Originally Posted by vasracer

You also have to take into account the material your working with. With high tensile steel you can bend it and it's elasticity will prevent it from bending or cracking under high load. We also have to remember that with many bikes the steel you see marked on the seat tube (or anywhere) is usually the three mani tubes and the rear being of a lower quality steel meaning that the elasticity and point of bending or craking is much less. 3mm might not sound like much but in frame building its the difference between having a good frame or a heap of scrap metal.