For the joy of running through your heart.
The Roadster aimed for a "Jinba Ittai" sensibility.

As any Roadster fan knows, the first Roadster (NA6CE type) did not start as a formal development project.At that time, Chief Hirai started the development of club activities, but in fact, it was around 1983 when I was on a business trip to the United States for the RX-22 (SAXNUMXC type).
MMA (Mazda Motors Of America, now Mazda North American Operations) at that time also had a product planning department, and Bob Hall there enthusiastically showed a design drawing at a certain concept stage and a 1/5 clay model. Gave me.

If you listen carefully, this is a reproduction plan of Lightweight Sports (LWS) that existed mainly in Europe in the 1960s.If the two-seater sports of yesteryear can be revived with modern production technology and an open sports car that runs quickly and lightly can be sold for about $ 2 (at that time $ = ¥ 7,000), it must be a big hit in the United States. It's a story.
Especially on the west coast, there were many people who restored open 2-seaters such as Spitfire and Triumph, and enjoyed running.I sympathized with such an atmosphere, and I strongly wanted to make this LWS that is lightweight and can run quickly.

After returning to Japan, I was still worried about it, and it seems that I have begun investigating whether open cars can really be produced locally in light of regulations.
There was a technical standard called roof crash, and I thought that it would not conflict with this, but when I looked it up, I found that the technical standard for roof crash was not applicable to open cars.I thought this would be feasible enough.
And if the project really started, I secretly thought that double wishbones would definitely be the best suspension for the front and rear in terms of dynamic sensitivity.

A young engineer named Norman Garrett, a subordinate of Bob Hall, was thinking of a light and cheap strut at the front, but I insisted on a double wishbone.
Eventually, this LWS concept came to be considered within the product planning of Hiroshima.It is a team called offline 55, but I do not know if it will be a thing, but it is a team of advance planning that I will consider if it is 5 minutes 5 minutes.

Then, Mr. Hirai was returned from being seconded to the outside, and he was raised as the project chief of LWS from this offline 55.Even though the project started, it was not yet a formal mass production-based project within the company.Not to mention the lack of human resources, the office also started by renting a small conference room.

Also, inside the company, the people who heard this project were when Why? FR-FF was in full swing? Why? 2Seater-Can only two people ride? Why? OpenBody-Is the occupant safe?It was a situation where many opinions such as.I knew the LWS concept in MMA in the United States, so I started working on this project with a lunch box from an early stage.

Eventually, the product planning in Hiroshima showed the keyword "integrated human vehicle" as the concept of this project.I could understand that it is a car that people and cars operate while interacting with each other with a sense of unity, but at that time, all the members including Chief Officer Hirai said that this LWS was not just a well-made machine car, but "blood and blood." I wanted to make it a "living car" like a "racehorse with a flowing horse", so I changed the name of the human car to "human horse".From the point of view of dynamic kansei engineering, this "Jinba Ittai" is a strange concept, and it will be pierced as a synonym for the dynamic sensibility of the roadster of "a fun car to operate" for the next three generations. have become.
I was in charge of suspension design, but I set the following five indicators in order to achieve good steering stability.

　XNUMX. Front engine and rear drive.

　XNUMX. The vehicle weight should be a light weight of around XNUMXt.

　XNUMX. The yaw moment of inertia is small.

　50. The weight distribution of the front and rear wheels should be 50:XNUMX.

　XNUMX. The center of gravity is low.

These five were not the design conditions for the chassis area, but the basic layout of the car itself, which is related to the entire car, and the source of dynamic sensibility, and were essential conditions for realizing a "car that is fun to operate."Therefore, I was actively involved in the adoption of aluminum bonnets, battery mounting positions, power plant frames (hereinafter PPF), and so on.

For my main suspension, I decided to use double wishbone from the concept stage.The decision to allow "stickiness" but not "compromise" in the creation of sports cars led to the selection of double wishbones, which are "the most flexible form of undercarriage setting."However, this choice has resulted in great annoyance for Chief Hirai to proceed with the project later.At the stage of installing the commercialization approval of the officer, there is no reason to approve the car that sells as cheap as $ 7,000, such as the front and rear double wishbone format, which is more expensive than the strut format.

