Bike test: Small fold, big ride

The Brompton is the quintessential urban folder, slotting so neatly into that space in the cycling market that it has become the standard by which other folding bikes are judged. Many are found wanting: over the years a number of rivals have folded (pun intended). While it’s very difficult to out-Brompton a Brompton, there are things other folders can do better. For example: ride better. 

Tern’s BYB and the new Brompton G Line both promise that. The Tern BYB P8 is a compact urban commuter with a better-quality ride. It is, Tern says, “perfect for mixed-mode commuters who do a stretch of their daily journey on a train or bus”. Although there’s also a sportier BYB, the P10 (£1,599, 12kg), Tern’s own Verge models cover that ground better. Ultimately – and possibly to its cost – the BYB is a Brompton C Line rival. 

The Brompton G Line is more of an alternative to the Birdy and Airnimal ranges than its 16-inch wheel stablemates. Brompton calls it “the all-new, one-bike solution for the city and beyond”, saying that it’s suitable for “gravel paths, towpaths, asphalt and well-maintained off-road routes”. With larger wheels, fatter tyres, disc brakes and an Alfine hub, it’s reminiscent of the modified Bromptons from Ben Cooper at Kinetics (kinetics-online. co.uk). There’s a 4-speed electric G Line (£3,499) and two unassisted models, one with the Roller Frame and mudguards, one without. 

Frame & folding: Brompton 

The G Line is basically a bigger version of the 16in-wheel Brompton. It folds the same way. You unclip the catch behind the seat tube and flip the rear triangle underneath, so that the bike sits on its haunches. Then you undo the frame hinge and fold the front end back on itself, hooking the front axle hook over the drive-side chainstay. Then you lower the saddle and the handlebar. The left-hand pedal quick releases and is stowed behind the fork crown. 

The folded package is bigger than a 16in-wheel Brompton but still small enough for a train’s luggage rack. It is a lot heavier so it’s good that it rolls along well on its casters. The extra weight also makes the folding process more cumbersome. It helps to use a foot to flick the rear triangle under or back out. 

There are three nominal sizes: S, M and L. There’s only one frame size (and type – the electric version shares the same frame). Bigger sizes get longer or telescopic seatposts and stems with more height and reach. The sizing feels more like M, L and XL. I’m about average male height (177cm or 5ft 9ish) and the medium G Line was too big. The extended seatpost was 15cm (6in) too long at full extension – too long to correct with Brompton’s saddle height insert – and the stem was too high. So I couldn’t instantly set the saddle height and I had to sit more upright than I wanted. By contrast, the small has a slightly shorter stem and a standard-length seatpost that should fit riders with an inside leg measurement of up to 33in. 

The G Line frame hasn't simply been scaled up. The geometry has changed, too: a slacker head angle combines with the bigger wheel size to give more trail. Additionally, the front wheel has a thru-axle, and the frame hinge catches have stops so they can’t fall out when the bike is folded and stashed in a car boot. 

Frame & folding: Tern 

Like the Brompton, the BYB’s frame folds into thirds rather than halves. That’s why it’s smaller than Tern’s Link models, which also have 20in wheels. 

First you lower the saddle – two clamps as it’s telescopic – and point it rearwards. Then you undo the two frame clamps so the front end can Z-bend back on itself, enabling you to push the bulbous Anchor Bolt on the fork into a hole by the rear dropout. There’s a catch here to hold the anchor bolt but I found that it never engaged firmly, so the folded bike could easily flop apart – and often did. 

The next step is to sit the bike upright, balanced on the wheels and the Metro Transit Rack’s casters. Then you lower the handlebar and secure it with a rubber strap. Finally, you remove the quick-release left-hand pedal and stow it on the chainstay. You’re left with a tall but narrow package you can roll into tight places. Note that for the smallest folded dimensions, you’ll also need to adjust the handlebar angle and position with the quick-release Andros stem. 

The folding procedure is clever but always felt awkward to me, and the fact that the anchor bolt was insecure was a source of irritation. I thought it might have been worn but the same thing happened with a replacement bolt that Tern sent after the original fell off while I was riding. 

Components 

If the G in the G Line name stands for gravel, the tyres would explain that. They’re 54mm Schwalbe G-One Allround tyres with a dimpled tread that suits light off-road use without dragging much on tarmac. Although the bike comes with tubes, tyres and rims are tubeless ready. It’s an upgrade worth making if you’ll stick with the G-Ones as, like other performance tyres, they’re relatively fragile. With innertubes, tougher Schwalbe Big Apples would be a better bet. 

I suffered two flat tyres during the test, though neither was caused by thorns or glass. One was a faulty valve. The other turned out to be a scattering of swarf that had somehow been left in the tyre. This secretly scored the tube and ultimately caused a puncture. 

Removing the rear wheel can be a bugbear with a Brompton. It is fiddlier to disengage/engage the G Line’s Alfine gear cable than a standard Brompton’s Sturmey-Archer, but the wheel itself goes on and off more easily as the chain tensioner stays in place. On the other hand, refitting this tubeless-ready, small-diameter tyre was a stiff challenge. 

