I was wondering what advantages 29" wheels give. From what I understand they are well suited to mountain biking. Are they any use on road bikes perhaps?
Regarding road bikes, the 29" rim size is actually the same as standard 700c wheels used on road bikes -- only the tire is different (larger).
One difference that I don't see mentioned often is that 29" vs 26" wheels change your effective gearing. If you have two bikes with the same gearing but one with 29" wheels and one with 26", the larger wheel will have a higher gearing overall because of the larger wheel diameter (one turn of the pedals move you farther down the trail). This means that the 29er can help you reach a higher top speed, but the 26er will have a lower climbing gear.
Also see the comparison on Wikipedia.
There is a lot of discussion about 29 inch wheels versus 26 inch wheels. I would summarize it as such:
Major advantage of 29 inch wheels: ability to roll over obstacles easier.
Major disadvantage: weight
There are other advantages/disadvantages but I think those are the main ones. Considering that weight reduction is a major focus of road bikes, and rolling over obstacles is not a great need, I don't see 29 inch wheels catching on for road bikes.
There are many places online that summarize the debate more completely, for instance. . .
They do roll over objects easier, if you google angle of incidence it will explain why.
They also tend to keep momentum better, since the weight is farther from the axle. That same distance is why they will accelerate slower. Think of a tennis ball on a string. short string easy to get going and stop. Longer, harder to get going but will keep moving easier.
There is also a traction advantange since the contact patch is larger.
The only noticeable difference between a smaller and larger diameter wheel is that larger wheels roll over obstacles slightly easier (~5%). Everything else, including speed, is negligible.
Otherwise, the main thing you should consider is if a bike with that wheel diameter will fit you. Bike fit is far more important; it affects not only your comfort, but your performance as well. If you can't fit a 29" properly, you're going to suffer far more than if you went with a properly sized 26".
Reference for "rollover" ability.
As for the physics, let's assume moment of inertia is
I=mr^2. With an increased radius, the moment of inertia will be larger. A larger wheel will also have more mass, so the moment of inertia will increase further.
An increased moment of inertia and mass requires a greater force (
F=ma) as well as a greater torque (
T=Ia), so will therefore be harder to accelerate. So with the same amount of force applied, the 29" will actually go slower.
The thing about taking longer to decelerate is also not necessarily true. The rate at which friction (
umg) decelerates on the same surface is always the same. Yes, momentum (
p=mv) and angular momentum (
L=Iw) may be greater, but only at the same velocity. When the same force is applied, they are equal, since the velocity is lower, so momentum ends up being irrelevant.
To be clear though, the difference of force required (to move at the same speed) is a small amount. The difference in radius (not diameter) between a 700c (311mm) and a 26" mtb wheel (279.5mm) is only 31.5mm, or 1.24", 1.11 times larger. 1.11 times might seem like a lot, but remember, compared to the rest of the bike and your body, the masses of the wheels aren't really that much, so the moment of inertia also isn't that much.
Given a wheel with a mass of 1.5kg and a radius of 279.5mm,
1.5 * 0.2795^2 = 0.117kgms. Multiply the mass of the 279.5mm by 1.11
1.5 * 1.11 = 1.665. Calculate moment of inertia for a 311mm radius wheel
1.665 * 311^2 = 0.161kgms. Subtract
0.161 - 0.117 = 0.0439kgms. A very rough estimation, but as you can see, the difference is small.
There are likely many other factors that affect the differences, but in terms of strictly wheel size, it doesn't really matter.