All About MTB Tire Sizing and Why It Matters So Much
Let’s get into it with a supposedly easy topic. The dimensions are clearly labeled on any tire by number combinations as 27.5×2.4 or 29×2.5 and so on. These measurements are the outer tire diameter in inches and the tire width in inches.
You can count on that piece of information to be always printed on any bike tire’s sidewall, even in the same format. This is because tire manufacturers have to adhere to the European Tire and Rim Technical Organization (ETRTO) and these are the standard measurements.
Cool story, but why should you even care about any of that?
Why tire size matters on a mountain bike
MTB tire size affects handling over rough terrain and when cornering, as well as rolling resistance. Bigger mountain bike wheels have higher pedal efficiency and handle better over obstacles. Smaller wheels on the other hand are more agile and handle better in corners. The common sizes 27.5″ and 29″ are equally viable, based on the intended use.
With tire width, it’s similar but not entirely the same. The wider a tire, the better it is over the rough stuff, but the less efficient it is. The opposite is true for narrow tires, but more on that further down. Let’s get cracking with wheel sizes first.
MTB Tire Diameter
These ETRTO labels are the standardized numbers you need to look at when changing tires. Turns out, changing tire width and therefore total volume also changes the tire’s height. After all, it’s a rubber balloon that expands in all directions. So your brand new 29×2.0 tire may actually not be exactly 29″ in diameter. Just as your 29×2.6 isn’t. They are both regarded as 29-inch tires, but actually different diameters if you took a measuring tape to them.
The actual, factually correct measurements are found elsewhere and are reported in millimeters. That’s called the Global Measuring System some manufacturers are using. For example, 61-584 is a tire that has a 61mm wide tire casing and has a 584mm inner “bead seat diameter” from bead to bead – which is of course also the rim’s outside diameter. That’s as far as I’ll go for here. But you can always nerd out with me on MTB tire sizes and dimensions here.
Here’s the takeaway: The range of dimensions you can actually mount on the wheel is dictated by the rim width and rim diameter. This takes a lot of guesswork out of the equation for you as the bike manufacturer provided you with the wheel dimension that best suits the bike’s intended use.
Optimal rim width
Rim width is in fact crucial to the tire profile. Only the optimal rim width, which a tire is designed for, gives it the shape it needs to work properly. Stans NoTubes have a very illustrative comparison of what happens if you run the same size tire on different rim widths on their website.
In the middle, you’ve got a tire that’s mounted on the rim width it’s designed for. This is how the profile of a tire on a rim should ideally look for it to be able to perform optimally in all its characteristics. It can absorb the most force laterally as well as vertically while conforming to the ground the best. The tread pattern has a good, sharp edge to it to dig in on a lean angle, while it’s still round enough to make the transition from center tread to the side knobs.
On the right of that, you got the bell shape on a too wide rim. With the tire pressing against the ground, it’s going to take more of a square shape with no real transition as the entire tread, including the shoulder tread is touching the ground all at once. There’s also no corner support since there are no actual side knobs (you know, on the side) left to lean on.
And on the left the lightbulb on a too narrow rim. If you were to mount a 2.5″ wide tire on a 20 mm internal rim width, you would get that very round profile on the top. The shoulder knobs are actually sticking out almost 90 degrees to the side and the tire tread can’t do its job properly anymore. On top of that, there is a drastic loss of stability from so much tire volume with the tire beadings sitting so close together.
This is why it’s quite important to factor in rim width as much as tire size. You want to have the recommended rim and tire widths go together for the best-performing wheel.
Here are the recommended internal rim widths for the common tire sizes in each MTB discipline:
|MTB Discipline||Tire Width in inches||Internal Rim Width in Millimeters|
|Gravel||1.8 in||19mm – 25 mm|
|Cross Country||1.9 in – 2.1 in||19mm – 25 mm|
|Trail||2.3 in – 2.4 in||25 mm – 30 mm|
|Enduro||2.3 in – 2.5 in||25 mm – 30 mm|
|Downhill||2.4 in – 2.6 in||30 mm|
|Plus-Size||2.7 in – 3.0 in||30 mm – 35mm|
|Fatbike||> 3.0 in||> 40 mm|
Can you put different size tires on your bike?
