A bicycle gearbox is a mechanical system that can change a bike's gear ratio, enabling you to adjust your pedalling effort and speed. Gearboxes use a clever spur gearing system and are integrated directly into the frame, in the bottom bracket area.
As the rider pedals, the cranks drive the gears directly, and the output is converted in the gearbox and then transmitted to the back wheel via a belt.
Although gearbox bikes are still a rare sight on the trails, there is a growing buzz around their benefits – such as improved suspension performance and unique shifting capabilities – which can’t be ignored.
In this guide, we’ll explain how gearboxes work, their pro and cons, and whether we think they will take over from conventional drivetrain design in the coming years.
How do bicycle gearboxes work?
Bicycle gearboxes use a complex design borrowed from the automotive industry. Two sets of cogs engage with each other to create different gear ratios, controlled by a bar-mounted shifter.
Here’s how Pinion, a German brand that’s leading the charge in bicycle gearbox design, describes its tech:
“Pinion gearboxes use spur gearing with two sub-units connected in sequence. The individual gear ratios are derived by matching the two sub-units with various cog pairings. The steps between gears are evenly spaced and an ergonomically sensible gear ratio range is built specific to the area of application.”
Once the rider’s input has driven the gears and been converted by the gearbox, it is transferred to the rear wheel using a belt drive. The belt replaces the conventional bike chain and accomplishes the same job – connecting the crankset to the rear wheel to drive.
Shifting gears, however, is somewhat different on gearbox bikes. Some gearboxes use a Continuously Variable Transmission (CVT) design, offering a seamless range of gears and adjusting the ratio without distinct 'steps'. Others, such as the latest Pinion gearboxes, offer 18, 12 or 9-gear options with equally spaced gears.
Bicycle gearbox pros and cons
Bicycle gearbox advantages
For riders looking for the best possible descending performance, the advantages offered by gearboxes are numerous and convincing.
Although gearbox systems are heavier than conventional drivetrains, their weight is located low on the bike’s front triangle, by the bottom bracket. This is partly why they're beneficial on downhill bikes, because they reduce the bike’s unsprung mass.
Reducing unsprung mass (the weight of the rear triangle and anything mounted to it, including the rear wheel, drivetrain parts, etc) results in a more sensitive suspension system and thereby more grip and control. With no derailleur or cassette at the rear of the bike, all else being equal, using a gearbox can improve the sprung-to-unsprung weight ratio of a bike.
Gearboxes are also potentially more durable and easier to maintain than derailleurs, being entirely contained in a sealed unit.
Compared to conventional drivetrains, which can suffer from damaged derailleurs and dropped or snapped chains – and need regular cleaning and servicing – gearboxes have far longer service intervals. They're more durable and reliable in challenging conditions too.
Drivetrains with derailleurs and chains need to be rotating for gears to shift properly. That means whenever you’re not pedalling, you can’t change gear. Gearboxes, however, can shift even when you’re on the brakes, freewheeling or ploughing through technical terrain. This is because the freewheel is inside the gearbox itself rather than the rear hub, so even when you’ve stopped pedalling, the belt continues to rotate.
While this might sound like a bit of a gimmick, 2023 downhill world champion Charlie Hatton says it offers genuine advantages on the track: “There’s limited time for gear shifts in a straight and I need to be in the right gear. [With the gearbox], I can shift mid-air if I need to.” Although he won his world championship title on a derailleur-equipped bike, Hatton will be riding a prototype gearbox bike in 2025.
Bicycle gearbox disadvantages
While gearboxes offer some clear advantages when it comes to weight distribution, suspension performance and overall durability, the system brings with it some trade-offs.
The most obvious obstacle is the fact that not many mountain bike frames are compatible with internal gearboxes. You can’t just slap one onto any bike — the frame needs to be designed to accommodate it.
Then there’s the weight. Gearboxes are heavier than traditional drivetrains. For example, Pinion’s 12-speed C1.12 weighs 2.1kg and costs £1,200. Shimano’s similarly priced, flagship XTR M9100 drivetrain weighs in at 1.54kg, saving more than half a kilo. While we know gearbox bikes put that weight in a more beneficial location, the extra overall heft is enough to put some riders off the idea.
Gearboxes also have an inherent drag, which causes a small loss in efficiency. Reporting seen by BikeRadar suggests a 4 to 5 per cent efficiency loss with the Pinion system, although this was measured at lower power ranges, rather than full sprints – where the loss is said to be less. Whether this is a real downside or more of a matter of personal adjustment depends on your priorities as a rider.
Who makes bicycle gearboxes and which bikes use them?
When it comes to frame-mounted gearboxes, there’s one clear market leader. Pinion has been pushing the design of bike gearboxes in recent years, and we’ve seen them specced on a number of interesting bikes, such as the Zerode G3.
Brands such as Zerode, Nicolai and Gamux have been Pinion advocates for some time. Excitingly, gearboxes are now also a hot topic in the world of downhill racing, with Atherton Racing trialling a Pinion gearbox-equipped version of its A.200.G race bike.
Suspension giant Fox Factory also looks set to enter the gearbox game, successfully filing a patent for a gearbox design that can shift under load – a crux for existing gearbox designs.
On a smaller scale, Scottish brand Intradrive is developing an eight-speed gearbox drive unit, which combines a gearbox with an ebike motor.
The internal gear hub market is somewhat more saturated, with brands such as Shimano and Rohloff catering for commuters, and long-distance and touring riders who need a highly durable and low-maintenance system.
What does the future hold for bicycle gearboxes?
While there’s still a way to go before we can start to imagine gearboxes taking over from conventional drivetrains, it’s exciting to see mountain bike brands testing them in competitions on the world stage.
Before the start of the 2024 World Cup Downhill season, gearbox belt manufacturer Gates announced a €100,000 prize purse for the first person to win an elite DH World Cup on a Gates-equipped bike.
As nobody managed to claim the prize last year, it's rolling on to the 2025 season and we’re seeing more teams than ever taking note, including Intense Factory Racing, MS Racing, Reece Wilson’s new AON team and of course, Atherton Racing, which has fully committed to ditching the derailleur.
Gearbox use in the world of commuter bikes is nothing new, and we expect them to become a popular choice for their reliability and ease of maintenance. While brand research and development is currently focused firmly on mountain bike tech, we’re sure innovations in this area will make their way to gearboxes in other applications soon enough.
