"Ability to talk with tires" is indispensable for realizing the ultimate "human-horse sense of unity".

Tires play an exceptional role not only in terms of comfort and safety, but also from the standpoint of enjoying driving.Railcars without tires cannot be driven like an extension of their own limbs.Because our car has tires, we can enjoy driving freely without being restricted by a certain rail by making good use of its performance.
However, when it comes to what kind of force works on the tires, what kind of force is generated, how it changes depending on the driving operation, and how it affects the behavior of the car, there are people who can confidently answer correctly. , Not too many.This time, let's take a closer look at the tire.If you can drive while talking with the tires by knowing the details of the tire performance that influences the subtle dynamic sensibility, not just the good or bad grip, you will have an exquisite sense of unity with the car and the fun of driving. I think it will deepen further.


■ The force that tires act on the car is finite
Needless to say, the source of the tire's function on maneuverability is the frictional force derived from the rubber material.Because it has a large frictional force that is incomparable to iron wheels, it is possible to receive the driver's driving operation and run while laying an invisible momentary rail between it and the road surface.
However, there is a limit to the frictional force of tires. Since F (friction force) = μW (coefficient of friction x wheel load), if more force is applied, the tires will not be able to hold the car to the road surface.In other words, the derailed Shinkansen state becomes uncontrollable.Therefore, we must always recognize that the frictional force of a tire is finite and try to drive with the limit in mind.To do this, you should always have the ability to infer the condition of your tires.

I want to be especially careful when cornering.As I mentioned last time, except at extremely low speeds, cars change their direction of travel and turn by obtaining centripetal force (centripetal force) that opposes centrifugal force from the tires.The cornering force is called CF (cornering force) and is generated by SA (slip angle: deviation between the direction of rotation and the direction of travel of the tire).
However, the CF also has its limits. CF that stood up vigorously in the minute state of SA stagnated as SA increased, and eventually reached its limit.Therefore, the driver must predict the condition of the tire at that time.How the tires respond to the driver's driving operations ... Without that assumption, there is no way to hope for a sense of unity between humans and horses.
So why and how does the tire CF change?To help you understand that, let's first talk about the mechanism of tires that generate CF.

■ Mechanism of CF generation
When the driver steers, the direction of rotation of the front wheels changes.Therefore, the direction of rotation of the tire attached to the rim is also forcibly changed.However, on the other hand, the tread of the tire tries to maintain its direction of travel due to friction with the road surface.Looking at the cross section of the tire, the forces from above and below are competing and pulling each other.As a result, the tread, shoulders and sidewalls are twisted and elastically deformed.The reaction force of the tire against this twist is CF.

The magnitude of the reaction force is related to the height of the profile and the rigidity of the wall material (the magnitude of the force that tries to return the deviation between the tire rotation direction and the tire traveling direction).For treads of the same area, the lower the shoulder and sidewall heights, the easier it is to ensure lateral rigidity of the tire, and a higher reaction force (that is, CF) is generated.That is why the flatness is a guide when selecting tires with an emphasis on maneuverability.
Next, let's take a closer look at the process in which CF occurs.The image is shown in the lower right figure (relationship between SA and CF).

The area where the tread surface is twisted is called the adhesive area, and the CF generated in that area increases as the amount of twist increases.

However, the tires are designed so that they cannot come off the rim, so they will eventually slip off the road and return to the direction of rotation of the wheels.This area is called the sliding area.Looking at the size of the CF generated, it is the largest at the boundary between the adhesive area and the slip area, that is, at the most twisted part.
Also note that as SA increases, the adhesive area of ​​the tread decreases.By twisting with a larger force, the deformation in the adhesive area progresses rapidly and approaches the limit.As a result, the ratio of the slip area increases, and the total amount of CF (the area of ​​the triangle formed by the distribution of Fy in the figure) does not increase as much as the increase of SA. I think you can imagine the delicate relationship between SA and CF.Therefore, there is a limit to CF.

