It's all about the temperature difference. Never wash it immediately after a ride, you're risking the chance of a crack. Even if it doesn't crack you're causing the metal to stress from the rapid cooling. You can crack cold aluminum that has been heated by washing even room temperature warm water over it. Any rapid cooling of metal is going to cause it to shrink at an abnormal rate. When you cool aluminum at a high temperature it causes the aluminum to become softer and often crack(stretch). In stainless and steels rapid cooling causes carbon to build up(heat discoloration area) around the hot/cooled area which makes it more likely to break since it becomes hardened and susceptible to cracking(breaking).