MXA TECH SPEC: WHAT’S SO GREAT ABOUT THE KTM CLUTCH?
People often ask why MXA raves about the KTM, Husqvarna and GasGas clutches so much. They want to know what makes theses clutches so different from Yamaha, Kawasaki, Suzuki and Honda clutches. Let’s be clear, the standard clutches on 2021 Japanese bikes are the best technology available—if this were 2006. There is only one truly advanced clutch on the track today, and it comes on the 2021 KTM, Husqvarna and GasGas machines. This clutch is head and shoulders above any other clutch for six reasons.
(1) It comes with a steel clutch basket that never notches and will last as long as you own the bike.
(2) Unlike a clutch with six separate coil springs, the Austrian clutch uses one large cupped Belleville washer that provides equal pressure across the face of the pressure plate.
(3) The Belleville washer can be preloaded for more or less tension by turning a stepped disc on the pressure plate.
(4) The KTM/Husky/GasGas clutch is activated by a hydraulic master cylinder and slave unit that self-adjusts as you use it. Husqvarna uses a Magura hydraulic system in 2021, but will switch to Brembo in 2022 to match the KTM and GasGas Brembo parts.
(5) The CNC-machined steel clutch basket has the primary gear machined into the billet of steel (instead of riveting a separate primary gear to the back of a cast aluminum clutch basket).
(6) Riders who burn up KX450, CRF450, YZ450F or RM-Z450 clutches in four motos will go months with a Belleville-washer clutch.
This is the DDS (Damped Diaphragm System) clutch that comes in the KTM 450SXF and 250SX two-stroke (it also comes on selected Husky and GasGas models). Six kidney-shaped rubber dampers (one of which is shown at the bottom of the photo) cushion the clutch from acceleration jolts.
Clutch models. There are two types of KTM/Husky/GasGas hydraulic clutches. First, the 450 four-strokes and 250/300 two-strokes come with the Belleville-washer-operated DDS (Damped Diaphragm System) clutch. It has a rubber-damped cush hub inside to lessen the jolt when the more powerful engines hit. Second, the 250/350 four-stroke engine’s clutch is called a DS (Diaphragm System) clutch. It differs from the 450’s DDS clutch in that it doesn’t get the rubber-damped cush hub. Why not? The 250/350 engines do not need to cushion the jolt of each gear because of their lower power profile.
Belleville washer. A Belleville washer is essentially a large cupped washer that provides spring pressure as it is compressed. The first Belleville washer clutch came with a 250 N/m spring back in 2012. It was upped to 280 N/m in 2014. With today’s more powerful engines, the 2021 450’s Belleville washer is now rated at 295 N/m, while the 250/350 washers are 270 N/m. The Austrian engineers do not actually classify the spring pressure of the Belleville washers in N/m, preferring to call it “slip torque,” but for clarity’s sake, slip torque is the amount of spring pressure needed to control pressure plate and friction surface grip.
Lever pressure. One big plus of the Belleville washer’s cupped shape is that it is at maximum stiffness in the initial part of the clutch lever’s movement, but the farther the rider pulls the clutch lever, the flatter the cone shape of the cupped washer gets, which means that, unlike a coil-spring-operated clutch, a Belleville-washer clutch gets easier to pull the farther you pull it in. This relieves hand fatigue. In 2019, Kawasaki used a hydraulically activated clutch and for 2020 borrowed KTM’s Belleville washer spring design, but called it a “coned disc-spring clutch” to avoid looking like they copied the KTM design, which they did. However, they did not copy the CNC-machined clutch basket/primary gear combo.
The notches in the tangs of this cast clutch basket are caused by the metal drive plates banging against the aluminum tangs during acceleration. The smooth action of a clutch in this basket would suffer greatly as the clutch plates get caught on the ridges.
Notching. The billet steel clutch basket will outlast any cast-aluminum clutch basket. Best of all, it will not notch where the metal drive plates hit against the tangs or suffer from tang breakage. The aftermarket clutch companies do not make a KTM clutch basket from either steel or aluminum. Why not? Most notably because the stock KTM basket is too good and practically because there are no bolt-on primary gears available. Hinson does make a steel clutch basket for the YZ450F and CRF450, but that is because they can use the stock primary gear.
Clutch fluid. KTM/Husky/GasGas clutches can use Dot 4 or Dot 5.1 brake fluid. Some older Magura hydraulic clutches require mineral oil instead of brake fluid, but if that is the case, it will be embossed on the master-cylinder reservoir cap.