To keep the chassis from doing the suspension's job, Mazdaspeed's engineers designed the wheel rate to be six to eight times softer than the chassis stiffness. In other words, the chassis deflects one inch for every six to eight inches of wheel travel. To calculate wheel rate, divide the chassis' torsional stiffness by the target ratio-7.5 in this case. Finding the appropriate spring rate is then a matter of knowing the suspension's motion ratio and doing the math.
Spring rates differ front to rear because sprung mass and motion ratios vary from front to rear. These differences mean this calculation must be performed for both ends of the car.
As a final check, natural ride frequency-the frequency at which an undamped vehicle will oscillate using a given spring rate-is targeted as a ride quality measure. Increasing spring rates contributes to higher ride frequencies. An overall natural ride frequency of 1 Hz to 1.5 Hz is the accepted norm for optimal comfort and is what most sedans are designed to meet.
A Ferrari Challenge car will have a ride frequency near 2.5 Hz, while most reasonably well-tuned street cars hover around 2 Hz. This was the target for the street coil-overs. Since front and rear spring rates differ, so will ride frequencies. Mazdaspeed spec'ed the street coil-overs for 2 Hz in front and 1.72 Hz in the rear. The higher front frequency helps account for weight transfer under heavy braking.
Once experimental spring rates are selected, a prototype suspension is built using five-way rebound adjustable dampers valved specifically for the chosen spring rates. Each shock is dyno'ed to provide empirical data to compare with subsequent damping profiles. The prototype suspension, which must go through a gauntlet of manufacturing and quality control validations, is installed on a stock car along with prototype anti-roll bars and tires specific to the Mazdaspeed vehicle. Ride height and alignment are initially set from the engineer's intuition.
At this point, the development process is about 70 percent complete and the vehicle hasn't even been driven yet. This is where seat-of-the pants experience kicks in. Chassis engineers depend on their experience and sensitivity to vehicle dynamics to fine-tune suspensions from the theoretically ideal to empirically fast.
Mazdaspeed's engineers employ the help of several GPS satellites and a telemetry data recorder to verify results. Test cars are put through skidpad, lane change, slalom and autocross tests while at the track. Everything from vehicle speed and lateral acceleration to yaw angle and suspension travel is monitored and recorded. Prototype suspensions are also tested on mountain roads to evaluate the car's driveability on real-world surfaces.
Spring rates, damping, roll stiffness and alignment specs are all updated and revised as this process is repeated several times. Dampers are revalved according to the test engineer's suggestions and dyno'ed for verification and comparison. Mazdaspeed also tests its vehicles at Mazda's Michigan Proving Grounds for ride comfort and high-speed evaluations. It takes at least two cycles of this process before a product is ready to go on market.
This use of O.E.-level engineering, repeated testing and extensive development experience makes many factory-approved suspension products superior. Mazdaspeed and its counterparts at other manufacturers have more resources to spread over fewer applications than most aftermarket companies. This inevitably leads to an extensively sorted final product that is quite often covered with a factory warranty and can be installed at the dealer. - Jay Chen
On the flip side, we were surprised to find that the torquey MZR mill, rated at 170 hp from the factory, only pumped out 134 hp on our Dynojet.
It's not just the power this engine generates that's impressive, it's the way the revs seem to climb as if you're free revving the engine.
The Mazdaspeed car is a brilliantly balanced cone-threading machine straight from the factory.
In the autocross it plays a constant tug-of-war with the MX-5, ending up quicker in four of eight sections, and only marginally ahead at the end.
Each of the three cars in our test is a driver's car built around focusing the experience down to its fundamentals.