Look at any lineup of bikes and you’ll likely see two different fork setups. Some forks have the skinnier, shiny portion at the top (conventional, or right-side up forks), while others have the chromed part at the top (upside-down, or USD forks). The general consensus is that upside-down forks are better, but how? Ari Henning explains in this video from the MC Garage.
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When it comes down to it, there’s really just one major difference between upside-down and conventional forks, and that’s rigidity.
When you clamp on the front brake or turn the bars to steer the bike, the fork is subjected to some serious bending and twisting loads. The whole fork is basically a lever, with the braking and steering force being applied at the tire’s contact patch, but acting on the rest of the bike way up where the fork attaches to the triple clamp. Any bending is bad, since it’ll screw up steering, suspension action, and mess up handling.
Since most of the bending force is focused right below the triple clamp, it makes sense to have that section of the fork be very strong. And the best way to make a tube stronger is to increase its diameter. So, just flip over your conventional fork and the larger-diameter female end is now clamped in the triple, so that fulcrum point is stronger and less likely to flex. This is a discovery that race teams made back in the late ‘80s, and by the ‘90s USD forks started to appear on sportbikes. These days, upside-down forks appear on all kinds of bikes.
Another thing that makes USD forks stiffer is the fact that the female slider is usually quite a bit longer than on a conventional fork. That means there’s more support for the exposed section of the stanchion, which helps it resist bending.
So, upside-down forks are definitely stiffer, but do you think you’re gonna feel the difference? Hell no! At least not on the street. Even so, folks want USD forks because it’s what top-of-the-line bikes have and because, well, they look cool. Case in point, even the lowly Grom, a distinctly non-performance bike, has an upside-down fork. On that bike, it’s all about the look. And honestly, a USD fork won’t look good on all bikes. I mean, can you imagine Triumph’s little retro Street Twin with a fat upside-downer? It just wouldn’t look right.
So stiffness aside, is there any other real performance benefit to an upside fork? Actually, yes. Generally speaking, and this is a huge generalization, upside-down forks have more sophisticated bits inside. Most are going to be fully adjustable, and even the non-adjustable ones will typically have a damping cartridge that offers better performance than the crude damper-rod assembly that’s found inside most traditional right-side up forks.
Since conventional forks are so much simpler, they’re also a lot easier to work on when it comes time to change the fork oil, swap out springs, or do a complete rebuild. When it comes to doing maintenance on a fully adjustable upside down fork, things are a lot more complicated.
One more practical difference that not many people consider is what happens when a fork seal fails, which does happen from time to time. On a right-side-up fork you’ll probably get some weeping fork oil and it might even drip down the slider. On an upside-fork, though, you run a much bigger risk of getting oil on your brake disc and pads since the seal is so much closer to those components and it has most of the fork oil sitting on top of it, just waiting to drip out past the seal.
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17 ноя 2016
