[*STR = So Cal Trail Riders, my handle, dwsand3r]
I went ahead and threw my two cents in and decided to share my response here, but I added a sketch for extra dork points!
“Most of the force from your fork (a really long lever arm*) is dispersed into the lower headset cup. That is why the lower is 1.5”.
Note: for all the engineering types, I did not show all forces, nor did I balance these forces. This is not an accurate free body diagram, just a quick conceptual sketch.
Remember many years ago, just before the switch to the 1.125″ standard, how common it was for a frame and/or lower headset cups to fail. The remedy was to move onto thread-less oversize (1.125″) headsets & steerer tubes. This also was during the time frame when 100mm of fork travel was huge and a 2 foot drop was gnarly! Also the fork was a pretty weak, noodle like member.
Most bikes offering the 1.5″ or tapered head tubes are designed for 140mm+ travel forks (even longer lever arm, that is why you’ll void many warranties by running a fork longer than the manufacturer specifies*), the forks are incredibly stiff (35+mm stanchions), and a 6 foot drop won’t even get you a photo in your friend’s blog.
The 1.125″ standard is fine for XC and 90% of trail riders out there.
The 1.5″ and/or tapered is a great thing for the rest of us. That is why 1.5″ stems are only available in shorter lengths. It is assumed if you’re running 1.5″ you’re a free rider or D/H and you’re running a short stem.
*lever arm : imagine if you were to put a pipe in your bike’s head tube and then pulled on the pipe in an attempt to break the head tube off the frame. It be a lot easier to do with a longer pipe than a short one.
“Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.” – Archimedes”
Comment with what you think, should the whole industry go to 1.5″? Should 1.5″ be abolished? Is taper the ultimate middle ground?