The first time I saw a reference to a USD fork, I had no idea why my country's currency would have anything to do with the leaking fork seal I was dealing with. Go ahead, laugh it up. I'm a hillbilly, and many technological advancements are lost on my simple self. It does beg the question, though. What are inverted forks and why are they in use?
As the nomenclature suggests, inverted forks (also called upside-down forks or "USD" forks) orient the slider (or fork tube), near the axle, and the outer slider up top, where it clamps into the triple tree. (This is also called a yoke, for our friends across the pond.) Conventional (or "right-side-up") forks typically have the the slider (the fat, outer part of the telescoping mechanism, also correctly labeled a fork lower in this context) near the axle, and the skinnier fork tubes (sometimes called stanchions) are the items clamped into the triple. That's the functional difference. Man, that was a lot of parentheses. If I lost you, just look at the illustration below.
Why is this fork upside down?
The reason for reengineering the conventional fork is two-fold. First, arguments have been made that it lowers the amount of unsprung weight on the motorcycle, which typically leads to better handling. This has been hotly debated, and any savings here are likely pretty miniscule. The larger benefit is said to be increased stiffness from the front end. To understand why, let’s study engineering for a minute with your favorite redneck professor.
Stiffness — or resistance to deflection — of a tube (in this case, the outer telescoping section of the fork) increases greatly with respect to its diameter. Because the outer tube is, by definition, larger than the inner telescoping section, it should be stronger and stiffer.
If you think about the front end of a motorcycle, it works just like a lever. The headstock of the frame (or the nearly parallel triple trees) acts as a fulcrum, so the greatest forces of deflection (or "bending," as non-engineering types might commonly say) are imparted upon whatever piece of tube is nearest to it. Since we know that larger-diameter tube is stronger, one can surmise that an inverted fork will deflect less, because the stronger part of the fork is installed in a way that allows it to cope with the strongest forces of deflection.
The stiffness leads to better feedback and feel, and greater responsiveness, due to the reduced stiction between the upper and lower sections of fork.
Do I need this?
Well, sort of. The engineering principles are sound. The problem is that most street riders don't ride hard or fast enough to test the limits of conventional forks. Like many other technological bits that come to the motorcycles we ride, a good part of why you see USD forks on everything now is for homologation purposes. Depending on series or the sanctioning body of an event, race bikes can only be so different from production models. The solution? Make the production stuff racier! This applies both to racer-replica sportbikes and motocrossers.
You'll also see USD forks on upscale adventure tourers. In offroad riding, those USD forks deal with big hits better, so if you're Lemmy-sized, or you're riding a Lemmy-sized adventure bike, or you just love the big, high jumps, they should offer you some rigidity that a standard setup cannot.
The drawback (other than paying for technology you might not use) is that the flipped nature of things makes service a bit trickier. For starters, most USD forks do not have drains in them. (Some conventional ones don’t, either, but that's an issue of the manufacturer not making things easy on the end user.) For those of you who change fluid often, you know how much easier those drain screws make that job! Thus, to change out fluid is a considerably more laborious task on a USD fork.
Another issue comes up when or if your fork seal goes kerflooey. On a regular fork, it’s going to leak a little, but it’s not really an immediate danger. On a USD fork, however, the seal sits over the brakes, and the oil is above the seal, so gravity and suspension pumping are both working to push fluid out. Once it gets out, it is perilously close to brake pads and your front tire, so a busted fork seal is a bit more serious on bikes with inverted forks.
Just forkin' around
The upshot? They're probably better in the most extreme of conditions. Will you be able to tell? Most likely not, but if it makes you feel more confident in a turn or through the whoops, and you ride with more aplomb, then the forks have made you faster, which is the goal. And let's not beat around the bush: They look trick. If that's not a good reason to have a set on your ride, I sure cannot come up with one!