Its just simply because the LHS tire cornering stiffness climbs with the reduced wheel load. LHS has considerably higher mu than RHS for D-coded tires used in all oval track series. No different than tires on a standard sedan, but different that tires on perf. sport cars (like Corvette, in which case stiffness climbs with added load. Its all about load rating, rim and pressure for the series the tire is developed for. Pressure also plays a part here,too, During the fuel run pressure increases and drops cornering stiffness and aligning moment stiffness. The reduced aligning moment stiffness loosens the car. So, as you can see, its a rather complex interaction between front and rear axle sideslip stiffnesses, but in the end the net understeer increases. Because the axle tire stiffnesses change, the systemn damping drops, hence the loose in, tightoff problem. Effects of tractive force on tire stiffness are a player, too. Keep in mind where the power is on and where it is off and it should be apparent to you what's happening. On speedway and superspeedway runs, the aero loading is a prominent factor,too, hence encouraging drivers to keep their speed up in spite of shortest distance line theory. In other words, keep the front grip up by keeping your Left front tire (highest mu) loads up using a downforce prominent setup.
