How to control toe changes?
I challenged the suspension of a full-scale sports car.

Following on from the hugely successful first RX-7, be sure to make the second one a hit!With that strong determination, the design and development of the second generation RX-2 (FC2S) started.At that time, Mazda was also recovering from the oil crisis, and we ran around the United States as a concept trip to fully observe the American market and solidify the direction of the second generation RX-7.By that time, I had already decided to go with the suspension independently.After Porsche 3, 2 came out, and I thought that I could not lose to these.At that time, Mazda used semi-trailing for Cosmo and Luce, so the second-generation RX-7 was also a candidate for semi-trailing.

2nd generation RX-7

Following the original packaging concept, we have set two major technical goals to start development.First of all, I would like to further improve the steering stability performance in order to dispel the image of the first generation, which was said to be "Too Nerous!" As a sports car.These two points are to make the underspring as lightweight as possible.

The first generation used front midship packaging to reduce the yaw moment and improve the responsiveness of yaw changes to steering operations.However, the problem was controlling the change in the toe of the rear wheels.

There was a view that if you do not stabilize here, sudden behavior changes will occur and it will be fun, but it was because it was felt by general drivers that it was difficult to control and nervous maneuverability.That's why the second generation challenged new technologies with the desire to somehow calm the rear.
While considering the overall suspension configuration, the front is a MacPherson strut type that is easy to take up space, is light in weight, and has a high degree of perfection.And, in order to reduce the unsprung weight, we decided to adopt the first aluminum suspension arm in Japan.
At first, I intended to produce this aluminum arm by casting.If you make it from casting, it is easy for "casting cavities" to enter the product, so it is necessary to inspect all of them with X-rays, but Porsche and others also adopted cast aluminum suspension arms, so I believed that it could be done.However, it took a lot of capital investment to make an X-ray inspection line, so we changed to forged aluminum, which is a technology for aircraft parts with excellent strength and stable quality.
However, the manufacturing cost of this was quite high. The cost of one was equivalent to one regular MacPherson strut.When I think about it now, I think it was often adopted.As a result of discussions with sales and promotion, it was decided to adopt aluminum for the first undercarriage in Japan as the second selling point.

Furthermore, a molten metal forged aluminum hub is also used for the rear toe control hub.This is to achieve the goal of making the spring lighter.Since this molten metal forging rear hub was Mazda's first technology, we had various troubles at first, but with the help of the Technical Research Institute, we crushed each problem one by one.The Technical Research Institute is proud and willing to cooperate with us because our research technology is reflected in mass production.
And next is production technology.At first, it was said that one mold could produce more than 2 aluminum hubs, but when I tried it, the mold became useless after about 5.Therefore, we reviewed the shape of the product and made improvements so that the mold life could be extended.We narrowed down our wisdom along with production technology.I designed it, I can make it, and I can't do anything very good.It can be achieved by thinking together and proceeding.
In addition to these, many aluminum parts have been adopted.Brake calipers, engine mounts and differential cases.

Aluminum parts group

Aluminum bonnet hood

At that time, there was a car called Porsche 928, and the Weissach axle was adopted, but this is one theory to realize lateral force toe-in.The idea is that if you loosen the accelerator while cornering, the front will tuck in and the rear will escape smoothly toward the toe-in, and the stability will be increased to the last.

Rear suspension assembly

For the rear suspension, we adopted an independent suspension semi-trailing link + multi-link type.In the first place, in semi-trailing, when a lateral load is applied, the arm bush bends and becomes a toe-out, and when the lateral force is released, it is tucked in this time and a big problem in maneuverability appears.Around that time, computer analysis was also introduced at Mazda, and when we analyzed this technology, we obtained good characteristic data of yaw gain (easiness of turning a car).If a semi-trailing arm can be used to counteract the toe-out caused by the load, it will be easier to maneuver.With that in mind, I worked on the idea of ​​the Toe Control Hub as product planning.Then, he took a patent with this mechanism and received the "Technology Development Award" of the Society of Automotive Engineers of Japan.The idea is to fix one point of the hub as a pillow ball (point C) around one point in order to generate a rear toe-in, and lateral force, braking force, and force to induce toe-out come into this. At that time, I designed the rear wheel to be a toe-in.

