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?
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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.
Here is a YouTube video explaining advantage of having bigger wheels. This is only part of the answer, most important reasons were explained by darkcanuck: