The recent introduction of road disc bikes is ushering in a variety of new technologies and “standards” that buyers must come to terms with. One of the more controversial refinements involves through-axle systems that complicate wheel changes and make things like fork-mounted roof racks and workstands obsolete. In this article, CTech editor Matt Wikstrom examines whether through-axles are truly necessary for road disc bikes.

When disc brakes were first introduced to mountain bikes in the ‘90s they were dismissed, because they were heavy, impractical, and, completely unnecessary. The pushback from rim brake stalwarts was considerable, yet disc brakes were destined to become ubiquitous for MTB.

Now road riders are facing the same shifting of sands as disc brakes penetrate their market. Interestingly, road traditionalists are expressing the same complaints and reservations as those made by MTBers during the late-‘90s. Like it or not, road riders are following in the footsteps of MTBers.

The major difference for road riders is that there is an established disc brake market and they are inheriting a lot of technology and standards designed to meet the demands of MTB. While there is no question about the effectiveness of the current state of the art, there is a question about how well it translates for road use.

Some concessions have already been made with the acknowledgment that road riders probably don’t need the same size rotors as off-roaders. Thus, 140mm rotors were developed for the road, whereas the smallest rotor used by MTBers is 160mm. Hub widths also seem settled at 100mm for the front and 135mm for the rear due to the use of relatively narrow rims and tyres.

The choice of axle systems is much more controversial though. The battle here is between the traditional quick release skewer (QR) and through-axle systems (TA) developed for MTB, especially suspension forks. The latter have been embraced by MTBers and thus, there is some expectation that road disc riders will need them too.

The rise of through-axle systems

TA resemble large pins that pass through holes in the frame and forks tips to secure the wheels. There are no slots in the dropouts so a TA must be removed completely to release the wheel. TA typically thread into the frame or fork tip and have a quick-release lever to assist with tensioning the axle.

The earliest TA were developed for downhillers about ten years ago at a time when every component was being fortified to withstand the demands of racing. Riders were breaking front axles (QR or solid) chiefly because long-travel fork legs weren’t compressing in unison.

By pinning the fork legs together, a TA synchronises compression and preserves the front axle by reducing (or removing) uneven loading (Figure 1A). TA also stiffen the lower legs by reducing or removing torsion at the front hub (Figure 1B). As a result, MTB riders consistently report a stiffer front end with improved steering and handling while the forks are more robust, and therefore, safer. TA also stiffen the rear end of the bike and provide extra strength and durability for the bike.

There is another issue that arises when the brakes are used: as the brake caliper bites into the rotor, it is carried forward by brake reaction forces, leading to torsion of the fork leg (Figure 1c) or stay. TA resist this torsion much more effectively than QR by providing a larger interface to contend with these forces. TA also ensure correct alignment of the disc rotor with the caliper, so overall, braking performance is improved, as is the quality of steering under brakes.

The effectiveness of a TA depends upon its diameter: at present, 20mm TA are favoured for the front end by downhill riders, while everybody else (except weight-weenie XC racers) uses 15mm TA (although there were some early misgivings over the smaller diameter). In contrast, 12mm TA are almost universally favoured for the rear end.

TA are relatively easy and intuitive to use, though they do require purpose-built dropouts and compatible hubs. As such, it is impossible to retrofit TA to a bike, and with no firm standards—just a popular consensus—there is a risk that any design will become outdated. Regardless, TA converts argue that the benefits easily outweigh all of their shortcomings.

Translation to the road

From the discussion above it’s clear that TA are well suited to suspension forks, but do they have anything to offer rigid forks? After all, there is no need to compensate for asynchronous compression. However, rigid forks are still prone to torsion at the hub and of the left fork leg during braking (Figure 2).

A recent comparison of QR and TA versions of a rigid off-road fork found that there was no significant difference in the performance of the forks. Minor improvements were noted though, suggesting that if there are any gains to be made on the road, they will be marginal.

No consideration was given to the long-term performance of the forks, and in particular, fatigue in the left fork leg after repeated braking. Carbon fibre is well known for its fatigue resistance, so perhaps the only issue is whether torsion of the brake caliper will detract from the quality of braking. At best, TA promise another marginal gain for road users.

What about the disadvantages? The extra weight is an obvious handicap, and while it is relatively minor it adds to the already significant weight penalty of disc brakes.

As noted above, there has been very little refinement of TA for road disc use, and there are indications that the industry is paying attention to weight concerns. 12mm TA have already been introduced as an early refinement for road disc forks and the smaller diameter appears to be gaining favour for road use.

