Introduction to Dynamic Kansei Engineering for Enjoying Tuning §4


To continue to be a true sports car.
It was poured into the suspension of the 7rd generation RX-XNUMX.

The first RX-XNUMX contributed not only to the domestic market but also to the extremely sluggish sales network in the United States, the reconstruction of the image of the rotary engine that had fallen to the ground, and the recovery from the business crisis triggered by the oil crisis. The second generation has established itself as a solid sports car, matching the market needs of Japan and the United States for full-scale sports cars.By the time we started developing the third-generation FDXNUMXS RX-XNUMX, the social situation changed drastically, and Mazda started to expand its domestic sales system to five channels in the midst of the bubble economy.Also, since the XNUMXrd generation RX-XNUMX was decided to be the flagship car of the Amphini channel, I felt that its mission would be significantly different from the XNUMXst and XNUMXnd generation.At the same time, the yen-dollar pair changed sharply, and what was $ XNUMX = ¥ XNUMX at the time of the first generation soared to $ XNUMX = ¥ XNUMX, and in addition, sports car insurance soared in the United States.
As I said last time, when I was in charge of the first suspension, I took on the job of a sports car within the truck team.I think this was, in a sense, fortunate for both the RX-XNUMX and me.That's because the work was done in a simple design and development environment without the shackles of passenger cars.And there was a lot in common with the strict sports car requirements and the development of a simple and robust truck.

The development concept of the XNUMXrd generation RX-XNUMX was to create a super-first-class sports car.As a designer, I was also thinking of putting in the best technology that Mazda could have.As long as it is a first-class sports car, styling is of course the most important factor.Low fender and bonnet, how to design and develop a compact suspension with sufficient stroke.

B360 truck3rd generation RX-7 (FD3S)

Looking back on the past suspension development, I went back to the beginning and created the concept.What is the essence required for the suspension, and the keywords I envisioned were words that were faithful to technology and physical phenomena, such as "light, straightforward, compact, and linear."

Of particular importance was weight reduction. For the XNUMXrd generation, it was the goal of the overall exercise performance of the vehicle to keep the power-to-weight ratio to XNUMXkg / ps or less, and it was a necessary requirement to keep the weight of the vehicle to XNUMXkg, so we made a thorough weight reduction.

B360 truckRX-7 (FD3S) components

During the development, I went to see the wreckage of the Zero Battle with the main developers, but I remember being amazed at the technical capabilities of the engineers at that time because of their thorough commitment to weight reduction.

With the goal of reducing the weight of the vehicle to 0 kg, we reduced the weight of the parts to zero and made a thorough weight reduction called Operation ZERO in analogy to the weight reduction of the zero battle, six times during the development period. I did it. In order to make it lighter than the second generation, we set weight reduction targets for each section, first discussed how much weight reduction is possible at the concept stage, and at the next stage, we put drawings of each part on the wall, and everyone Then, hold the red pen and write on the drawing whether there is any useless part or if there is room for making holes for weight reduction.

B360 truckPerspective view of RX-7 (FD3S)

Furthermore, we will disassemble (tear down) the parts of Porsche and sports cars of other companies into pieces, and consider whether we can make it lighter by using the lightest parts as a model.The designer in charge thinks that it is the best part to draw, so it is surprising that the designers in other departments will correct it with a red pen.However, all the participants discussed and considered it, and found a solution for weight reduction.
In this way, Operation ZERO, which aimed at -XNUMX% of the weight of the second generation, worked to reduce wasteful meat, including the thickness of the window glass and the sharing of mounting brackets for parts.

Exterior design is also an important factor in terms of vehicle weight.The chief designer has requested that the front fender line be lowered on the XNUMXmm design.This saves XNUMXmm of fender steel weight, which is welcome as a vehicle weight, but this time the suspension design layout will be very strict.Therefore, we reviewed the internal structure of the fender without being bound by the customs and reduced the number of steel parts.
Now, as it has appeared frequently, let's consider how the weight of the vehicle is important for the dynamic sensitivity of the vehicle.

The equation of motion of an object is "F = mα"(F:Power,m:mass,α: Acceleration).Mass for all car conditions such as accelerating, turning, stopping, etc.mIs deeply involved.For example, massmIf is a large value, the output for speeding upFWill be a large value, and the power required for the engine will be higher.FWill be big.Similarly, the braking force when stoppingFIs the massmIf is large, more braking force is required.To put it the other way around, powerFIf they are the same, the massmThe smaller the accelerationαIs to grow.And this acceleration is the interface that appeals to the driver's sensibility, and it can be said that the heart of dynamic kansei engineering is here.
So, I would like to take a closer look at the kinematics of cars.Car movement is not just about front and back, left and right, and up and down.(Figure 1)In the car likeX, Y, ZThere are motions around the three axes of, and together with the motions of these rotating systems, it is defined as XNUMX degrees of freedom.And the movement of the car is always the center of gravity(CG)Occurs around.
Next, each equation of motion(Figure 2)Let's look at.You are not an automobile engineer, so you do not need to understand it exactly, but in the motion of a rotary system, force is called moment, acceleration is angular acceleration, and mass equivalent is moment of inertia. Please note that it is.It's a bit confusing, unlike linear exercise, but the basics are the same.
The force to rotate is the product of the resistance of rotation and the acceleration of rotation. It may be easier to imagine if you remember the "principle of the lever".As a result, if the rotational moments are the same, the angular acceleration increases as the moment of inertia decreases.
* Click the figure to open the enlarged image.
Figure: Car movement
Figure 1 Car movement
Fig. XNUMX Equation of motion when considering a car as a rigid body
   