The Brompton’s Alfine hub gives a good spread of gears, and it’s nice to be able to shift between them while stationary. But at nearly 1,700g, the hub adds a chunk of weight to what is already a heavy folding bike. A 4-speed derailleur setup like the P Line Urban’s, which the frame has mounts for, would save around a kilo. 

Braking performance is excellent, thanks to TRP hydraulic discs and tyres with a large footprint. Contact points are comfortable and, befitting the longer rides you might do on a G Line, there are stem mounts for a bottle cage. To avoid increasing the folded size, I'd suggest a Fidlock or Monkii Cage bottle mount. 

The Tern BYB P8 is £1,000 cheaper than the Brompton G Line so it would be unrealistic to expect equivalent components. Its 8-speed derailleur drivetrain works fine, however, and gives a comparable gear range to the Alfine hub. On wet or unsealed surfaces, the Acera derailleur picks up grime easily because it’s so close to the ground. 

The Tern’s V-brakes are more powerful than a standard Brompton’s sidepulls, thanks in part to the Jagwire compressionless outers, but they’re no match for the G Line’s discs. The front brake is tucked behind the fork. Its pads aren’t perpendicular to the brake arms, so uneven pad wear could be a problem. 

Although there’s only one size of BYB, it should fit a very wide range of rider heights as the seatpost is telescopic and the unusual twin-armed Andros stem is adjustable through a wide arc. The seatpost has height markings so can be set accurately if not instantly. 

The BYB’s quick-release MKS pedal goes on and off easily but is worse for pedalling than the G Line’s. It’s a quill-style pedal with only one flat side; the other is convex and provides an awkward footing. A different quick-release MKS pedal would be better. 

There are lots of accessories available for the Tern BYB, ranging from bags, racks and baskets to lights and covers. I tried the Luggage Truss CMT (£65) and the Go-To Bag (£140), which provide the BYB with front-mounted luggage similar to a Brompton’s. I’d recommend them. 

The ride 

The G Line has better handling than a standard Brompton. Its slacker head angle and bigger wheels yield more trail, and the longer wheelbase and wider tyres improve stability further. I could just about ride it no-handed. It reminded me of the Tern Node D8 (cyclinguk.org/less-famous-folders), in that it feels more like a normal bike that can be folded. 

The G Line’s off-road capability is comparable to a gravel bike’s. Technical singletrack is out, but forest fire roads and the like are fine. I took it along the Scarborough-Whitby Cinder Track, where it performed well as long as I kept an eye out for deeper holes and ruts; 20-inch wheels, even with wide tyres, are more easily balked than bigger ones. 

I liked the G Line’s broad pedals for their more secure, more supportive footing. I was less keen on the unusually high handlebar. Switching to a size S would shave 10mm off the bar height, while a flat bar would reduce the height further. 

The Tern BYB has a similar wheelbase to a standard Brompton but its head angle is less steep and the wheels are larger. With more trail, the handling is noticeably more stable. I wasn’t able to ride it hands free like the G Line. 

I took the BYB down the Cinder Track as well. It coped OK as I rode cautiously. The frame hinges are solidly built and flex free, and the tyres are just wide enough. On tarmac, these Schwalbe Citizens felt stodgy and rather slow compared to the higher pressure Schwalbe Marathons on my own Brompton. The sportier BYB P10 gets Schwalbe Kojaks, which would surely transform the P8’s ride. 

Like the G Line, the BYB has an upright riding position that’s fine around town and frustrating when you want to up the pace. While the Andros stem is adjustable, it moves the handlebar through an arc so you can only lower the handlebar by increasing the reach.

Verdict 

The BYB P8 has an ingenious fold and a stable ride but lacks an obvious USP. As an urban folder for daily use, the lighter, more compact, better integrated Brompton C Line beats it hands down. And if you just want a 20-inch-wheel folder that rides nicer than a Brompton, a Tern Link B8 does that for half the price of the BYB P8. 

The G Line isn't a direct rival to the standard Brompton, although it can fill the urban commuter role. It’s more of an all-rounder like the Birdy – and a good one. Standout features include the wider tyres, hydraulic disc brakes and much improved steering geometry. Yet the sizing of the bike feels off, like it’s been designed for the Dutch, and at 15kg it’s a weighty folded package. I’d like it more if it were 2-3kg lighter.

BIRDY CITY £2,579+

At 12.9kg, this full-suspension folder is a couple of kilos lighter than the G Line, while the derailleur-equipped Birdy Touring is lighter still. Folds to 80×62×34cm. 

TERN LINK B8 £700

A fold-in-half commuter that, unlike the cheaper Link B7, will take a front bag. It folds to 38×80×68cm and weighs 13.6kg. 

First published in Cycle magazine, December 2024/January 2025 issue. All information correct at time of publishing.