The basic way a motorcycle fork functions is pretty easy to understand: a smaller-diameter tube slides within a larger-diameter tube, and their telescoping action enables the assembly to compress and expand. This telescoping action, supported by a spring, is what absorbs the energy transferred to the front axle by the front wheel as it rolls over bumps, thus reducing the impact of those bumps on the motorcycle chassis.
The oil within damps this telescoping motion, which keeps the wheel from bouncing several times after hitting a single bump. Simple, yes?
But why are there two different types of telescopic forks? There's the conventional type, where the front axle attaches to female sliders; and the inverted, or upside-down (USD), fork, where the front axle attaches to male sliders. Is the latter just a marketing gimmick, or are there benefits and drawbacks of each one?
The conventional fork has been around much longer than the USD fork and is simpler in design. Most use a rudimentary damper-rod damping system, though it's not the damping system that spawned the USD fork; it was the advances made in the development of sportbikes in the early '90s, and their rigid chassis setups led to the introduction of the USD fork. The name, as you can guess, is derived from the USD's inversion, or upside-down installation, compared with a conventional fork. The easiest way to stiffen a fork assembly in the pre-USD era was to add a fork brace. This worked to some extent, but it was the stanchion tube (male tube) diameter that ultimately limited a fork's resistance to twist and flex; the larger the tube, the stiffer the fork. Increasing the stanchion diameter stiffened a fork somewhat, but the larger steel tube also added weight.
By inverting the fork, the larger-diameter female tube now became fixed to the triple clamp, which also grew in diameter. A larger triple clamp is no only stiffer thanks to its larger size, it also has greater clamping power on the larger tubes. Stanchion tube diameter being equal, it's easy to see why the inverted fork setup is therefore stiffer than its conventional counterpart.
Inverted fork also have longer female tubes, allowing them to have a greater overlap over the male sliders. This places their respective bushings farther apart, which increases rigidity. And because the exposed part of the male slider is short, it also has much less of a tendency to flex beyond the female slider's outermost bushing. This added rigidity has also allowed motorcycle manufacturers to use thinner-walled tubing on the female sliders, which reduces weight.
Because most USD forks use larger tube diameters, this makes room inside for cartridge-type damping units. This is among the reason why ll USD forks, even non-adjustable ones, use cartridge-type dampers. The cartridge dampers do nothing to increase a fork's rigidity, but they do provide better damping characteristics over a broader range than do damper-rod forks.
As you can see, there are many advantages to using USD forks on motorcycles, which raises the question: Why don't all motorcycles have them?
Well, there's the cost. The least expensive type of fork on the market is the conventional damper-rod fork, which is why it's more commonplace. And although the USD fork outperforms the conventional fork in most circumstances, the differences are usually most advantageous on the racetrack. The average rider doesn't need the added complexity and cost of a USD fork for everyday riding.
Then there's the issue of maintenance. A damper-rod fork is straight-forward, with a minimal number of components, and it's easy to disassemble and assemble. Many damper-rod forks include oil drain plugs to ease fork oil changes, and if the springs need replacing, they are easily accessible by removing the fork caps.
An inverted fork, with its internal cartridges, is much more complex, often needing special tools to take it apart. The forks must be removed for simple tasks like changing the oil, and the cartridge must be bled when doing so. Removing the springs also requires further disassembling, as they are assembled onto the fork cartridges.
Another disadvantage of a USD fork is that if its seal fails, it can be hazardous. This is because the seal sits at the bottom of the female tube, and is closer to the brake disc than on a conventional fork.