Andrew R Stewart 

There is flex within any structure. With a solid fork the two blades have a high amount of stiffness in their lengthwise directions (since most fork materials are pretty much compressionless). So with a solid fork blade any flex is largely that of side or fore/aft directions. And i think we all know that these directions need to be limited to a high enough degree so the fork keeps the front wheel pointing/placed in a manor that lend it's self to control and long life.

Now with a suspension fork the blades are designed to flex, or as more people will say "compress". This is not necessarily going to be the same on both sides/blades. If the wheel sees a tilting force (say when the rider presses down on one side of the bars while lifting up on the other end of the bars) then the two sides of the fork will tend to flex/compress to different amounts. Only the top brace and the axle/clamping will resist this. The lower cost forks tend to not have as stiff a brace and also use the standard axle securing designs (that solid forks use). The standard axle securing designs are axle nuts or QR skewers and are only originally meant to hold a wheel in place, NOT keep the two blades/drop outs aligned with each other. Why should the securing design need to do that, after all with a solid fork this is handled by the blades' materials. But with a suspension having blades that can change length the standard axle securing methods are overwhelmed and there is flex between the drop out and the axle (or wheel).

This flex is seen as the wheel tilting off parallel to the fork's center line. Yes, this means that the top brace is bending some what. This means that the drop outs are wiggling to a degree between the axle lock nuts and the securing device (QR or nuts), or the axle it's self is bending along it's length. (This is why suspension equipped bikes see more bearing wear the solid forked bikes do. But that's another topic...)

My experiment was an attempt to see easily and quickly this flex and why it often causes the pads to rub the rim when pedaling or cornering hard. The stiffer axle securing designs using drop outs that better clamp a larger axle (through axle designs) so to counter this flex. They are found on the more expensive bikes typically. A visit to your LBS will offer you a chance to view the increasingly stiffer but more expensive axle/fork designs. And you could preform the paper between the rim and pads test to confirm these designs greater stiffness. (If you can find a rim braked bike with through axle designs. These days pretty much any higher performing mountain bike will be using disk brakes. Although the same issues apply to disk brakes. Just with less distance from the axle to the pads, with disks, there will be less measured amount of movement all things otherwise being equal). Andy.