A basic tuning recipe to enjoy the "ride" of the suspension.

This time, I would like to think about the specific specifications of the suspension.It is generally said that the point of automobile engineering is the balance between "ride comfort" and "maneuverability", but if the same thing is expressed in Kansei engineering, it means the creation of "ride taste". ..It does not mean that the performance as a car is good or bad, but that it is trying to create a "taste" that seems to be delicious from the viewpoint of the sensibility of the rider.The main ingredients that are the target of the recipe are the vehicle height (stroke), spring rate, and damper damping force.How will the ride quality change if you change the way they are cooked? That is the theme of this time.First of all, as usual, we will start with general automobile engineering, but if you do not understand the basics, it will not be applicable to tuning, so please keep in touch.

XNUMX. XNUMX.What are the conditions for setting the vehicle height?

For mass-produced vehicles, there is a large margin in the vehicle height in consideration of ride quality and chain mounting.However, in modern road environments, I think that percentage is often overkill.The lower the styling, the cooler it is, and the lower center of gravity is effective for maneuverability.Therefore, the lower the vehicle height, the better.There are some conditions, but if you lower the vehicle height, the roll moment will decrease and the roll amount will also decrease.If the amount of roll is reduced, the stability of the car will increase when turning, and the ride will be solid.On the contrary, if you raise the vehicle height, the roll moment will increase, the amount of roll will increase, the driver's anxiety will increase, and the ride will feel sloppy and slow.The same applies to the pitching direction.Therefore, the vehicle height should be as low as possible, if practical usability allows.

However, if the vehicle height is lowered, the stroke of the tires and suspension arm (distance that can move up and down) cannot be taken sufficiently, and it becomes easier to contact the vehicle body.Given the rough roads and high-speed cornering, such a situation is absolutely unacceptable.Therefore, here comes the limit for setting the vehicle height low, or the conditions for that.It is to limit the distance that the arm can move shortly ... that is, the spring needs to be strengthened accordingly.Then, the specifications of the corresponding damper will also change.The changes due to individual settings will be described later, but thus changes in vehicle height affect the specifications of the entire suspension and further change its ride quality.Therefore, I think that setting the vehicle height should be considered as part of the basic design of the vehicle.When tuning a mass-produced vehicle, it is important to consider these relationships properly.Excessive lowdown leads to insufficient stroke and has the potential to deteriorate both ride comfort and maneuverability.Especially when the vehicle height drops by -40mm or more, it is difficult to secure the stroke on the compression side, so unless you use a very hard spring, you may end up running with a lot of shrinkage of the bump stopper, which is the final safety device. Hmm.I hear that there is a method of cutting the bump stopper to secure the stroke, but it is probably out of the question.There is also the possibility that the piston inside the damper will bottom out.I said earlier that "the height of the car should be as low as possible", but in the tuning of mass-produced cars, unless the whole is specially redesigned, it is "taken on the safety side of mass-produced cars". In principle, it should be done within the range of the margin.


XNUMX. XNUMX.What are the factors that determine the spring rate?

The basic job of a spring is to keep the body weight (1G) on the static spring at the target vehicle height.On top of that, it is important to absorb vibrations from the road surface, ride comfort, movement in the pitch direction in the running state, that is, bump during braking, squat during acceleration, and the ride quality of the car in the left and right rolls. It is a part that plays a key role.Its performance is determined by the force required to shrink the spring by 1 mm, the spring rate (spring constant), which is generally expressed as hardness.
At the time of design, as a common sense in automobile engineering, the spring rate is calculated assuming that the maximum load increases by 90G.In the worst case, that much input is possible.For example, if the stroke of the suspension arm is XNUMXmm on the bump side, the force of the spring and bump stopper will prevent it from moving any further.The bump stopper is an elastic body made of rubber or resin, and it is a part that regulates the stroke by becoming a completely rigid body at the end while leaving the minimum elasticity even when the spring can not handle it.Ideally, there should be little change in hardness when moving from the area of ​​the spring to the area in charge of the stopper.

So, it is a concrete calculation. One wheel load 250G (1250kg when one wheel spring load is 0.5kg) is a setting of an emergency, so roughly, 4.5G is the spring and 306G is the bump stopper.For example, if the load on one wheel is 90 kg and the stroke until the spring hits the stopper in the bump stroke of 60 mm is XNUMX mm, the required spring rate can be calculated as follows.

The load carried by the spring is
"306kg x 0.5G = 153kg x 9.8m / s2 = 1450 (N)"
So if you divide 1450N by a stroke of 60mm
It will be "1450 ÷ 60 = 24.2 (N / mm)".
■N = Newton: A unit of force converted by multiplying kg, which is a unit of weight, by gravitational acceleration G.

