Every manufacturer has its own rubber formulations, and they are a closely guarded secret. We ask manufacturers about the rubber compound all the time but few disclose any information. What we do know is the softness or durometer of the rubber; that’s something we’re able to measure. What makes it slightly confusing for the consumer is not all tire manufacturers state the durometer; they often call the rubber a name like ‘super tacky’ or ‘gumX’.
Durometer is only one part of the story but to give you a rough guide, a soft compound tire is around 45-55 on the Shore A Hardness scale, a medium tire is around 55-65 and anything above that is a hard compound.
On a trail bike, you generally want to run a medium compound on the rear and a medium/soft on the front. That way you’ll have good cornering grip combined with quick-rolling and acceleration. The firmer rear tread will also wear better, saving you money.