Naturally, a tyre can't work if it doesn't fit, so, on the sidewall of your tyre there will be some numbers that will look like this.
There are two different sizing formats in bicycle tyres. There is a metric and an imperial format. The metric format is called the ETRTO (European Tyre and Rim Technical Organisation)system, while the imperial format is the traditional sizing system. Most people tend to use the imperial, traditional system although the metric system is more accurate, particularly for width (more on that later).
In the ETRTO size, the first number is the width of the tyre in millimetres. The second number is the rim diameter. In the traditional format it's reversed. First number is the rim diameter in inches, second is the tyre width, also in inches.
Almost all mountain bike tyres are covered by three different diameters.
27.5 or 650b
29 or 700c
The new tyres need to be the same diameter as the old ones to ensure they will fit.
The vast majority of mountain bike tyres typically range from 2.1” up to 2.7” although it is possible to get tyres as small as 1.0” up to as big as 4”. Each rim will have a certain range of width that it is compatible with, so, it's worth having a quick google search of your rim model to confirm your getting a tyre within the range.
We'll break the tyre widths into three categories.
Small - Less than 2.25” wide - The advantage of a smaller tyre is that it is lighter and will generally roll faster. A lighter tyre is quicker to accelerate, and easier to maneuver. Typically tyres in this range are used for cross country riding and have small knobs and very thin casings. This is the size for you if you like to climb fast or are doing some longer distances.
Medium - 2.25” up to 2.5” - This will cover your trail, all mountain and enduro styles of riding. Like these styles of riding, you need a tyre that offers good grip so you can push it on the way down and through the corners, but is still light enough that you can climb up the other side. It's not uncommon for riders in these disciplines to run a larger more aggressive tyre up front that will hook up (grip!) on the descents, with something lighter and faster on the back. These tyres typically have bigger knobs and tougher casings as the riders are going to be pushing them harder and faster.
Large - 2.5” and bigger - Tyres of this size are suited towards the gravity crowd. They aren't suited very well to climbing or riding on the flat, but point them downhill and they will come into their own. These tyres typically have the biggest knobs, and the thickest, toughest casings. They are typically quite heavy and therefore slow to roll, but they do provide excellent grip and are very robust.
Pro Tip: If you're comparing width of tyres between different models, it can be handy to have a look at the ETRTO sizing. Two tyres might both have the same traditional sizing measurement, but a different ETRTO and therefore one will be slightly bigger than the other. This is simply because the ETRTO system is more accurate.
3. Tubes or Tubeless
Identify what you're currently running. When you take your tyre off, if it has a bladder (tube) between the rim and the tyre you running a tubed setup. If there isn't a tube, your running tubeless. Tubeless has very much become the new norm for three main reasons.
You can't get a “pinch flat”. (a pinch flat is when you hit something so hard the tyre and tube are squashed into the rim with such force it cuts through the tube)
You can run lower tyre pressures, enhancing grip and rolling efficiency over rough terrain
Small cuts or holes in the tyre are normally sealed by the sealant inside the tyre without you even noticing.
Each brand has it's own way of labelling tubeless or tubeless compatible tyres. The more common labels on the side wall of the tyre are: UST (Universal System for Tubeless), LUST (Lightweight Ultimate Sidewall Technology), Tubeless Ready or TR and Tubeless Easy.
If you're not sure whether your rims are tubeless or not, Google the make and model to find out for sure. If your rims aren't tubeless compatible you can convert them with these kits.
4. Terrain and Tread Pattern
The terrain that you are riding will help to determine what tread pattern you will run.
The softer the dirt, (think sand, loam or mud) the more open you'll want your tread pattern to be, and the taller the knobs that you'll want. The taller the knob, the more they are able to “dig” into the soil. In muddy or sticky conditions, a more open tread pattern allows the mud to clear out of the knobs so your tyre doesn't end up clogged.
The Maxxis Wetscream is a full on sloppy mud tyre. Very tall knobs spaced a long way apart.
On the other hand, if the terrain you are riding is very hard pack, you'll want something with a lot of smaller knobs very close together. Since the knobs don't have anything to dig into (as the ground is too hard), you might as well have a lot of them contacting the dirt to give the most grip. Also, if you run tall knobs on hard pack, the tyre can give a vague feel as the knobs tend to squirm under load. The further the knobs are apart, the slower the tyre will roll (the less “round” the wheel becomes).
