At will, snappy! Commitment to "dynamic sensibility".

Fortunately, I've been involved in the fascinating job of developing sports cars for many years, but it's time to make a baton touch for young people after the age of fifty.A new chief was selected and the development of the third generation Roadster was about to begin.However, looking at the new concept, I thought, "I can't agree with this project as a former chief examiner."Because, at the direction of the management from Ford, the work to carry over the chassis parts of RX-XNUMX and finish it as a roadster was in progress at the upstream of development. The RX-XNUMX weighs XNUMX kg. By diverting the chassis parts of a vehicle that is XNUMX kg heavier than the XNUMX kg of the current roadster that embodies "Jinba Ittai", the lightweight sports (LWS) as intended, that is, the dynamic sensibility of "Jinba Ittai" It was clear that it would not be finished.
However, one day my boss told me, "I've decided to let Mr. Kijima be the chief of the next roadster, so thank you!" And I was very confused.Even from my age at that time, I thought that the development of the XNUMXrd generation Roadster would be my last job as an engineer at Mazda, so I didn't want to make it an unwilling work.I was worried about what to do every day.Since we carry over the main chassis parts, the development team was also assigned the minimum required development man-hours.

3rd generation Roadster (NCEC)

At that time, the kamikaze blew.The economic situation in the world has changed drastically.The exchange rate against the dollar has greatly increased, and $ XNUMX = ¥ XNUMX to ¥ XNUMX.Then, a cost reduction (C / R) requirement of XNUMX% was imposed on each program.For Mazda as a whole, a stronger yen will lead to a large loss of profits, but I think it is a great opportunity for me and for the development of the third-generation roadster. I thought about developing a new roadster exclusive part because of C / R.As mentioned above, it is a chassis part developed according to the RX-XNUMX's XNUMX kg vehicle weight, so it will be over quality for a XNUMX kg roadster.

NCEC monocoque

This will greatly impair the dynamic sensibility of "Jinba Ittai".Immediately, we started the work of completely reworking the development plan.
In the development activities, we called it "Operation Gram" to reduce the weight, and despite the increase in vehicle size, the weight of the vehicle was reduced to +XNUMX kg compared to the second generation.As for the monocoque, it was ultra-lightweight at -XNUMX kg, and we were able to finish it with a highly rigid body.It goes without saying that this greatly contributes to the good packaging (weight distribution) and the reduction of the yaw moment of inertia, which are the genealogy of successive Mazda sports cars, and is a factor that emphasizes the goodness of LWS. is not.

The chassis parts of the XNUMXrd generation Roadster look the same as the RX-XNUMX, but there are no parts that are the same no matter which one is taken, such as the suspension arm, hub, and subframe. Developed to have sufficient strength and weight for LWS.Furthermore, even with a XNUMX-speed transmission, the second generation was purchased from Toyota company A, but the price is high and the shift feeling is well suited to the dynamic sensibility that I advocate. It wasn't.Therefore, we decided to in-house a XNUMX-speed transmission that has a more dynamic sensibility.

<Front suspension>

<Rear suspension>

<XNUMX-speed transmission>

Now, this time, I would like to focus on the creation of "dynamic sensibility," which is the most important requirement for the dynamic sensibility of "Jinba Ittai."Modern car manufacturing is a tool that uses CAE technology (Computer Aided Enginennring: computer-aided design) to perform simulations such as preliminary examination, structural analysis, and stress analysis for steering stability performance of around XNUMX%. ) Can now be developed.Then, by actually making full use of benches and experimental vehicles, we will improve the commerciality such as performance, function, reliability, and dynamic sensitivity.

<Fishbone Chart> * From Mazda Technical Report

Based on the premise of utilizing this tool, an expert team called "Unification Task" was formed to unify the taste of "human-horse unity" felt from the dynamic characteristics of LWS, and the elements of the sensory area were also quantified. I continued to train.On the fishbone chart, "run, turn, stop, listen, touch", these five pillars are arranged in a well-balanced manner, shared by all development team members, and a unified taste is created to ensure "human-horse integration". I poured it into.And that taste is defined as the balance between "enjoyment" and "manipulation at will = accurate and linear" and "light maneuvering = lightness".In addition, the direction we should aim for is "light and human", and the common keyword for the taste of each performance area is "quick and light = response, linear, and accurate".

Therefore, regarding the theme of this time, it goes without saying that even if we say "dynamic feeling" in a word, various elements are intertwined to create the overall kinetic performance of the car.Among them, I will take up "Yorate gain" and "Accelerator response" as typical elements.

