Mountain Bike Tire Casings Explained (TPI, Ply, Armor)
Mountain bike tires are the rubber connecting bike and ground, making them one of the most important parts. With modern technology, they are more than just a piece of round rubber with knobs. The basis of any bike tire is its casing, which gives it its shape and is fundamental for riding characteristics.
When talking about tires and their characteristics, tire pressures and tread patterns come to mind. They are instrumental to the way the tire (and therefore your bike) handles on the trail. Casings may be the dark horse here, fundamental for any tire.
Tire casings may not be the most talked-about topic ever, but understanding the basics to be able to select the optimal casing option for your riding style can vastly increase your performance and joy out on the trail. Especially when you experience unwanted characteristics like unreliable grip or reliable punctures the casing may be the issue. That, or maybe your tire pressure is off.
What is an MTB tire casing?
A tire casing is the main body of the tire and where the tire tread sits on top of. It’s constructed of one or two layers of plies of nylon cloth covered in rubber. The tire casing acts as the fundamental structure of the tire, giving it its shape, stability, flexibility and puncture protection.
As you can see in the illustration above, the tire casing is smack dab in the middle of it all. It is surrounded by other parts, even woven into it, that all rely on the casing as a supporting structure. It joins the two sections of beading, that sit on the rim itself, together. The tire tread, the bit that grips to the dirt, sits right on top. The sidewalls are the home of another feature: puncture protection. And on them is all the information you need to know about your tire, but deciphering the labels on an MTB tire is a whole other story.
The tire casing and the two beads are the foundation of the tire. No matter how good the rubber compounds are, if you don’t get the basics right, it’s simply not going to handle well and you will end up with an unreliable tire.
Now, let’s go through the individual components of a tire casing so we know how it’s constructed and what parts actually matter for what type of riding.
The tire beads are what hold the tire on the rim. As the only part of the tire actually touching the bike, it’s really important for a tire to get right. Thankfully most manufacturers seem to have this nailed these days. But they still use some different materials that affect the handling in slightly different manners.
There is steel beading, that was used for heavy-duty downhill tires, and is still seen on some cheaper-end tires. Other than the weight, there is nothing wrong with steel beads – except if you try to get your tire on or off the rim. Rigid beads tend to require extra effort, patience and good technique on a tire change.
Higher-end tires have Kevlar or aramid beads on them. These can be folding beads, also called folding tires. You can clearly make those out in your local bike shop. Those are the ones curled up in their packaging as opposed to hanging around as a full tire-shaped circle.
Now, let’s get onto the actual casings, the meat of any tire.
Single-ply or dual-ply casings
Tire ply is a term that refers to the casing construction method. Most tire brands offer mountain bike tires in single-ply or dual-ply tire casings. What this refers to is simply the number of plies of cloth used in the casing construction. It doesn’t say anything about the cloth itself, only that it’s folded over itself to create a dual-ply casing.
More on what that cloth actually is, later on in the section about TPI or threads per inch.
So, there is singly ply, which is typically used on lighter tires. Cross Country and Trail tires are all about minimizing rotational weight and tires with only a single ply casing are definitely lighter. The downside of a single ply tire compared to a dual ply is simply the fact that it’s far less puncture-resistant.
A dual ply tire is much thicker in comparison. You can actually see and feel this yourself when the tire is deflated, or even better when it’s off the rim. A dual ply will keep its general shape without folding over. It’s a very tough, thick tire and exactly what you’ll want for gravity-oriented biking like Downhill and Freeride. That more rigid structure also tends to make it harder to get onto the rim in some cases. The downside of dual ply is that they weigh more.
That increased sidewall stiffness of a dual ply helps the handling of the tire in aggressive cornering situations where folding, wallowing and even burping can become an issue. Yes, your tubeless tires can burp. That sort of support definitely something Enduro, Downhill and Freeride riders look for in a tire. Tire weight takes a back seat here in favor of tougher, reliable casings.
