Stiffer clutch springs for modern four-strokes.

WHAT’S IT COST? $69.95 (most models), $79.95 (2013–’15 CRF450, 2005–’15 RM-Z450, 2010–’15 YZ450F).

CONTACT? (951) 738-8050 or

WHAT STANDS OUT? Here’s a list of things that stand out with Pro Circuit’s works clutch springs.

(1) Function. Do you know how a clutch works? It is a simple mechanism. A clutch consists of 16 spinning discs. Half of the discs are coated with fiber, and the other half are bare metal. The metal discs are connected to the transmission, and the fiber discs are connected to the engine. When the 16 discs are pressed together, the power from the engine is transferred to the transmission. It goes without saying that when the 16 discs are separated, the tranny and the engine are disconnected. To make a bike move, the eight fiber discs have to grab onto the eight metal discs and stay connected via friction. The firmer the contact between the 16 plates, the more positive the drive to the rear wheel.

(2) Common bond. What holds the 16 discs together? Five or six little coil springs. No matter how much power your motorcycle produces, if it weren’t for those springs, you wouldn’t move a single foot. This clutch arrangement has worked flawlessly for decades, but modern four-strokes have added a new dimension to the life of a motorcycle clutch. With its massive increase in torque, flywheel weight and crank inertia, a four-stroke puts a greater load on a motorcycle clutch than a two-stroke. The problem is that most 250cc four-stroke race bikes use a clutch designed for a 125cc two-stroke, while 450 four-strokes run 250 two-stroke clutches. Not good, and especially not good if the four-stroke rider tries to apply clutch-heavy two-stroke tactics to his thumper. Can you smell what you’re cooking?

(3) Spring rate. Pro Circuit’s clutch springs are 15 percent stiffer than the stock clutch springs. What does that mean? Pro Circuit’s springs press the 16 clutch plates together with 15 percent more pressure than the stock springs. A rider can also change the overall pressure on the clutch plates by mixing and matching the stock clutch springs with Pro Circuit’s stiffer clutch springs. For example, on a Honda CRF450 we run three Pro Circuit stiff springs with three stock CRF450 springs. This produces a middle-of-the-road rate change. The Honda really needs all six stiff springs, but if you get the springs too stiff on the CRF450 the narrow actuation window of the CRF450’s clutch arm becomes like a light switch.

(4) Clutch pull. The Pro Circuit clutch springs make the clutch lever harder to pull, but this is only a concern for a 10-year-old Girl Scout, not a manly motorcycle racer.

WHAT’S THE SQUAWK? If you are a clutch abuser, stiffer springs won’t be enough; you’ll need to call Hinson for the high-priced solution.

The manufacturers are so concerned with having an easy-pulling clutch on the showroom floor that they compromise performance on the track. The bikes that benefit most are the Kawasaki KX250F/450, Suzuki RM-Z250/450 and Honda CRF250/450. KTM doesn’t need help.

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