■ Tire performance that changes depending on the situation
The figure shows a concrete example that the CF of a tire is not simply proportional to the increase of SA. It shows the tire characteristics equivalent to XNUMX / XNUMXRXNUMX.

CF in the region where SA is minute rises almost linearly, but as SA increases, CF does not increase so much.
For example, you can see that the amount of increase in CF due to the same 0 ° is different between when SA increases from 1 ° to 4 ° and when it increases from 5 ° to 1 °.
This CF increase rate is called CP (cornering power).In the previous lecture, we simply defined CF (N / deg.) Per SAXNUMX °, but it is not just CF / SA in a certain situation. It is a numerical value that indicates the momentum of CF change, that is, a value differentiated by SA.
This is the amount of CF change corresponding to SAXNUMX °, assuming that CF changes with the same momentum.Think of it as an instantaneous change, not an average change.

Even with the same tire, the CP changes depending on which point on the graph you look at. The reason CF is not proportional to SA is that CP is not constant and decreases towards the limit.The CP in each SA is shown in the figure on the right (CP characteristics of the tire).

Another thing to note is the four curves shown in the graph. CF and CP also change depending on the wheel load applied to the tire.The lower right figure (flatness and CP characteristics) shows the change in CP when SA is 1 °.

With modern tires, it grows in a gentle curve in response to the load. The region where CF increases linearly (linearly and directly proportionally) is called the "linear region", and the region beyond that is called the "non-linear region".

 

In addition, the situation where CF does not grow in the non-linear region is described as "saturating" or "saturating" in industry terms.
Both are words that are often used when discussing automotive engineering, so it is useful to remember them.
In any case, in order to drive the car in a fun and safe way, it is essential to use the CF of the tire as close to the linear region as possible.If you enter the non-linear region, the CF will be damaged from that point onward, so you must be aware of this and drive carefully.

■ CP that is directly linked to dynamic sensitivity
CP is not power. N / deg.As the unit indicates, it is an index showing "the magnitude of the reaction to the operation".How the CF of the tire changes when you step on the accelerator or turn the steering wheel in the middle of cornering ... The key is CP.
Therefore, if you drive without assuming the CP in that state, you will not be able to keep up with the increasing centrifugal force, the front and rear wheels will be out of balance and the direction of travel will be disturbed, and the sense of unity between the person and the car will be lost. .. CP is a performance that is directly linked to dynamic sensitivity.CF as a total amount and CP as a momentary change amount.I would like you to clearly understand the difference in meaning.
However, if the tire CP is not constant as described above and it does not make sense without a condition such as "SA is around XNUMX °", it is inconvenient when comparing the performance of different tires.So, with that in mind, please review the graph above again.
The CP value (CF increase rate) from 1 ° to 1 ° SA is almost constant.In the practical range, there is almost no effect of load.In addition, the CP value in this area is a numerical value that represents the potential determined by the structure and material, so using this numerical value also means comparing the performance of tires.Therefore, use the CP value in this area unless you have a particularly tedious discussion.When you say CP without notice, please think that the condition "at around XNUMX ° to XNUMX °" is always set.
Also, in this area, CF = CP × SA, and CP works as a proportionality constant, so in technical data, etc., K, which is an idiomatic expression of the proportionality constant, may be used and described as KP.Of this seminarLast lecture (§XNUMX) However, in the explanation of NSP (Neutral Steer Point), Kf = CP for front tires and Kr = CP for rear tires. It means that KP = CP (SA is around 1 ° to XNUMX °).In normal driving, SA uses a linear region from XNUMX ° to XNUMX °, so there is no big problem even if it is calculated by KP.

■ Review of steering characteristics
With the explanation so far, you should have almost understood the force that the tire acts on the car when cornering.Therefore, I will reexamine the "steering characteristics" mentioned in §XNUMX.Even if you didn't understand the previous explanation, I think you can make a point this time.

