A few questions about tyres: Pressure - I know you can read this off the side of the tyre, how do I translate the number system there into PSI? Small cracks are appearing in my rear tyre, maybe a millimetre or 2 in length, there aren't many, but i presume this is a bad sign? And should I get a new one soon? Tread, how do I tell when a tyre needs replacement because the tread is no longer sufficient? Or how do I measure the tread, and what's good, what's bad? Thanks!