Derailleurs Explained: The Bike Gear Shifting Evolution
Though plenty of die-hards live on single-speed rigs, for most cyclists, gears are the sweet elixir that makes riding fun.
It’s easy to take for granted the ease with which riders can cycle through gears on bikes these days. Derailleurs are the mechanisms that make changing gears on bikes possible. They move the chain from cog to cog on the rear or between chainrings on the front and manage the required chain slack.
Likewise, it’s easy to overlook or fail to appreciate the wide-ranging gear ratios on modern drivetrains. These make rides that were all but impossible for riders even a couple of decades ago accessible to nearly anyone.
Since the first bicycle, inventors and manufacturers have been slowly modifying and tweaking designs to perfect the derailleur and the larger drivetrains of bikes.
Some would argue that the industry is now near the peak of performance and capability with derailleurs and drivetrains. Others, however, view a brave new world on the horizon.
Early Bicycles: No Derailleur Required
It may be hyperbole to say that early bicycles amounted to death traps; they were a bit dangerous. The earliest bikes from the beginning of the 1800s did not have drivetrains or even pedals.
Fast-forward to the late 1800s, and the most common bikes came with a giant front wheel that lofted riders far enough from the ground to crack plenty of bones in the event of a tumble. These bikes, known as “penny-farthings,” were pedal-powered, but the pedals were fixed to the front wheel. They did not need a chain or a drivetrain.
The dangers noted with the penny-farthing and other similar big-wheel bikes of the time led to the invention of the “safety bicycle.” These bikes allowed riders to hop on with a degree of confidence. Should they fall, at least they weren’t looking at a 10-foot gap or more between their heads and the ground.
The safety bicycle included wheels of the same size and a single-speed drivetrain that formed the basis for modern bicycle mechanics.
The Beginning of Gears on a Bicycle
Once riders were fully pedaling their bikes (rather than running while perched on top of them like children on wobble bikes), the need for more gears quickly became apparent. Early attempts included wheels that had different-size gears on either side. The wheels could be flipped over to give riders more or less power.
Likewise, hubs gave riders a couple of gear options that were integrated inside the housing. However, both of those options were challenging to work on, especially on the fly. They also only offered a couple of gears, so the benefits may not have been worth all the added time to make the switch.
According to Sports Fitness, the groundwork for the modern derailleur was laid out in a patent from Jean Loubeyre. The Frenchman patented a two-speed system known as “La Polyceler” in 1895.
You can see a diagram of the design at desraeligear.co.uk. A paddle laterally pushes the chain across the two speeds on the rear wheel, while a spring “expander” manages the required chain slack.
Derailleurs Go Racing
Despite these options and many others that sprang up from the likes of Tullio Campagnolo and others, derailleurs didn’t explode onto the race scene until 1937, when the Tour de France allowed derailleurs for the first time.
After that, many manufacturers began tinkering with different systems that allowed riders to shift gears while riding. One option had riders reach toward their back wheel to manually change their gears using a rod-actuated system that moved the chain. That act was extremely dangerous, as a wrong move could result in lost fingers and horrible crashes.
In 1938, Simplex launched the first cable-actuated derailleur, which allowed mounting the shift levers on the downtube. Riders still had to reach down to shift, but it beat dangling fingers near spinning spokes.
This shifting was done by feel, meaning the rider manually placed the shift lever where they felt the correct gear was engaged. In the 1940s, the “dual parallelogram” style of derailleurs came into play and is still in use today.
In 1984, Shimano introduced the Shimano Index System (SIS). As the name suggests, the shift lever would click into position when the chain was correctly positioned on the rear sprocket of choice — no more feeling around for the ideal lever position.
Shimano sparked another seismic change when it integrated ratcheted shifting levers into brake levers on mountain bikes in the late ’80s. This Shimano Total Integration (STI) allowed riders to keep their hands on the grips while cycling through gears for the first time. The brand applied the same STI logic to road bicycle brake levers in 1990.
Modern Derailleurs
Since shifting moved up to handlebars and manufacturers around the world dialed in their groupsets into the derailleurs we see today, companies have been refining the basic design of those systems to maximize efficiency.
Across cycling disciplines, derailleurs now have massive ranges that put previous offerings to shame. For example, SRAM’s Eagle groupset features a 12-speed cassette with a 10t to 52t range. Similar wide gearing ratios are found on road and gravel bikes as well.
To make derailleurs glide more smoothly from gear to gear, companies began integrating features like ramped teeth on cassette cogs to help chains snap into place more easily. They also worked to move cogs closer together to incorporate more gears.
The gearing range also necessitated managing longer chains and more chain slack. Rear derailleurs had to incorporate designs that could “wrap” much more chain than previous close-ratio rear cog sets.
But even that wasn’t enough. Until the last couple of years, the most common derailleur systems from SRAM, Shimano, Campagnolo, and others have relied on cables to move derailleurs up and down a cassette.
Electronic Shifting
Electronic shifting had been in the works for a long time before gaining steam in the past several years, but it was more of a gimmick than a viable tool.
That has since changed. All major drivetrain manufacturers now offer popular wireless or partially wireless options that eliminate cables in favor of a remote, wireless interface. This saves on weight and creates snappy, reliable shifting that isn’t susceptible to cable stretch. Cable stretch necessitates derailleur adjustments and can cause missed shifts.
The most common among these are the SRAM AXS and Shimano Di2 systems.
Derailleur Weaknesses and Future Prospects
While derailleurs have become one of the most refined and efficient pieces of equipment found on bicycles, they certainly are not without their weaknesses.
Derailleurs attach to bicycle frames low at the rear of the bike. They are in a prime position to take hits from trail debris or bear the brunt of the damage during a fall. Most attach via derailleur hangers designed to break away when they take a massive blow to protect the derailleur itself. Still, that often isn’t enough to prevent significant damage.
Other issues include the limitations posed by current bicycle frame and wheel designs. The long-standing architectures hinder the number of cogs that cassettes can accommodate.
Some manufacturers have tinkered with internal gearbox drivetrains that remove the derailleur from the equation altogether. Instead of running on an exposed device that can snap off and get thrown into a wheel, these systems run embedded in bicycle frames or boxes that protect them from the trail and the elements. Those features make the need for servicing rare, while allowing for reliable shifting in widely varying conditions or even under power.
While gearboxes are out there, the derailleur still is the primary shifting component across all bikes, from the lowest end of the spectrum to professional WorldTour setups.
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