The early adopter dilemma, part 1: Disc brakes on road bikes

In May 2014, I bought my Giant TCX SLR 2 cyclocross bike, Hyro. Back then, disc brakes on drop-handlebar bikes were not on most people’s minds, save for fans of cyclocross. The unique demands of that sport – racing drop-handlebar bikes across mud, grass, and sand, and shouldering them over unrideable obstacles – granted it leniency by the UCI to be the first to adopt disc brakes in 2010. At the time, it was one of the few avenues available if consumers wanted a combination of drop handlebars and disc brakes.


I was in a position to follow the growth and eventual adoption of the technology, beyond cyclocross and branching into full-on road racing bicycles over the next four years. It has not been an easy journey, with the UCI dilly-dallying over pro peloton approval in 2015 and 2016, until finally giving the green light in 2017.

At the beginning, the bike industry simply took the disc brake technology and form factor that had already existed for mountain bikes, and repurposed it for use on road bikes. So,

  • 135 mm rear hub spacing got mated to hubs with either 140 mm or 160 mm rotors,
  • sitting in open dropouts,
  • secured by quick release skewers, and
  • clamped down upon by disc brake calipers attached by the Post Mount standard.

At the time, I thought this was a very smart move. Why reinvent the wheel when you’ve got an already serviceable technology? And, while such road bikes were meant to carry a 160 mm rotor at maximum, with Post Mount hardpoints, it’s easy enough to accommodate a 180 or 203 mm rotor up front if you so desired.


A Shimano BR-RS505 flat mount disc brake caliper, as installed on an Eddy Merckx Mourenx69 endurance road bike.

When Shimano announced its ST-RS505 STI levers and BR-RS505 brake calipers, it announced a new method of attaching them to frames: Flat Mount. This eliminated the gap in between frame/fork and caliper while streamlining the entire interface. However, I feel this was a step backwards, and reintroduced division between road and mountain bikes yet again. Shimano would ultimately discontinue all Post-Mount brake calipers for road bikes except for the BR-RS785. Worth noting is that, unlike with mechanical brake parts and their different cable pulls, any mountain bike Post-Mount Shimano brake caliper will work with a hydraulic STI lever.

Photo credit: CyclingTips.

The second, more damning innovation was much more fundamental: it involved the interface between hub and frame. Fears arose about quick-release skewers and open dropouts being insufficient to combat unintended ejection of the wheel under the loads of a disc brake caliper. (Further investigation of such wheel-ejection accidents revealed that quick-release skewers were not being used properly, though.) Frame makers decided to adopt closed dropouts and through-axles from mountain bikes. Because the dropouts technically cannot let the hub drop out of them unless the through-axle is removed, this was seen as a safer alternative.

Photo credit: CyclingTips.

The through-axle itself, around 2.5x thicker than a quick-release skewer, also stiffens the entire frame, becoming the fourth side of the rectangle made by the fork legs or rear triangle.

The bike industry being what it is, it took a long, long time to settle on an acceptable standard…which eventually became 100 x 12 mm up front and 142 x 12 mm at the rear for road bikes. Fortunately, wheel and hub makers played nice with everyone and developed interchangeable end caps on their hubs, which allowed the same wheels to be used with quick-release skewers, 142 x 12/100 x 12 mm through-axles, and 142 x 12/100 x 15 mm through-axles – the latter and fatter-axled of which came directly from the mountain bike world, and is still used on some cyclocross bikes.

With this post, I’ve laid out the background. In the next installment, I go over what all this means when deciding to upgrade an early disc brake bike.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.