Gran Turismo creator Kazunori Yamauchi says tire simulation remains Polyphony Digital’s most difficult physics challenge, with contact-patch behavior and tire graining still not fully reproduced. Almost three decades after the original Gran Turismo, Polyphony Digital is still trying to solve one of the most complex problems in racing simulation: how a tire behaves at the point where it meets the road. Gran Turismo creator Kazunori Yamauchi discussed the subject in a wide-ranging interview with evo, published on August 22. Although the interview was conducted earlier in the year during the FAT Ice Race in Austria, Yamauchi’s comments provide a rare insight into the areas where Polyphony believes its current physics simulation still falls short of reality. According to Yamauchi, suspension systems, transmissions, differentials and other mechanical components can now be simulated with relatively little discrepancy compared with the real world. Tires remain a fundamentally more difficult problem. Tire Contact and Graining Remain Difficult to Simulate Modern tire simulation involves significantly more than assigning a fixed amount of grip to each compound. Polyphony must account for deformation, different tire dimensions and compounds, temperature changes, wear, track conditions and road surfaces. Complicating matters further, Yamauchi notes that tire manufacturers do not necessarily disclose everything about how their products behave, leaving developers to construct mathematical models from the information and measurements available to them. One particularly difficult area is how the tire’s contact point with the road changes dynamically. Heat and wear alter the available grip, while different conditions can produce physical effects that are difficult to reproduce accurately in software. Yamauchi highlighted tire graining as one example that Gran Turismo does not yet fully simulate: “The level of grain that happens when you have certain conditions on the track, it’s something that we just don’t have yet.” Graining occurs when stresses on