Do thru-axles and disc brakes have a place on triathlon bikes?

Jordan Rapp has raced triathlon professionally for the past 10 years. He is the 2011 ITU Long Distance World Champion and a six-time Ironman champion, and holds a mechanical engineering degree from Princeton University. 


Having now spent about six months riding an Andean, I can say without reservation that the two most significant features of the bike are - to me - the 12mm through-axles and disc brakes. Each of these features would, on its own, be a small-but-notable improvement to what I had been using before, but together they represent a clear step forward as compared to bikes I've ridden in the past, both because of how they work together and also because of the flexibility they offer with regards to selection of wider tires.

Andean Thru Axle
The Combination

Looking at the current state of the art in mountain biking, it's easy to forget that disc brakes and through-axles are not inherently related technologies. In particular, mountain bikes - especially on the cross-country side where weight is at a premium - made use of through-axles in the front while sticking with quick releases in the rear (or, occasionally, vice versa), both of which were paired with disc brakes. But the two technologies do form a natural synergy, and it's now increasingly uncommon - especially on higher-end bikes - to not find them together.

There are a few good reasons for this. Disc brakes are obviously more sensitive than rim brakes - where the pad pitch can be adjusted - to alignment. Through-axles necessarily force perpendicular alignment of the hub to the frame or fork (assuming that the through-axle guide and threads are aligned properly). While you cannot make minor adjustments to the alignment of the wheel in the dropouts as you can with a quick release, you also don't need to worry about such variance in alignment when putting your wheels on.

With pad travel on most disc brake calipers being only a few millimeters, a rigid and consistent interface is of paramount importance. The calipers - and their frame mounts - are designed to offer significant adjustability, allowing you to position them just so in order to prevent rotor drag. But once they are set, no part of the system is designed to move, and the reliable - and robust - through-axle anchor works especially well for this.

One thing that the system is remarkably intolerant of, however, is bent rotors. So make sure to take your rotors off when traveling. A 6Nm torque key helps with reinstallation on ISO 6-bolt hubs, and Shimano makes a dedicated tool for the higher torque and unique splined interface of centerlock rotors.

Overall, though, the combination of disc brakes with through-axles provides a simple, reliable, and repeatable method of attaching wheels to your bike for with perfect alignment and optimal braking performance.

The Case Against Disc Brakes and Through-Axles

Overwhelmingly, the case against both disc brakes and through-axles is limited exclusively to the professional peloton. Potential safety issues related to rotors are valid in a field of 100+ riders regularly traveling at speeds in excess of 30mph (50kph). Likewise, ease of wheel changes are an issue with differing rotor standards. And speed of wheel changes is at least possibly slower with a through-axle. But none of these concerns is valid for a typical recreational rider, especially a triathlete.

The main issue for triathletes is related to standards, or a lack thereof. The UCI is moving towards 160mm rotors in the front and 140mm rotors in the rear. Whether or not those standards will find their way to triathlon remains to be seen. With a greater concern about aerodynamics and generally less technical courses, 140mm rotors front and rear have proven more than adequate in my case. However, a heavier rider (there's no clydesdale division in the TdF...) might opt for 160mm front and rear. But the issue of neutral support and ultra-critical wheel swaps is just less of an issue for triathletes as well. Neutral support at most triathlon races is much more likely to help you change a flat tire than they are to give you a new wheel.

The spacing of the rotors is supposed to be standardized (in terms of offset from the left end cap), but given the limited pad travel on most calipers, it doesn't take much variance from manufacturer to manufacturer to cause issues when swapping in neutral support wheels. But this also works out for triathletes, since - as mentioned previously - neutral support typically prefers to help you change a flat rather than giving you a new wheel.

Likewise, while 12x100mm in the front and 12x142mm in the rear is becoming the standard on road bikes, it's still not universal; 12x135mm also exists, and - most frustratingly - there is no consistency with regards to thread pitch. While that's not an issue for wheel changes, it is totally unnecessary that the industry cannot agree on a single thread pitch for through-axles. It does no good in terms of easy accessibility to spare parts if the 12mm diameter of through-axles becomes standard but thread pitch varies.

All in all, there's just a lot more variance with a system that is less tolerant of such variance. All of this is a nightmare if you're trying to get a spare wheel or, if you're a race organizer, trying to figure out what spare wheels to carry, especially since most competitors will still be on quick-release equipped rim-brake wheels. With a generous storage compartment, though, the Andean nips this issue in the bud. You can carry a generous array of spare supplies so that you aren't reliant on neutral support at all, except for maybe an extra pair of hands.

Andean

Concern of varying standards is not a new issue though. During the change from both 9 speed to 10 speed and then 10 to 11, the number of cogs on cassettes on neutral support wheels was likewise a headache. Periods of changeover are never easy. Ultimately, though, reticence about accessibility to neutral support is not a good reason to choose - or not - a given technology.

