The desmodromic system avoids this problem, because it does not have to overcome the force of the spring. It must still overcome the inertia of the valve opening and closing, and that depends on the effective mass of the moving parts. The effective mass of a traditional valve with spring includes one-half of the valve spring mass and all of the valve spring retainer mass. However, a desmodromic system must deal with the inertia of the two rocker arms per valve, so this advantage depends greatly on the skill of the designer. Another disadvantage is the contact point between the cams and rocker arms. It is relatively easy to use roller tappets in conventional valvetrains, although it does add considerable moving mass. In a desmodromic system the roller would be needed at one end of the rocker arm, which would greatly increase its moment-of-inertia and negate its "effective mass" advantage. Thus, desmo systems have generally needed to deal with sliding friction between the cam and rocker arm and therefore may have greater wear. The contact points on most Ducati rocker arms are hard-chromed to reduce this wear. Another possible disadvantage is that it would be very difficult to incorporate hydraulic valve lash adjusters in a desmodromic system, so the valves must be periodically adjusted, but this is true of typical performance oriented motorcycles as valve lash is typically set using a shim under a cam follower.