I was asked by expert Kijima to explain directly, but I declined.Even if you show the materials and explain in front of the officers, the strut type, which is cheap, has a small number of parts, is light and durable, cannot be seen from a profitable point of view.I asked Mr. Hirai to push it through because it is absolutely necessary for "Jinba Ittai".And it was worth the persuasion of the chief examiner, and in the end it was decided to hire him.

Human resources were limited for this project, so I thought about proceeding efficiently.There are various specifications related to the suspension to achieve the desired steering stability performance, and the characteristics influence each other.
It is necessary to repeat trial and error and determine the characteristics so that we can understand those entanglements and achieve the desired performance.

The design work is to create the shape of the parts that realize the characteristics.In order to efficiently perform the conversion procedure from this software to hardware, we created the chart shown above.This chart has greatly contributed to the realization of steering stability that creates the enjoyment of maneuvering.

PPF structure / roadster

Japan's first PPF worked on how to make this LWS into a bloody horse suitable for dynamic sensibilities.In some European cars, the transmission is called a transaxle mechanism, and the transmission is separated from the engine and connected to the differential case of the rear wheel (hereinafter referred to as the differential case) to wind up the engine and the differential case (the twist response delay of the engine and the differential case). There was a mechanism to hold it down (mainly for weight distribution).I wanted to incorporate the same effect into LWS.Originally, I was thinking of using it for the 3rd generation RX-3, which is a rotary engine with a thin low-speed torque, but before that, I was involved in LWS, so I decided to use it first. I made a prototype with the FCXNUMXS RX-XNUMX and confirmed its effect, so I was convinced that it was a perfect technique for "Jinba Ittai".

Since it is a small engine with 1600cc and a little 100 horsepower, how to transmit the small output to the rear wheels as driving force with good response.When the car starts and accelerates, the torque reaction force from the road surface causes a torque opposite to the direction of rotation of the tire to enter the differential case, causing a swing phenomenon in which the front end of the differential case is lifted.Only after this swing stops will the driving force pushing the car forward be transmitted 100% to the road surface.This leads to a delay in the response of the driving force, that is, deterioration of the transient characteristics.The same applies to engine braking.The dynamic sensitivity of a car is an important factor not only in the transient characteristics when turning, but also in the transient characteristics during acceleration and deceleration.

In the case of the PPF structure, the engine mount rubber doubles as the mount on the front side of the differential case because the long arm called PPF is extended forward from the differential case and connected to the transmission case.Therefore, the mount span that receives the torque reaction force has expanded to the same length as the wheelbase in the front-rear direction.

Compared to the case without PPF, the mount span is 6 times, and the pitching rigidity in the swing direction of the differential case is 36 times. By installing the PPF, the delay in the response of the acceleration response from the engine to the car body was eliminated, and the feeling of "human-horse unity" with excellent dynamic sensibility was poured in, which allows you to operate the car at will with a small displacement. is.

 

Since PPF was a large structure, we decided to make it from aluminum for the most important issue of weight reduction.Although the cost of a single item is certainly high, it has become possible to reduce the transmission members, rubber mounts, and rubber mounts on the front side of the differential case, and the total cost and weight have been reduced compared to the conventional type.

The concept of this PPF was proposed by the chassis design department to the drive system design in charge by designing a structure with a deformed lightweight hole with less stress concentration by computer analysis.However, as a result of our own simulation analysis from the drivetrain design, we received the answer that the engine block would be damaged in the PPF structure.

