Besides being made up of rubber, an MTB tyre is also made up of a layer of nylon, cotton or aramid. This layer is called the carcass. The quality of the carcass is also called the degree of TPI. TPI stands for Threads Per Inch and represents the number of threads the carcass has per inch. The higher the TPI, the smoother the MTB tyre is and the lower the rolling resistance is. A high TPI value is not necessarily a positive thing. In fact, it is also true that the higher the TPI is, the lower the puncture resistance is.