When actually tuning the spring rate, there is no basic problem as long as it is greater than or equal to the value obtained in the above calculation.For the driver's sensibility, those who like a hard ride should make it hard, and those who do the opposite should make it soft.In the example of a sports car, the front wheel spring rate is 34.2N / mm in mass production specifications, but if you increase it by 11% to 38N / mm, the basic character does not change much, but it is sporty. Will increase and the ride will be harder.Furthermore, when it is raised to 108N / mm (316% of mass production), you can feel a considerable sense of rigidity, and the ride will be like no roll.
However, if a stiff spring is used without a stroke, the ride quality will be poor due to the lack of supple spring characteristics, and sudden load transfer will occur, which will also tend to impair steering stability.Driving on rough roads can be dangerous as your car may jump.Therefore, it is necessary to take appropriate measures with a total design such as how to take strokes and combinations of dampers.I think that tuning with an extremely high spring rate should be strictly avoided.As a guideline for tuning, it is a value that considers the balance between steering stability and ride quality, that is, the hardness that can realize the desired "ride taste", but it is difficult to guess, so carefully consider it in the tuner catalog etc. Please check the image after mounting.


3. XNUMX.What is the factor that determines the damping force of the damper?

In parallel with the spring rate, the damping force of the damper must also be determined.When a spring starts to vibrate, it has the property of continuing to vibrate at a constant frequency unless an external force is applied.Therefore, if the car body begins to shake, it will continue to vibrate indefinitely.It is the role of the damper to properly converge this vibration.The resistance generated when viscous oil passes through the orifice of the damper piston converts the kinetic energy of the spring into heat energy, which is then cooled and released into the atmosphere.In short, the damper slows down the movement of the spring, so the effect can be used not only for suppressing vibration but also for controlling the speed of rolls and pitches.Focusing on these two points, you can see that the character of the damper has a great influence on the ride quality of the car.The figure below shows the performance curve of the damper.The vertical axis is the damping force N (Newton), and the horizontal axis is the piston speed (m / sec).

If you look at this, you can see the character of the damper at a glance, but how should you set this damping force value to create the ride quality of each car?Since the weight, spring, and purpose of use are different, it is meaningless to simply compare the numerical values ​​of the damping force.
So, first of all, it is basic automobile engineering.It is known that once the mass on the spring and the spring rate are determined, the critical damping force of the vehicle when the piston speed of the damper is 1 m / sec is theoretically determined.The following is the calculation formula.


■k = sum of spring rates m = mass on spring

Think of this critical damping force as the critical value (the last-minute value) that determines whether or not to vibrate.If you set this damping force, it will be possible to converge the vibration in the shortest time, but the ride quality of the car will be very hard and hard, and it will not be usable.Therefore, in the actual design, one guideline is how much (damping ratio) the damping force should be below this critical value.

From here on, it's a bit confusing.Since the damping force of the damper is almost proportional to the piston speed except for the very low speed range, it can be considered that the damping force = C (proportional constant) x V (piston speed).

This proportionality constant is called the damping coefficient.Applying the above critical damping force to this equation reveals the following relationship.

In other words, although the unit is different, the critical damping force = critical damping coefficient.
Therefore, the critical damping force of the car can be represented by a straight line like the dotted line in the figure.The ratio of this value to the actual damper damping force (solid line in the figure) is the damping ratio.Once this is obtained, it is possible to compare the emotional data of the hardness of the damper for the car, rather than simply comparing the absolute value of the damping force.Imagine the relationship in the figure on the right.

Next, let's take a concrete example of a sports car with the following specifications.In order to simplify the calculation, I made a trial calculation only for the front wheels here.

　　・ Weight on front wheel spring = 6017 (N)
　　・ Fr spring rate = 34.2 (N / mm)　
　　・ Fr rebound damping force = 1630 (N)
　　・ Fr compression damping force = 1330 (N)　
　　■Damping force = 0.3m / sec　

First is the calculation of the critical damping force.
　　
　　　■The spring rate is converted from / mm unit to / m in order to align the unit system.

Next, calculate the critical damping force of the damper at 0.3 m / sec.It's a simple proportional calculation.

　　　Critical damping force at 0.3 m / sec = 40574 x 0.3 = 12172

Next is the calculation of the damping force of the damper at 0.3 m / sec.
Since it is the sum of the two left and right wheels, multiply the sum of the rebound side and the compression side by 2.

　　　Damping force at 0.3m / sec = (1630 + 1330) x 2 = 5920

Therefore, the attenuation ratio = 5920 ÷ 12172 = 0.49 = 49%.