Conventional structure / RX-7 (FC)

PPF structure / roadster

Since PPF was the most important component for "human-horse integration" as a dynamic sensibility, we confirmed what kind of calculation was done for the drive system design.Then, even in the worst case, it was supposed to fix the engine and input all the torque from the differential case.It was natural that the engine block would be damaged.Once again, I requested a simulation with a structure that supports both ends with the rigidity equivalent to that of a rubber mount, but it was not accepted.It wasn't a formal project in the company yet, so I remember being frustrated that I couldn't get it seriously.
Meanwhile, Chief Hirai decided to rent the garage on the 1986th floor of the Design Center along the Goji River as a dedicated office for the project.Although it was just a garage with bare concrete, we named the office the "Riverside Hotel".I went to the "Riverside Hotel" during normal work and overtime, but finally in July 7, the LWS project got official development approval from the management meeting and got off to a good start. ..
First, we aimed to reduce the yaw moment of inertia and optimize the weight balance as the basic concept of "Jinba Ittai," which is an index of dynamic sensitivity.In order to improve responsiveness, turning performance, and control performance at the limit, the yaw moment of inertia has been reduced and the front-rear weight balance has been optimized.To that end, we have set the following eight items as shared goals for the entire team.

　XNUMX. Adoption of front midship layout.

　XNUMX. Shortening the wheelbase.

　XNUMX. Weight reduction of tires and wheels.

　XNUMX. Weight reduction of bumper reinforcement.

　XNUMX. Installation behind the seat at the gasoline tank position.

　XNUMX. Aluminum bonnet hood.

　XNUMX. Lightening the battery and installing it in the trunk.

　XNUMX. Stainless steel exhaust manifold.

Comparing with the case where the same design as before is performed without setting these items, the reduction of the yaw moment of inertia is 1,536 kgm2 → 1,340 kgm2 (also called the rotational weight, which is a value indicating the difficulty of bending the car. The lower this number is, the easier it is to bend), which reaches 12.8%.

In order to eliminate the influence of the vehicle weight on the yaw moment of inertia, the weight difference between the front and rear is plotted against the value obtained by dividing the yaw moment of inertia by the vehicle weight, and compared with other sports cars, the figure on the right shows. It will be like.It can be seen that this LWS has a small yaw moment of inertia, a good weight balance, and excellent dynamic sensitivity when the steering is turned.
I began to concentrate on LWS double wishbone suspension design and was driven by the desire to make the real thing.

It was Mazda's first front and rear double wishbone suspension, and I personally wanted to own a car that would allow me to adjust the suspension alignment myself.
I have declared to Mr. Hirai that I will develop a high-performance double wishbone as low-cost and lightweight strut suspension, so I have wisdom in every detail from the arms to the mounting part of the chassis member. I proceeded with the design while issuing.For example, the top mounts of the four-wheel dampers have been standardized, the arms have been devised in shape and shape, the left and right sides have been reversed and shared, and the mounting parts of the members have also been standardized.

* Click to open the enlarged image.
Fr upper arm	Rr upper arm	Rr lower arm	Rr cross member (Left and right / common mold parts)	Rr cross member (Front and back, common mold parts)

In addition, there was a design standard that the plate thickness of suspension-related parts should be 2 mm or more, but at that time, a cation coating that was introduced with new technology and was effective against rust was applied, and the bag-shaped part of the rust-prone arm was covered with water. By providing a hole for removing mud, the plate thickness was changed to 1.8 mm to reduce weight and cost.Furthermore, as a requirement for protection of oil pans and floors, there was a standard that the minimum vehicle height was 150 mm or more, but I knew that it was possible to produce even at a vehicle height of about 120 mm on the factory production line, so I was forced to car in the middle. I was able to secure a low center of gravity height by lowering the height to 135 mm.

In the development of the suspension, we also paid attention to the rigidity of the body, frame, arms, hub carrier, etc. in order to realize a feeling of rigidity like a sports car.As a result, we were able to achieve sharp characteristics with little delay in steering.In addition, by increasing the suspension rigidity, we were able to efficiently use compliance steer (a characteristic in which the toe angle changes according to the lateral force by utilizing the elasticity of the bush when lateral force is applied to the suspension). ..Since this suspension enhances responsiveness and turning performance by reducing the yaw moment of inertia, it is under compliance from the beginning (setting that emphasizes stability in the understeer direction by lateral force compliance. Lateral force toe out at the front, lateral force at the rear The direction of the toe-in.) Was set and balanced.As a result, we were able to ensure stability when changing lanes at high speeds and when cornering.