Last time, I explained the equation of motion in the yaw angle direction.This time, we will focus on the yaw rate gain and explain the development of the dynamic taste of the vehicle, that is, the performance development around the yaw as a dynamic feeling.The yaw rate shown in the figure below means the angular velocity in the direction of yaw rotation.In other words, the speed at which the car tries to turn.Yaw rate gain is the "magnitude of yaw angular velocity per steering angle of the steering wheel".In other words, when turning quickly with a small rudder angle, the yaw rate gain is said to be high.
In order to achieve a sense of unity in dynamic performance, we adopted the following method of setting goals. First of all, regarding the characteristics that determine the taste of handling, the figure shows the characteristics of the first roadster and EU competing A car (relationship between yaw rate gain and steering force) minus the "unity line". I set it as "Harmonic Gain Line".
Both the first and second generation roadsters had a light taste, but the first generation was a little too sensitive based on customer feedback and development evaluation results, so the second generation was in a slightly stable direction. Therefore, the XNUMXrd generation has decided to aim for a yaw rate gain = XNUMXdeg / s / deg and a steering force (SWF) = XNUMXN (Newton), which is a little more agile than the middle between the XNUMXst generation and the XNUMXnd generation.	* From Mazda Technical Report
Next, I would like to talk about another point of "dynamic feeling".Accelerator response is a typical factor in the quality of the dynamic sensitivity and transient characteristics of a car.In order to emphasize the crisp and light "running feeling" taste, it is necessary to specialize in the DNA "Lively" of the running feeling.It is a characteristic (response) that reacts quickly the moment you step on the accelerator.Similar to the relationship with the steering force above, the yaw rate gain was set to XNUMX in the "unity line", so the target value of ΔG (change in acceleration per unit time) is XNUMXG / It becomes sec.	* From Mazda Technical Report
This is not only the responsiveness of the engine itself, but also the linear response performance of acceleration and deceleration in the entire drivetrain system from the throttle pedal → engine → drivetrain → tires, and the definition of response is "the moment when the accelerator is depressed". The responsiveness of the acceleration generated in the engine is good.Three factors are important: the time from the moment the accelerator is stepped on until the acceleration G rises (TG), the amount of change in acceleration (ΔG), and the average value of the rise of G (G_Ave).As a matter of course, the response is good or bad by shortening the time for the acceleration G to rise after stepping on the accelerator and raising the rising acceleration at once.	* From Mazda Technical Report

As mentioned above, modern cars are CAE technology, and development can be simulated to a considerable extent.Similarly, for drivetrain response development, the digital model is input to the computer and the rigidity of each part is changed to calculate and select the optimum response.Specifically, the rigidity of the engine mount, the twist of the propeller shaft, the windup of the differential, the twist of the drive shaft, the rigidity including the bush of the suspension, the rigidity of the tire, etc.

<Analysis data by CAE technology> * From Mazda Technical Report

As a result of such CAE development, the size of the propeller shaft has been increased from φXNUMX to φXNUMX, and the drive shaft has a hollow structure that achieves both high torsional rigidity of XNUMX Nm / deg and weight reduction, improving drivetrain transmission performance. Higher response performance has been obtained.

We also pursued Linear in order to realize "Jinba Ittai".In order to achieve the desired acceleration, it is important to have torque characteristics that allow the engine to be used up without stress from low speeds to high speeds.In this new model, the intake system has been tuned in detail such as air-fuel ratio control, aiming for the best output characteristics by suppressing torque valleys and losses as much as possible.

<Comparison when accelerating at full throttle in XNUMXrd gear> * From Mazda Technical Report

Since the acceleration of the XNUMXnd generation Roadster (BP type engine) drops sharply at high rpm, it feels like it has reached a plateau well before the red zone.Therefore, in the new model, we aimed to keep the flat acceleration until the end like the first roadster (BXNUMX type engine) and to enjoy using up the engine.In addition, by increasing the displacement to XNUMX liters of the MZR type, the drop in acceleration and the absolute value, which were the drawbacks of the second generation, have been improved, and the characteristics of extending flat to the red zone of the first generation have been created.With the new model, as shown in the figure, we were able to achieve engine output characteristics that are extremely dynamic and have excellent transient characteristics.

As an aside, it was when I reviewed it with North American officials during the development process. "Why is the engine a XNUMXps MZR type? XNUMXps is absolutely necessary to be successful in North America!"Therefore, we decided to conduct a performance comparison test with a XNUMX-seater open car (XNUMX kg) that has a XNUMX cc variable valve timing mechanism from competing company H.The test situation was set to full throttle acceleration of XNUMX mph (XNUMX km / h) → XNUMX mph (XNUMX km / h) in XNUMXrd speed, assuming the case of merging to the main line from the entrance of the highway (expressway).In this situation, the new Roadster reached XNUMXmph earlier than the competing vehicle, as the variable valve timing mechanism had not yet begun to work on the high output side.This fact convinced all North American officials.You can see that the quality of the transient characteristics of the vehicle weight and engine torque from low speed to high speed is not just the numerical value in the catalog, but the point to discuss the potential as a sports car, especially LWS.
In this way, the new roadster was introduced to the world by Mazda's meticulously inherited sports car development philosophy.