Bentley in the pits at the 1930's 24 hours of Le Mans Race [1]

Why Does A Tyre Grip The Road? Part 6 - Grip & Tyre FAQ

By Trav Mays

Here I have compiled a list of common grip and tyre questions, you can jump to any of the questions by simply clicking on the question in the table of contents. If you are in a rush check out the quick reference table at the top, it might have what you are looking for.

Within the answers, whenever I am talking about what happens if you increase of decrease something, for example increasing inflation pressure, I am assuming you are starting for a reasonable base, going too far in either direction will have negative effects on your handling. There is a fine balance, one that changes depending on temperatures, tyre wear, driver etc, optimum settings will require testing and diligent note taking.

If you have any of your own questions, feel free to ask them in the comments or email them over and I'll answer them and update the list as I get time.

Table of Contents

General

- What is a tyre made from?

- What is a contact patch?

- What creates grip?

- What is the grip equation?

- What is a tyre’s slip ratio?

- What is a tyre’s slip angle?

- What effect does the tread of a tyre have on grip?

- What effect does tyre compound have on grip?

- Why do wider tyres create more grip?

- What is a tyre’s load sensitivity?

- What is a tyre’s lateral stiffness?

- What is a tyre’s cornering stiffness?

- What is a tyre’s overturning torque?

- What is self aligning torque?

- What causes a tyre to wear?

- What causes a tyre to blister?

- What causes a tyre to grain?

Wet Weather

- Why is there less grip in the rain?

- What causes a tyre to aquaplane (hydroplane)?

- How to reduce aquaplaning (hydroplaning) and increase grip?

Things that affect grip

- What effect does a tyre's temperature have on grip?

- What effect does speed have on grip?

- Why does increasing downforce create more grip?

- What effect does ambient and track temp have on grip?

Inflation Pressure

- What impact does inflation pressure have on Grip?

- What impact does inflation pressure have on the Contact Patch?

- What impact does inflation pressure have on the Lateral Stiffness?

- What impact does inflation pressure have on the Cornering Stiffness?

- What impact does inflation pressure have on the Overturning Torque?

- What impact does inflation pressure have on the Self Aligning Torque?

- What impact does inflation pressure have on the Relaxation Length?

Vertical Load

- What impact does vertical load have on Grip?

- What impact does vertical load have on the Contact patch?

- What impact does vertical load have on the Lateral Stiffness?

- What impact does vertical load have on the Cornering Stiffness?

- What impact does vertical load have on the Overturning torque?

- What impact does vertical load have on the Self Aligning Torque?

- What impact does vertical load have on the Relaxation Length?

Rim Width

- What impact does rim width have on Grip?

- What impact does rim width have on the Contact patch?

- What impact does rim width have on the Lateral Stiffness?

- What impact does rim width have on the Cornering Stiffness?

- What impact does rim width have on the Overturning torque?

- What impact does rim width have on the Self Aligning Torque?

- What impact does rim width have on the Relaxation Length?

General

What is a tyre made from?

Tyres are made from a combination of materials, the exact combination is tyre specific, but the thing they all have in common and is of most importance for a tyre’s gripping ability, is vulcanised rubber.

Rubber in its pure form is far too soft to be used for tyres, the harsh conditions would mean they wouldn’t last very long. Vulcanisation of the rubber causes the polymer strands within it to attach to one another, it does this by connecting sulphur between the polymer stands, these connections are called sulphur bridges. The sulphur bridges restrict the movement of the strands, hardening it and reducing the deformation of the rubber, making it an excellent material for tyres.

Click here if you’d like to learn more.

What is a contact patch?

A contact patch is the name given to the area of the tyre that is in contact with the ground.

Click here if you’d like to learn more.

What creates grip?

Grip is created in two main ways, molecular adhesion and hysteresis. It is also created in another way, through wearing of the rubber, but current theories have these two creating the vast majority of it.

Molecular adhesion creates grip through a process called Van Der Waals bonds, where the polymer strands stick to the road surface. This type of grip is created in dry smooth sections of the track and typically at lower speeds. Hysteresis on the other hand creates grip in the rougher sections of the track and is dominant at faster speeds. Rubber is what’s known as a visco-elastic material, it is both viscous and elastic depending on the circumstances and conditions. You can think of a tyre’s rubber flowing over the holes and bumps in the track. As the rubber hits the bumps it creates energy within the tyre, this energy is then released as the tyre flows into the next hole, however not all of the energy is able to be released due to internal friction. This difference in energy creates a horizontal force which opposes the tyre’s slippage, it creates the grip.

