The authors' objective was to determine the effects of bar diameter on the stiffness and cost of a knee-spanning external fixator. The authors studied 2 versions of an external fixator with a difference in bar diameter (small bars, 8-mm diameter; large bars, 11-mm diameter). Fixators were tested using frame dimensions and a synthetic fracture model appropriate for a tibial plateau fracture. Five configurations of each fixator were tested: standard, cross-link, oblique pin, double stack, and super construct. The construct stiffness of each configuration (n=60) was measured in anterior-posterior bending, medial-lateral bending, axial torsion, and axial compression. Cost analysis allowed for calculation of the stiffness per unit cost. In the large bar group, an increase in construct stiffness was noted for all constructs and testing modes. Magnitude of stiffness increase ranged from 24% to 224% (P<.05 in all cases), depending on the configuration and loading mode. Increase in stiffness was so large that double-stack small bars performed similarly to standard construct large bars. Considering that the frame components have similar costs, the double-stack small bar fixator results in a 66% increase in cost for the same stiffness provided by the standard large bar. Bar diameter seems to have a large effect on knee-spanning external fixators. The authors observed an increase in stiffness of up to 191% under anterior-posterior bending despite an increase in bar size of only 37.5%. This finding might allow clinicians to use less expensive frames constructed of larger bars without sacrificing construct stiffness.