As if all the different wheel dimensions out there weren’t plentiful and sometimes confusing enough, there is a particular range of mountain bike tires called plus size. Plus size is a term used to describe larger volume tires, not to be confused with fat bike tires. It is often associated with 27.5-inch wheel size. But besides tire size 27.5+ there are also 29+ options. So, what exactly are plus MTB tires?
Anything from a 2.8″ to a 3.5″ tire width is considered a plus-sized mountain bike tire. These widths sit between the ranges for Trail / Enduro and Fat Bike tires, combining traction and comfort with higher rolling resistance. Since most bike frames don’t fit tires this wide, they are quite uncommon.
So plus MTB tires aren’t considered common sizes. While there is no one single mountain bike tire size, there are certain sizes that are established for individual types of disciplines like XC and All-Mountain. You can read about common mountain bike tire size here.
Plus-sized MTB tires 101
The standard sizes 27.5 and 29 refer to the outer diameter of a tire mounted on a wheel, not the tire width. Plus sizes blur this line because the additional volume of a plus tire comes from an increase in both tire height and width.
Remember, the total wheel size is a combination of tire size and rim diameter. The rims aren’t what is 27.5 or 29 inches, only the outer diameter of the tire, mounted on the correct rim for it. Plus size actually makes a lot of sense. For example, a 2.3″ wide 27.5″+ tire is actually similar in size to a 29″ at 2.3″ width. 27.5+ was actually seen as a steppingstone to 29-inch wheels defining the current trend. Today 29-inch wheels have become widely excepted and part of the reason for that is down to the plus size.
Back in the mid-2010s riders were skeptical of a wheel size change. Fast forward to today, when the winning bikes in cross-country, enduro world series, and downhill world cup all are equipped with 29-inch wheels. In front art least since the advent of mixed-wheel (mullet”) bikes. But that trend is the topic of another discussion. We mostly see the trends set by professional athletes being followed by the weekend warriors, so it’s become common to see 29ers on the local trails and bike parks.
Are wider bike tires better?
There has been a trend over the last decades towards wider and wider tires, rims, hubs, handlebars, and bottom brackets. And with good reason.
Generally, mountain bike tires benefit from a wider profile and more volume. The increased surface area allows them to provide more traction, control and comfort over rough terrain. Going too wide comes at the cost of traction in sloppy conditions, corner stability, energy efficiency and slightly heavier weight.
The main characteristics attributed to plus tires come from the harder compounds they are able to run while keeping the same level of grip due to increased surface on the ground. This stiffness results in stronger performance under breaking than a narrower tire with a soft compound, with the added benefit of less rolling resistance and overall durability.
Due to the larger footprint and usually lower tire pressures (between 17 and 35 psi), they tend to feel less precise. Which is more noticeable the better you know a trail. On the flip side, plus sizes can significantly compensate for rider mistakes and provide comfort on unfamiliar trails. So, if you tend to ride in many different places, this is a point you might benefit from.
So, wider mountain bike tires have proven benefits and shortcomings. With further and further increased width come fewer and fewer marginal benefits. On the extreme of the spectrum are fat bike tires, that can get up to 5 inches wide! Which is twice as wide as a big downhill tire. They have some legitimate homerun benefits, but their window of utility is small comparatively.
Pros and Cons of plus MTB tires
- faster rolling speed
- more grip
- harder, durable compounds
- lower profile, easy-rolling tread patterns
- comfort and forgiving
- reduced vibrations and small bump feedback
- thinner, more vulnerable sidewalls
- less support in corners
- not suitable for muddy conditions
- slightly heavier
Notice, that overall faster riding is on neither side, as this depends on much more than rolling speed. The tire is only half of the grip-equation in mountain biking. It still needs to hook up to the ground, and if that’s particularly sloppy, wet, or soft, a wide tire simply can’t dig in as well as a narrower can. So overall speed depends on the type of terrain and conditions you are riding most often.
Size and weight compared
Let’s look at some of the facts surrounding 27.5+ in terms of the weight and size, particularly rim widths, and tire compounds. It’s important to know these because you might be wanting to swap your 29-inch wheel bike to 27.5+ for all the benefits listed above.
So first, let’s get a clear picture of the different heights of different wheel and tire combinations. If you compare a 27.5″ wheel with a 2.8“ tire to a 3.0“ tire, the outer diameter is actually quite similar despite the increase in width. Comparing that same 3.0″ 27.5+ wheel to a 29-inch wheel with a 2.3″ tire, they’r again quite similar as well. The 29×2.3 is only marginally larger by the size of the knobs.
