Is the Single Chainring Drivetrain the New Trend for Road Bikes?
Unless you are preparing for professional races with a committed team of mechanics at your side, Jonas Vingegaard's single-chainring Cervélo bike joyfully lifted on the Champs-Élysées in 2023 to celebrate his second Tour de France win is not truly for everyone.
A few teams and riders employed single-chainring drivetrains during the classic races this spring, which spurred another round of debates on whether this approach has any place in road cycling.
Indeed, single-chainring systems have advantages in some cases, and the most recent developments in hub gear technology might be revolutionary. To be honest, though, a complete single-chain revolution in road cycling still seems unlikely.
Modern front derailleurs are simply too fantastic to lose, in my view. Usually, the advantages of leaving them outweigh the trade-offs most of the time.
When Does a Single Chainring Drivetrain Make Sense for Road Bikes?
On road bikes, single-chain drivetrains have advantages (and disadvantages). A single chainring system can give all the gear ratios you require while being simpler to operate—no front derailleur hassles—if you are riding on reasonably level ground and do not need a huge gear range.
It may also enhance aerodynamics. Depending on the front derailleur you remove, AeroCoach in the UK claims using their ARC 1x aero chainring can save between 1 and 4 watts at 30 mph (48 kph). Though this improvement seems little, more and more time trial and track bikes are embracing it.
1x aero chainrings are becoming increasingly popular on time trial bikes.
Of course, adding a chain guide to prevent chain drops may impact some of the aerodynamic benefits. However, this setup can offer greater chain security compared to a 2x system, which can be crucial in rough races like Paris-Roubaix. For instance, Wout van Aert used a 1x SRAM Red eTap AXS drivetrain in the 2023 Milan-San Remo and Paris-Roubaix, likely for this very reason.
Depending on the setup, a single chainring drivetrain can also reduce bike weight. Dedicated climbing bikes, for example, almost always feature a single chainring system. For riders chasing every gram of weight savings, this is a significant bonus.
Climbing bikes are often set up with a 1x drivetrain because it’s lighter and doesn’t require two chainrings.
The Drawbacks of Single Chainring Drivetrains on Road Bikes
Although in some cases single chainring drivetrains have significant benefits, they also have some clear disadvantages.
Eliminating your front derailleur and one chainring obviously drastically lowers the variety of gear ratios and gear count on your bike. Using a wide-range cassette or carefully choosing the chainring size to fit your ride will help you partially offset this; but, trying to match the adaptability of a 2x system usually requires concessions.
More compact cassettes used with 2x drivetrains offer greater flexibility to find the optimal cadence.
For example, wide-range cassettes meant to replicate the gear range of a 2x arrangement typically weigh more than more compact cassettes. More critically, however, they sometimes have bigger gear gaps, which results in a clunky shifting experience. On the road, where gradient variations typically are more subdued than off-road, this can be very annoying. The more gaps between gear ratios, the more difficult it may be to find the ideal cadence for certain circumstances.
Wide-range cassettes used with 1x drivetrains, such as the Campagnolo Ekar 9-42t, offer an impressive gear range but also come with compromises.
Smaller gears bring still another problem. Systems like SRAM AXS or Campagnolo Ekar tend to be rather less efficient since they use 10-tooth or even 9-tooth sprockets coupled with smaller chainrings. This is so because the chain must bend more abruptly around smaller sprockets than it would around bigger ones, therefore generating higher energy loss. Although SRAM contends this isn't "true cross-chaining," a 1x system does pull the chain into more extreme angles at both ends of the cassette compared to a 2x system. All else being equal, these elements raise drivetrain friction, so less power is transferred to the tires than in a 2x arrangement.
The original 3T Strada's 1x drivetrain might be more aerodynamic, but it could also be less efficient.
How then much efficiency is wasted? VeloNews and CeramicSpeed ran experiments in May 2019 contrasting the friction characteristics of 1x and 2x drivetrains. Their results were fascinating; we will examine their test procedures and data closely next, then run some calculations to discover how these friction losses might affect your riding speed.
Test
VeloNews/CeramicSpeed tested two different drivetrain setups:
1X drivetrain: Using SRAM Force 1 rear derailleur, 48-tooth single chainring, PC-1170 chain, and 10-42t cassette.
2X drivetrain: Using Shimano Ultegra rear derailleur, 53/39t chainring, HG701 chain, and 11-34t cassette.
These drivetrains were selected since their gear range lets one compare friction losses under the same gear ratios. Installations of both drivetrains on a test setup capable of computing friction losses for every gear combination The machine delivered 250 watts of power while simulating a rider pedaling at 95 RPM.
Chain line alignment:
Whereas the 2X drivetrain linked the 53t chainring to the 5th smallest cog and the 39t chainring to the 8th smallest cog, the 1X drivetrain was arranged to have a straight line at the 5th smallest cog.
Ceramic Speed also removed the factory lubrication and re-lubricated using the same mineral oil. The test run for both chains lasted the same length.
Note: CeramicSpeed already discovered that utilizing a 1X chainring instead of a 2X chainring with uniform teeth does not cause friction to rise. They also came to find that friction is not affected by rear derailleur pulleys.
Results
Some conclusions from the results:
- Friction losses generally increase with higher gear ratios.
- The 2X drivetrain is more efficient in every gear (assuming you shift to the larger chainring after 39x21t).
