Cairn E-Adventure 650B review

Cairn has rebooted its Fazua-powered E-Adventure bike to make it more user-friendly and added a carbon-wheel-fatter-tire option. Guy Kesteven has been finding out how all that adds up.

Cairn E-Adventure 650B 2021 review
(Image: © Guy Kesteven)

Bike Perfect Verdict

Neatly evolved, unobtrusive but usefully power-assisted and versatile gravel all-rounder with a firm emphasis on fun. You’ll need to go low on the pressures to make it comfy though.


  • +

    Boisterously playful character

  • +

    650b tires add to the fun

  • +

    Precise and balanced handling

  • +

    Carbon wheel upgrade boosts responsiveness

  • +

    Great price for the package

  • +

    Fazua motor is smooth, silent but usefully powerful

  • +

    Tons of tire space

  • +

    Discreet top tube motor switch/indicator

  • +

    Neater cable routing including dropper post

  • +

    Optional ‘motor out’ storage


  • -

    Needs low tire pressures for comfort

  • -

    Motor rattle when freewheeling

  • -

    Limited on bike motor/battery info

  • -

    Less range and power than a full-fat e-bike

  • -

    Mixed transmission is noisy when dirty

Why trust BikePerfect Our cycling experts have decades of testing experience. We\'ll always share our unbiased opinions on bikes and gear. Find out more about how we test.

The original Cairn E-Adventure bike was a seriously fun and usefully power-assisted adventure gravel bike, but the new bike tidies up some of its original design flaws. Plus, the 650b version promises plump rubber fun for those who want to tread the MTB terrain borderline. The result is a very capable and enjoyable all-terrain bike that blends the best electric mountain bikes and e-gravel bikes, and again proves why the Fazua is one of our favorite lightweight motor systems. Carbon wheels add noticeable pep to its playful character too, but tire pressure is crucial to get some smoothness under the solid baseline ride.

Design and geometry

The basics of the frame haven’t changed since the first-gen bike. The Fazua motor and battery still lock into the underside of the large down tube with a soft connection into the driver transfer gearbox in the bottom bracket. The simple three-color, five LED mode/battery state display and power button have been moved to the slightly curved top tube for much easier accessibility on the move. 

The top tube has also been dropped so standover clearance is improved, especially for smaller riders. Cable routing has been tidied up with optional ports for an internally routed dropper post, and there’s room for two bottles inside the mainframe. A forged chainstay yoke removes the need for a cross brace, so there’s plenty of tire room around the 51mm wide rear rubber, and the Fazua speed sensor is tucked inside the offside chainstay. The kinked dropouts make room for the flat-mount rear brake and keep it clear of the rear rack mounts, which are matched at the top of the seat stays. You’ll need to add the bolt-on cross brace to run ‘proper’ fenders though. 

The full carbon fork is a deliberately stout piece complete with three 'Anything mount’ bolts on each leg. It’ll take full 'guards too, and there’s plenty of space even though it runs a bigger 2.25in (56mm measured) tire upfront as standard.

It’s not just the tires that are cross-country MTB standard either. A 71-degree head angle and 73-degree seat angle were the default numbers for MTBs for about three decades. The plate chainstay ends mean the back end is relatively short at 430mm. Reach is reasonable at 384mm for the medium I tested, while bar width and stem length are size-specific with a 460mm/70mm setup on the medium. 

Components and build

The heart of the E-Adventure is the Fazua Evation motor and 252Wh battery that sits in the same ‘tube’. A plastic three-lobe connector transfers the motor power to the chain via a Praxis single ring crank, and from there the transmission is conventional.

It’s a right mix and match though, with a top-spec GRX 810, controlled via mid-range GRX 600 shifters moving a KMC chain across a Sunrace cassette. It all works fine as long as you keep it clean (it gets a bit noisier than a full Shimano set up when filthy), and it keeps the overall Cairn cost-competitive. My sample was slightly off-spec with the cockpit branding (it should be Cairn branded, I got Ritchey), and I ran the bike with a fixed carbon seat post and Fabric saddle as well as the standard-fit Trans-X 70mm dropper post. 

Ride, handling and performance

While the Fazua system is comparatively light (the whole battery, motor and transfer drive setup is 4.6kg), the E-Adventure still weighs in at nearly 16kg with pedals fitted. Together with the chunky frame and chubby tires, that gives it a really stout, unstoppable feel straight away. In the two higher power modes (River is 210W default, Rocket is 250W), there’s easily enough grunt to make light of the mass when you’re accelerating or heading up reasonable slopes. You will overpower it and lose the sense of obvious assistance if you start cranking out of the saddle, but it certainly feels like it’s helping a lot more than the Mahle E-bike motion system that’s its obvious competitor. If you want full power, Cairn also does the Shimano Steps powered BRAVe. 