Within a tyre’s contact patch, both molecular adhesion and Hysteresis are simultaneously creating grip. The proportion of each depends on the tyre itself, the smoothness of the road, speed, and a whole host of other variables.

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What is the grip equation?

The equation typically used to approximate a tyre’s grip is the same as the friction equation

Where: F = friction force
μ = coefficient of friction
Z = normal force

This equation is a simple way to think about the amount of friction force being generated, but it isn’t correct. As you probably know, this equation assumes that despite the size of the contact area there is no change in friction force. That would mean a bigger tyre doesn’t create more grip but as we all know, that isn’t correct. The above equation also says that if we doubled the normal force we would double the friction force, but again this isn’t true, we have tyre sensitivity which means we do get more grip from higher normal forces, but the rate with which they go up is not 1 to 1. It’s a great starting point, but is far from the correct equation.

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What is a tyre’s slip ratio?

A tyre’s slip ratio is the percentage of the tyre that is slipping in the longitudinal direction at that moment. It is an important consideration, as slippage is what grip is opposing, there is therefore a slip ratio that maximises grip. Knowing what slip ratio it is and designing the car to operate in this region is extremely important. The formula for the slip ratio is

Where: G = Slippage rate
ω= Angular speed of the wheel
R = Rolling radius, combined (ωR) they are the rolling speed of the tyre, and
V = Vehicle's speed

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What is a tyre’s slip angle?

A tyre’s slip angle is the angle between the direction the car is travelling and the direction the tyre is pointing. When we turn the steering wheel to navigate a corner, we point the tyre further into the corner than we travel, the difference between the two is due to the tyre deforming within the contact patch.

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What effect does the tread of a tyre have on grip?

A road tyre is expected to drive in all conditions, it therefore needs tread, or grooves in the rubber to expel the water from between it and the road, reducing the chances of hydroplaning and increasing grip in the wet.

Race teams on the other hand usually have a range of tyres they can use, they are able to pick the best one for the conditions. In some series only one tyre is allowed, when that is the case, the allowed tyre will look more like a road tyre, it will be have a tread pattern. Race teams use tyres with no grooves (slicks) in the dry to maximise grip, they are getting as much of the rubber touching the road as possible, and wet tyres in the rain with grooves to expel the water, again reducing the chances of hydroplaning and increasing grip.

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What effect does tyre compound have on grip?

Softer compounds allow the tyre to be more viscous, this means it is able to flow into the tiny valleys in the road better increasing the hysteresis grip mechanisms. It also helps to ensure as much of the rubber is touching the tarmac as possible, more rubber touching the tarmac will increase the other main grip mechanism, molecular adhesion.

There is however a downside, whilst you will have more grip from a softer compound, the tyre will wear out quicker, that’s why in race series such as F1, they have a soft, intermediate, and hard tyre. They can do less pit stops with the hard tyre, but at the detriment to lap time, whilst the opposite is true for the soft compound.

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Why do wider tyres create more grip?

This is a good question and one that I can’t answer satisfactorily. If we just look at the tyre and ignore all of the other car effects, we have a couple of things going on. The first is that we have increased the size of the contact patch, we have therefore increased the chances that the different grip mechanics have found their optimum roughness level. The other major thing is that wider tyres usually have a higher cornering stiffness, they are able to produce more lateral force for a given slip angle. They are therefore creating more grip for a given corner with the same slip angle. This means we could take the same corner with a lower slip angle. The lower slip angles also mean low temperatures, we can therefore run slightly softer compounds and we are producing less induced drag, unlocking some traction currently being wasted.

These are my understandings of why we have more grip with wider tyres, but as you can see, what is going on between the tyre and the ground is not fully understood yet. The simple grip equation would state that there is no difference in grip, it is down to the vertical force alone, as we know, this isn’t true.

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What is a tyre’s load sensitivity?

The amount of grip a tyre generates is influenced heavily by the vertical load placed on it, more load more grip, see the grip equation above. Whilst we get more grip, the relationship, despite what the equation may make you think, isn’t linear, there is decreasing returns for increasing loads. A great way to visualise this is with the lateral force coefficient, which normalises the lateral force by dividing it by the vertical load. If you have a look at the graph in this post you will see that increasing the load decreases the lateral force coefficient.