Next, I will focus on the "bending performance", which is most closely related to the enjoyment of driving, and proceed with a little more detail.ZIt is about the rotational movement (in the yaw angle direction) around the axis and the force that changes the direction of travel of the car.The speed at which the direction of travel of the vehicle changes (angular acceleration) with respect to the steering operation is faster as the yaw moment of inertia is smaller or the input from the tire is larger, when applied to the above equation of motion. is.Controlling the speed at which the car changes direction is extremely important in terms of dynamic kansei engineering, so let's do our best to calculate the yaw moment of inertia.The equation of motion in the yaw angle direction is shown below.Before dealing with the entire vehicle, let's focus on the front and rear bumpers and engine.
Yaw angle map
First is the calculation of the input from the tire.Here, the moments of the four wheels are calculated and the total is calculated.The larger this is, the easier it is for the car to turn.This is mainly determined by the tire performance, steering angle, vehicle speed, etc.Next is the calculation of the yaw moment of inertia.It's a difficult formula, but the point is that you cut the car into round slices, calculate each moment of inertia, and add them up.
* Click the figure to open the enlarged image.

Figure 3 Moment equation due to tire force		 Yaw inertia calculation
Figure 4 Formula for calculating the yaw moment of inertia
In this way, the moment of inertia is the moment of inertia of the unit and the mass of the part.mIt is determined by the sum of the square of the distance L from the center of gravity and the yaw moment of inertia of the entire vehicle.IzIs the sum of each partΣThe above formula shows that.Complicating compared to the basic luck equations listed above, each unit is not a point, but each has its own size and mass, and has its own moment of inertia around its own center of gravity. Because.This is called the "parallel axis theorem".Therefore, in order to keep the yaw moment of inertia low, the weight of the vehicle and the distance from the center of gravity of the heavy object are important factors.In short, in order to control the vehicle at will, it is desirable from the viewpoint of Kansei engineering not only to reduce the weight of the entire vehicle but also to concentrate the mass near the center of gravity of the vehicle.The story has become a little hard, but did you understand?

Here, I would like to talk about the development of the third generation suspension.There are two major features of this suspension structure. XNUMX. XNUMX.The basic format should be double wishbone. XNUMX. XNUMX.A sliding bush and a pillow ball bush are used for the pivot of the arm.As I mentioned in the Roadster episode, the double wishbone suspension has the most freedom to provide optimal geometry control for various inputs and displacements of vertical stroke, left and right turning lateral force, front and rear braking drive force. It can be said that it is high, lightweight, and has high rigidity.

* Click the figure to open the enlarged image.
Front suspension diagram Front suspension Rear suspension diagram Rear suspension
Front suspension Rear suspension

In the XNUMXrd generation, we focused on optimal control of camber and toe.
For camber control, we adopted ultra-flat and wide tires of XNUMX / XNUMXRXNUMX with the aim of improving steering stability, so the vertical spring constant of the tires will increase.Therefore, camber control is even more important in order to equalize the contact patch pressure while ensuring a sufficient contact area of ​​the tire.Regarding this point, the optimum bump camber change was realized by making the upper and lower arms unequal in length.

B360 truckFr suspension Toe control operating principle

In addition, we aimed for a weak understeer in order to obtain excellent exercise performance when turning, which is typical of a pure sports car.By swinging the geometry control of the front suspension to the toe-out when turning, the weak under characteristic as intended is realized.

The lateral force when turning is generated behind the distance of the tire's ground contact center point pneumatic (difference between the center of the ground contact surface and the center of the load).By setting the kingpin axis forward with respect to this input point, toe-out is generated.We also created a toe-out in roll steer.On the other hand, for lateral force, toe-in is generated by the rigidity distribution of the suspension mounting part, and the toe-out amount is relaxed.

* Click the figure to open the enlarged image.B360 truckSuspension spherical body theory (model catalog)

Moreover, the spherical body theory is adopted as this suspension.For details, please refer to the car model catalog at that time.
I will explain another major feature, the adoption of sliding bushes and pillow balls.In order to increase the rigidity of the suspension, it is necessary to increase the spring constant in the direction perpendicular to the axis of the bush used for the fulcrum of the suspension arm and the link.On the other hand, in order to secure the working torsion angle and reliability of the bush, it is necessary to take a large amount of rubber volume of the bush.


* Click the figure to open the enlarged image.B360 truckInternal structure of sliding bush and pillow ball bush

For this reason, the spring constant in the direction perpendicular to the axis cannot be set higher than a certain level (the rigidity of the suspension becomes low), but by adopting this slip bush and pillow ball, it is related to the operating torsion angle of the bush. I was able to set the axis right angle spring constant to the optimum value for steering stability performance.As mentioned above, the RX-XNUMX has been cultivated by incorporating Japan's first technology, such as adopting slip bushes and pillow balls for all suspensions.I took over the chief examiner from my predecessor Kobayakawa in XNUMX and have been involved for nine years until XNUMX, but it is unusual to be in charge of the chief examiner for one car model for such a long time. increase.I am very happy to be able to develop and nurture a true pure best sports car by putting all my own skills into it.

Good weight distribution, low center of gravity, low moment of inertia, etc., which have been stubbornly adhered to as requirements for the realization of Mazda sports cars, are all factors that determine the goodness of steering stability.Suspension is what makes the best use of its features, and I am convinced that we will steadily develop technology (light, rigid, small) that is faithful to the basics.As an engineer, looking back on the development process of the RX-XNUMX for three generations, it can be said that "the person who drives the car" is a deep area that cannot be talked about by modern theory, that is, dynamic Kansei engineering. Does that mean there is an area?

Next time, I will talk about the development of the NCEC Roadster.