Specifically, the high degree of freedom in setting the geometry of double wishbones has made it possible to keep the tires in good contact with the ground in all situations.Both the front and rear are composed of a short upper arm, a long lower arm, and a hub carrier that connects them, and when the suspension strokes, the camber changes due to the difference in the trajectory drawn by the upper and lower arms.The optimum value of this change in camber angle was determined by simulation analysis at the time of design and actual running evaluation using a prototype vehicle that can change the geometry.

Toe control mechanism

Rear suspension toe change (when lateral force is applied)

The LWS inherits the concept of the FC3S RX-7 rear toe control suspension and Luce's E type multi-link suspension, and incorporates a toe control mechanism in a simple form.As shown in the figure, by making the hardness of the rubber bush on the hub carrier side of the lower arm close to the H shape different in the front and rear, a virtual kingpin axis that passes through the hard rubber bush on the outside of the upper arm and the rear of the lower arm is set.When lateral force is applied to the tire during cornering, the turning outer ring compresses the soft front bush more than the hard rear bush and sets the tire to face the toe-in. As I mentioned in the FC3S RX-7, at that time I thought that stable maneuverability could be obtained by making the rear toe-in.However, it turned out that the dynamic sensibility was difficult to be satisfied if there was an area where the yaw generation and the rider's sensibility did not match due to the movement of the toe, so in the subsequent second generation roadster (NB2C type) , Toe control will be changed to a setting that further reduces.

As I mentioned quickly, the bloody "Jinba Ittai" car, Roadster, was raised in this way, and in the fall of 1988, the entire company was steadily preparing for development, production, and launch. rice field.We, the developers, were confident that we made the car we wanted to drive, and we were convinced that it would be a big hit.

In addition, one of the great attractions of this roadster is the structure that allows the user to easily tune the suspension.For alignment adjustment, the suspension mounting bolts have a cam-type structure, and are designed to enable camber angle adjustment and caster expandability.

It is now possible to select the optimum alignment by yourself according to the adjustment of the tire wheel offset, tire size, and vehicle height selected by the user.This is an important element of the dynamic sensibility that I advocate, and it was also the real thrill of tuning.
And as you all know, this LWS was given the brand name Miata, MX-5, and Roadster in Japan, and it became a big hit in the United States, Europe, Japan and all over the world.

And I was appointed as the development chief for the latter two years of this first roadster.So I started to review the alignment, especially the kingpin tilt angle.The experimental team's settings emphasized low-speed agility, and chose a direction that was more sensitive and nervous at high speeds than I intended.We reviewed the Kingpin tilt angle and shook it toward higher speed stability with an emphasis on driving in Europe.We also reviewed the steering force of the steering wheel.

The first roadster continued to sell, so I didn't have to change the model to the second one.However, due to the stricter restrictions on collision requirements and the change in regulations for retractable headlights in Europe, we were forced to make a model change.As a result, we had to increase the weight to meet the collision requirements, and changed the headlights to a fixed type.

* Click to open the enlarged image.

In addition, we have started to improve the rigidity of the body and monocoque in order to polish the "Jinba Ittai" as a dynamic sensibility.As shown in the figure, as a body reinforcement common to all grades, the front and rear dimensions of the side sill inner have been expanded, and the tunnel gusset has been changed from a short skirt to a deep skirt, improving flexural rigidity by 13% and torsional rigidity by 6%.Furthermore, RS and RS16 have been given special body reinforcement.A suspension tower bar that enhances the coupling rigidity while reinforcing the suspension tower head is adopted, and a corner gusset at the rear bulkhead is added, and two rear crossbars, a truss member, a tunnel member, and a rear performance bar, are under the floor. Added.As a result, compared to the original NA22C roadster, the flexural rigidity has been significantly improved by XNUMX% and the torsional rigidity has been significantly improved by XNUMX%, further enhancing the feeling of "Jinba Ittai".

Next time, I will talk about the development of a pure sports car that incorporates the essence of dynamic Kansei engineering that I advocate for the 3rd generation RX-7 (FDXNUMXS type).