Newbie here, I bought a new K2 9 speed Shimano 105 road bike this past fall. It has the OEM Alex R500 wheels and OEM 700x23 Vittoria Zaffiro tires. The bike has 400 miles on it. I noticed today that the both tires, but especially the rear tire, has hundreds of small cracks (splits) in the sidewall. They are about the same length and are lined up around the fatest (the bulge) part of the tire. There still is a lot of tire tread remaining. I may be heavy for these tires, I'm 240 lbs. I'm running 90 lbs in the front and 100 lbs in the rear. The tires are good for 120 lbs. Does this happen often? Is this normal? Are the tires being stressed from my weight? Does this mean blowout time is coming soon? I was going to buy a pair of 700x25 and switch when the tires tread wears out but now this has me worried. I was thinking now maybe I should be buying 700x28 tires.