Bend in the Road: Road disc axle shuffle

Road-disc axles: 12mm, quick-release, 15mm or something else? Time will tell

A few of my friends are eager to buy a new road bike with disc brakes, but they are hesitant about buying a frame that could soon be incompatible with new wheels, as thru-axle hubs seem to be a likely future trend. I checked in with Shimano, SRAM, Reynolds and a few major bike companies for a look down the road.

Conversations with a dozen experts on the subject revealed two main takeaways. One, there will not be a single standard. And two, wheel companies are eager to make compatible and even convertible solutions, so you likely won't be "stuck" with an abandoned format in a few years.

Although a few cyclocross bikes have debuted with thru-axles, most road bikes with disc brakes now have standard quick releases. But that is quickly changing.

A similar progression to mountain bike wheels?

Years ago, mountain bike wheels had similar quick-release axles to those found on virtually all road bikes today. When mountain bikes moved to thru-axles, there was a short transition period while the industry moved jerkily forward. Downhill and big-hit bikes jumped to burly 20mm-diameter thru-axles, and there were a few trail bikes that followed this before most settled into the 100x15mm front and 142x12mm rear thru-axles that are common today.

On the road side, we will likely see a similar transition period before the major players settle into their own respective directions.

Concurrently, we are also seeing an increase in bike brands making their own — and, sometimes, increasingly high-quality — wheels. While house-brand wheels are certainly a money-maker/money-saver for bike brands, a systems argument can be made that integrated design can result in a better bike overall.

Road thru-axles: who is following whom?

In determining the dimensions of road disc wheel axles, who is calling the shots? Well, that depends on who you ask. Wheel companies say bike companies will drive it. Bike companies say component companies (Shimano and SRAM) will control it. And the truth is somewhere in between. Potential options at the moment are quick release, 100x15mm or 100x12mm up front, and quick release, 135x12mm or 142x12mm for the rear.

Specialized is pushing 100x12mm and 135x12mm; Trek is holding the line on MTB-heritage 100x15mm and 142x12mm for its road disc wheels. And Giant isn't abandoning quick releases until a lighter thru-axle standard arises. Wheel companies are scrambling to make everyone happy.

"We are all dancing around one another, waiting for a couple big companies to publicly settle on a standard,” said Cervelo marketing manager Heather Henderson, voicing the situation of many smaller players in the industry.

Trek/Bontrager: 'MTB has shown what works'

With substantial amounts of research in mountain bike wheels already behind us, Bontrager brand manager Alex Applegate said there is no need to reinvent this particular part of the wheel. Trek is going with MTB standard dimensions for road disc-wheel axles.

"Years of disc brake wheel innovation from the mountain bike world have helped flushed out what works, and what is needed to make wheels as strong and light as possible," Applegate said. "We are all for innovation to make things stronger, faster and lighter, but introducing new standards with negligible weight and strength benefits just doesn’t make sense. We have taken our cues, done the research, and a 100x15 in the front and 142x12 in the rear are the way to go."

"Replaceable end caps that easily step down to QR should keep wheels QR-compatible going forward, but a thru-axle is going to make the most of the bike, brake and wheel interaction," he said.

Specialized/Roval: 'Both QR and thru-axle offer many benefits'

The product teams at Specialized disagree with Trek.

"We believe that 100x12mm will be the front standard for road thru-axle disc," said Specialized performance road and wheel manager Chris Wehan. "We believe that 135 rear ends are the future of road disc bikes. They allow the chainstays to remain narrow, allowing for heel clearance, while still giving enough stiffness to the rear end and wheel."

Specialized designed a system called SCS 135x12mm, which is a 135mm wheel with a hub offset like a standard 130mm road wheel. "This system allows Specialized to build bikes with short chainstays while maintaining great shifting. This standard is completely open and we have discussed this system with multiple other companies; we do expect some of them to launch SCS 135 systems in the future," Wehan said. The downside to this system is incompatibility with other wheels. In the case of the S-Works Tarmac Disc we recently tested, the bike works great with the stock wheels, but you can't crosschain on other 135mm wheels as the chainline is too extreme on Specialized short-chainstay disc frames for which this SCS standard is designed.

Specialized isn't abandoning quick-release hubs, either. Wehan says QR hubs remain "the lightest and fastest way to change a wheel on a road bike and in race situations this can make a huge difference.

