March 21, 2012
By Jim Kerr; photo by Grant Yoxon
Mazda has just introduced a new compact SUV named the CX-5. Known as a company that provides a sporty feel in its passenger cars, Mazda has upped the ante with its new SkyActiv powertrain, body and suspension design for the CX-5. This new vehicle is the first to incorporate the company’s complete SkyActiv concept, and a few laps of a slalom course and a few more trips around Mazda Raceway at Laguna Seca showed that the SkyActiv design is about much more than just engine and transmission. The suspension and body have been redesigned to provide both improved ride and handling.
Ride means different things to different drivers. A few drivers prefer the floaty soft suspension that isolates you completely from the outside environment. A few others prefer a stiff setup that lets the driver feel every crack and pebble back through the steering wheel and in the seat of the car. Most of us prefer a ride somewhere in between, that provides comfort but still delivers feedback to the driver. Tuning the suspension for ride qualities is accomplished with spring rates, shock damping, rubber bushing hardness, suspension pivot points, sway bar design, tire selection and body structure. All of these also affect vehicle handling, so let’s take a look at the body structure first to see how it affects both ride and handling.
When it comes to vehicle bodies, the stiffer the better: a body that flexes can’t control the position of the suspension and the ride and handling is mushy and imprecise. Imagine floating in a swimming pool in a cardboard boat. As a wave passes, the cardboard flexes and the passenger has to shift positions to stay upright. This quickly fatigues the passenger, plus it is difficult to paddle a flexing boat. Now, make the boat out of wood or fibreglass and the solid structure allows the passenger to ride over the waves securely and maintain steering control as they paddle. Mazda, as with many other auto companies, continues to stiffen the body structure by changing the thickness or strength of materials and using more bracing and box sections. It works very well on the CX-5, as this vehicle really performs on the race track –- an environment that shows up body flex very quickly.
Many people think you need stiff springs to provide handling. This isn’t true. Handling improves when the tire stays in contact with the road and the body doesn’t lean or roll a lot, so that the tire can stay upright. To accomplish this, a softer spring will allow the tire to lift easily over a bump without disturbing the rest of the vehicle, and by adding a stronger sway bar, body roll is minimized. Suspension parts can’t be designed in isolation, so along with the softer spring rate, a shock absorber with damping rates that will allow a tire to travel upwards rapidly over a bump, but controls how fast it moves back down so the tire doesn’t bounce, needs to be included. Keeping the tire in contact with the road more helps handling and safety, while the softer springs and correct damping provide a comfortable ride.
With a stiffer body structure, suspension bushings can be made more flexible for a better ride, but the location of the suspension pivot points can also make a difference. In most vehicles, the rear suspension pivot points are low, to maximize cargo and passenger space. When the tire went over a bump, the arc of the suspension moving up caused the tire to move slightly forward. By moving the pivot point higher, as done in the CX-5, the tire now moves slightly rearward when it moves up over a bump. The forces transferred to the body are lower because the tire is now moving in a natural direction, and ride comfort is better. Moving suspension pivot points higher may seem like a simple task, but it takes careful engineering to do it on vehicles like the CX-5 so that there is still good cargo and passenger space.
Finally, tires make a big difference and are the simplest way to change vehicle ride and handling. Low profile tires give a stiffer ride but improve handling and steering response because there is little flex in the tire sidewalls. Tires with a higher sidewall can soak up bumps better but have a slower response to steering inputs and flex more on corners.
While drivers can modify the handling of their vehicles dramatically by changing tires and tire pressures, a good factory design is really important for the best of ride and handling. Mazda’s CX-5 has got it right.