Tailfin Tech: Axle Deep Dive

One of the distinctive features of a Tailfin Rack or AeroPack is how we use the wheel axle to secure the system to the bike. Whilst it’s true that you can also mount a Tailfin system using frame mounts, it’s when using the axle that the full strength and security of our design reaches its peak.

Before we focus on the design and construction of the Tailfin Axles it’s good to get a little bit of background as to the history of the humble wheel axle. Without wanting to confuse matters from the off, it isn’t the actual wheel axle we replace when fitting a Tailfin system. The actual axle that the wheel bearings spin on is part of the wheel hub and doesn’t get touched.

What most of us refer to as the axle is more specifically a wheel fastener; it’s the component that holds the hub axle securely in the frame or fork dropouts. Holding the hub axle firmly in place allows the hub bearings to take the load and spin freely and efficiently. That’s enough of that, for the rest of this feature we will resort to the popular nomenclature and refer to the wheel fastener type as the axle – hopefully this will be the end of any further confusion!

Tailfin axle deep dive

Barring the last four or five years the most common type of wheel fixing was the quick release skewer. This simple system relies on a cam activated mechanism operated by hand and provides a quick and effective solution for holding wheels in place. The widespread adoption of disc brakes, firstly by mountain bikes and more recently by road and gravel bikes highlighted some specific failings of the quick release (that of inconsistent clamping forces and poor alignment between hub and frame dropouts resulting in brake alignment issues) when used with this more modern style of braking. 

To mitigate this issue the bike industry developed what we call the thru axle (or bolt thru axle). This larger diameter wheel fastener threads securely into the frame itself through well toleranced circular dropouts and provides much more consistent alignment and clamping force that minimises frame/fork compression and the resultant annoying brake rub.

Being fueled by competition the cycle industry is notoriously reluctant to settle on universally accepted standards and brands are keen to adopt unique and specific designs in order to stand out from the crowd. The thru axle is a victim of this process and one of the roadblocks we encountered when designing the Tailfin axle was the multiple standards that have been adopted by various manufacturers. Not only does the thread pitch (the angle and spacing of the raised thread) change between designs but also how the axle fits within the frame itself. Which is why the Tailfin axle is available in four different threads pitches and why each axle comes with four differently shaped end caps. 

We didn’t have any of these issues with the quick release – it’s a design so old that the cycle industry has settled on a single standard. Result: if you have a quick release there is just one Tailfin product for you.

Designing the Tailfin Axle

The anatomy of a Tailfin Axle

Using the rear wheels ‘axle’ as a key attachment point for the Tailfin system was the result of identifying the strongest part of the bicycle in terms of load bearing. Spreading the forces of any additional load to the axle isolates it from potentially causing stress on the relatively delicate rear triangle of the frame and in case of a catastrophic failure – either through completely overloading or an accident the axle is a relatively inexpensive item to replace and should not result in writing off your precious frame.

When coming up with the idea of utilising a wheel ‘axle’ to use as an attachment point for the Tailfin System there were several considerations to factor into the design process.

  1. How to attach a Tailfin system to the axle
  2. What materials were optimum in terms of strength and durability without being overly heavy
  3. How to fasten the axle in the frame
  4. How to create an axle that will accommodate the many different thru axle and frame designs

How to attach a Tailfin system to the axle

You will by now notice that the replacement Tailfin axle has a smaller diameter extension at each end of the main axle. These extensions are designed to stick out further than the existing axle and provide a solid mounting point for each side of the Tailfin Arch. The length of these extensions has been carefully chosen to ensure there is zero risk of fouling when riding your bike without a Tailfin fitted, meaning the tailfin axle can be seen as a permanent replacement for your old axle.

You will also notice that the extensions feature machined ribs on the mounting surface. These are designed to prevent the Fast Release Dropout from sliding about. It’s also why the dropouts feature nitrile bushings that conform to these ribs and create an incredibly secure connection without rattle or movement.