Back to the E-Adventure, the latest Black Pepper OS means it picks up smoothly anywhere between 50 and 120rpm, but it definitely feels more efficient being spun rather than stomped. The GRX gears make light work of moving up and down the wide range block though, so that’s no problem. The Fazua is also impressively smooth in the way it transitions almost silently and imperceptibly between powered and unpowered with zero tangible motor drag above the 15mph legal cut-out speed (US bikes are rated to 20mph). Any weatherproofing concerns were eased after I plowed through a hub-deep lake during testing without a hint of a hiccup from the electrics. 

While on-bike information and interaction are very limited, you get full custom tuning options and ride data recording through the Fazua smartphone app. Uniquely for Fazua you can also remove the whole system and ride it motor and battery-free with the space used for storage instead if you get the optional cover plate. The downside is that there was significant clunk and clatter from the separate motor and battery pack on my sample when I was freewheeling, but that went away immediately if I pedaled.

While it’s easy to look at the fat tires and presume you’ll instantly get a much smoother ride than the 700x40mm version we tested before, that’ll only happen if you understand how pressure works in bigger volume rubber. There’s a fascinating series of blogs about it on the Silca site if you really want to go deep, but the basics are that the fatter the tire, the softer you need to go or it’ll feel like concrete. Both the boxy frame and the Hunt carbon wheels are stiff too, so even when I started at 25psi it wasn’t until I went to 20psi that I really started cruising smoothly. The big advantage with the fatter tires though was that I wasn’t remotely concerned about what I was clattering over in terms of flats. 

Cairn E-Adventure 650B 2021 review

The overbuilt fork adds precision and accuracy to the handling (Image credit: Guy Kesteven)

Tire grip and girth, plus the precise feeling fork meant I could also pick much more aggressive or slippery lines on singletrack and rutted cart tracks than I could on 700c. The lighter carbon wheels, centralized motor weight and 71-degree head angle mean the Cairn is really responsive to direction change or a quick power kick. Swapping over to the Trans-X dropper post gave me just enough space between shorts and seat to start really leaning and leaping without feeling like a pencil topper too. Despite being a hench hooligan off-road, the Mezcal tires spin well on Tarmac too, and I regularly outran the motor cut-off when tapping tempo between the gritty bits. 

Compared to the Merida eSilex I rode recently, the Cairn encourages a muscularly confident attitude off-road rather than a meek one, but the Merida was definitely smoother even with smaller tires, and big hub aside, it’s less aesthetically obvious you’re on an e-bike.


I already liked Cairn’s E-Adventure bike, but having the motor mode switch right on the top tube and having more standover space are significant gains. The combination of Hunt carbon wheels and the fatter 27.5in tires don’t just match the bike really well aesthetically, if you go low enough with the pressures they also fully liberate its potential as a proper off-road play bike. Most surprisingly they do it without obviously dampening its easy on-road feel, and the comprehensive cargo mounts make this a properly mixed surface explorer. While it can rattle freewheeling across rough off-road sections, the Fazua motor system is otherwise a great mix of unobtrusive when you don’t need it, but usefully powerful when you do. Pricing is very good for a small direct-sell brand too, but keep that mongrel transmission clean and well lubed to keep it quiet.

Tech Specs: Cairn E-Adventure 650b 

  • Model name: Cairn E-Adventure 650b 2021
  • Discipline: Gravel/adventure
  • Price: $4,069.44 / £3,735
  • Head angle: 71-degrees
  • Frame material: 6061-T6 custom alloy frame with full carbon fork.
  • Size: S, M (tested) L, XL
  • Weight: 15.3kg
  • Wheel size: 27.5x2.25/2.1in
  • Motor: Fazua Evation motor and battery 
  • Drivetrain: Shimano GRX 810 rear mech, GRX 600 shifters, Sunrace 11-42T 11-speed cassette and KMC chain
  • Cranks: Praxis 42T chainset
  • Brakes: Shimano GRX 400 brakes with 160mm rotors
  • Cockpit: Ritchey 460mm bar and 70mm stem
  • Wheelset: Hunt Adventure Carbon Disc wheels
  • Tires: Vittoria Mezcal 27.5 x 2.25in front and 27.5 x 2.1in rear tires
  • Seatpost: Trans-X 70mm dropper post
  • Saddle: Fabric saddle

Test conditions

  • Temperature: 12-16 degrees,
  • Surface: Road, back road, gravel track, cobbles, rural singletrack.
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