"Thru-axles provide extra safety and security for the wheel, as well as helping the rider properly center the rotor in a disc caliper, cutting down on disc rub noise. At the end of the day, the intended use of the bike needs to be taken into consideration," Wehan said.

Giant: 'QRs are the best solution currently available'

While Giant now has thru-axle disc hubs on its cyclocross bikes, the Taiwanese manufacturing superpower is holding off on road bikes until a lighter solution emerges.

Giant isn't showing its hand on future plans, but a 12mm thru-axle seems more likely than 15mm. Giant road spokesman Doug Barnett said "component manufacturers will ultimately be the deciders of what exactly the new standard will be. However, we expect that a truly 'road-specific' thru-axle standard would be less-robust than the current options developed for off-road use, such as QR15. There has to be a middle-ground between current road and off-road axle standards."

"Current QR15 thru-axles — both the frame design and the axles themselves — would add over 150 grams over an open-dropout system," Barnett said, adding that for the Defy endurance bike this was "not an option when our goal was to build the lightest disc-specific road frame on the market."

Additionally, current QR15 thru-axle designs slower than quick-release hubs to remove a wheel, Barnett said, and overbuilt for road riding. That said, Barnett kept stressing that Giant's decision was based on current axle standards available, which clearly seems to point to something else coming down the pipe.

Wheel companies: 'The challenge is an interchangeable solution'

Contacted for this column, Shimano was tight-lipped, giving a generic response: " Shimano is always looking at ways of improving performance and thru-axles for road bikes holds promise. We are currently investigating options for a road-specific standard but cannot comment on a timeline.”

At SRAM/Zipp, spokesman Dan Lee said 100x15mm and 12x142mm now "seem to be the norm, however, it could certainly shift to 100x12mm and 135x12mm as more disc brake road bikes hit the market."

"A lot of this will come down to the frame manufacturers. The challenge for wheel companies is to have an interchangeable solution," Lee said.

And the main wheel companies do have solutions. "Wheel companies can go a long way to ease the rider’s minds by making their systems to handle both QR and thru-axle," Specialized's Wehan said. "Roval for example, does offer many of their disc brake wheels with multiple end caps so the rider can choose which system is best for them now without worrying."

At Reynolds, founder Paul Lew sits on the wheel committee for the World Federation of the Sporting Goods Industry, where a consensus has yet to be reached.

"At Reynolds we continue to have multiple end-cap/ axle options. WFSGI has formed a working committee among bike manufacturers to create a standard. I think it will be 3+ years before the industry sorts out a standard," Lew said. 

Bottom line: You won't be left out in the cold

"I understand the hesitancy of your friends," Lew said. "Both consumers and industry folks have the same concern. Considering my role in the cycling industry, and my inability to give you a definite answer regarding a standard — well, that should tell you something about where we are as an industry when it comes to a standard."

Industry veteran Nick Legan, however, has a broader view. "Quick-release wheels are a transition thing, but they were in mountain bikes too," said Legan, a former pro team mechanic. "That doesn’t mean that once thru-axles become more prevalent that you won’t be able to find replacement wheels for a bike you bought with quick-release hubs. I think this worry is exaggerated. The bicycle industry has done a good job of continuing support of older standards. And at a point, you can trade up. By then other systems will have advanced as well, making it, if you’re so inclined, time for a new bike."

What do you think: Will the lack of an axle standard for disc road bikes keep you from buying your next road bike? Sound off in the comments section below.

Ben Delaney

US Editor-in-Chief
Ben has been writing about bikes since 2000, covering everything from the Tour de France to Asian manufacturing to kids' bikes. The former editor-in-chief of VeloNews, he began racing in college while getting a journalism degree at the University of New Mexico. Based in the cycling-crazed city of Boulder, Colorado, with his wife and two kids, Ben enjoys riding most every day.
  • Discipline: Road (paved or otherwise), cyclocross and sometimes mountain. His tri-curious phase seems to have passed, thankfully
  • Preferred Terrain: Quiet mountain roads leading to places unknown
  • Current Bikes: Scott Foil Team, Trek Boone 5, Specialized S-Works Tarmac SL4, Marinoni fixed gear, Santa Cruz Roadster TT bike
  • Dream Bike: A BMC Teammachine SLR01 with disc brakes and clearance for 30mm tires (doesn't yet exist)
  • Beer of Choice: Saison Dupont
  • Location: Boulder, CO, USA

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