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What is a tyre’s lateral stiffness?

Lateral stiffness is a tyre’s ability to hold shape whilst being subjected to a lateral force, by tyre I mean the rubber, belt, carcass, the whole thing. A super high lateral stiffness, will cause the tyre to breakaway (lose traction) quicker, whereas a low lateral stiffness will make the tyre deform too much.

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What is a tyre’s cornering stiffness?

A tyre’s cornering stiffness is the linear portion of the lateral force vs slip angle graph. A steeper curve (high cornering stiffness) will allow you to generate high lateral forces from a low slip angle, but it will mean the grip is a lot more sensitive, small changes in the slip angle will change the amount of grip dramatically. As you can imagine race tyres have a high cornering stiffness, whereas road tyre’s have a lower cornering stiffness, they require more slip angle to generate the same lateral force, but will be a lot easier to drive.

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What is a tyre’s overturning torque?

When a tyre has a lateral force placed on it, the contact patch shifts across in the direction of the lateral force and with it the resultant force, the upward force opposing the vertical load. The distance between the shifted resultant force and the centre of the rim is the overturning torque.

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What is self aligning torque?

Self aligning torque is the name given to the tyre’s desire to be pointed in the direction of travel. It is created by two things, a mechanical trail, castor angle and kingpin inclination, and a pneumatic trail, deformation of the tyre. When a tyre experiences a lateral force, the contact patch deforms, this moves the resulting force away from the centerline of the wheel and creates the self aligning torque. The mechanical trail is essentially static, whilst the pneumatic trail is dynamic, it changes with the slip angle and vertical load.

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What causes a tyre to blister?

A tyre blisters when it has been overheated, the overheating of the rubber causes the sulphur bonds to break down, it goes back into its rubbery state. The overheating occurs where the rubber meets the belts. The rubber, now softer, is able to be easily torn away from the belt.

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What causes a tyre to grain?

Graining, like blistering, is caused by overheating of the tyre. The high temperatures cause parts of the rubber to be worn away, similar to what you see when you use an eraser. These rubber bits are then restuck to the tyre due to the high temperatures.

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Wet Weather

Why is there less grip in the rain?

There is less grip because water is forming a barrier between the road and the tyre. This barrier stops molecular adhesion, the tyre is therefore only able to generate grip through one of its main grip mechanisms, hysteresis and this grip mechanisms can also be reduced if the water level is high enough.

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What causes a tyre to aquaplane (hydroplane)?

As a tyre travels through the water on a road, it creates a bow wave at the front, this bow wave creates an upward pressure on the tyre. If the bow wave’s pressure equals or exceeds that of the pressure the tyre is putting into the ground, then the tyre will be picked up, that tyre will be hydroplaning.

How to reduce aquaplaning (hydroplaning) and increase grip?

The best thing is to run wet tyres, wet tyres have grooves cut into them to dispel the water, reducing the size of the bow wave and the amount of water between the tyre and the road, increasing grip.

The next thing to do is to increase the tyre pressures, this reduces the contact patch size, which increases the pressure the tyre is pushing into the ground, see equation below.

After swapping to wet tyres and increasing pressures, if you still are struggling and you have fresh rubber on, you can reduce the stiffness of the sway bars. This allows the car to roll more, reducing the lateral force being put into the tyres and reducing the likelihood the tyres will breakaway. Depending on which you are struggling from, oversteer, understeer, or a combo of both, you might want to just reduce one and see how you go, especially if the track could potentially dry out during the race.

Where: F = Force
A = Area

If you are still struggling, you could try changing other suspension settings to stop breakaway, but if nothing is working, the next thing to do is slow down. Not very practical in a race, but slowing down a little more will reduce the size of the bow wave, increase the effectiveness of the tread, and through these two increase grip.

Things that affect grip

What effect does a tyre's temperature have on grip?

Tyres are designed to work within a temperature band, known as the zone of maximum hysteresis. Within this temperature band the tyre is able to create its maximum grip level. Heating the tyre up enough that it moves out of this zone will make the tyre too rubbery. Whereas running a tyre that is below this zone, by not warming it up correctly or selecting the wrong tyre, will cause it to be too brittle. Running the right tyre for the conditions combined with proper tyre management is paramount to maximising grip. The temperature band is partly the reason why we have tyres designed specifically for summer, winter, and rain.

