Bike test: Sonder Frontier Rigid 27.5+ mountain bike

A rigid mountain bike stands in the middle of a woodland trail
Sonder’s Frontier Rigid is a good all-round off-road bike
An off-road all-rounder for bikepacking, rough stuff and even technical trails. Dan Joyce tested this versatile mountain bike for those on a budget

Sonder’s Frontier Rigid looks like an aluminium version of the Genesis Longitude: the frame and fork bristle with brazeons and there’s room for 29er or 27.5+ wheels. But the Frontier’s geometry is based on a taller fork and its head tube takes a tapered steerer, so you can fit – or buy it with – a modern 100mm suspension fork.

The rigid version is cheaper, lighter and still capable off road, especially with 27.5+ wheels (a zero-cost upgrade over 29in wheels). Plus-tyres absorb bumps well enough by themselves so long as you’re not chasing trail centre Strava KOMs or racing choppy descents.

It’s just as well because the beefy aluminium frame and fork have basically zero flex. I’d recommend wider rubber even if you’re going for larger diameter 29er wheels: 29×2.5in will fit the rear triangle; the fork has ample room for 29×3in. That’s how I’d build up the bike if I were starting from a Frontier Rigid frameset.

There are à la carte options for complete bikes. You can specify different wheels, handlebars, saddles, a dropper seatpost, lights… For regular technical riding, a dropper post is a must. While you can lower a rigid seatpost with a quick-release clamp, this post bangs into the seat tube bottle bosses (unless you saw it shorter).

Gearing is good for a budget bike thanks to the trickle down of SRAM’s 1×12 Eagle groupset. That 50t large sprocket yields an 18in bottom gear, which combined with the traction of wider, softer tyres will winch you up very steep slopes indeed – if you can keep the front wheel from lifting. The Frontier is a little short in the top tube, so the front end isn’t always well weighted. If in doubt, go up a size.

I found the 780mm handlebar and hard grips uncomfortable, big tyres notwithstanding. A hacksaw, different grips and bar ends would solve that, as would an On-One Geoff handlebar.


Versatile, upgradeable and good value, the Frontier Rigid is best suited to ‘natural’ trails but is well worth considering by any non-jumpy mountain biker on a budget.

Other options

Genesis Longitude £1,199.99

Steel all-rounder with 27.5+ wheels, 2×10 Shimano Deore gearing, and lots of frame mounts.

On-One Bootzipper 29er SRAM SX £719.99

Another option, the On One Bootzipper 29er SRAM SX

Steel 29er with 1×12 SRAM SX gearing. Not meant for a suspension fork, though the head tube is 44mm.

First published in Cycle magazine, June/July 2021 issue. All information correct at time of publishing.

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Tech spec

Sonder Frontier SX Eagle Rigid

Price: £849 (frame & fork £399)

Sizes: S, M (tested), L, XL

Weight: 12.82kg/ 28.2lb (no pedals)

Frame & fork: 6061 aluminium frame, 148mm bolt-through dropouts, 73mm threaded BB, one triple & two double cage mounts, mudguard mounts. Aluminium fork with 110mm bolt-through dropouts, two sets of triple cage mounts, mudguard mounts

Wheels: 67-584 WTB Ranger tyres, Sonder Nova 27.5+ tubeless-compatible wheelset (Sonder i38 rims, 32×2 pg spokes, Sonder hubs)

Transmission: 170mm SRAM SX Eagle Powerspline chainset, 32t chainring, SRAM Powerspline BB, SRAM PG1210 12-speed 11-50t cassette. SRAM SX Eagle shifter & derailleur. 12 ratios, 18-82in

Braking: SRAM Level hydraulic disc, 180 & 160mm rotors

Steering & seating: 780×31.8mm riser handlebar, 55mm stem, FSA Orbit C-40-ACB headset. Love Mud Abode saddle, 400×31.6mm Love Mud seatpost