A much larger concern is that TA will slow down wheel changes during a race. Nobody has ever argued that TA allow quicker wheel changes, but current disc brake systems already slow down a wheel change. That’s because the disc caliper must be aligned precisely with the rotor otherwise the brake pads will rub and slow the bike down. The clearance between the pads and the rotor is tiny (<1mm) and very sensitive to minor variations in the position of the rotor, which occur often even for otherwise identical wheels.

Thus, World Tour mechanics have been opting for spare bikes rather than attempting to fit a new wheel in the event of a puncture. Whether this continues to be a practical solution when an entire team is racing on disc-equipped bikes remains to be seen, though it seems unlikely, especially for an event with a high incidence of punctures (eg. Paris-Roubaix).

Putting aside the needs of the professional peloton, the influence of TA on the speed of a wheel change is largely academic for an everyday user. On balance then, the advantages and disadvantages of TA appear to cancel each other out.

That is, until the intrinsic safety of QR and TA are compared.

When dropouts become popouts

As disc brakes achieved widespread use by MTBers during the early years of the new millennium, a safety issue emerged that undermined faith in QR.

In 2003, James Annan warned that there was a risk that QR could unwind and the front wheel could be ejected as a result of disc brake use. The problem was a direct consequence of the position of the caliper behind the fork leg combined with the near-vertical orientation of the dropouts.

I’ve already mentioned that brake reaction forces are responsible for torsion of the left fork leg during disc braking; they also cause the front hub to be pushed downwards. Repeated braking can slowly loosen the QR with the worst-case scenario that the wheel could escape the dropouts (Figure 3).

At first, most considered it unlikely that a disc caliper could generate enough force to eject a QR hub from the dropouts. That view slowly changed with anecdotal reports of QR working loose, followed by independent testing of the phenomenon.

More than a decade later QR are still in widespread use on MTB with disc brakes. Annan’s risk wasn’t as profound as he feared, but support for TA has grown considerably. Interestingly, Annan’s simplest remedy — re-positioning the brake caliper in front of the fork leg—was ignored by the industry, however some fork manufacturers have re-oriented the dropouts to reduce Annan’s risk (Figure 4).

While the latter has some relevance for road disc bikes, there is much bigger safety issue that continues to undermine QR.

Let’s get litigious

QR have been in use for over 80 years. Tullio Campagnolo’s elegant design may have revolutionised professional cycling, but it isn’t an intuitive device and novices struggle to use them properly.

As a consequence, there have been a multitude of accidents after the front wheel has been ejected from the forks, and a lot of litigation (especially in the USA). Indeed, one industry representative suggested that QR “have generated at least as much litigation over the years as all other bicycle parts combined”.

Human error was a common cause in these complaints, yet bicycle manufacturers have been held accountable because they failed to warn owners of the risk associated with improper use of QR for the front wheel. Furthermore, such accountability extends to the provision of a fail-safe mechanism (designed to retain the front wheel if the QR is used improperly) hence the appearance of lawyer tabs (or lawyer lips) on fork dropouts during the ‘90s.

The problems with QR are ongoing. Trek recently announced a recall for an estimated 900,000 QR with a defect that affected the lever. When used correctly, there was no added risk; instead, there was extra risk when the QR was used incorrectly in conjunction with a disc brake.

Some believe it is time for QR to be re-designed, but as discussed above, the system is not designed to contend with all of the risks associated with disc brakes. After all, QR pre-date disc brakes by more than half a century. In contrast, TA were invented in the context of disc brakes, and hence, go much further than QR in addressing disc-specific risks.

This is an important distinction because liability laws in some states of the USA can compel a manufacturer to adopt a new design if it is safer. Thus, one test case may be enough to decide the matter. Otherwise, manufacturers will take the initiative and insist on TA for all disc-equipped bikes to avoid litigation.

Summary and final thoughts

The traditional road bike with rim brakes has steadily evolved over the last fifty years with a variety of refinements such as clipless pedals, indexed shifting, integrated brake/gear levers, and carbon fibre construction. The recent introduction of disc brakes marks the next step in this process, and it may signal a new era for road bike design.

Rim brake stalwarts are likely to note with a measure of smugness that all of the arguments for TA also serve to question the wisdom of disc brakes altogether. After all, current road bike design appears to meet the needs of riders and racers perfectly without any threat or significant risk. However, there was once the same level of satisfaction with toe clips, friction shifters located on the downtube, and heavy steel bikes.

To embrace through-axles is to acknowledge that road bikes are evolving.