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What effect does speed have on grip?

Tyres have a stress frequency that they work best in, going too fast for a tyre or too slow will mean the tyre is not operating within this ban, and therefore grip will be reduced. If the stress frequency is high enough, the polymer strands become immobile, morphing the rubber into a glassy state. The polymer strands also have less time to attach to the road surface, reducing the likelihood and strength of the bond; molecular adhesion is losing its power. If however the frequency is too low, the polymer strands will move too freely, morphing the rubber into a rubbery state.

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Why does increasing downforce create more grip?

Increasing downforce creates more grip by increasing the normal force, as the equation for grip is the coefficient of friction multiplied by the normal force, any increase in it will increase grip.

Downforce is especially good because it is almost free, you get all of the added grip that increasing the weight of the vehicle would give you, but you don’t have any of the negative effects increased weight have. It is almost free, you do pay somewhat in acceleration and top speed through increased drag, but the cost is nothing like a heavier car.

If you’d like to learn more, check the first drop down in this post.

What effect does ambient and track temp have on grip?

Increases in either or both the ambient and track temp will increase the chances the tyre will run too hot, pushing it into the rubbery state. They will also increase the pressure within the tyre, the pressure you set it at (cold temp) will need to be adjusted for the increases in the temperatures. Higher ambient and track temps reduces the temperature differential between them and the tyre, which slows down the cooling of the tyre.

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Inflation Pressure

What impact does inflation pressure have on Grip?

Inflation pressures are one of the most important things when it comes to grip, getting them correct for a given rim and tyre package, a given temperature, a given track roughness, is paramount to optimising grip. Diligent note taking and testing are your best friends when it comes to finding the correct pressures.

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What impact does inflation pressure have on the Contact Patch?

Over inflating a tyre reduces the size of the contact patch in both directions, length and width. Increasing the inflation pressure can be good for when you have a heavy load on, you want to reduce the chances of aquaplaning, or if you have a side of the vehicle that will experience a lot more vertical load, a banked oval track for example.

Under inflating the tyre also reduces the contact patch, but this time instead of making the tyre skinnier, the centre retracts upwards towards the rim, the stress distribution becomes concentrated on the edges of the contact patch. The centre will most likely still touch the ground, but you will have a non uniform distribution of the load, a large portion of it will be borne by the edge of the tyres.

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What impact does inflation pressure have on the Lateral Stiffness?

An increase in inflation pressures will increase the lateral stiffness of the tyre, the higher pressure makes it harder for the tyre to deform. Decreasing the pressure does the opposite, it will deform under less force, or more for a given force.

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What impact does inflation pressure have on the Cornering Stiffness?

Increasing the inflation pressure increases the lateral stiffness of a tyre for high vertical loads, if we refer to the brush model equations in this post, we can see that a higher lateral stiffness increases our lateral force for a given slip angle. Our lateral force vs slip angle graph has now gotten steeper, we therefore have a higher cornering stiffness.

However, if we have low loads, the higher inflation pressures will decrease the length of the contact patch. If we again refer to the equations in this post, we can see that the lateral force for a given slip angle is dependent on the lateral force, slip angle, and contact patch length. At low vertical loads, whilst the tyre will have a higher lateral stiffness, the reduction in contact patch length will dominate, lowering our lateral force for a given slip angle and with it our cornering stiffness.

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What impact does inflation pressure have on the Overturning Torque?

The amount of overturning torque being developed for a given vertical load is very dependent on the lateral stiffness. Increasing the pressure will increase the lateral stiffness, the tyre will deform less, there will therefore be less overturning torque. The opposite is true for less tyre pressure.

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What impact does inflation pressure have on the Self Aligning Torque?

Increasing inflation pressures results in a shorter contact patch length, this reduces the self aligning torque of the tyre. If you have a look at the equations in this post, we can see that the equation for self aligning torque has half of the contact patch length cubed in it. Any decrease in the contact patch length will have a large effect on the self aligning torque. The same can be said for decreasing pressures, which along with the longer contact patch will increase the self aligning torque.

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What impact does inflation pressure have on the Lateral Stiffness?

