How do disc brakes change the design of road bike wheels?
Now that hydraulic disc brakes are available for road bikes and manufacturers are embracing the new format, what will happen to road wheels? In this article, CTech Editor Matt Wikstrom considers the effect disc brakes will have (and are having on) wheel design, while considering a prototype disc-specific carbon wheelset built by Wheelworks.
A little more than a decade has passed since disc brakes first appeared on mountain bikes. At the time, there were complaints about the extra weight and cost, as well as safety concerns, but function quickly triumphed over all else and they have become ubiquitous for MTB.
Since their introduction, off-road disc brake systems have undergone significant development to make them lighter, more reliable, and easier to service. They have also been refined to suit different riding disciplines. Hydraulic systems dominate on the basis of their performance, offering powerful braking in all conditions with a very light braking action and a lengthy service interval.
Now that road cycling is poised to adopt disc brakes, it will benefit from many of the lessons learned in MTB. At the same time, it will also adopt many of its standards. Thus, disc-equipped road frames have inherited 135mm rear axle spacing from MTB, and as a consequence, can take advantage of the wide range of disc-ready hubsets that are already on the market.
There is one drawback to all of this ready hardware: most of it has been designed to meet the demands of off-road use. Early adopters can expect robust and reliable products rather than lightweight and aerodynamic designs. Shimano is already working on a new standard for road disc brakes while smaller rotors (140mm) have already been introduced as an early refinement. I expect there will be plenty more refinement and innovation in the coming years as the industry embraces disc brakes for road use.
What About the Wheels?
Shifting the braking surface from the rims to rotors that attach to the hub has enormous ramifications for wheel design. Disc-specific rims no longer need a braking track, allowing extra freedom for the design of new profiles and some reduction in weight. However, clincher rims will still need to be strong enough to withstand 120psi in the tyres, so the amount of weight that can be shed will be modest at best.
Any weight lost from the rims will offset the extra weight of the hubs and rotors. Shimano’s Centre Lock system allows for lighter hubs than standard six-bolt mounts, but extra material is still required to ensure the hub can withstand substantial braking forces.
In addition, a minimum of two-cross lacing is required to contend with the forces of braking as well. Front wheels will suffer the biggest weight penalty due to heavier hubs and more spokes. Thus, disc-equipped wheelsets will always be heavier than traditional wheels in the same way that clincher wheelsets will always be heavier than tubulars.
At least carbon rims are well suited for disc-equipped wheels. Riders tempted by carbon rims will be free to indulge their desire and enjoy some weight-saving without having to worry about the dangers of sustained braking on long descents or reduced stopping power in the wet. Of course, the rotors and pads will still be at risk of overheating but at least it won’t result in rim failure and/or a tyre blowout.
Wheelworks Prototype Road Disc Wheels
Tristan Thomas, the owner of Wheelworks, is excited about the potential of disc brakes for road bikes. He’s been using disc brakes on the road for around four years on the strength of their performance, especially in unpredictable weather conditions. Now he’s starting to experiment with lightweight builds.
“It’s no secret I’m a disc brake fan, and as a wheelbuilder, disc brakes open up some interesting pros and cons for wheels. In general terms the front wheel needs to have more spokes and they need to be crossed to deal with the huge force from the disc. 70% of your brake force comes from the front brake so whether it’s rim or disc there is a lot of load under hard braking. The extra spokes actually help a little by allowing a lighter rim to be used as the lateral stiffness requirements for the rim are lower.”
Tristan is currently testing pre-production 40mm carbon disc-specific rims produced in Taiwan by BDOP.
“BDOP are a Taiwanese based company that work as an OEM and ODM source for companies of various sizes — they’re a true middleman in every traditional sense of the word. They’re the connection between a Western company who wants a product made in Taiwan, and the factories that will be making it. The DTO-40 is the first rim they’ve designed and produced themselves using their existing factory contacts and relationships.”
The DTO-40 is 40mm deep with a 19mm internal width and weighs around 405 grams offering a 10% weight saving when compared to carbon clincher rims with a braking track, such as November’s Rail 34, Enve’s 3.4, or Reynolds’ Assault.
“The lack of brake track opens up the rim shape a little bit, however it’s not drastically different from a modern ‘wide’ rim. The lack of a brake track does mean the rim’s bead hooks can be better supported by the central/internal brace, and thus lower weight.”
Tristan matched the DTO-40 rims with a White Industries CLD (Centre Lock Disc) hubset and DT Aerolite spokes (24 spokes, two-cross lacing, front and rear).
“The CLD was designed as a CX- and road-disc hub from the outset so it can accept a Shimano or Campagnolo 11 speed freehub. The modular nature of the White Industries products means that these hubs can easily be converted to 15mm thru-axle front, or 142×12 rear, which is useful as road-disc heads towards through-axles. This is a lightweight hub — Center Lock really helps with that as it keeps the flanges small and doesn’t require a ton of meat for the six-bolt rotors to mount to — but they haven’t skimped on bearing sizes, and the freehub body is the same proven titanium unit which ships with the T11 rear hubs.”
The final weight for the wheels including rim tape and without skewers was 1,348g (670g front, 678g rear). Shimano’s Ice Tech 140mm rotors add another 204g, making for a combined weight of 1,552g.
“We’ve intentionally gone a little light on the rear rim. For a guy my weight we’d normally use a 28-hole. One thing that fascinates me is wheel stiffness and the concept of enough or not enough. On a rear rim-brake wheel ‘not enough’ means the rim rubs the brakepads, but what about on a disc wheel? The rim won’t rub on anything, so how much stiffness is needed? Do you feel rear wheel stiffness other than brake rub?”
Wheelworks has demonstrated with its prototype wheelset that even at this early stage of road disc development, it is possible to build performance-oriented clinchers at a very competitive weight. The extra weight of the rotors and mounting hardware was partially offset by lighter rims — not enough to impress weight-weenies, perhaps — but more than enough to serve performance-oriented riders. Furthermore, there is the promise that disc-equipped wheelsets will realise the full potential of carbon rims.
It is worth noting that disc brakes still suffer from a couple of unresolved issues. Most pressing is the tendency for discs to squeal. A bunch of disc-equipped riders has the potential to terrorise a sleepy neighbourhood and years of development for MTB has failed to eliminate this issue. Likewise, the risk of brake fade (brought on by overheating the rotors and brake pads) continues to haunt disc brakes.
Regardless, it will be very interesting to see how wheelsets evolve for road disc use in the coming years.