Some manufacturers have found ways to strike the middle ground between single ply and dual ply. Sounds like exactly what you want to have on an Enduro bike. They do this by making their, lighter, softer single ply casings more resistant by putting some extra armor into the sidewalls or even around the entire surface of the casing.
Protection in tire casings
You might have come across the EXO protection variants from Maxxis. That’s exactly that trick to combine the benefits of single ply and dual ply into one casing: light weight with better protection.
In our example, the casing’s puncture protection or flat-resistant armor is indicated by the little square that says “EXO Protection”. Tire casing protection refers to the added layers the manufacturer weaves in between the cloth of the tire casing when is being manufactured.
Manufacturers put protection in the sidewalls because when you’re riding your tires squishes together, pushing the sidewall further out, making it prone to hit rocks and roots on the trail. That’s the EXO part, colored green in the picture below.
All the tire brands are doing something like this and calling it something strong-sounding like: EXO, MaxxShield, Kevlar Composite, Silkshield, MaxxProtect for Maxxis, or Raceguard, Double Defense, Dualguard for Schwalbe, or Hardshell, DuraSkin for Continental. You get the idea.
There is no shortage of marketing slang, but the principles are the same. Every tire manufacturer is putting some sort of inserts either in the sidewall of the tire or some sort of casing protection all the way across the top to keep you from tearing holes in your really expensive mountain bike tire by riding through those pointy rocks.
One thing that’s particularly helpful about sidewall inserts is when you get one of those treaded pinch punctures. Also called a snake bite, that sort of cause for a flat tire can also happen on tubeless tires by a hole near the tire bead. This is almost impossible to fix by sealant exiting or even a tire plug. It just can never seem to seal properly in those areas. So having an armor insert in the sidewall even a single ply is incredibly resistant to getting tears even right down to the bead.
This kind of protection does not actually help you in the case of pinch flats with inner tubes. That’s when you compress the tire so much that it folds and gets pinched between the rim and ground. This is how you get the two holes on either side of the tube resembling the bite marks of a snake.
Why protective rubber matters
When I first tried out a tubeless configuration on the e*13 rims my YT Tues downhill bike came with, I went with a pair of tubeless-ready Minion DHR with a 120 TPI casing and the light EXO protection I had laying around. Needless to say, it didn’t exactly go my way.
As you can see from the EXO illustration, sidewall armor doesn’t protect the top of the tire, only the sidewalls. For additional protection you would need on hard-hitting downhill tires, there’s more armor available. In our case that would be EXO+, which comes in with the regular EXO layer in addition to their “silk shield” technology.
This thinner protective layer runs from the bead on one sidewall all the way up, under the tread, and down to the bead on the other side. Again, that’s another layer woven into the tire casing combined with the sidewall protection.
So while one layer of protection (EXO) only protects the sidewalls, another casing insert (silk shield) covers the entire casing bead to bead to give provide better protection along the top of the tire.
If you ride aggressively with heavy hits, save yourself time, money and your patience by going for a heavy-duty tire casing at least in the rear of your bike. Due to the higher weight and harsher impacts the rear tire has to endure, punctures are more frequent there. So, remember:
Thin rubber feels great, but offers minimal protection.
What TPI on a tire means
TPI stands for threads per square inch, meaning, how many threads are in one square inch of cloth the bike tire casing is made of. These are commonly nylon or cotton threads. The higher the TPI number, the smaller the individual threads are, making the tire more compliant and comfortable to ride.
Bike tires used to be manufactured from cotton thread back in the day. Some high-end road tires still are because of the ride quality that only cotton can offer. A major downside of cotton thread used in a tire, that has to endure a lot of abuse, is – you might have guessed it – durability. This is why bike tire manufacturing has moved into using nylon thread. With these nylon threads, you tended to get 60 and 120 threads per inch on a tire casing. Up to 320 TPI is not uncommon in road cycling because there they use much thinner threads to produce a tire that is as lightweight as possible while still somewhat comfortable to ride on hard concrete.
