You’re learning about bicycles and want to understand what the abbreviation TPI stands for on the tires.
You may have even read a little about it and seen that some cyclists swear by it, while others say it doesn’t make as much difference as tire manufacturers claim it does.
TPI stands for Threads per Inch, a feature of your bike’s tires that affects its speed, safety, and comfort. Tires with a low TPI are heavy and stiff; tires with a high TPI are light and supple.
Most of us are used to hearing about “thread counts” in the context of bed linen. Rest assured, though, that the same concept applies to bike tires.
A tire’s casing comes in various qualities and thread counts.
The Parts of a Bike Tire
The typical bike tire consists of a carcass, bead, and treads:
The carcass is the inner part that consists of fine textile strands, such as nylon, cotton, or silk, woven together. It’s the part that gives structure to the tire and that absorbs the air pressure, the weight of the rider, and the shocks of the ride.
Treads are the rubbery parts on the outside of the tire that stick out and come into contact with the road. They protect the carcass and provide grip. When the treads are worn, the tire’s carcass is exposed, making it susceptible to punctures.
The beads are the parts of the tire that touch the rim when the tire is mounted and inflated. They hold the tire in place and provide protection against snake bite punctures (also known as “pinch flats”).
The purpose of a tire is to roll as many miles as possible at high speeds while absorbing as much shock as possible to protect the rim, frame, and rider. The TPI is one of the characteristics of a tire that greatly affects its ability to fulfill this purpose.
The Role of TPI in Bike Tires
The TPI is basically a trade-off between protection against punctures and the ability of a tire to reach high speeds:
Tires with low TPI have fewer threads. These threads are bigger, so they require more rubber and make the tire heavier. For this reason, tires with low TPI are bulky and sluggish, but they’re also better protected from punctures and therefore more durable.
Tires with high TPI have more threads. These threads are smaller, so they require less rubber and make the tire lighter. For this reason, tires with high TPI are supple and speedy, but they lack good protection from punctures and are susceptible to flats.
If you need durability, especially on rough terrain, buy lower TPI bike tires. If you need speed, especially on smooth roads, buy bicycle tires with high TPI.
Of course, this is an oversimplification and there’s a lot more to tire science than that. The participants in this Google Groups thread go into great detail about how threads count affects the overall safety, speed, and ride comfort of your bike’s tires.
(For the purposes of this post, we’ll keep it high level, since I’m not a tire engineer and I don’t understand all the details myself.)
How You Count TPI Matters
Generally speaking, a 120 TPI tire is faster and suppler than a 60 TPI tire, but it’s also more prone to cuts and punctures. But, as we will discuss in a moment, not all TPIs are created equal.
You may hear about 300, even 400 TPI tires. However, these are misnomers.
Some bike tire manufacturers count TPI based on a single ply of the carcass (i.e., 120), while others count the TPI by adding up multiple plys of the carcass folded together (i.e., 60 + 60 + 60 = 180).
For example, a 300 TPI tire may sound impressive, but it’s really a tire that’s triple wrapped with a 100 TPI casing. (And a 400 TPI tire has an additional fourth ply for puncture protection.)
When you come across a bicycle tire with a TPI over 100, check how the TPI was counted and whether it refers to a single ply of the carcass or multiple plies taken together. A single ply with more threads per inch is almost always better than multiple plies with fewer threads each.
TPI Isn’t the Only Factor
The number of threads per inch isn’t the only factor that makes a difference in bicycle tires.
With fine threads, for example, the carcass tends to become brittle. Therefore, some manufacturers choose to apply more rubber to make their tires more robust… but it also makes them stiffer. So a 110 TPI tire from one manufacturer may look, feel, and perform different from another manufacturer’s tire.
Incidentally, the above logic may seem a little backward to some, who will argue that a high TPI is usually preferred in order to use less—and not more—rubber. But some manufacturers feel differently on this issue, wanting to focus first and foremost on creating a robust tire.
The material of the threads can also make a difference.
Silk is more flexible than cotton, which, in turn, is more flexible than polyester.
To truly pin down the difference, you would need to compare apples to apples. For example, it’s kosher to compare a 100 TPI tire threaded with silk to another 100 TPI tire threaded with cotton; but it would be difficult to claim that a 100 TPI silk-threaded tire is better than a 110 TPI polyester-threaded tire.
For example, consider the following thee tires:
|A||400||Four-ply||Dense weave||Stiff threads in the casing with more rubber in each layer.|
|B||180||Tri-ply||Medium weave||Fine thread of the casing fabric with less rubber in each layer.|
|C||90||Single-ply||Loose weave||Superfine thread of the casing fabric with the fewest amount of rubber.|
The table above illustrates that:
- The TPI of the tire, as declared by the manufacturer, is not useful in and of itself. Tires with multiple plys can report higher TPIs, but that doesn’t necessarily indicate a scale in which the quality of the tire increases with the TPI.
- All things being equal, Tire C is the best among the options presented in terms of both the speed and the quality of the ride, as long as that the ride is not being used over a rough terrain where cuts and shocks can easily damage it.
- On rough terrain, Tires A and B could be superior to Tire C because they feature a high enough single-ply TPI with enough protection. This makes them less susceptible to bursts and punctures given the nature of the terrain.
Simply put, choose your tires based on the type of bike and the roughness of the terrain, with thread material and single-ply TPI being one of your best ways to compare apples to apples.
Why this matters is very simple. The higher the thread count of a tire, the steeper its price. And yet, as we saw, higher stated thread count doesn’t necessarily mean better quality and more durability. When in doubt, speak to someone at the local bike shop you trust, and they’ll help you come to the right balance.