First, the figure on the right (a vehicle at a steady circle / constant speed turn) shows the state of a vehicle rotating at a constant speed on a circumference with a constant radius R.This is the case of a steady circular constant speed turn of an FF vehicle with a heavy front wheel axle weight.To make it easier to understand, the model is drawn with the center of gravity (CG) extremely biased toward the front wheels.
Since it rotates at a constant speed and maintains a constant radius, the centrifugal force applied to the front and rear tires and the CF generated by each are in balance.Centrifugal force is proportional to weight distribution, so the corresponding CF must be the inverse ratio of the distance from the center of gravity (CG).Therefore, CFf / CFr = b / a.To achieve this, the front wheels generate a larger SA (βf> βr) than the rear wheels.

In addition, the fact that the car is stable in a posture that maintains a constant rotation speed means that the yaw moment due to the CF of the front and rear wheels is balanced like a balance.The calculation formula is XNUMXCFf × a = XNUMXCFr × b, so CFf / CFr = b / a.The car is rotating at a constant speed with the CF of the front and rear wheels being the inverse ratio of the distance from the center of gravity.

In this way, CFf / CFr = b / a is the steady circular constant speed rotation.Since the car just keeps spinning, the acceleration is constant and there is no change in motion.If the weight distribution of the car is different, the CF required for the front and rear wheels and the SA that can obtain it will also be different, but there is no problem.All you have to do is keep the balance of power.All you have to do is cut the steering wheel until that happens.

The required steering angle (θ) is βf + βr including the direction of travel of the vehicle body, which is deviated by the SA of the rear wheels (because the rear wheels are fixed to the vehicle body). θ = βf + βr.As a result, the traveling direction of the front and rear tires and the angle of the vehicle body are equal (βr), and the tangents are the same circumference, so the turning radii of the front and rear wheels also match.

Next, we will examine how the car turns when increasing the speed (shifting to the acceleration state) while maintaining the steering angle from the state of steady circular constant speed rotation, so-called steering characteristics. The conclusion of §XNUMX is reprinted in the figure on the right (accelerated turning of NS characteristic vehicle volume).

In this case, there is a change in motion that accelerates (centrifugal force increases), so the rate of change in force corresponding to that change becomes an issue.How does the momentary increase in CF (CP) that occurs in the front and rear wheels affect the rotational moment around the center of gravity?
Last time, as shown in the upper right figure (accelerated turning of NS characteristic vehicle volume), I explained that the agreement between CG and NSP is a condition of NS (neutral steer) characteristics using the concept of NSP (neutral steer point).That is, δ = XNUMX.The simplified formula is as follows. (The reason why the CP of the front and rear wheels was set to Kf and Kr was mentioned in the previous section.)
Since the numerator needs to be 0 for δ = XNUMX, aKf-bKr = XNUMX.If you go further, it will be aKf = bKr → Kf / Kr = b / a.In other words, the CP of the front and rear wheels is the inverse ratio of the distance from the center of gravity.Compare with the conclusion of steady circular constant speed rotation. Only CF and CP have been swapped.You can see the position of CF, which is the total amount of force, and CP, which is the rate of change in force.

Let's continue this calculation a little more.The wheel load of the front and rear wheels changes depending on the weight distribution, but as mentioned above, it has almost no effect on KP in a minute area.If the front and rear tires are the same, Kf = Kr.Therefore, Kf / Kr = b / a = 1.In other words, a = b.So, if you don't consider other factors, a XNUMX:XNUMX weight distribution is a basic condition for a car with NS characteristics.Did you agree?

So far, we have treated it as a model without weight transfer for the sake of simplicity.However, in the actual three-dimensional four-wheel model, the load applied to the tire moves due to rolling and pitching.And if it exceeds the linear region, the change in CP due to the load cannot be ignored.

If you apply this to the inner and outer rings during cornering, it will be as shown in the figure on the right (CP and load change of the inner and outer rings).To make it easier to understand, to explain with extreme load transfer, for example, if a load transfer of XNUMX N each occurs on the inner and outer rings, the CP value of the inner ring is A point and the outer ring is B point, and the inner and outer rings are total. The average value C'of the CP generated in is lower than C (CP before weight transfer) as the potential of the tire.

