At the center of every hub (from a car hub to a freecoaster hub) lies a spindle. It is the link between the frame and the hub (via the bearings). The axle is very important in BMX because we like to beat out bikes to hell and expect them to keep on spinning. Pegs apply enormous amounts of force to the axle, trying to tear it off the frame(or at least bend it). For any style of BMX the axle is a very important consideration.
Axle vs. Spindle
These two are very similar to one another, and so they are often confused (or just learned in different ways). This how I learned to two (I am on a plane right now,
so I cannot check the dictionary’s definition, and I will most assuredly forget to do so later tonight):
- Axles are central axis which the hub spins around. This will usually be associated with thread-on axles (which will be covered in the next section).
- Spindles are a type of axle where the threads are on the inside of the axle. This is commonly associated with cranks. But there are bolt-on spindles for hubs.
This article will henceforth use the word axle as synonymous to spindle. If you have a problem with that… Then that is nice…
Types of Axles:
Thread-on axles: Commonly referred to as bolt-on axles in the MTB and road worlds, these do not actually use a bolt to fasten it on, but rather, a nut. There are generally two existing standards for this (with a variety of sub-standards)
- 10 mm Diameter. Also goes by 9 mm (on some lighter road hubs), and 3/8” for many American made (and some Taiwanese made) hubs. The metrics generally have their thread pitches down. Between different countries, it will sometimes vary (changing from 1.25 mm pitch to 1.5 mm, or other such changes). However, with 3/8” it is different It was fairly well decided to change to 26 tpi (threads per inch) from the old 24 tpi (I may have those switched up), but recently some odd manufacturers have been trying to bring back the old standard (this makes finding the right axle nuts a bit annoying).
- Oversized diameter. The Primo Bomb freecoaster was the first of the freecoasters to go with an oversized axle. It was 12 mm. Some hubs still use 12 mm, but it has fairly universally
been replaced by the 14 mm standard. There were some 15 mm axles floating around, but they have pretty much faded because they required grinding your dropouts. There aren’t any imperial size versions of this in the Thread-on axle arena (thank the freecoaster gods).
Bolt-on axles: These have been around for a while, but G-sport has really repopularized these. They provide much more thread contact with the internal axle, can be lighter (because the internal axle can be made with aluminum or thinner steel, because of its larger diameter), has been claimed to be stronger, and is much more easily available. There are 9/16”
versions (thanks G for betraying the metric system) which is approximately equal to 14.29 mm, and the 10 mm (3/8” counterpart) Generally American companies (such as Profile Racing) go with Imperial sizing, while most Asian and European (excluding Great Britain in most cases) companies prefer the more intelligent Metric sizing (Do I have a bias? Yes, but for good reasons. I will compile an argument against the Imperial system of measurement some other time). But even within this, they vary in pitch. It all depends on the manufacturer, and the differences are more common than even the thread-on axles.
Quick-Release axles: These are prominent in road and mountain biking because of their ease of use. You just flip a lever and it either goes on or comes off. No tools necessary. They use a hollow 10 mm (or sometimes 9 or 3/8” or 11, or some other random ones) axle which has a quick release skewer run through it. This uses an eccentric pivot system to tighten the dropouts against the cone nuts. There are problems with this system though:
- It prohibits the use of pegs
- It does not hold as steadily as other methods (though vertical dropouts have certainly aided this)
- They do not transfer torque very well.
- In the front hub they make the steering feel less responsive and a little sluggish.
20 and 24 mm through-axle: These axle systems are used on stronger mountain bikes. They use a 20 mm (or some use a 24 mm) skewer (effectively a tube) that uses a thin bolt to tension the bearings and pinch bolts on the ends of the fork legs to fasten the hub. The internals of the hub are mad to accommodate this large axle. This provides a very responsive feeling from steering. Also, these axles can be stiffer (and generally stronger, but the Hollow vs. solid argument will be presented later) because of their larger outside diameter. The axle system itself is not necessarily heavier than a normal thread-on system, but the hubs are usually built up to be super durable. From my measurements, I would not rule out the possibility of using pegs with this style of system (in fact it seems plausible), it would just require a certain amount of modification. Quick release through-axles have been developed in order to meld the two axle styles together. Thus far, there have been no rear mounted through axles, but that would bring about other issues, such as single speed chain tension.
