First, we'll look at it in its unloaded state, with 1500 Ibs on the front tires and 1200 on the rear. Remember that lateral acceleration is nothing more than the percentage of a vehicle's weight that the tires generate in lateral force. To find it, you just divide lateral force by weight.
If you look at the curve in Figure A, you'll see that each of the front tires has the capability of generating 600 Ibs of lateral force when loaded at 750 Ibs (half of the total weight on the front). So, by dividing the front tires' combined 1200 Ibs of lateral force by their 1500 Ib. combined loading, you'll see that they can generate 0.800 G of lateral acceleration (this can also be called cornering power or cornering capability). Following the same pattern, you'll see that the rears can generate 0.833 G.
In this example, the rear of the truck has 4.1 percent more cornering capability than the front. That indicates an understeer condition. In other words, the front would lose traction (break loose) before the rear. Now let's put 600 Ibs of additional weight in the truck's bed, right over the rear wheels, to keep the loading on the front tires the same.
Since nothing has changed up front, those tires can still generate 0.800 G. However, with the rears now loaded to 900 Ibs apiece, they can generate 630 Ibs of lateral force each (check the chart). By dividing 1260 Ibs by 1800, you can see that the rear can now generate only 0.700 G. So our pickup has suddenly gone from having 4.1 percent more "bite" at the rear to 12.5 percent less.
Simply by loading this truck with an extra 600 Ibs, we have turned what was a mildly understeering vehicle into one with a pronounced oversteer...that is, the back would lose traction well before the front. Adding still more weight would make things even worse. So what can realistically be done to remedy the situation? Stiffen the front springs, or anti-roll bars, or both? This would reduce the front cornering level and decrease the oversteer by forcing the outside-front tire to carry more of the load; but it would require adjustable springs and anti-roll bars like they do in NASCAR racing. Not very practical for a pickup or street hot rod.
Put larger tires on the rear? That would increase the cornering forces and would decrease the loaded oversteer, but the truck would then understeer heavily when empty or lightly loaded. Not good. Adjust air pressure? Since all tires gain cornering power with an increase in inflation pressure (up to a point), you could increase the lateral force that the rear tires can generate by boosting their inflation pressures. Not only is that the most convenient method, it also allows you to optimize the handling to the loading at any time. So that is the recommendation for this truck. (It is also the recommendation of most truck manufacturers).
This phenomenon of being able to modify handling characteristics simply by adjusting inflation pressure applies to any kind of vehicle that you're likely to encounter, and it is used extensively in racing to fine-tune a car's performance. So understanding the relationships of load, inflation pressure and tire cornering power can be extremely beneficial to you both on the race track and in day-to-day driving as well.
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