Upside-down (USD) fork

An upside-down fork, also called an inverted or USD fork, works exactly like a conventional telescopic fork but with the tubes the other way round. The large-diameter outer tube is clamped in the triple clamps, and the slimmer inner tube points down to the front axle. Most USD forks also carry a cartridge damper, a sealed piston-and-valve unit, instead of a simple damper rod. Once reserved for superbikes, it now appears on mass-market Indian motorcycles such as the KTM 390 Duke, Yamaha R15, Bajaj Pulsar NS200, TVS Apache RTR 200 4V and the Royal Enfield Himalayan 450.

• The front wheel rises and the inner tube slides up into the outer tube, compressing the spring.
• The cartridge piston pushes oil through shim stacks that open progressively with speed, giving finer control of compression than drilled holes.
• Under hard braking the fork bends backward; the stiffest part of the assembly, the big outer tube, is where that bending load is greatest, so the fork flexes less and the wheel stays pointed where the rider aims it.
• On rebound a second valve set controls the return.

The rider notices sharper, more consistent steering under braking and a more controlled feel over sharp bumps, partly because the lighter inner tube and axle clamp reduce the weight that has to move with the wheel. The downsides are cost and exposure: the seals face upward, so dust and water sit on them, and a scratch on the inner tube, which is now at the bottom where stones hit, can tear a seal. Many riders fit fork guards or neoprene gaiters for Indian roads.