MTB 101 - Gear »
Demistifying the 29″er: Why the wheel works
In my first post, I discussed how the 29 inch wheel got its name and touched on its background a bit. This post will reveal the main reasons why the 29 inch wheel works for mountain biking. It is these benefits that drive the 29 inch wheel movement.
Angle of Attack
This section is not a treatise on military maneuvers. Rather, it is a description of how a wheel rolls over an object.
As a wheel makes contact with a trail obstacle, it travels up and over it. The diameter of the wheel determines at what point on the circumference of the wheel the object will make contact with the wheel. A smaller wheel will contact a given object at a point on the tire that is higher off the ground than that of a larger wheel (see Figure 1a)
Drawing a line from the trail obstacle’s point of contact to the tire to the point where the tire touches the trail describes the “angle of attack”. A high angle of attack slows the forward momentum of your bicycle more than a lower angle of attack would. The 29 inch wheel has a lower angle of attack than a 26 inch wheel does, translating into a smoother ride (see Figure 1b).
Tire contact patch
The “footprint” of the tire as it touches the trail is called the tire contact patch. The outline of this patch is shaped differently for 26 and 29 inch wheels. The shape of the 29″er’s tire contact patch is slightly thinner and more elongated than a 26 inch tire’s contact patch, when comparing tires of similar width at similar air pressure. The tire contact patch works in concert with the 29″er’s low angle of attack to give you more knobs in contact with the trail surface. This translates into more grip for cornering and climbing.
The Flywheel Effect
The fact that a larger diameter wheel will keep spinning for a longer time than a smaller wheel does is quite evident when you take a look at bikes built around the 29 inch wheel format. Some of this ‘flywheel effect’ can be attributed to the slight increase in the weight of a 29 inch wheel over a similarly equipped 26 inch wheel.
The momentum that a 29 inch wheel develops can be put to use more effectively than it can by a 26 inch wheel because of the better angle of attack and the better grip, as well. This translates into more speed into and through corners, with less need to accelerate afterwards.
Recap
There are three things that make going with a 29 inch wheel format a good choice for many riders. These things — a low angle of attack, the 29″er’s tire contact patch, and the flywheel effect — translate into handling traits and ride qualities that can’t be had by bikes running 26 inch wheels.
In the next post, I will discuss what effects that the 29 inch wheel has on the geometry of a trail bicycle. I will also describe how these ‘wheel benefits’ work in concert with the 29 inch specific geometry to produce a great riding bike.
1. Posted by Richard Burt | 2:11 am, 16 March 2006
Interesting article- I hope you discuss some of the benefits of the 26 inch wheel too- e.g. shorter wheelbase for more manoeuvrability, and possibly more appropriate for small frame sizes?