The final issue, of course, is the pushback whenever planned obsolescence comes up. Many people will have invested resources - sometimes significantly so - in a given system, and whenever there is a total incompatibility moving forward, people want to be convinced that the new technology is "worth it."

But new technology is rarely - if ever - "worth it" at the time. Value comes over time as an ecosystem of support grows. If we all waited until it was the "right time" to switch, we'd still be riding horses and sticking ice blocks inside our iceboxes to keep our food cold.

Through-Axles

One argument against through-axle equipped wheels is that wheel or tire changes are slower. With a hex key of the appropriate size handy, I haven't found this to be the case. Of course, having a hex key handy is critical, but it's simple to position one for easy access, especially with the Andean's generous storage options. Likewise, there are numerous offerings for hand-adjustable/tighten-able through-axles for people that don't want to mess with tools.

Thru Axle

But this brings up another topic that's little discussed - hex-key through-axles are much lower profile than quick-releases. While there are plenty of low profile quick-release options out there, none of them approach the overall low profile of a hex-head through-axle. While the aerodynamic savings aren't significant, they are nevertheless measurable. Certain niche manufacturers like Tune and View-Speed have made low-profile hex-key end cap quick-releases that are - essentially - 9mm through-axles, but here it seems you are gaining all of the hassles of a through-axle (the need to use tools to change a wheel) without gaining most of the benefits (stiffness and consistent anchoring)

The two most obvious benefits to through-axles are security and stiffness. How measurable are these? It's impossible to say quantitatively, especially when discussing something like safety that is inherently subjective. Quick-releases have been the subject of CPSC recalls by both Trek and Zipp, and a 2005 article on SFGate.com quotes FSA's Eric Hjertberg as saying, "quick releases on bicycles have generated at least as much litigation over the years as all other bicycle parts combined." [1]

Quick-releases have been used for almost 100 years at all levels of the sport without issue, and yet it's clear that they are at least somewhat more prone to user error - and more unforgiving of such errors - than a through-axle design. The typical response of veteran cyclists is, "Well I have never had an issue..." discounts the fact that many novices still struggle with things that many of us never even think of - such as changing a tire. A through-axle that is not adequately tightened is simply more secure than a quick-release that is not adequately tightened.

But there are some additional safety concerns with disc brakes that through-axles are best designed to address. Because of the position of the rotor and caliper, disc brakes apply a force that wants to pull the axle down and out of quick-release dropouts. Fork and rear dropouts can be re-oriented - so that they aren't pointing straight down - to resist this force, but a quick-release and the open design of dropouts are inherently less suited to resisting the rotational forces at the hub than a through-axle - which anchors through two holes which are fully closed in the direction of these forces.

When it comes to measuring stiffness, there's no good data, simply because the age of aftermarket forks is largely over on the road side. Most companies make their own proprietary forks, especially for triathlon/time-trial bikes. Further, some of the additional stiffness is a direct requirement due to the asymmetric loads placed on a fork due to disc brakes. Because the caliper and rotor exist only the non-drive side of the bike, braking introduces a torsion on the fork legs that simply isn't present with rim brakes. Through-axles provide additional stiffness right where it's needed most for disc brake applications. So there's no easy way to say that a through-axles make forks X% stiffer. But it's clear - in particular - the larger axle diameter and the more secure anchoring method both work to improve the stiffness of the complete fork assembly.

Triathlon/time-trial bikes have relatively low stiffness requirements, especially in the front end. Standing sprints are just not all that common. So slender fork legs for the purpose of aerodynamics are relatively easy to realize anyway. Can you make them even more slender due to the additional stiffness of a through-axle? It's hard to say. The Andean has an extremely slender fork built on a 1" steerer tube, so the ability to stiffen that package with the use of an inherently stiffer through-axle is an obvious win. But the stiffness argument, as in most cases when talking about a bike that is almost exclusively used for sub-maximal efforts, is largely moot. Compared, for example, with potential safety, stiffness is a distant second.

Andean Fork

But it's actually neither safety nor stiffness that I - personally - enjoy the most about a bike with through-axles. For me, the biggest advantage is repeatability. Through-axles simply only can mount in one way. And there's virtually no play in this system. A wheel on a quick release can easily be skewed slightly to the left or right. Especially if you travel a lot - as most triathletes do - and are taking down and building back up a bike, it can be a real nuisance to try to figure out whether the brakes are slightly misaligned or if your wheel is not perfectly aligned in the dropouts. That worry is eliminated with through-axles. The wheel is anchored securely and identically at the hub every time.