What materials were optimum in terms of strength and durability without being overly heavy

Tailfin thru axles are individually CNC machined from solid stock 7075-T6 aluminium alloy and further processed with an anodised finish in classic Tailfin Black. 7075-T6 alloy was chosen as it has the ideal attributes for this application: high strength to weight ratio, resistance to fatigue and corrosion plus excellent overall toughness. The fact it was originally developed for aircraft space frame construction should tell you all you need to know.

On average a Tailfin Axle with additional hardware weighs 69 grams – standard bolt-thru axles weigh between 45-75 grams – so the trade-off is very minimal.

How to fasten the axle in the frame

Due to the unique shaping of the Tailfin thru axle it is very difficult to incorporate any form of lever or handle into the design and still retain simplicity and light weight. Instead the ends of the Tailfin axle are machined to accept a standard 5mm Hex or Allen style tool, something every rider will have on their multitool. The 5mm recesses are machined deep enough to provide a very positive fit with all tools to reduce the chances of slippage and potential rounding off of the axle end which can happen with some axles.

Whilst we will always recommend you to tighten the Tailfin axle ‘hand tight’ in your frame (using a 5mm hex key of course) individual frame manufacturers will recommend differing specific torque settings. If in doubt refer to your friendly local bike shop for guidance on how tight an axle should feel in your frame – you shouldn’t need two people and a huge metal bar to undo the axle but equally the axle shouldn’t be able to be loosened just by hand.

How to create an axle that will accommodate the many different thru axle and frame designs

Probably the most difficult hurdle during the development of the Tailfin axle was catering for the myriad of standards that are encountered within the thru axle family. Key considerations were covering different thread pitches, axle seat shapes and frame spacing/widths. As it stands we produce four different axles based around the four most common thread pitch standards: 1.0mm, 1.5mm, 1.75mm and 1.0mm Speed Release.

Each version of the Tailfin axle ships with four different axle seat spacers: 2mm flat, 2mm cone, 8mm flat and 8mm cone. These primarily are to make sure the axle sits flush with the frame and creates a solid interface for a strong and reliable connection. The secondary reason for providing the different heights of spacer is so you can maintain an equal amount of axle showing at each side of the bike. This helps centre the Tailfin system on the bike and prevents uneven loading.

The differing width spacers also allow us to produce a single length of axle that will easily fit to the majority of common rear hub/frame spacing. The axle will fit most 135, 142 and 148mm spaced rear ends. At the moment we cannot accommodate the rarer 157, 177 or 197mm rear hub spacings. If this is your bike we recommend frame mounting if you want to run a Tailfin. 

A simple but incredibly helpful feature of the Tailfin axle is the inclusion of a rubber o-ring to keep the chosen spacer in place. This is not only a secure way of keeping things together but gives the rider who likes to move their Tailfin system between bikes the ability to quickly swap between spacers without needing any tools.

The final thing to note about the axle seat spacers is the importance of using the included stainless steel washer. This fits between the axle and spacer and prevents the two aluminium parts from galling (sticking) together when the axle is tightened. This is vitally important as allowing the two parts to spin freely enabling the axle to tighten freely without risk of damaging the frame.

Additional design considerations

In some instances specific bike brands also adopt additional proprietary features within the rear thru axle design that has led us to provide a variety of additional adapters to ensure further compatibility. Focus and Cervelo have adopted a keyed R.A.T. axle system requiring additional parts. Salsa and OPEN also have further requirements in the form of replacement nuts or additional locking nuts. We now also produce an adapter for brands that have recently adopted SRAM’s new UDH (Universal Derailleur Hanger) system. 

Tailfin Axle Finder

we currently produce different Tailfin compatible axles to suit all five. But the bad news is this still provides a confusing scenario for many riders as unless you have a complete and thorough understanding of all parts of bicycle design, you are able to accurately measure a thread pitch or your axle has the correct standard measurements printed on it – how do you know which axle to choose?

It has been one of the biggest sticking points for Tailfin customers and we have been working hard to make the experience as painless as possible. We have now gathered an extensive database of bike brands, specific models and model years and placed it into an easy to use axle finder to enable riders to take the uncertainty out of getting the right thru-axle.

You can find the Axle Finder when choosing the options for your new Tailfin RackAeroPack or when ordering an additional axle.

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