Sonder Cortex NX Eagle Trail 29er review

How good is Sonder’s affordable short travel 29er Cortex trail bike? We got a ton of pre-lockdown miles in - and a bit of component swapping - to find out

Sonder Cortex NX Eagle Trail
(Image: © Guy Kesteven)

Bike Perfect Verdict

Excellent baseline chassis and kit for the cash but benefits from a shock upgrade if you want plush grip not just pedalling grunt


  • +

    Well-balanced, affordable, lightweight trail chassis.

  • +

    Mostly well-sorted very cost-effective componentry

  • +

    Cane Creek shock upgrade makes it feel great

  • +

    Other components switchable on request

  • +

    Very solid pedal response with the stock shock


  • -

    Stock shock clatters rather than grips and blows through travel

  • -

    Wooden rear tyre emphasises lack of shock sensitivity

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Affordable outdoor gear specialists Alpkit already had a great reputation for no-nonsense bike packing bags so adding an actual bike arm - Sonder - seemed a logical step. They knocked it out of the national park with their first wave launch that included the multi award winning Transmitter (aggro plus tyre hardtail) and Camino (gravel bike). Can the Cortex add to the brand momentum with its 120mm of rear suspension and 29er wheeled trail bike?

Design and geometry

As you’d expect from a bike range that starts at £1,599 for the SX Eagle build, the frame is all alloy. The main tubes are still double butted and pressure shaped though and you get internal cable routing. At 3.2kg for a medium with a shock, it’s competitive with some similar travel carbon frames that cost more than the whole bike too.

The standard-fit Rock Shox Deluxe shock uses up to date Metric shock sizing with a relatively long 190mm body, short 45mm stroke ratio giving extra overlap for longevity and smoothness. Suspension layout is a classic 4 bar with a rear pivot on the chainstays and rocker plates on the curved seat tube driving the shock. That leaves room for a large bottle inside the frame and there’s room for up to a 2.6in rear tyre between the asymmetric chainstays. You get a neat Syntace style vertically bolted replaceable gear hanger behind the bolted Boost width rear axle and a top of seatstay brake mount means no limit on rear disc size.

Sonder Cortex NX Eagle Trail

The Sonder Cortex comes standard with a 190mm long metric Rock Shox Deluxe shock  (Image credit: Guy Kesteven)

Geometry is similarly sorted and practical with a 66º head angle, 74.5º seat angle and a 465mm reach on the large with a 130mm travel fork. Chainstays are relatively long at 447mm but BB drop is a happy medium 40mm. The frame comes in 4 sizes in 3 different colours (metallic brown, off white and the military green here) and frame and shock are available separately for £999. You can also change to a Cane Creek DBAir Inline shock for an extra £150 but more on that later.

Components and build 

Cortex comes in three levels of build, available direct from Sonder or through Alpkit’s four shops. Either way, as each bike is built by hand at Sonder HQ in the UK you can use their bike builder menu to tweak some more personal component aspects. More options outside those listed are potentially available if you ask nicely too so if you want to add your own favourite kit or just upsize the 180/160mm rotors on the adequate but not amazing Guide T brakes, that’s an option too.

Going off the books the SRAM NX Eagle bike here is the mid-price option with a RockShox Revelation fork and an X-Fusion Manic dropper post as standard. There’s also an SX Eagle version for £1,599 with the dropper as an upgrade and a GX Eagle versions with the awesome Pike Ultimate fork for £2,499.

Sonder Cortex NX Eagle Trail

Sonder's own brand Love Mud supplies much of the finishing kit including a 45mm stem and 780mm low riser handlebar (Image credit: Guy Kesteven)

They all get the same appropriately-short 45mm stem and 780mm low riser from Sonder’s Love Mud range and lock-on grips and saddle are Love Mud branded too. Our sample came with Love Mud Hobo 29er wheels due to a supply hiccup but bikes normally come with Nova wheels. Either way, you’re getting tough 29mm internal ‘Hobo’ rims, bombproof plain gauge spokes threaded between brass nipples and good quality hubs. Tyres are WTB with a toothy 2.5in ‘High Grip’ compound, ‘Light’ carcass Vigilante upfront and a closer-spaced 2.4in ‘Fast Rolling’ compound, ‘Tough’ casing Trail Boss at the rear. It comes fitted with inner tubes but tyres and rims are tubeless ready so it’s a simple swap (or a £40 upcharge including valves and sealant when you order).

Ride, handling and performance 

The tyres reflect the overall character of the Cortex really well too in that everything is where you want it and you can go out and ride really hard straight away. The front end is slack enough for decent stability in loose or slippery situations without feeling like you’re trying to negotiate a wheelbarrow around every slow speed switchback. Reach is similarly balanced between roomy without being too radical for riders coming off older bikes. The grippy Vigilante digs confident traction out of any surface from loose rocky to wet slop and the short stem and wide bar give good weight to responses. Given the choice, we would fit a shorter offset fork to increase the feeling of stability at speed while lightening up the bar feel.