On most bikes, differently sized tires can be installed. The rim’s width is given but allows for slight variations in the tire width while still mounting optimally. With wider width, the overall outer wheel diameter including the tire increases also, making the wheel bigger while using the same rim dimension. Inner tire diameter can only be changed with different rim diameters.
As you can see in the table above, a wheel with a very common 30 mm internal width can accommodate a wide range of tire width well. Even the latest plus-size tires are designed to fit on 30 mm rims since larger rims are too uncommon. Even a 25 mm rim has a wide range of possible tire widths even if riding styles can vary drastically between Cross Country and Trail or Enduro. Any bigger than 30 mm is definitely going into plus-size territory or even all the way into the fat bike realm.
Optimal Tire Width
So, while you can’t size up or down in (inner) diameter, you can play around with different tire widths – even for the same rim. Incidentally, wider tires with more volume also mean a bigger overall wheel diameter.
Leaning into extra-wide tire dimensions are plus-size tires, that can get wider and bigger than regular frames can accommodate. Fat tires are in a league of their own, needing a fat bike to be designed entirely around those huge rubber whoppers.
While most mountain bike disciplines, regardless of the type of riding, have trended towards 29″ wheels from Cross Country to Downhill, the real difference-maker in terms of dimension is the tire width. Only looking at tire width there is a clear preference for narrow tires when energy efficiency and rolling speed are favored. The more gravity-oriented disciplines like Enduro and Downhill generally use wider tires with trail bikes in between to take advantage of both worlds.
There is a point of diminishing returns, however. MTB tires have become wider over the years, but at some point, more volume only brings marginal benefits with increasing negative effects. Fat bike tires on the far extreme take full advantage of what maximum tire width can offer, but at the cost of increased negative side effects.
There is a window between minimum and maximum tire widths viable for the rim width you got.
|Mountain Bike Discipline||Optimal Tire Width|
|Cross Country||2.0 inches|
|All Mountain or Trail||2.3 inches|
|Enduro MTB||2.4 inches|
|Downhill MTB||2.5 inches|
Why do mountain bikes have wider tires on the front?
Rear MTB tires are usually the same width as front tires, or 0.1 inches smaller in order to roll a little faster. Especially on uphill sections, the weight distribution to the rear wheel increases its resistance in relation to the front wheel. This can be mitigated by higher pressures, low profile tread patterns or narrower widths.
I personally prefer predictable grip over rolling resistance and always run both my tires in the same dimensions. The only difference is the tires’ tread pattern on my Enduro bike, which is more aggressive in the front, and easier rolling in the rear for those uphill pedals. In my downhill bike, I see no real reason to run different tire dimensions or tread patterns. They are always the same tires front and rear.
Are wider MTB tires faster?
Wider mountain bike tires can run faster on downhill trails compared to a more narrow one, given the same tread pattern. This is because going downhill relies on traction on obstacles and through corners, rather than low rolling resistance. Wider tires have more surface area in contact with the ground, thus providing more control at higher speeds.
Are wider MTB tires slower?
Generally, wider mountain bike tires do roll slower compared to more narrow ones. This is because rolling resistance is higher on a wider tire, given the same tread pattern. Wider tires have more surface area in contact with the ground, which generates more traction for corners or braking but also creates more friction when going straight.
Schrödingers tire width
So, wider tires are faster and slower at the same time??
Yes, because varying tire widths have varying strengths and weaknesses. Narrow tires go faster in a straight line, while wider ones carry more speed through corners and allow for heavier breaking. As you can see in the table above, tires for disciplines that focus on going uphill and energy conservation use narrow tires while downhill-oriented disciplines employ wider casings.
Are bigger MTB tires faster?
Let’s complete the circle here and go right back to the first topic of tire diameters.
Wheels that are bigger in overall diameter are faster, with all else being equal. This is due to higher pedal efficiency, meaning more distance is traveled with the same power input at the pedals. They also roll more steadily over obstacles preserving forward motion.