Toe control hub structure diagram

Toe control characteristics

Toe control patent

With this mechanism, when the steering is turned due to a lane change, a load is applied to the tires on the outside of the rear, the toe faces in, and the lane can be changed without generating yaw on the rear.In addition, the engine brake, braking, and driving force were also devised and set to toe in.By doing this, you can always maximize the grip of the rear tires.

* Click the figure to see the enlarged image.
When turning, a force is applied to the center of the rear wheel from the outside, and when the bush at point B with a stopper exceeds XNUMXG, it deforms, promoting the deformation of the bush at point A and turning the rear wheel toward the toe-in.	The engine brake is applied from the front to the center of the rear wheels.Normally, it would be a toe-out, but the B-point stopper does not cause a toe-out, but it is a force that pushes the wheel hub backward around the pillow ball C point.

At the same time, the camber control mechanism consisting of a control arm with a control link and a lateral link ensures the optimum camber angle and improves cornering stability.The trailing arm moves up and down in an arc when stroked, but the control arm, which normally moves at this time, does not follow the trailing arm because the movement is restricted by the control link, and the arc is drawn. It works to correct the vertical movement by applying a force to the trailing arm to be drawn by twisting it outward.This will always maintain proper ground camber.

* Click to open the enlarged image.
Point C swings so as to draw an arc around the axis connecting points A and B.At this time, the control arm also moves, but because it is restrained by the control link, it does not follow the trailing arm, but pushes the trailing arm that draws an arc outward and works to keep it perpendicular to the road surface.

As mentioned above, the front suspension uses a forged aluminum A-type arm, and the rubber bushes at both ends have a double bush structure to ensure compliance in the front-rear direction.At the same time, we have introduced a new technology for handling information when cornering.It is to increase or decrease the amount of assist in response to strong lateral G in tight corners.

It is not just a speed-sensitive type, but the reaction force from the road surface is read by the change in hydraulic pressure, the friction coefficient of the road surface is calculated, and when it is judged that the grip force of the tire is being lost, the assist amount is increased and the steering force is lightened. To do.This allows the driver to sense that the grip limit of the front tire is approaching.In this way, we actively incorporated consideration not only for the rigidity of the steering system but also for dynamic sensitivity.

And when I analyzed the toe control of the rear suspension, I got quite good results in the damping data of the yaw gain when turning the steering wheel.Since the rotary engine is laid out in the front midship in the genealogy from the first generation, the weight distribution is close to the ideal value (50.5: 49.5 / 2 passengers), and the yaw gain of the vehicle is a fairly good value.In other words, in addition to the advantages of the original front midship, the toe-in control mechanism that suppresses the yaw itself quickly and low makes the second generation RX-2 agile and excellent in all scenes such as high speed turning, engine braking, braking. The steering stability was achieved.

However, there were some reflections. When I tried riding this FC3S RX-7 with the feeling that "humans operate the car", that is, from the viewpoint of dynamic Kansei engineering, I felt that the rear toe should not move.Specifically, if there is an area where the yaw generation and the rider's sensibility do not match, the dynamic sensibility is difficult to be satisfied.The driver feels that it is good for stability to look in the same direction as the car body.Therefore, if the rear wheels steer and move differently from the direction of the car body, the driver will feel uncomfortable.

The definition of dynamic Kansei engineering is difficult, but in a nutshell, "Jinba Ittai".The car reacts closely to the driver's intentions, predictions, and expectations.In that sense, the state of yaw gain for steering operation is a typical issue.
Therefore, in the subsequent minor changes of the FC3S RX-XNUMX, we reduced the amount of rear toe control and aggressively set it in a direction that does not cause toe-in.Especially in the Amphini version, the bush and arm will be hardened so that the toe-in amount to the car body will be almost zero.
It was a new technology that was realized with great effort, but in the next 3rd generation FD7S RX-XNUMX, we finally returned to a suspension that does not move.If toe-out is not good, toe-in is not good.The toe control of the rear wheels was effective in terms of automobile engineering, but the result was that the dynamic sensibility remained uncomfortable.This prompted me to pursue research in dynamic Kansei engineering.

By the way, the cars that will appear next time are not only the suspension development but also the NA type and NB type roadsters that served as the chief examiner.I would like to talk about its dynamic Kansei engineering, such as the first front and rear double wishbone suspension and the adoption of a power plant frame.