In my never-ending search for decreasing weight, increasing reliability and, in this case, reducing the ease with which my very nice wheels can be stolen from my bike, I was looking at buying thru-axles that require an allen (hex in North America) key to cinch up. Who knew that there were so many different options and key technicl characteristics.
The upside of the search is that the few options that are available are almost works of art from beautiful machining allowing reduced weight and hopefully increased deterrence. However I hit an obstacle when I looked to see which rear axle I would need to replace my DT Swiss RWS Thru-axle.
At first I went to my bicycle frame manufacturer’s website and the FAQ section. I was helpfully informed that my bike required a 12x 142 rear thru-axle (thanks I think I had that bit worked out by myself) but there was no information about the specific type required. An email to the Tech Department was followed up with an unexpected reply on a Sunday (very prompt and slightly odd, I think they should have been out riding) indicating that the axle type I had thought least likely (and the one that is least stocked) was “the axle that looks like will work best”.
So I thought that if the confusing multiple standards were news to me that they might well be causing confusion to other riders too. On closer examination, helped in part by very thorough tech diagrams on the Carbon-Ti website, there are a variety of axle lengths and, most importantly, thread pitches and threaded section lengths that determine which axle should be used.
The variety of standards for 12 mm rear thru axles (at this time) are: E-Thru; Maxle; X-12; Scott; Maxle 150mm; Maxle APB and Maxle APB Boost. With Boost spacing there are currently 14 different rear axle types one can buy from Carbon-ti.
12 x 142mm E-Thru:
Flat 1.5mm washer; Axle length: 171 mm; Thread length: 17 mm; Thread pitch: M12 x 1.5mm.
12 x 142mm Maxle:
Flat 1.5mm washer; Axle length: 174 mm; Thread length: 20 mm; Thread pitch: M12 x 1.75mm.
12 x 142mm Scott:
Flat 1.5mm washer; Axle length: 168 mm; Thread length: 16 mm; Thread pitch: M12 x 1.0mm.
12 x 142mm X-12:
Tapered 3.5mm washer; Axle length: 159 mm; Thread length: 16.5 mm; Thread pitch: M12 x 1.0mm.
The front axle situation is a little easier to understand, there are four options: Rockshox 15 x 100mm; Fox 15 x 100mm; Rockshox 15 x 110mm (Boost) and Fox 15x110mm (boost).
Flat 1.5mm washer; Axle length: 147.5 mm; Thread length: 10 mm; Thread pitch: M15 x 1.5mm.
Fox 15 x 100mm:
Flat 1.5mm washer; Axle length: 145.5 mm; Thread length: 11 mm; Thread pitch: M14 x 1.5mm.
Note that the unusual bit here is the smaller diameter thread ie 14mm on a 15mm bolt. I am sure that Fox have good reasons for this design but it essentially means that the front axles are not compatible across the two brands.
Some things that stand out include why Rockshox decided that the front and rear Maxle standard should have different thread pitches. This is beyond my non-engineering brain.
In general terms someone chooses to use a coarse thread in order to reduce machining costs and quality control percentages as it is easier to accept less precision when machining tolerances are not as tight. SRAM will no doubt argue that the coarser thread is harder for the home mechanic to cross thread and wreck.
In my opinion, based on tolerances and resource use the X-Scott rear (simple delrin washer) and Fox 15 mm front (again a simple delrin washer) should be adopted as the industry standard.
I have no idea what one does for other brands but it all comes down to washer shape, washer width, total axle length, thread length and diameter and thread pitch. Simple!!
Diagrams courtesy of Carbon-Ti.com