The casing of the tire I took the picture of has 120 TPI, which sounds like a lot of threads and it is. Some other TPI numbers, that you might see in mountain biking, are 60 threads per inch which is half the amount. Incidentally, some manufacturers fold a 60 TPI cloth over to create a dual ply casing, essentially doubling the TPI and calling it 120 TPI. Which is technically true, but fulfills none of the characteristics you would expect from a real 120 TPI casing.
What does 60 or 120 TPI mean on a bike tire?
60 or 120 TPI refers to either 60 or 120 nylon threads per inch used in the tire casing’s construction. 120 TPI and 60 TPI are the most common casing options you can choose from in mountain bike tires. Most higher-end tires usually have 120 threads per inch, whereas downhill MTB tires and cheaper tires tend to have 60 TPI. Low TPI makes them heavier and stiffer, but also less able to conform to the ground.
With the fewer 60 threads per inch, you don’t start to see holes between the individual threads – they are just as closely packed. But the threads themselves are much thicker and thus more durable. This resistance from the casing construction makes the tire better suited for high-impact riding like downhill or freeride riding and for rocky terrain.
The downside to having bigger, but fewer threads, is the loss in compliance with the ground. The stiffer rubber simply can’t conform as well to the ground as a 120 TPI tire can. This is where a harsh, wooden feeling can come from.
Is higher TPI better?
Higher TPI bike tires offer more riding comfort and compliance with the ground. More threads per inch also mean thinner individual threads used in the tire’s casing. On the flip side, they offer less stability, durability and puncture resistance compared to lower TPI casings.
So, what does that mean for our two most common options in mountain biking?
Is 60 TPI or 120 TPI better?
120 TPI is better for biking disciplines, where weight saving and better conforming are important aspects. 60 TPI is better for high-impact riding, where puncture resistance and handling stability are key. High TPI MTB tires are often enhanced with puncture-resistant inserts or armor.
Now, what does that mean for your favorite type of riding in particular?
What’s a good TPI for MTB tires?
120 TPI tires are used for Cross Country, Trail and Enduro while 60 TPI tires are only used in Downhill and Freeride. Gravity-oriented disciplines need tire casings that can resist great impacts at higher speeds. For regular mountain bike riding, more ground compliance and comfort from softer casings are beneficial.
With the details in the casing construction covered, let’s get over to the tubeless-ready part of modern mountain bike tires. After all, we have been talking about cotton and nylon threads. How can any of these even hold air without an innertube?
What makes a tire casing tubeless
Because of their construction, tire casings are still threads at the end of the day, as referenced in the TPI measurement. For this to get air-tight or tubeless ready, you have to make that nylon material sealed.
A tubeless ready tire has a layer of latex and ammonium mix among other materials that effectively seals in the inside of the tire. So you can put a tubeless tire on a tubeless compatible rim with an air-tight rim tape and valve, and inflate it. You actually don’t need any sealant to make the system air-tight. Rim and tire are designed to do that.
Still, I wouldn’t recommend to keep tire sealant out of the equation entirely. You still can get punctures and the sealant is one of the three main benefits of tubeless tires as it manages to keep air in your tires in case of a puncture with minimal pressure loss. The other benefits are of course lower tire pressures and lower weight
Let’s tie it all together now with what we know about casing construction, puncture protection and tubeless readiness.
How does TPI relate to puncture protection?
Bigger, but fewer threads (lower TPI) in the cloth of tire casing are more resistant. While TPI or threads per inch refers to the tire casing’s stiffness, any additional puncture protection is in fact not a part of the casing itself. It’s inserted in areas in need of extra protection like the sidewalls and below the tread pattern.
That’s done by weaving it in between layers, starting down at the tire’s bead and going up to just below the side knobs, or all over the casing. Manufacturers put protection in the sidewalls because when you’re riding your tires squishes together, pushing the sidewall further out, making it prone to hit rocks and roots on the trail. That’s especially the case for higher TPI tires, that are more conforming to the ground.
Lower TPI casings are less squishy (technical term) so their sides are pushed out not so much, making them less prone to slices through the sides. The bigger threads also protect from any pointy rocks going through the tire profile. The same is true for dual ply casings, that double the layer of nylon cloth.