Vehicle Handling


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OPTIMUM VEHICLE HANDLING

     Vehicle handling is a subject that requires dealing with a large number of variables. Attaining optimum vehicle handling requires the adjustment of these variables to achieve the desired result. Cars and trucks require the same techniques to achieve the optimum setup, but the moment that any loading takes place that balance is disturbed.
     Cars don't change their handling characteristics drastically when passenger loading and trunk loading changes but trucks experience radical changes in handling as loading varies. Looking at a truck going through these changes gives a more dramatic illustration of these effects. A trucks load bias (front to rear) shifts more radically than that of the average car because of the variable created when a load is placed in the rear of the vehicle. Original suspension design characteristics and tire selection will determine how the vehicle will react both loaded and empty, but some adjustments can be made to compensate for this.

     To get an idea of what takes place under these conditions and what can be done to compensate for them let's look at the case in point presented by Bob Strange, advanced tire development engineer for BFGoodrich, regarding the classic example of widely varying vehicle loading of the pickup truck. Because the changes in loading can be so great, and their effects on handling so large, it is very easy to see what goes on...and why. So let's consider an imaginary pickup truck like the one in Figure B.

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.


	Of course, there are a variety of chassis adjustments that can be made to induce more or less understeer/oversteer:


	To Increase Understeer

		Lower front tire pressure Increase rear tire pressure Reduce front tire width Increase rear tire width Increase front wheel positive camber Increase rear wheel negative camber Increase front spring rate Decrease rear spring rate Stiffer front anti roll bar Softer front anti roll bar Decrease front tread width Increase front weight bias Decrease front weight bias


	To Increase Oversteer

		Increase front tire pressure Lower rear tire pressure Increase front tire width Reduce rear tire width Increase front wheel negative camber Increase rear wheel positive camber Decrease rear spring rate Increase rear spring rate Softer rear anti roll bar Stiffer rear anti roll bar Incrase rear tread width Decrease rear tread width Decrease rear weight bias Increase rear weight bias


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