12 mm rear through-axle: Free ride, downhill, and other forms of mountain biking that put a lot of shock load stresses on the rear wheel have developed a 12 mm through axle. Basically it is just a long bolt that runs from one end of a very thick drop out with a hole through it, through the hub, and out the other drop out. At which point it is simply tightened into a nut. This has similar advantages to the front through axle designs, but has proven to be quite heavy.
Skewer-style system: Sym-Hub uses what they call a skewer. Basically it is just a fully threaded, 10 mm rod that uses two long nuts to fasten the hub to the frame. This has been successful in isolating the hub into specific sections, but has not proven to much of a technological leap, nor much of a performance advantage.
Stud System: Profile Racing used a stud system on their SS hubs. The KHE Geisha uses, what they call the EAS (equal Axle System). These are designed to make the over all axle assembly lighter, more customizable, and replaceable. The idea that there needs to be some mechanism to create equal amounts of axle on each side of the hub is rubbish. Both of these hubs use sealed bearings, which require bearing seats to machined into the axle itself. These force the entire hub assemble to self center when tightened down (even without cone nuts, as has been shown by the G-sport hubs). Unsealed hubs are easily adjusted to make the appropriate amounts of axle on each side, and with some axles that are not fully threaded, this is even less of a concern. It is a nice idea, however, that you are (or were since the aluminum versions were discontinued) able to use a steel (or ti) axle on your peg side and an aluminum (meaning lighter) axle on your non-peg side.
- Axle Nuts: There are not that large of a variety of axle nuts out there. They only really vary in their material composition. Steel nuts are the most resilient, Ti nuts are nice and trendy (and fairly light and pretty), and Aluminum Nuts are even lighter, come in anodized, and a variety of sizes. I am a big fan of the Eastern Aluminum axle nuts. They have more than proven themselves to me. I use a torque wrench and get them down to 60 Newton-meters (which is actually quite a lot) and they do not move. You must be careful not to cross thread them though. For the 3/8” versions, you can get ones with built in adapters to fit 14 mm dropouts and 14 mm pegs (at the same time!), which is a nice bonus for Nankai owners.
- Unsealed Cone nuts: These are pretty basic and really any one will do. Some have deeper bearing races than others (which tends to make those ones stronger, but have ever so slightly more resistance). Many non-drive side cone nuts, such as Nankai and Taska, use an elongated cone nut that also aids in the resistance of your spring (as described in the other article. These work well, but have their drawbacks. They are more easily moved inadvertently and can cause your hub to fall out of adjustment. In order to fix this, just tighten the non-drive side cone nut very tightly against the lock nut and adjust your hub from the drive side cone nut. Or, Biz house makes the Nonstop axle which helps to prevent such an occurrence. This is a one piece axle that really does wonders for your hub.
- Sealed Cone nuts vs. sliding collars: Many bolt on style hubs use sliding collars to tension the bearings (that is to hold them in place) rather than a threaded nut typically seen on thread-on style hubs. This makes assembly much easier, and sometimes makes manufacturing cheaper. The down side to such a system is that if the dropouts are not parallel (as used to be common) they would apply uneven pressure on the bearings and lead to premature wear. On hubs like the Odyssey Reloader, the threaded cone nuts are aided even more by a locking set screw. I have found this does very little, nor is it necessary with modern frame manufacturing being as good as it is. But the idea is to make your hub never need to be adjusted, without machining the axle entirely differently to accommodate a sliding collar. A variety of different hubs utilize titanium and aluminum cone nuts successfully. They do need to be threaded on with more care, and require the use of the appropriate sized tool (which you should be doing anyway), but they can withstand the same pressures as other cone nuts. Remember! Cone nuts and sliding collars are only supposed to contact the inner race of the bearing, and that is it!!!
