Originally Posted by
mickey85
The only purpose that sidewall rubber has is to offer some protection to the cloth that makes up the actual tire casing. There are some tires (I'm thinking specifically of tubular track tires) that have no rubber at all on the sidewalls. In a pinch, I've ridden a recently-purchased bike 10 miles home on tires that have no sidewall rubber at all. If there is a crack or two in the sidewalls, you'll be fine. It doesn't affect the structural integrity of the tire at all.
+1.
I don't know how many times someone has told me a bike is going to need new tires, because they're flat and cracked and dirty and terrible looking, but after I pump them up there is nothing wrong with them. If the tube holds air, it's fine. If the tire holds the pressure of the tube, with no visible gashes or bulges or whatever, it's fine. If you're not sure, pump them up to full pressure and wait a week or two, then inspect them again. If you still can't find any problems, relax. The tire is probably fine.
Would I go on a month-long cross-country tour with 30 year old tires? No, of course not. But if I'll be staying within a hundred miles of home, I wouldn't worry about it.
Can I really be sure? No, of course not. Tires can fail, for various reasons, and you can never be 100% sure it won't. A brand new tire can turn out to be defective, and blow off the rim or something after ten miles or less; or a rock or something can hit a hidden weak spot and cause catastrophic damage to a tire of any age. But cracks in the rubber don't mean much, if anything.