Mountain Biking for Dummies, Part 3

For reference, here are parts 1 and 2.

Wheels, hubs, and axles OH MY!

Here we are in our third installment of The Mountain Bike Series!  We are now deeply educated individuals when it comes to the basics of mountain bikes in general and the basics of their suspension systems.  Along those lines, we’re going to take today to look into the basics of wheel systems.

The first step is to look at the various wheel sizes offered with off-road bikes, which come in varieties of 29 inch wheels, 27.5 inch (or called 650b), and 26 inch wheels.  For quite a long time, 26” was the only option.  You will still find 26” sized wheels on entry level bikes, youth bikes, and even some (adult sized) “downhill” bikes.  With the exception of the downhill bikes, you will really only find 26” on what could be classified as “entry level” bicycles.  It has been generally agreed upon that “bigger is better” when it comes to mountain bike wheel sizing.  

So most of the choices we - as new mountain bike purchasers - will make are going to fall into either the 650b or 29” category.  Let’s examine some pros and cons of each, shall we?

When 29” wheels first came out (which was before my time) they were - like many new things in the cycling world - met with skepticism.  These huge new wheels weighed more, looked silly, and couldn’t possibly be better, could they?  Well usually: yes, they are better than 26” wheels in almost all applications.  Some of the things you might not think about at first glance are the bigger contact patch with the ground and the shallower angle of attack these wheels will have as compared to their smaller brethren.  The bigger contact patch is simply due to the larger circumference of the wheels, which results in more of the tire coming into contact with the ground.  The benefits of this are obvious, as there is more grip for cornering and braking.  The shallower angle of attack is a little more difficult to explain, but it essentially means that every object encountered on the trail is smaller as it relates to the wheel itself.  As a consequence, a 5 inch tall rock which, with a 26” wheel, would’ve been met a bit more “head on” is now lower on the 29 inch wheel.  This picture makes it more obvious, although the size disparity is a little dramatic:

So the bigger wheel gives the rider a little more room for error.  It makes obstacles “ride” smaller, it makes drop-offs easier to deal with, and in general it feels as though you cover more ground more quickly as compared to smaller wheels.  The wheels do, in fact, generally run a bit heavier.  Is this a big deal? In my opinion definitely not, but some may argue that the bigger wheels are more difficult to “spin up” or get up to speed, but to be honest I have not compared a 29” bike to a 26” bike so can offer no personal perspective.  It IS noticeable, however, when doing tight and twisted singletrack at slow speeds that a 29” bike can have more trouble than a smaller wheeled bike.  The bike has a longer wheelbase and bigger wheels and, as a consequence, can be more difficult to maneuver around tighter, smaller radius turns.

The 27.5” or 650b standard is newer and was created in no small part to resolve some of the concerns mentioned in the previous paragraph (or maybe it was created just to create more bikes to buy??).  Without taking into context the size or style of a bike, the 650b standard is most often viewed as a “best of all worlds.” It has many of the benefits of the larger wheels, such as better angle of attack and bigger contact patch, but with a lessening of the drawbacks such as weight and maneuverability.  It is still a bit newer but has mostly caught up in terms of availability of tire selection and wheel brand choices.

In my personal opinion, if I am looking for a cross country or trail bike I am probably going to say go with whatever size wheel “feels” appropriate.  For example, on a medium or large (or extra large) bike, 29” wheels are likely NOT going to feel “big” to you.  On the other hand, if you ride a small or extra small frame then 29” wheels likely ARE going to feel big, and as a consequence I’d say focus more on 650b bikes lineups.  If, on the other hand, you are looking for a big hitter bike (all mountain, DH, etc) then it’s much more normal to see 650b wheels on those big travel bikes.  

Let’s also take a few moments to talk about hubs.  Not just the hub itself, but how the wheels are bolted/attached to your frame.  We are ALL used to quick release skewers, as these have been used on bikes for quite some time.  The idea is pretty simple as it’s basically an axle that slots into your dropouts and is tightened using a cam pressure system.  For a long time, this was the most common thing seen on mountain bikes as well.  Nowadays, however, the system is completely different.

Most off road bikes (this is now beginning to include cyclocross, gravel, and even some road bikes) now use what is called a “thru-axle.” The desire for this system began with a desire to increase the overall rigidity of a bike. Dropouts (the openings into which a wheel’s hub fits) are wider on a mountain bike to help achieve this.  The thru axle is another mechanism that aids this rigidity goal, as the axle itself literally bolts the dropouts together THROUGH the wheel’s hub.

Quick releases are usually 9mm, whereas thru-axles are generally much bigger.  At first, mountain bikes began utilizing thru-axles only in the front while still keeping a quick release in the rear.  Now, however, that has changed to be thru-axles on both the front and rear.  Whereas a road bike has dropout spacing (basically the width between the dropouts) of 100mm in the front and 130mm in the rear - with 9mm quick relases - modern mountain bikes are most commonly 100mm in the front (with a 15mm thru-axle) and 142mm in the rear (with a 12mm thru axle).  

Interestingly, the rear hubs themselves are still 135mm, but the frame has 3.5mm cutouts in the rear to allow the hub to be more easily guided into place before the axle is inserted through the frame and hub and bolted tight.

All of this is to say that thru axles represent a stiffer, more robust form of wheel to frame attachment that, in general, results in a more responsive and predictable ride through the trails.  The “size” of your thru axles depends entirely on the type of bike you buy, with 15mm being a common front axle for everything except the biggest hitting (mainly downhill) bikes, which use 20mm.  In the rear, you will most often find 12mm thru axles. The only real downside to a system such as this is that it can take a bit longer to change a flat.  Especially with the axle type that’s in the middle in one of the above pictures, as it requires carrying allen keys. A little extra preparation never hurt anybody, however, and the increased stiffness and precision is DEFINITELY worth it.

Now that I’ve said that, I feel bad for my next and final section.  Like with anything in the bicycle world, nothing remains “standard” for very long.  Enter what is called “boost” spacing.  Most simply, this “standard” (ha. ha.) replaces 100/142mm with 110/148mm.  Big whoop, you say? I agree, mostly. But let’s dive into a little deeper to see what this difference REALLY means.

One of the changes between 26” and 29” wheels is the size of the wheel (umm, duh.). This larger diameter wheel has many benefits, which we’ve already highlighted.  One of the things it has that is potentially a negative, however, is longer spokes.  These spokes, despite being attached to the same width hub (100/135) are significantly longer, and are attached “further out” in the triangle.  The boost standard is designed to allow the triangle to have a wider base - as each spoke is attached to the hub a few mm further outboard - which results in a stronger, stiffer overall wheel.

What may seem like a subtle, small change actually has a huge effect on other parts of the bike.  You must now have wider frame dropouts and a crankset that moves your chainring a bit further out to accommodate the different rear hub spacing (this is done to maintain what’s called the chainline, which is chain deflection - or how straight or not straight a chain is - in the middle of a cassette).  The frame itself has a bit more clearance, which allows for wider tires and better mud clearing.  But the endgame here is that this new “standard’ (I still chuckle every time I hear that in the bicycle industry) really requires entirely new bicycles designed around the standard.  New wheels, new bike frames...it’s almost like the bike industry wants you to buy new stuff!!? Who would’ve guessed it...