- The difference in friction losses ranges from as little as 1 watt (48x21t / 53x23t) to as much as 6 watts (48x10 / 53x11).
- The 1X drivetrain has greater friction loss on a perfect chainline (48x18t) compared to the 2X drivetrain (53x19t).
- The efficiency of the 1X drivetrain ranges from 96.0% to 92.4%, with an average efficiency of 95.1%.
- The efficiency of the 2X drivetrain ranges from 96.8% to 94.8%, with an average efficiency of 96.2%.
The 1X drivetrain has an average friction loss of 12.24 watts, CeramicSpeed found. Summing the drivetrain power losses of every 11 gears and then dividing by 11 helped one to ascertain this. For the 2X drivetrain, 15 best gears were totaled and split by 15. With 9.45 watts, the average difference between the two drivetrains is rather less than 3 watts.
Why Is 1X Less Efficient Than 2X?
A chain generates four basic forms of friction. The most obvious one is the chain's bending angle, which increases the scraping of chain plates against the cogs. Crucially important tasks also fall to chain tension, chain articulation, and chain speed.
Chain stress: Especially in smaller chainrings and cogs, more tension results in more pressure between the chain and the chainring or cassette.
Chain articulation: This relates to the degree of bending at the chain pins. Higher friction results when one wraps the chain around smaller cogs.
Chain speed: This is the gear interaction count per minute that rises with a reduced chain size.
A 1X drivetrain cannot ultimately keep a precisely straight chainline across its gear range. Its smaller chainring generates a higher chain speed, requires the chain to flex more on lower cogs, and raises chain tension.
These extra elements help to explain why a 1X drivetrain still runs over 2 watts more power than a 2X drivetrain even on straight-chain line gears. It also explains why the 48x21t gear shows the lowest friction even though its chain line is not exactly straight: slowed chain speed and reduced chain articulation save more friction than the straight chainline could.
What Is the Speed Difference Between Drivetrains?
Now for the interesting part!
Using BikeCalc, we can determine the bike’s speed at each gear ratio for 700x44C wheels with a cadence of 95 RPM. Then, by inputting the power output (231–242 watts, depending on the gear) and rider+bike weight (85 kg) into Bike Calculator, we can calculate the speed and time differences over 100 km.
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Minimum efficiency difference:
At 48x21t (1X) and 53x23t (2X), you’ll ride at 29.12 km/h @ 95 RPM. A 1-watt difference results in a 0.06 km/h speed advantage for the 2X drivetrain. Over 100 km, the 1X drivetrain will take 25 seconds longer (0.2% slower). -
Largest gear:
At 48x10t (1X) and 53x11t (2X), you’ll ride at 61.28 km/h @ 95 RPM. A 6-watt difference results in a 0.14 km/h speed advantage for the 2X drivetrain. Over 100 km, the 1X drivetrain will take 14 seconds longer (0.3% slower). -
Smallest gear:
At 48x42t (1X) and 39x34t (2X), you’ll ride at 14.49 km/h @ 95 RPM. A 2.5-watt difference results in a 0.15 km/h speed advantage for the 2X drivetrain. Over 100 km, the 1X drivetrain will take 3 minutes and 50 seconds longer (0.9% slower).
Front Derailleurs Are Already Amazing
One main reason I'm not going to 1X on my road bike anytime soon is simple: front derailleurs are just too good now! Both positives and drawbacks of 1X!
Modern road bike groupsets have front shifting action that is simply amazing. Particularly with electronic groupsets, which front shifting's "gold standard" now is. Though many people—including myself—consider Shimano as the top in this industry, SRAM and Campagnolo are not far behind.
Even mechanical groupsets today provide outstanding front shifting. Of course, wrong shifting or poor setup can still lead to problems; nevertheless, front shifting has progressed to the point where it no longer feels like a problem needing attention.
Shimano's flagship 105 R7000 drivetrain delivers flawless front shifting at an affordable price.
Is the Classified Powershift Hub a Game-Changer?
The Classified Powershift hub is an intriguing technology that appears to perform well but is currently quite expensive.
The Classified Powershift hub is an intriguing piece of technology that seems to perform well, but it doesn’t come cheap.
This hub gear system features a wireless-controlled 2-speed planetary gear integrated into the rear hub. Often hailed as a "front derailleur killer," I don’t see it taking that crown anytime soon. However, it does address many of the drawbacks associated with single-chainring drivetrains—perhaps giving us the best of both worlds in the future.
For most riders, though, the issue is its price. For example, the Classified R50 and G30 Powershift wheelsets retail at £2,300—only £25 less than the complete Trek Émonda ALR 5 bike mentioned earlier.
So, what do you really gain from spending all that money instead of sticking with a 2X drivetrain? A minor improvement in aerodynamic efficiency... and that’s about it.
Of course, if Classified’s hub technology gains popularity and widespread adoption, its cost might come down over time.
For an all-round road bike, it’s hard to beat the versatility offered by a 2X drivetrain.
2X Is Still the Best Choice
For an all-round road bike, it’s hard to beat the versatility of a 2X drivetrain.
While all my mountain bikes use single-chainring setups, transferring that system to a road bike doesn’t make much sense to me. For most riders, 2X remains the better choice.
With the performance and versatility of modern front derailleurs, the minor potential advantages of a 1X drivetrain just don’t seem worth it.
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