TPI and Tire PerformanceJan Heine
Tires with supple casings are faster, more comfortable and simply more fun to ride. Most cyclists know this, but how do you measure “suppleness”?
A measure that often is used to describe the quality of tires is “threads per inch” (TPI). The idea is that tires with higher thread counts usually have finer weaves that make these tires more supple.
The reality is more complex, and TPI is of limited use when comparing tires. Here is why:
1. How do you measure? Ideally, you look at the TPI of the casing fabric before it is made into the tire. Casing fabrics vary between 15 TPI for coarse utility tires to 120 TPI for very high-end tires.
What about the tires with 300 TPI or more? These makers count every layer of the tire. Most tires have three layers of overlapping casing, so by that method of counting, a 100 TPI fabric will make a 300 TPI tire. And if you added a fourth layer for added puncture protection, you’d make the tire slower, but you’d bump up the TPI to a record-setting 400! So if a tire makers claims a TPI of more than 130, you have to divide the number by 3 to get the TPI of the fabric.
2. What is the diameter of the threads? The reason high-TPI tires usually are more supple is that the threads are thinner. If you keep all things equal, thinner threads will mean more threads per inch. However, if you make your weave denser, you also get more threads per inch, but actually a stiffer casing.
Panaracer, who makes our Compass tires, offers a 120 TPI casing. However, they found that if they use the same super-fine threads, but space them out a little further, they get an even more supple, and even faster, tire. So the Compass Extralight tires use that casing, which only has 90 TPI.
If you go by TPI alone, the Extralight casing looks inferior, but it’s in fact the more supple, faster casing.
3. How much rubber? Fabrics with very thin threads are fragile. They have to be handled very carefully during production. Some makers of budget tires compensate for this by covering the fabric with more rubber, which protects the threads. Of course, this makes the casing stiffer, and thus less performing. So one maker’s 120 TPI casing may be a lot less supple than another maker’s 120 TPI casing.
4. What material is used for the threads? With hand-made FMB tubulars, you get a choice of cotton or silk threads. The silk is much more supple than the cotton (which already is more supple than most polyesters). Even among polyesters, there are great differences in the thread materials. It makes no sense to claim that a 90 TPI silk casing is less supple than a 100 TPI cotton casing.
These are just a few of the factors that determine the tire’s suppleness. Let’s compare two hypothetical tires:
Tire 1 uses a stiff and relatively large-diameter thread. The fabric has a super-dense weave and is slathered with rubber. The maker counts every layer of the casing, and thus arrives at a 300 TPI tire.
Tire 2 uses a supple, superfine thread, woven into a relatively loose weave. The manufacturer keeps the rubber coating to a minimum. They report the TPI of the casing fabric, and arrive at a 90 TPI tire.
It’s easy to see that the Tire 2 above is superior to Tire 1, even though it has less than 1/3 the TPI. Suppleness, like so many important things, is hard to quantify, but you’ll notice it when you ride the tires.
Acknowledgments: Thank you to the engineers from Panaracer, Francois Marie of FMB tires, and Challenge Tires for the information contained in this post.