What is Load Transfer?
Vehicles experience many kinds of acceleration both on the racetrack and on the road. These are either longitudinal (from braking or speeding up), lateral (from turning), or vertical (from bumps and dips).
During each type of acceleration, the weight of the car, which is supported on springs, is shifted one way or another. This is known as load transfer.
Due to the nature of vehicle suspension, a change in load on a wheel or pair causes compression or expansion of the vehicle’s springs and therefore creates a change in the tire contact patch.
With changes in the tire contact patch come changes in grip and turning behavior. These changes are undesirable and cause the need for methods of resistance.
Anti-Geometry: Drive, Squat, and Roll
When you apply the brakes while driving, you can feel yourself being forced forward in your seat. The vehicle does the same by compressing the front end. This is known as dive. Inversely, as you apply the gas, the vehicle compresses the rear end. This is known as squat. Finally, as a vehicle takes a left turn, the right side is compressed. This is known as roll.
All these compressions or expansions of the vehicle’s springs lead to changes in the contact between the tires and the ground. Auto manufacturers predict this movement and compensate by employing anti-dive, anti-squat, and anti-roll suspension geometry.
This is known as “anti-geometry.” It is important to note that all mentioned methods do not minimize load transfer. They only minimize the dive, squat, or roll that accompanies it.
Although roll is important to notice and minimize, the methods of doing so are rather simple. The best way is to include a stiffer spring, called an anti-roll bar. As there’s not much to show for this, the following measurements will only include dive and squat.
How to Measure and Use Anti-Geometry
There are many proposed methods of measuring the anti-geometry of a vehicle. However, they all show anti-geometry as a percent. One method uses the crankshaft as an estimate for the center of gravity, then measures the instant center’s distance away from the 100% anti-dive line. Although this is a good estimate, it does not give an exact value.
Another method factors in the wheelbase of the vehicle as well as the height and length of the blue lines in the image below. This will yield accurate percentages but is a bit more complex and requires more measurements.
A much simpler way is one proposed by legendary racing driver and engineer Carrol Smith in his book “Tune to Win.” We’ll cover this below.
Center of Gravity
The center of gravity is important for all load transfers and spring compressions. The center of gravity is a point on any object that could be thought of as the concentration of mass. If the object were able to be suspended from the said point, the object would not rotate or move.
This point is especially important because it can be thought of as the point by which all accelerative forces act upon. In the image below, the center of gravity can be seen as the red dot just below the door handle.
Instant centers are points on a vehicle that do not actually exist in the suspension, although they are crucial in calculating anti-geometry. They are calculated differently depending on the suspension types. While there are many varieties, MacPherson strut and four-link/double-wishbone systems are the most widely used suspension types.
In both cases, the intersections of each pair of green lines show the instant centers from a side view. In the front (MacPherson strut), the lines come from the center of the top damper mount (damper shown in orange) and the lower mounting points (shown in yellow). In the rear (double wishbone or four-link), the lines are simply made by following the mounting locations of the upper and lower control arms (dark green).
Note: The shown instant centers and control arm angles may be unrealistic, especially in the front, as they have been exaggerated for clarity.
Finding a Percent
With the instant centers located, the next step is to draw a line from the center of the contact patch through the instant centers. This new line is called the side-view swing arm (shown in blue). The point at which the swing arm crosses the vertical line from the center of gravity to the road (dashed red) is the percent of anti-geometry.
In the case shown above, the front shows 100% anti-dive (as the blue line passes directly through the center of gravity), and the rear shows about 50% anti-squat (as the blue line lies about halfway from the ground to the center of gravity).
The front swing arm determines anti-dive, and the rear swing arm determines anti-squat. All swing arms that pass above the center of gravity will have greater than 100% effect, and all that fall below the center of gravity will have less than 100% effect.
Why Not Always Run 100%?
As shown with the MacPherson strut setup in the front, 100% anti-geometry can easily be achieved. If the diving or squatting of a vehicle causes negative effects on handling, why not eliminate all that movement?
The methods shown and discussed angle the suspension arms in a way that causes them to absorb a portion of the load transfer. At 100% anti-dive, the car does not dive because all of that forward momentum is directed into the suspension arms rather than the springs.
This makes the suspension incredibly stiff, and if a bump were to be hit while under this load, it would likely bind solid and minimize traction with the road.
As this technique makes the suspension extra sensitive and unpredictable, the maximum amount of anti-geometry run is typically about 20-30%. Racing cars run much less, as their suspension is much stiffer, which already limits most of the diving or squatting.
Next time you go for a drive, pay attention to the load transfer as you brake, turn, or speed up. Your car likely employs some amount of anti-geometry and recognizing the way that each vehicle reacts differently to driving inputs can increase overall driver skill.