Of course, this places an additional onus on frame manufacturers to make sure that the frames and forks are perfectly aligned, but it's hard to see how this is a downside. And, at least in the rear, the through-axle anchor can be given some float to account for the complexity of making everything perfectly true in a frame as compared to a fork.

Andean Drive Train

Disc Brakes

Disc brakes simply work. Regardless of conditions, the stopping power of organic [2] pads on steel rotors is just there. But hydraulic rim brakes and textured braking surfaces also offer a remarkably good braking experience. While I'd say that disc brakes offer a better experience than any rim brake setup I've ever used, I've also never found had a safety issue where I could not stop with a properly trimmed rim brake setup.

But therein lies what I'd consider - for triathlon bikes - the biggest advantage of disc brakes. Disc brakes are simple to work on and to properly trim and adjust. The location - unlike hidden rim brakes - is in probably the most - rather than least - accessible location. Working on a braking system that's centered at the hub is simply easier than working on brakes that may be hidden deep inside the frame/fork.

Traditional dual-pivot calipers work extremely well. But increasingly, in the quest for aerodynamic gains, manufacturers have moved to various single-pivot or otherwise proprietary designs that simply do not have the stopping power of a good dual-pivot brake. While some athletes may be keen to trade stopping power for aerodynamic benefit, the net result of this trade outside of a wind tunnel is often less clear cut than is revealed by a simple analysis of drag. The ability to stop quickly and reliably has performance benefits beyond the simple - and not unimportant - aspect of confidence.

Wider Is Better: Expanding Your Tire Choices

But most critically, the shift to disc brakes also allows for wider tires. Fitting a tire inside of a brake caliper no longer is a concern.

The aerodynamics of tire selection is now starting to become more widely understood. But aerodynamics is only half of what makes a tire "fast." Rolling resistance can be even more important, depending on the quality of road surface.

As a general rule, wider tires offer lower rolling resistance for a given tire construction methodology but worse aerodynamics. However, on very rough roads, the primacy of wider tires can quickly become apparent. The differential in power expressed by Crr (Coefficient of Rolling Resistance) goes up as road surface quality deteriorates. Tires that, in drum testing, show a difference of 5w at 25mph (40kph) can easily show double that on rough roads.

Josh Poertner, the long-time technical director at Zipp and current owner of Silca, the venerable pump and tool brand, said that if people truly understood the impact of rolling resistance and tire width, we'd all be riding 28mm or 30mm tires on the road, not just for training, but for racing as well.

Andean Heattube

This shift is already showing up at the highest levels of racing, where 26mm tires are common in places where even a few years ago you might have seen 20mm tires. The "typical" road is just a lot rougher than people realize, meaning that wider tires - and the lower pressures they allow for - are simply that much more beneficial. Especially when it comes to bikes that typically - due to aerodynamic reasons - have relatively poor vertical compliance, the extra comfort of wider tires can have a very real impact over 56mi or 112mi (90km or 180km) of racing, and even more so when you have to run afterwards.

Wider tires also complement disc brakes in that a larger contact patch allows for greater braking force. The width of a tire - it's plenty easy to lock up a wheel on a road bike with rim brakes - is a significant factor in the braking power of the total bicycle system. In order to fully realize the additional braking power of disc brakes, you need more rubber on the road. The fact that wider tires allow for this only makes them an even more logical choice for most applications.

In my own experience, the ability to run 28mm tires for training at 75psi has made the Andean more useful as a training tool, because I find it's comfortable to ride more places. It stops well and is comfortable to ride. All because of the disc brake platform.

But What About Aerodynamics?

The aerodynamics of rotors and calipers must be considered within the context of the entire system. What seems clear after a variety of manufacturer and independent studies is that disc brakes need not be inherently slow. A bike designed intelligently around disc brakes can equal - or even exceed - the aerodynamic performance of a bike designed intelligently around rim brakes. Disc brakes are neither inherently slow nor inherently fast. They obviously require a different approach to optimize the aerodynamics of the entire bicycle - the head tube can be made more slender, for example, without the need to mount calipers at the top of the fork.

Andean Cockpit

But with appropriate consideration to both the requirements of a disc brake - calipers and rotors located at the hubs - and the flexibility that offers - no caliper at the head-tube-to-fork junction, there's no reason that bikes with disc brakes need to pay an aerodynamic penalty.

The only place disc brakes are inherently slow is when you're relying on them to stop you. And that's exactly what you want.


[1] Source of this article is the excellent 2015 CyclingTips.com article on through-axles: https://cyclingtips.com/2015/10/road-bikes-are-headed-towards-through-a…

[2] Sintered pads - which contain metallic flecks - and rotors designed for use with them are common on motorcycles. In the bicycle world, they are exclusively the province of downhill bikes.