(057) CO-EFFECIENT OF FRICTION & SLIP ANGLES (7 of 13) Intelligent Racing, Minimal Spending: The Definitive Introduction To Racing Concepts

Introduction

Welcome to the seventh part of this racing concepts series. Remember: the most dangerous words in the English language are “I already know that.”

Alright. Here it is.

Co-efficient of friction is just a brainy term for the relationship between a tires vertical load and its ability to grip–grip being either a lateral force (side to side) or longitudinal force (forward and backward).

Up to a point, higher vertical loads allow racing tires to provide more traction (Figure 16).

 

coefficient of friction `

 

If the vertical load surpasses the tires maximum ability to grip, traction will begin to decline.

When enough traction is removed from a tire working on a lateral axis (during cornering) the tire will begin to slip.

In an interesting hiccup to Newtonian physics, a road racing tire has the ability to produce a greater lateral force than the vertical load being applied to it.

For example, a theoretical tire experiencing a vertical load of 400 pounds is capable of producing a 650 pound gripping force.

Up to a point, higher vertical loads increase the potential force of a tire. (Figure 17).

 

coefficient of friction 2

 

If the vertical load surpasses the tires maximum ability to grip, traction will begin to decline.

When dividing the potential force by the vertical force, you can see that this tire has a coefficient of 1.625 gravities (or G forces)–a huge amount of potential force.

This is the mathematical cornering (or accelerating or braking) force that this tire can produce.

You must keep in mind, however, that even though this theoretical tire is capable of producing 1.625 Gs during a corner, its ability will likely be stifled by the friction limitations in the driving surface, as well as the limitations of the vehicle.

It is very important to realize that a tire is capable of producing more grip while experiencing a combination of lateral and longitudinal forces.

But we’ll get to that later in the series.

 

Slip Angles

A slip angle is the angular difference between the direction a wheel is pointed and the actual path of the wheel (Figure 18).

 

slip angle 1

 

Now we’re getting to the good stuff–the application stuff.

Slip angles are another key component in going fast.

Yet, like tire pressure, are often ignored by the amateur racer.

Understanding how slip angles work is imperative and will allow you to drive your car at its maximum potential.

The slip angle concept is easily grasped when considering a tire during cornering.

As a tire works to change the direction of the vehicle it is attempting to alter the course of the vehicle’s mass.

This means that even though the tires are pointing in a direction other than the direction of the vehicle, the vehicle will attempt to travel in a straight line.

As the two forces act on the vehicle, two things happen:

  1. The vehicle will begin changing direction.
  2. The actual arc of the tire path will be larger than the arc made by the direction of the wheels.

This is because the vehicle’s moving mass is forcing the tires to slip sideways across the driving surface (toward the outside of the corner) during the corner.

As you can see, the angle between the direction of the wheel and the actual path of the wheel is the slip angle.

It’s important to understand that every tire experiences some degree of slip during cornering.

On the road at regular highway speeds, a tire may only need a slip angle of 1 or 2 degrees to navigate a corner.

On the racetrack, however, it’s not uncommon to see slip angles between 8 and 14 degrees.

Tire manufactures are very aware of the slip angle phenomenon and have designed their tires to operate most efficiently within a certain range of sip angle.

As you can see by the accompanying graph (Figure 19), a tires co-efficient of friction (remember, that’s just the tires ability to grip) is directly related to slip angle.

 

slip angle 2

 

Let’s look at the three theoretical tire categories from the contact patch post (jump there by clicking here).

  1. The dotted line is a factory tire. It has a low coefficient of friction, so it won’t be producing nearly as much grip.But just the same, its peak gripping ability can be reached over a long span of slip angles- about 10 to 20 degrees.
  2. The dashed line is the extreme summer performance tire. It has a higher co-efficient of friction, but a significantly lower range of slip angles where it produces peak grip.The driver of this tire must be on the ball to keep the angle within the tires optimal range.
  3. Last is solid line–the purpose-built racing tire. This tire has a high co-efficient of friction and is capable of producing much more grip than the others.In addition, the range of optimal slip angles is relatively narrow. This allows the driver a bit less tolerance to produce maximum tire adhesion.But if he can get there, he gets the payoff.

You can see that, while a tire can produce consistent amounts of grip over the range of its optimal slip angles, it’s worth remembering that a driver who can remain at the lower end of the range will have better results.

Lower slip angles generate less lateral force-therefore less friction and less heat.

This means that a tire will last longer, degrade slower, and provide maximum grip longer before it wears outs.

The driver is entirely responsible for consistently staying in the optimal slip angle range.

Otherwise, if he has too low or too high of a slip angle as shown by the graph, his tires will not be in their peak range of grip.

In addition, it’s important to understand that the rear tires must also fall into the optimal slip range.

And while the rear wheels don’t do any active turning, they are still a large part of cornering, and must slip across the driving surface like the front tires.

In a well-balanced racecar, this will happen naturally so you don’t have to be too concerned about it.

 

NEXT WEEK: My Custom Tire Pressure Program (How To Find Your Perfect Tire Pressures)

With this program you’ll be able to:

Determine if you’re driving the car hard enough
Determine if you’re driving inside the optimal slip angle range
Determine if you’re running optimal tire pressures, and how to adjust them as necessary
Determine if your car is well balanced or if it has a natural tendency to understeer/oversteer

 

The next part in this series is only a week away. Go back and read through this again. Make sure you truly understand what you just saw, because the series progressively builds as it continues.

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