"Durability" of a tire is the culmination of all the aspects of the tire including casing material, rubber compounds, under tread layers, and tread thickness / pattern. When durability is discussed we have to also look at what kind of durability we are looking for. Are we looking for high mileage tires for touring or adventure riding? Or are we looking for optimal puncture protection for reliable commuting? All of these aspects of a tire need to be considered when looking for a "durable" tire.
Let's look at this one feature at a time and keep in mind that each one of these features is comparing the performance with all things being equal.
TPI:
Durability is related to TPI in that the lower the TPI, the thicker the threads as they need to fill the same space as a higher TPI casing tire. These thicker threads mean that they are more tear and puncture resistant than the thinner threads of the higher TPI tire and that there is more rubber required to fill the voids and cover those thicker threads, thereby creating a thicker, heavier casing which also can reduce the likelihood of punctures or sidewall tears.
The extra puncture / tear resistance does come with with some considerations however. The tire ends up being heavier due to the increased mass of the threads as well as more rubber to cover them. The lower TPI casing will result in a stiffer / harsher ride. Think about ride quality compared to bedsheets. Higher thread count bedsheets will move more easily and feel softer whereas a lower thread count sheet will feel stiff comparatively.
The reason the 120tpi tires are typically only offered in premium tires is that they are the most expensive to make but offer the best ride quality as they are more supple and able to conform to the terrain with less resistance. This translates into lower rolling resistance equating to more speed and better grip.
Rubber Compounds:
When a tire is designed for a specific purpose we will use a combination of rubber compounds to produce the qualities desired for that purpose. For instance a downhill mountain bike tire that is designed for optimal grip may feature softer, grippier rubber compounds on the side knobs to ensure those knobs will hold their grip on roots and rocks. This softer compound however may wear at an accelerated rate compared to a harder compound used in the center tread to improve durability.
These rubber compounds can also be extruded in different ways for different purposes such as a Single Layer compound, Dual Layer compounds or a Dual Tread Compounds.
We have a nifty article on our help center that gives a visual representation of the construction of these compounds linked below:
Tread Layers:
Typically premium tires will include extra layers under the tread that will not be featured in the lower priced tires. These under tread layers can offer increased puncture protection, tear protection, tubeless air retention, rim strike protection and more. Each layer that is added to a tire will increase the weight and price of a given tire (all other features being equal).
For puncture protection for example we offer some commuting tires with our KS (K-Shield) puncture protection layer. This 3mm puncture protection layer is under the center portion of the tread to prevent common road debris from moving its way through the rubber to the inner tire / tube. This KS layer is an intermediate puncture protection layer and we offer tires with KS+ (K-Shield Plus) which increases this layer to 5mm. This sounds great however the tire weight can be up to 15% heavier for this protection.
A comprehensive list of these casing constructions can be found on our help center site at the article linked below:
Tread Thickness / Pattern:
This is fairly self explanatory but the thickness of the tread will affect how deep debris would have to reach the inner tire / tube to cause a puncture as well as how much wear a tire can sustain before the grip and protection is compromised. A race oriented road tire such as our Valkyrie for example will feature a thin and therefore lightweight tread thickness whereas a commuting style tire such as our Drumlin features much more thick tread which will improve tread wear from high mileage commuting. This makes the Drumlin great for commuting but slow and heavy for racing.
As you can see the TPI is only one piece of the "durability" puzzle and the other components of the tire as a whole should be considered as well when making a purchase for your specific needs.