The shock absorber usually consists of two parts: a spring and a damper or dashpot. The spring may be implemented with a steel or titanium coil, an elastomer, or even compressed air. The choice of spring material has a fundamental effect on the characteristics of the fork as a whole. Coil spring forks are often heavier than designs which use compressed air springs, but they are more easily designed to exhibit a linear, Hookean response throughout their travel. Replacing steel coils with titanium coils in a design can decrease the weight of the design but leads to an increase in expense. Air springs utilize the thermodynamic property of gases that their pressures increase as they are compressed adiabatically. As the "spring" is provided by air rather than a coil of metal, forks with air springs are often lighter. This makes their use more common in XC designs. Another advantage of this type of fork design is that the spring constant can be adjusted by adjusting the air pressure. This allows a fork to be tuned to a rider's weight. One disadvantage of this design is the difficulty in achieving a linear response, as pressure varies approximately inversely (not linearly) with volume in a gas. As the fork compresses, the effective spring constant of the fork increases. Increasing the volume of the air inside the spring can reduce this effect, but the volume of the spring is ultimately limited, as it needs to be contained within the dimension of the fork blade.