Actually, kevlar amd other materials are stronger than steel. My tyres are quite heavy and certainly seem thorn and glass proof.
It is hard to avoid glass here plus I always seem to pass farmers hedge cutting.
Lighter tyres do not use kevlay protection on sidewalls.
Some tyres claim 'steel puncture protection' but maybe that is simply the Chinese use term for the aramid type materials used. The material no doubt just needs to be the correct thickness for 'steel' protection.
That old Kevlar vs Steel comparison is BS. Strength isn't equal to strength unless you know what you're comparing. There's compressive, tensile, torsional and bending strength. Weight for weight, Kevlar outperforms steel only in tensile strength and tensile strength is not what you're after when providing puncture protection.
Since puncture protection is the issue, let's consider this little thought experiment.
Take a small sheet of CroMo steel, say, 6 inches by six inches and about 0.3mm thick. Weight is about 250g.
Now take a similar size sheet of woven kevlar (weft and weave, which is the only way to get a flat surface with Kevlar since it is a fibre) which equates to 7.7mm in thickness. Place the steel sheet/tile on your thigh and stab it with an awl. using X Force. How safe did you feel?
Now stab, with the same force, the kevlar sheet with an awl. Do you think the awl will penetrate? Yes it will, because tensile strength does not prevent sharp objects from finding a pathway between woven fibres.
When penetrative forces are considered, steel is superior to kevlar by a big factor.
Kevlar is an excellent replacement for steel beads in tyres but a poor puncture protector and a leads to high rolling resistance.
Both Kevlar and its derivative (often used in tyres) Vectran, can be formed in almost monolithic sheets but even so, it is more like felt or paper, not uniform like steel or glass. Penetration is always a problem for these "strong" materials because they only offer tensile strength. Penetration is prevented by hardness, compressive strength, density, work hardening, plastic deformation and load spreading.
Puncture protection in tyres is purely a function of material thickness and it matters very little what that material is. All , tough, pliable materialswill work but they increase rolling resistance massively. A very good example of that is a PVC liner. Excellent at preventing punctures because the material is thick and tough. But damn, does it make pedalling difficult!