- Washers: These simple pieces contribute a lot to installing your wheel on the frame. They allow an independent surface to slide between the frame and the axle nut so that your wheel does not move when tightening (chain tensioners help with this also). They can also be of aid if you do not have the appropriate spacing anywhere in the hub, alignment with bearings (as is common in Geisha hubs), and tightening of cone nuts. Just throw them where needed and torque them down. Some of the compressive force will aid in keeping your hub feeling tight. Aluminum can be used in some washer and spacer situations, but is generally seen as not wear resistant enough to survive long. Titanium is much to expensive to justify use That is why steel (and actually some plastics, though not in BMX) is almost unanimously used.
Hollow vs. Solid
I absolutely hate when this argument comes to message-boards (as do others, which is what lead to “stickies” referring back to them, so the thread will never appear again). Mostly it just comes down to semantics. People are often confused about Strength vs. Stiffness, and Flexing vs. Bending. The strength of an object refers to the amount of energy required in order to PERMANENTLY deform the object (i.e. bending or breaking the axle). Stiffness, on the other hand, refers to the amount of deflection (movement from its original, static state) of the object, under a certain specific load or force. Typically, bending is related to the axle’s strength. If you bend a part, you permanently deform it. Flexing, respectively, is associated with the stiffness of an object. When you flex an axle, it moves in one direction, but then when the force or load is removed, it moves back to its static state. Titanium is generally regarded as more “flexy” than steel, and Aluminum more stiff than both (per weight). However, steel is seen as the least likely to bend, then titanium, then aluminum (being the most likely). OK, so we are going to take a look at the hollow argument vs. the solid argument here. We will compare a hollow 14 mm, a solid 14 mm, a hollow 10 mm (3/8”), and a solid 10 mm (3/8”).
Solid 14 mm:
- Weighs 2X ounces (just some arbitrary weight for this).
- Has a strength of 2X
- Has a stiffness of 1X
Hollow 14 mm:
- Has a weight of 1.2X
- Has a strength of 1.3X (hollow axles in the past were manufactured with better tolerances and materials, so this will be slightly higher than directly proportional.
- Has a stiffness of 2X
Solid 10 mm (3/8”):
- Has a weight of 1X
- Has a strength of 1X
- Has a stiffness of .6X
Hollow 10 mm:
- Has a weight of .6X
- Has a strength of .3X (because most of its volume is taken up by the threads)
- Has a stiffness of .8X
There are a few places where friction comes into play in a freecoaster hub. The first and most important is if there is a spring running on the axle. It must be smooth (as to provide good consistency) and hard to prevent wear. The second is in the bearing seats. They must be lubed so that when the bearing is installed, and tension is introduced into the system, it will properly slide into place without causing any issues. The third is on the threads. Yes, you should lube your threads. But wont that aid in the axle nut coming loose? Yes, but the added amount of tension you can achieve by having a well lubed axle counteracts, and over compensates for this. Just make sure you don’t have any grease on your dropouts, or the bottom of your nuts (as this can aid in the wheel slipping in the dropouts.
Chromoly: This is the traditional choice for axles because it is cheap, widely available, and strong. The downsides are that it is heavy and will rust if not coated.
Stainless Steel: Stronger than chromoly, and resists corrosion. Stainless steels do cost more than chromoly, but there is virtually no weight penalty.
Titanium: The material of the rich. Titanium costs many times more than steels. Also, titanium is highly susceptable to electro-galvinetic corrosion (in which it will sieze to pretty much any metal, including other ti parts). The corrosion can be overcome by using some sort of anti-sieze (such as Finishline Ti-Prep). Ti can be just about as strong as standard chromoly and is about 60% of the weight.
Aluminum: Lighter than titanium and 40% the weight of steel, aluminum can shave some of the precious grams off your bike. There have been major advancements in the production and quality of aluminums (such as has been seen with 7075 and the movement away from simply 6061). However, aluminum will always be weaker than either steel or titanium, but it
does cost significanly less than stainless or ti.
B-T-P, a light weight road bike company, produced a front hub under 40 grams. The main reason for such a light hub was that they used a polymer similar to Lexan for the axle. The axles would last for quite a while, but eventually succumbed to fatigue. The current state of polymers is not ready to be implemented into BMX.