A decrease in the inflation pressures will result in a disproportionate increase in the lateral and cornering stiffness. The Lateral stiffness increases more and over all vertical loads, if we have a look at the equations in this post, we can see that a disproportionate increase in the lateral stiffness over the cornering stiffness will lower the relaxation length. The opposite is true if we increase inflation pressures.

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Vertical loads

What impact does vertical load have on Grip?

Increasing the vertical load will increase grip, it won’t be a direct correlation due to tyre load sensitivity, but it will increase it.

What impact does vertical load have on the Contact patch?

Increasing the vertical load increases the length and width of the contact patch, changing it from an ellipse to a more rectangle shape. It also moves the point of maximum stress from the centre to the outside edges. The opposite is true for reducing vertical loads, the contact patch will become smaller, the tyre will now be over inflated for this vertical load.

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What impact does vertical load have on the Lateral Stiffness?

Increased vertical load on a tyre increases the length of the contact patch. The lateral stiffness of a tyre is directly related to the contact patch length, increasing it increases the lateral stiffness, decreasing it reduces the lateral stiffness.

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What impact does vertical load have on the Cornering Stiffness?

Increasing vertical loads on a tyre increases the length of the contact patch. The cornering stiffness of a tyre is directly related to the contact patch length, increasing it increases the cornering stiffness, decreasing it reduces the cornering stiffness.

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What impact does vertical load have on the Overturning Torque?

Increasing loads increase the overturning torque. As the vertical load is increased, the tyre deforms more, this deformation moves the resulting force further from the centre of the wheel, and with it increases the overturning torque. Under low loads the overturning torque is positive, it creates a torque that opposes the torque being created by the lateral force.

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What impact does vertical load have on the Self Aligning Torque?

Self aligning torque increases with increasing vertical loads. The higher vertical load creates a longer contact patch, the self aligning torque is directly related to the contact patch length. Increasing it increases the self aligning torque, decreasing it decreases it.

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What impact does vertical load have on the Self Aligning Torque?

Increasing the vertical load on a tyre reduces the relaxation lengths.

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Rim Width

What impact does rim width have on the Grip?

Correctly sizing a rim to a tyre is paramount to optimising grip, too small and you lower all of the stiffnesses, making the tyre deform too much, too large and again the tyre is in a weak position, allowing the force to deform it too much.

What impact does rim width have on the Contact patch?

Increasing the rim width for a given sized tyre results in a more squared, slightly wider, but shorter contact patch. The wider rim pushes the tyre’s sidewalls further out, the wider and more squared edges results in a shorter length needed for a given force (vertical laod) (P= F x (LxW)), thus the contact patch length reduces.

What impact does rim width have on the Lateral Stiffness?

The pushing out of the side walls leads to a higher lateral stiffness, the tyre sidewall is now in a better position to support the lateral forces, it deforms less. If however you go too far, the sidewall moves back to a weakened position, allowing the tyre to again deform too much.

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What impact does rim width have on the Cornering Stiffness?

The cornering stiffness is increased at mid to high vertical loads, whilst there is little to no change at low loads. I believe the reason for this is the higher lateral stiffness compensates for the reduction of contact patch length at low loads. Whereas at higher loads, the difference in contact patch length is reduced, which when multiplied by the increased lateral stiffness results in a higher cornering stiffness at mid to high loads.

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What impact does rim width have on the Overturning torque?

The wider rim increases the lateral stiffness of the tyre, this means it deforms less, reducing the distance between the resultant force and the centre of the rim, and with it the overturning torque.

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What impact does rim width have on the Self Aligning Torque?

This one I’m not 100% sure whether self aligning torque would go up with increasing rim widths or down. Lateral and cornering stiffness go up despite the reduction in the contact patch, but self aligning torque is more sensitive to changes in the contact patch length than those two. The equation for self aligning torque has half the contact patch cubed, whereas the other two have it squared. If anyone can shed some light on this or has a link to a paper that has measured this, please send it over.

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What impact does rim width have on the the Relaxation Length?

Increasing the rim width, disproportionately increases the cornering and lateral stiffness, with the lateral stiffness increasing further across all vertical loads. This results in a decrease in the relaxation length, wider rims will give you better handling.

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By Trav Mays

References:

[1] Bentley in the pits at the 1930's Le Mans Race, image, Link to original