This could be one of the most over used phrases in any industry. However, in the case of 27.5+ sized tires, this phrase is entirely true for mountain bikers! Whether you choose a hardtail or full suspension mountain bike, 27.5+ tires will give more traction and control while allowing you to push your limits on any trail you ride. Let’s take a look at how these tires work.
This new category of tires can go by a variety of names- 27.5+, 650+, 6Fattie, or even just “Plus tires”. In all of these cases, the wheel being used is a 27.5” or 650b rim with a 2.8-3.0” tire. Up until now, a typical mountain bike tire has varied in width from 1.9” to about 2.3”. How many of you have been on a 29er mountain bike? Did you notice the increased rollover? With a 27.5+ tire, you effectively get the same outside diameter and rollover capability. If you are unfamiliar with 29” wheels or “29er” mountain bikes, they have the ability to roll over objects in the trail much easier than their smaller wheeled cousins. While the 29” wheel has the benefit of fast rolling, you are limited to tire widths. This is where the 27.5+ tires fit in- all the benefits of a 29” tire but with the advantage of a wider tire!
We have all seen a balloon on a table before, only a very small area comes in contact with the table when inflated. If you push on the balloon, the area that touches the table increases. If you have an object like a book on the table, the balloon will conform around the book edge and the top of the table. This is the same principle with a Plus sized tire!
The increased width of a Plus sized tire allows for more tire contact with the trail – over 20% more tread is in contact with the trail! With the increase in width comes an overall increase in volume (over 50% more volume!) of the tire and that allows you to run significantly lower tire pressures – up to 25% less! Both of these factors add up to a tire that “hugs” the terrain and handles more intuitively, no matter what trail you are riding.
When you roll into a rock or root, a normal width tire that has less volume and higher pressure will want to roll up and over the obstacle – a slower and possibly rougher experience for the rider. A Plus size tire, when tackling the same terrain, will absorb the initial impact more easily due to the higher volume and lower pressure tires while maintaining more speed and a smoother experience.
With the lower tire pressure above, the lower pressure allows the tire to spread out over a great area giving you plenty in contact wiht the ground for greater control. Below is the tread pattern of the Specialized Purgatory. The dark section would be the contact path for a 29 X 2.3" Purgatory. The lighter grey is a 27.5 X 3.0" Purgatory. Over 60% more contact with the ground.
Below are some test statistics on 29er vs 650+ tires
Tire Pressure can effect the handling of these tires. Rider preference rules this area, but here is a recommended range to follow for each tire setup.
|29er Tire Pressure with Tubes||26-30psi|
|29er Tire Pressure without Tubes||23-25psi|
|27.5 Tire Pressure with Tubes||26-30psi|
|27.5 Tire Pressure without Tubes||23-25psi|
|27.5 Plus Tire Pressure with Tubes||16-18psi|
|27.5 Plus Tire Pressure without Tubes||14-16psi|
Everyone who wants to have fun. It truly is the setup that everyone can appreciate the benefits of. You don't ride fast, but you like the confidence the tires give you going over objects. You ride on loose trails, so have a tire that has more in contact with that trail. You push it to the limits every ride, so go, go, go through the knarliest line you want and let the tires take care of much of that. So many reasons to have this tire setup.
On new bikes, they are taking extra measures to give the rider the best experience possible. To accomdate that wide of a tire, the rear on frame had to be widened in sections for clearance. For dual susepnsion bikes, pushing the pivots wider allowed for greater stiffness. This also opened the door to widen the angle of spokes from hub to rim creating more stiffness and stability. Stand with your legs shoulder width apart and have someone give you a push from the side. Fairly stable, but your legs probably rocked side to side a bit. Now stand with your legs 6" wider than your shoulders and take that same push. Fairly solid, right? That is essentially what is going on with Boost. Instead of being 135-142mm wide at the rear hubs, Boost takes the width to 148mm. The front end also benefits from wider spacing going from 100mm wide to 110mm.