Both Trek and Specialized have jumped onboard with the plus size wheel/tyre standards for mountain bikes. The new standards use the now familiar circumferences of 29 inches and 27.5 inches, but expand beyond the maximum 2.5 inch width of the tyres. The increase in popularity of fat bikes, such as the Specialized Fatboy or the Trek Farley has meant that people have been enjoying the benefits of wider tyres, and now people are calling for them on more standard trail bikes. These previous fat bikes have tyres as wide as five inches, and have brought the focus back to fun when out on the trail. The new standards are called "27.5+" and "29+" and add extra width without the extreme widths associated with fat bikes.
The idea is that wider tyres mean that there is more of a contact patch between the tyre and the ground, giving more traction for driving forwards, and more grip in corners. The added amount of air that wider tyres and rims brings also adds to the "natural suspension" you get from air-filled tyres.
It's not as simple as adding wider tyres to normal rims though; there are issues that wider tyres bring up. Firstly the gap between the chainstays where it meets the bottom bracket is limited by the width that your feet and pedals cycle at comfortably. So, to avoid tyres rubbing the chain stays; the chain stays have to be moved out, but then the risk of riders clipping their heels on the stays also increases. This means that frames have had to be redesigned around wider tyres and rims. Trek have even raised the drive-side chainstay on their 29+ bike: The Stache, by joining the chainstay to the seat tube about 10 cm above the bottom bracket, and avoiding the chain line altogether.
Secondly, wider tyres need wider rims to support them, to stop them ultimately rolling off the rim. This then brings up the issue of the hub. To support a wider rim, wider hubs are needed, so in certain circumstances a wider hub standard is used also.
Specialized have answered the new wider tyre revolution with the the new "6Fattie" bike, a full suspension 650b+ bike. It's designed from the ground up with clearance for the new wide tyres, but still offers 150mm of travel up front and 135mm on the rear. It's all kept simple around the bottom bracket area by the frame being 1X specific, and doing away with any front derailleurs.
We're looking forward to getting our hands on these new semi-fat bikes, we'll be interested to ride them for ourselves and feel the benefits of the added natural suspension and increased contact patch.