However, when you go from a 29×2.3 up to a 29×2.5 the height difference is significant. So, these are both standard large 29″ wheels, but gap further apart than a narrow 29″ and a 27.5″+ wheel.
As you can see, the biggest difference in diameter is due to tire width, followed by the size of the knobs on the tread pattern.
Before the “plus” name stuck, these wider tires were also called 28″. As you can see, it’s good that this confusing name didn’t last as plus refers to tire width, not the overall height. Even tires on the same rim can make for bigger or smaller wheels, solely by the width of the tire.
Are plus size wheels heavier?
So, when they are so similar in height, surely the weight of the extra rubber is where the big difference lies. This is also the reason why manufacturers started out their plus tire line-ups with thinner sidewalls to save excess weight (and probably also bad press). Fully aware that the compromise in durability would lead to more puncture-prone rubber. A weaker casing also has less support in corners, when the tire has to hold up to lateral g-forces.
This way of thinking came from looking at the tires in isolation – understandable for companies focused on making this part the best they can. But the rubber is only on the outer end of a wheel, which is made up of much more components like the rim, spokes, hub, and disc rotor. And it’s the weight of the entire wheel that matters at the end of the day.
Since plus tires now fit on the common 30mm wide rims, the additional weight from simply more aluminum or carbon material is a thing of the past. So fair comparison is entirely possible.
Still, the tire weight is only one part of many. A pair of wheels without tires can weigh in a range between 1.500 g for an All-Mountain set to over 2.000 g for a burly downhill set. Wheels also have a rotor and on the rear the cassette to take into account. The latter by itself will be either about 360 g for a Shimano XTR or SRAM eagle, to over 600 g on a more affordable drive-train.
The weight differences between various options are why the actual weight of a wheelset is only partly affected by the tire.
Anyway, what about an apples-to-apples comparison between the same wheel types, but different sizes in 27.5″ and 29″. On average, the 29″ wheel-set will be slightly heavier by about 100 g, simply by more material being used.
Now, what about the rubber. Let’s stick with one of my all-time favorite tires, the Maxxis Minion DHR.
In a comparison between 2.3″ and 2.8″ widths, in exactly the same compound and casing, the plus tire is heavier by about 175 g. In detail, it’s 804 g for 2.3″ and 980 g for 2.8″ tire width, which is comparatively light for either of them. Still, in this comparison, the plus-size tire is heavier.
To put this in comparison, the more aggressive Maxxis Assegai comes in at a whopping 1334 g for a 29 x 2.5 size. No surprise, as this one is suited to downhill riding. So, not only size but also tire casings matter if you want to save weight on your conversion.
Overall in this example of as similar as possible parts, the entire plus-sized wheel would be 75 g heavier than its 29″ counterpart. Depending on your wheel and tire setup, the plus-sized wheel may just end up very similar in weight and rotating mass as the 29er.
Can you put bigger tires on your mountain bike?
So, what about if you’re thinking of switching to plus size.
The first thing to take into account is if your frame is able to accommodate a larger volume tire, both in height and width. These things are huge rubber whoppers. They are so wide, that only bike frames designed for them can actually utilize them. They require the fork stanchions to be wider apart as well as a wider rear end so that the tires don’t rub on the frame.
Secondly, a swap to plus tires may result in a rather significant change in bike geometry if you go from 29 to 27.5+. On average, 29“ wheels have a diameter of about 750 mm, while a 27.5+ wheel is smaller at 725 mm in outside diameter. This 25 mm difference brings the bottom bracket 12,5 mm lower to the ground and impacts the handling overall, especially if you are used to that bike.
Those are numbers for rims with a 30 mm inner width. A wider 38 mm plus-sized rim would give the tire a different shape, resulting in an even lower overall height of about 715 mm for a 27.5+ wheel, which can be a difference in diameter of up to 35 mm.
Some newer bikes come prepared for this kind of wheel swap by employing a flip-chip. This lets you switch the frame geometry slightly to make of for different wheel sizes. In any case, you need to check the frame measurements or user manual first and foremost before starting such a project.
If you are thinking about swapping to completely oversized fat bike tires, you’ll definitely need a different bike entirely that is designed around these huge rubber whoppers.