l have cracks in the sidewall on my front tire, how important is it to replace with cracks in the sidewall when the tire has so much rubber on it, the tire is also a tube type does that matter? Also, about the size, can l put something other than the OE that came witht the bike? It says 30x90x21. Is there another size that will fit? thanks

If the cracks r just on the surface then u r okay for now, if they actually have some depth then replace the tire immediately. A tire that size is under $100, so why put ur life at risk? 30 is the maybe the profile(height of the wall of the tire, though 30 sounds way to low), 90 is the width of the tire & 21 is the rim diameter. You could certainly go with a 100/110 width, but it has to be the same 21, they may have other profiles too. Lots of great tire prices here http://ronayers.com/index.cfm