Then, how should we evaluate this attenuation ratio of 49% (at 0.3 m / sec)?By the way, it is said that around 40% is good for passenger cars, but this car was called on-the-rail feeling because of its handling performance.The ride is very sharp and is completely different from the fluffy ride of a family car with a damping ratio of about 30%.I think this is a good example of the relationship between the desired ride quality and the damping ratio.
By repeating the above calculation for each piston speed, the change in the damping ratio of the damper can be seen.The figure on the right is an example.By doing so far, I think that the difference in the individuality of the damper will become more and more noticeable.

For tuning mass-produced cars, detailed specifications may not be available, but damper performance may be announced, so if you are interested, please make a trial calculation.It should be a clue to know the degree of change in ride quality.


XNUMX.Damper characteristics settings such as expansion / contraction side, low speed range / high speed range, etc.

After deriving a rough damping force value, the next step is to season the rebound (extension side) and compression (contraction side) with different damping forces.This is because it is necessary to suppress the effects of "pushing up" and "harshness" caused by compression.Therefore, the total damping force value is not changed, and the contraction side is soft and the expansion side is hard.For example, the total damping force at a piston speed of 0.1 m / sec is 1500 N, which is 500 N on the extension side and XNUMX N on the contraction side.The reason why the sum is not changed is to maintain the energy absorption of the amplitude of expansion and contraction.If only the damping force on the contraction side is suppressed in order to avoid riding comfort and the feeling of pushing up, the absolute value of the damping force will decrease, the damping ratio will also decrease, and the total amount of energy absorption during rolling and pitching will also decrease. It will end up.

Another important seasoning is the change in characteristics due to piston speed.The key point for the initial response of cornering and S-shaped turning is the damping force in the low speed range of 0.1 m / sec or less.Therefore, if the damping force in the 0 to 0.1 m / sec range of the damper performance diagram is suddenly increased, it will have a favorable effect on the initial responsiveness of steering.When the steering is turned on, the damping force of the damper works with the jiwa in the initial stage of the roll, and it becomes a linear reaction to the input steering angle, and the yaw and the roll are connected well.
Sensitivity evaluation is important in areas where piston speed is high.The range of piston speed of 0.3 to 0.6 m / sec is the piston speed that is not often used in daily life, such as when falling into a hole, but this evaluation confirms the characteristics of push-up feeling and energy absorption.The point of judgment is how much the feeling of pushing up is suppressed.
In tuning mass-produced vehicles, it is difficult to evaluate the damper alone, but in areas where the piston speed is high, I think it is better not to make the damper unnecessarily hard.Especially when the spring rate is extremely increased, soften it so that the damping ratio does not become too low.Tuning that corrects the feeling of pushing up due to the increase in spring rate by reducing the damping force of the damper is also worth the taste as a solid ride with high roll rigidity depending on the overall balance.
When changing the damper of a mass-produced vehicle, it is important to examine the concept of each tuner by adding such fine seasoning and saltiness, not just the magnitude of the damping force.


XNUMX.If I were to tune the suspension for myself

Automakers carefully determine the specifications of sports cars so that they do not interfere with the usability of diverse users around the world.That's why car enthusiasts need a way to tune to their liking.The Mazda sports car that I developed has many options in the aftermarket.However, it is also true that some of them, as developers, make you want to get angry.Which one to choose is discussed with your own purpose and wallet, but if you ask the developer, I would like you to enjoy tuning that makes the best use of the DNA of the base vehicle.

If it is in the center of the street, the spring should be kept at 110-130% of the standard specification, and the damping force should be reduced to about 85% by increasing the spring rate, and the damping force should be 115% + α on the rebound side. Set the attenuation ratio to about 50% without the total sum of the above falling below the standard specifications.Furthermore, considering the improvement of maneuverability and practicality by lowering the center of gravity, we think that the best vehicle height is around -15mm.Some adjustments are, of course, left to the individual owner's choice.

Actually, the other day I had the opportunity to evaluate such specifications with an actual vehicle.I tried street tuning of Mazda sports cars, including the ones I developed myself (usually used on a daily basis, but occasionally tuned to enjoy driving on the highway cruising).
With the cooperation of Auto Exe, several types of suspensions with specifications close to the above specifications were prototyped and brought to the test course, and the height-adjustable suspension kit was set by repeating the test for a full two days. Is secured, it rolls in a jerky manner, and it is possible to perform firm and stable cornering while maintaining a weak understeer.The steering and yaw generation can be felt linearly, there is a firm feeling, the ride quality is secured to the last, and I think that it is a setting that does not have a feeling of pushing up that makes a tingling sensation in the belly.

In addition, I think that the movement of the car was able to deepen the dynamic sensibility and the sense of unity between humans and horses with a ride that met the expectations of people who wanted to enjoy driving more positively.It is said that it will be unveiled as a new product of Auto Exe in the near future, so I hope everyone can experience the "ride".