Here, I discuss contact patch and related factors across the 3 common wheel sizes. Once again I will be taking the wheels and tires from Part 1 for consistency.
What is the contact patch, and how does it effect grip and rolling resistance?
|Fig.1 Contact patch on simplified tire represented in blue|
A larger tire contact patch area represents more rubber on the ground, which increases friction and therefore grip (good). However, the larger the contact patch area the greater the rolling resistance (bad). So, as with most things, there is always a compromise, and you just have to pick the right balance between grip and rolling resistance to suit your needs.
Shape and area:
For this section on contact patch shape, let's look at a basic representation of each wheel size (no tread, and no tire stiffness) each with 2.3" width , based on 50kg of weight (assuming 50:50 weight distribution, and bikes + rider = 100kg), and 2Bar (about 29PSI or 200,000 N/m²) of a perfect gas on a flat surface for all wheel sizes. Since the pressure is the same in each tire, the contact patch area will be the same for this scenario as Pressure=Force/Area. This is not very realistic as pressures will change a bit with wheel size (I will go into that later), so this is just to give an idea of patch shape.
|Fig. 2 Contact patch shapes for same tire pressure.|
One way of measuring optimal tire pressure is actually as 'tire drop', which is a percentage of original tire height (a little like suspension sag) as seen in Fig.3.
|Fig.3 Explanation of tire drop|
As you can see in Fig.4, the contact patch area and lengths change as tire pressure changes, but the width remains the same due to same tire carcass width and cross sectional shape. So for the same tire drop of 6% the 29" wheel has a 2.7% bigger contact patch than 650b, which in turn is 1.85% larger than 26". The difference in contact patch area and shape is far less than most marketing would have you believe, but it is present.
|Fig.4 Contact patch dimensions for 6% tire drop, and tire pressure for each wheel size|
These factors are the reason that mountain bike tires are wider than road bike tires. For road cycling, traction is less important than minimising rolling resistance (and weight) and so they run narrow low volume tires at high pressure. Mountain bikes run lower pressure, larger volume tires to increase traction as well as shock absorption. It's a case of picking the best tool for the job, by optimising what you want, and compromising on factors that are not as important to you.
Tire tread and compound:
The tread pattern changes the contact area far more than wheel size will!
So when thinking about grip, rather than think too much about wheel size and exact tire pressures, you'd be better off spending that time and effort picking the best tire tread pattern and compound for the riding conditions and experimenting with different tire pressures.
A softer rubber compound (lower durometer) will not only deform more to 'grip' the ground, but will also help damp the ride by compressing more easily under impacts. If you use a new soft compound tire you will be able to brake later, accelerate faster, and corner harder because the tread will bite into the ground with nice sharp edges, and the soft compound will have a higher coefficient of friction, and absorb the shock to stay in contact with the ground better.
For you to consider:
From all the information above, you can see that a bigger wheel will offer a slightly larger contact patch area due to the fact that you can run a slightly lower tire pressure. Therefore, a larger wheel will offer a bit more grip than a smaller wheel with same tire drop, but the increase in theoretical traction of larger wheels is probably less you were expecting.
With the larger tire contact patch comes more rolling friction, and efficiency is reduced. So smaller wheels are more efficient than larger wheels in this area for same tire drop. On a perfectly flat surface with a slick tire, smaller wheels with equal tire drop will lose less energy when rolling along than bigger wheels.
But let's be real... mountain biking isn't about just rolling along flat surfaces and we certainly don't use slick tires! It's about carrying speed through rough sections, cornering hard on the edges of tires, finding traction when climbing steeps and many, many more fun things. For most of these things, tire tread pattern and tire rubber compound are FAR more important than wheel size when it comes to grip. So my advice to you is not to get too lost in these wheel size numbers, instead pick a good tire choice and just enjoy riding your bike!