UDH hanger supports a 12-speed derailleur.

Pricing & Availability

The ARC8 Evolve FS is available from today as a frameset retailing at 3,299 €, or as a complete bike with three build options to choose from. The most affordable of those three is the Evolve FS XT, priced at 6,199 €. That one gets a 12-speed Shimano XT drivetrain with XT brakes, Fox 34 SC Float Performance 120mm travel fork with FIT4 damper, a Fox Float DPS Performance shock, and a Newmen Wheel Evolution A.30 wheelset. It weighs a claimed 10.7 kg.

Middle of the range is the Evolve FS XTR retailing at 8,999 €. That one gets Shimano XTR drivetrain and brake upgrades, with a Factory level Fox suspension and a Newmen Advanced SL X.A. 30 wheelset. It weighs a claimed 9.8 kg.

The Evolve FS XX1 is the range topping model that will set you back 9,999 €, weighing a claimed 9.5 kg. This one comes with 100mm travel front and rear, with a RockShox SID SL Ultimate fork and SID Luxe Ultimate shock. It gets an electronic shifting in the form of the SRAM XX1 Eagle AXS drivetrain, Magura MT8 SL brakes and Newmen Advanced SL X.A. 30 wheelset. 

For more information on Evolve FS models and pricing, head to the ARC8 Bicycles website.

Arc8Bicycles.com