In terms of suspension, the 130mm travel Revelation fork is very user friendly with a wide bandwidth on acceptable damping setup and good tracking stiffness. The air spring is soft and grippy off the top, supportive in the centre and usefully slam proof if you really smash into/off something. In other words, you have to be going pretty hard to overwhelm the front end.

The back end isn’t quite as balanced though. On the plus side, the default tune on the Deluxe shock gives a firm initial feel that translates into very stable pedalling, even when you’re out of the saddle and mashing it hard. Add the harder compound closer spaced rear tyre and while the wheel and rubber weight takes some grunt to get turning, it rolls really well once you’re going. That means the Cortex surprised us with how well it hustled for a 15kg-plus bike, even on hardback and it regularly put others in the hurt locker when things got competitive. The armoured rear tyre means no worries about bursting sidewalls when 120mm isn’t enough to keep the rim off the rocks.

The flip side is that the firm initial feel stops the shock tracking the ground as well as it could. The Tough carcass of the tyre feels wooden even at low pressures too and makes for a higher sprung to unsprung weight so it’s more likely to slap rather than smooth over stutter bumps. Even with three bottom out tokens as standard, we needed a lot of pressure (20% sag) to stop the shock from slamming through its travel very easily. This obviously impacts sensitivity too and after the first set up ride we actually pulled the shock out to check if the pivot bearings were moving properly (they were). 

Sonder Cortex NX Eagle Trail

The added adjustment when upgrading to the Cane Creek DBAir Inline shock is well worth the £150 extra  (Image credit: Guy Kesteven)

Thankfully you can order your Cortex with a Cane Creek DBAir Inline shock for £150 more which we reckon is money really well spent. Separate high- and low-speed compression and rebound adjusters mean you need to know a bit more about what you’re doing with your shock and we’d keep a 3mm Allen key handy to tweak regularly on your first few rides. With minimum low-speed and mid-level high-speed compression damping, the back end is grippingly, comfortably sensitive but the shock stays happily mid-stroke for consistent cornering and jump-pumping support. It stayed well off full travel unless we properly slammed it too and while there’s a bit more movement when pedalling that’s easily sorted by flicking the ‘climb’ switch across. The ride-feel and acceleration improved even more when we switched to a lighter carcass Tritech compound Trail Boss too, although that does obviously come with potential flat risk consequences.


Cortex is a light, well-shaped frame that balances instant familiarity with extra confidence on faster, rowdier trails. Add an easy to set up fork, decent stop, go and finishing kit rolling on sturdy wheels with a properly surefooted front tyre and you’ve got a great package for the price. If you’re a hard rider (particularly one coming off a hardtail) who hates rear flats and pedal bob then the default shock and rear tyre spec makes sense too. Upgrading to the Cane Creek DBAir IL gives a much more control rich and tuneable result though. Swapping to a less wooden rear tyre really unleashes the full potential of what’s basically a really great modern trail all-rounder and the fact that all these aspects (and more) can be sorted before you buy is a big bonus in Sonder’s favour too.

Test conditions

  • Temperature range: -2 to 12 degrees 
  • Surface: Mixed dirty woods, red and black grade rocky trail centres, moorland backcountry day and night
  • Route: Stainburn trail centre black and reds, Blubberhouses moor singletrack, local technical trails

Tech spec: Sonder Cortex NX Eagle

  • Price: £2,049.00
  • Frame: 6061 T6 alloy mainframe and swingarm 
  • Size:  Large 
  • Weight: 15.25kg  
  • Groupset: SRAM NX Eagle 11-50T 12 speed gearing and shifter with 32T DUB chainset
  • Crankset: SRAM NX
  • Wheels: Love Mud Hobo Boost 29er wheels
  • Tyres:  WTB Vigilante 29 x 2.5in front and Trail Boss 29 x 2.4in rear tyre 
  • Brakes: SRAM Guide T brakes with 180mm front and 160mm rear rotors 
  • Bar/stem: Love Mud Aspect 780x31.8mm bar and 45x31.8mm Piskie stem 
  • Seatpost:  X-Fusion Manic 150mm dropper post 
  • Saddle: Love Mud Abode saddle
Guy Kesteven

Guy Kesteven has been working on Bike Perfect since its launch in 2019. He started writing and testing for bike mags in 1996. Since then he’s written several million words about several thousand test bikes and a ridiculous amount of riding gear. He’s also penned a handful of bike-related books and he reviews MTBs over on YouTube.

Current rides: Cervelo ZFS-5, Specialized Chisel, custom Nicolai enduro tandem, Landescape/Swallow custom gravel tandem

Height: 180cm

Weight: 69kg