The Maxxis Race TT is a cross country tyre. Low profile ramped knobs packed very close together
The most common terrain is generally referred to as “loose over hard”. This is where the ground is hard at the base, but tends to have a soft layer of dirt sitting on top. For these conditions you want an intermediate sized knob that will cut through the soft top layer and dig down to the hard dirt underneath.
The other thing to consider with tread patterns is rolling resistance. The taller the knob and the further they are spaced apart the slower a tyre will roll. Manufacturers often try to get the best of both worlds here by ramping a knob so it isn't a square edge that is contacting the ground. This helps to improve the speed of the tyre, while still giving a taller knob that will bite through the loose soil.
Pro Tip: If you mostly ride on hardpack terrain or medium soils, you might want to consider a more aggressive (eg large wide spaced knob) tread pattern for a front tyre, and a smoother faster rolling (tight knob spacing) tread pattern on the back. This gives confidence in your front wheel when cornering hard on loose terrain, whilst lowering rolling resistance on the back tyre.
The compound of rubber or durometer refers to how soft the rubber is. A softer rubber will grip significantly better as it is able to conform more to the ground. The downside is that a softer compound tyre will wear out faster and typically roll slower as it is stickier. Durometers range from 40a (very soft) to 70a (very firm).
Tyres can also come in single, dual or triple compound. A single compound tyre is the same durometer the whole way across. A dual compound tyre will have a softer rubber on the side knobs that don't get used as much as the knobs in the middle of the tyre that are firmer. Triple compound tyres tend to have a firm compound at the base of each knob, then a bit softer rubber in the centre on top, and the tops of the side knobs are softer again.
Triple compound tyres are the most expensive as they are the most time consuming to manufacture. They are the best though, as the knobs hold their shape the best - due to their firmer base - but still grip the best due the softest rubbers actually contacting the terrain.
Pro Tip: A good compromise here can be to run a softer tyre at the front where there is less load and it doesn't need to cope with any skidding forces (hopefully!). Then run a harder compound tyre on the rear where it does have more weight on it and does cop skidding forces.
6. Bead, Casing & TPI
The Bead, casing and TPI are the three factors that make up the carcass of the tyre. These three factors also play the biggest part in the weight and reliability of the tyre.
Bead - Tyres come with either a folding kevlar bead or a wire bead. The wire bead is heavier and more robust. It is a more solid tyre to ride on and will hold its shape better as the very stiff bead is less likely to to move around in the rim. The folding bead option can also come in a TR - tubeless ready - version that is optomised at the bead to seal tight against the rim.
Casing - Tyres tend to come in a few different casings ranging from very thin and light to very thick and robust..
TPI - Threads Per Inch. This is how many strands per inch are in the casing of a tyre. A tyre with a higher TPI will use more thinner strands that is therefore lighter, but thinner also. This thinner tyre allows the tyre to conform to the ground better, but isn't as tough against cuts and abrasions. Most mountain bike tyres range in the 60-120TPI range.
In Maxxis the different carcasses on offer are:
Wire Bead - These are the cheapest tyres. Wire bead and 60TPI.
Exception Series - The lightest tyres. These run a folding bead and a 120TPI sidewall.
EXO - Very popular. They are a 60TPI side wall with an additional densely woven layer to protect the tyre from abrasions and cuts. These have a standard folding bead which keeps them nice and light too.
EXO TR - These are the most popular tyres. The same as the standard EXO but these also feature the Tubeless Ready bead.
LUST - These are much less common now. They feature a tubeless specific bead with an additional layer and a special rubber through the carcass of the tyre that is airtight for ultimate tubeless reliability.
Double Down - These are essentially a downhill sidewall on a Tubeless Ready folding kevlar bead. This makes them an exceptionally robust tyre while not being as heavy as an all out downhill tyre
Downhill - These are the most robust. A wire bead and the thickest sidewall offer a tyre with unrivaled reliability. It does come at a cost though as they are quite heavy.
So there you have it, everything you need to know about choosing mountain bike tyres! If you're in the market for a new set, you can click through here and put your new knowledge to the test while checking out the range.