Therefore, if the load transfer is large, including vehicles with NS characteristics in the basic layout, the steering characteristics will change accordingly.At the design stage of mass-produced cars, it is used to positively season the dynamic sensitivity by taking advantage of it, but when you think about tuning the suspension, the tires such as the balance before and after the roll rigidity that affects the load transfer Please do not forget the viewpoint of making the best use of.Basically, if the roll rigidity of the front wheels is increased, it will change to the underside, and if it is the rear wheels, it will change in the opposite direction.

■ "Maximum circle of forces" showing the limit as a total force
So far we have talked about the power of tires to bend a car, but in a real car, in addition to this, the tires need to have the power to move forward and the power to stop.However, the frictional force of the tire is finite.This is because there is a wall with the "maximum circle of friction", which is the limit of the frictional force with the road surface that I mentioned at the beginning.

The figure on the right (maximum circle of forces) shows the force that can be generated by the contact patch of the tire with a red vector.In the actual vehicle, the accelerator is lightly depressed and the steering wheel is turned.When the vector at this time is decomposed into a CF component and a T (traction) component, μW (generated frictional force) is XNUMX, and if the vector angle θ is XNUMX °, CF = μW × sinXNUMX = XNUMX, T. It means that = μW × cosXNUMX = XNUMX can be generated.

If the numerical values ​​of other elements are known, the available CF (Fmax) can also be calculated by the following equation using Pythagorean theorem.

The absolute value of the diameter of the maximum circle of forces is determined by the μ (coefficient of friction) of the road surface and the wheel load W. Become.Also, if you move the load by braking when entering a corner, W will increase on the front wheels and the circle of forces will expand, so you can get a large CF.However, as a matter of course, the opposite phenomenon will occur on the rear wheels, so be careful.

(* The circle of forces is generally shown as a perfect circle for ease of understanding, but the measured value is an ellipse as shown in the figure on the right.)
To understand the maximum circle of forces, I will explain it a little more concretely.If you step on the brake suddenly from the state shown in the upper right figure (maximum circle of forces), most of the grip is used in the front-rear direction, so even if you turn the steering, CF does not occur and the car does not turn (left side of the right figure: braking) And CF) state.On the contrary, even if the accelerator is fully opened, the CF component in the lateral direction is lost at once (right side of the right figure: accelerator operation and CF), and it becomes out of control.

Running while talking with the tires can be rephrased as driving while imagining the size of the maximum circle of forces of the four tires and the situation of the vector.Some people say that the right driving technique is to use the power of the tires well, like turning the vector along the circumference without pushing the limits.As a matter of course in automobile engineering, it is true both in terms of dynamic sensibilities and in terms of enjoying driving.

■ Characteristics of tires and sensibility of riders
Now it is possible to select tires with various specifications.However, it is not possible to determine which tire is better without deciding on a specific measure, such as an automotive engineering test or circuit time.

Especially from the viewpoint of dynamic sensibility, the tastes of riders vary widely, so it cannot be decided unconditionally.Performance that contributes to maneuverability, such as quick response to steering, changes due to CP size and SA, CF limit height and predictability, etc., may be a trade-off with ride comfort and wet performance.I think that what kind of tire characteristics are emphasized is meaningful only if it fits the dynamic sensibility of the rider.
If you have the opportunity to choose the next tire, it's not easy to get technical information, but you should check it online or consult a reliable shop as much as possible.Also, in my experience, I think it is important to pay attention to the difference in tires, such as when comparing with a friend's car, and to improve the sensitivity to talk with the tires.

"Cars talk about people," he says, but tires also have the power to talk about a person's personality.There must be the best tires for that person, who both self and others admit that they are truly that person.If possible, I would like you to enjoy your own tuning with such eyes, knowledge and experience.