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Tire Traction

Assuming equal horsepower and driver skills, the fastest car around the track will be the one with the most tire traction. Traction aids in acceleration, braking, and cornering.

"Handling" is all about maximizing tire traction. Regardless of how much advanced hardware the car has, the bottom line is that the car's entire braking, accelerating, and cornering performance has to be translated through the four small patches of rubber in contact with the road. Think about it. Ignore absolutely everything about a car except for how much rubber is in contact with the road. Maximizing the performance of these four small patches is what "handling" is all about.

Maximum traction, of course, is affected by the suspension design, the type of tire, it's rubber compound, its contact patch size, and several other factors. Once a given car and tire is selected, there is still the task getting the absolute most out of that specific tire.

After-market products which can help improve tire traction include anti-roll bars, shock tower bars, stiffer springs, adjustable shocks, wheel alignment kits, and others. Used properly, these items are designed to maximize the contact patch of the tire during dynamic conditions. Used improperly, these same components will actually deteriorate traction and handling under race conditions.

Another major factor in tire traction, often overlooked, is the driver. A practiced driver having very smooth control of the car, and high sensitivity to the tire's traction performance can improve the car's lap time as much as just about any single after market hardware modification, and it's free. Give a pro driver your car for a 30 minute session, and he'll likely best your times by an amount you thought impossible.

To do this, the driver must fully understand the tires, how their maximum performance is achieved, and have enough practice time developing a sensitivity to how the tire is performing at any given moment on the track.

Three factors determine the maximum potential grip of a tire: the coefficient of friction provided by its rubber compound (stickier is better), the amount of rubber as determined by the tire size (bigger is better), and the amount of downforce applied to the tire (pushing down adds to the total friction applied). Of course there is a limit to all of these, and a point is reached where more is not better.

Slip Angle

There are two factors in reaching and sustaining the maximum traction performance from the tire. First, a tire's maximum traction potential is actually reached when there is a small amount of slippage. This "slippage" is translated differently for braking, accelerating, and cornering.

Under braking, the peak performance of the tire is reached when the tire is turning slightly less than a one-to-one relationship of the distance traveled. In other words, if the car were at a steady state, and the wheel turned 10 times to cover a certain distance, under braking, the wheel would now turn perhaps only 9-1/2 times to achieve the peak slippage performance. It is possible to learn how to feel the car through the brake pedal, steering wheel, and seat and sense this tiny bit of extra braking force from the tire.

In acceleration, the tire should travel slightly more distance than the distance of the acceleration (spin just a bit faster than normal). The tires will actual slip; not a lot all at once to result in free wheelspin, but ever so slightly during the whole acceleration phase. When you can sense this slip, and control it, this is when you're getting maximum acceleration from the vehicle.

In cornering, this slippage is present when the wheels are actually turned just slightly more than the actual amount required to go the intended path (this difference is an angle and is where we get the term "slip angle" from). To accomplish this slip angle the car must actually be sliding ever so slightly during cornering. Not a big power slide, just a little extra slip. At first, this can feel very uncomfortable, as though you are starting to lose traction. In fact, this is where the car has its greatest traction. The tricky part is approaching this limit and not crossing it. If the car is not sliding at all, then it isn't going fast enough. If it is sliding enough to actually drift (or have noticeable understeer, or oversteer), the tire is being used beyond its limit. The corner speeds will be slower, and the tire will wear out much more quickly.

In each of these cases, we emphasized the "slightly" aspect of this slippage. Too little, and the tire does not reach maximum performance. If the car feels "hooked up on rails," then the car is not being driven fast enough. Until you feel that tiny bit of slip, you can go faster. Knowing how to approach that point without exceeding it takes a great deal of practice. Too much slippage and you'll exceed the tire's limits and the tires slide excessively resulting in locked-up braking, wheel spin in accelerating, excessive sliding during cornering. Ultimately they will overheat, get slippery, and wear out much faster.

The amount of slippage required is different for each tire, but we're talking in the range of 4 to 10 percent. When cornering, the steering angle input is perhaps 5 to 6 degrees more than required to negotiate the turn. However, to keep this from turning the car too much for the corner, it has to be pushed with speed to generate that little bit of slip to compensate. Racing tires will perform best with a little less slippage than a street tire. If you learn to race on a street tire then switch to racing tires, you'll have to learn to back off a little, and be smidgen more gentle with the race tire.

How should this slippage concept affect your driving? If you look at the graphs which illustrate the affect of slippage on traction, you'll see that at the peak traction point, there is actually a pretty wide margin of slip that will generate maximum traction. If you tend to drive in the latter part of that band, you'll acheive good cornering performance, but being closer to the edge the tire's limits, you'll build more heat and generate greater tire wear. The tires will feel great for a while, but they will wear out sooner, and the last several laps in session will have poor performance. If you drive within the earlier part of that peak traction band, your traction performance will remain consistent over a longer period of time. In time trials, you won't see much of a difference (except in replacement tire costs), but in racing, this will put you farther ahead of the competition in longer runs.

A time that you may need to drive in latter part of the traction band is when the track is cold, and you must push the tire harder to keep heat buildup in the tire.

Driving Smoothness & Traction

One more principle to learn. A tire's maximum traction potential will not be realized unless it is brought to that point gradually. This is true of just about everything dealing with frictional traction, and you experience it regularly in everyday occurrences.

Imagine this experiment. Place a piece of paper on a table, and an ordinary breakfast bowl on the paper. Start pulling on the paper slowly, then gradually faster. The bowl remains on the paper and is dragged along with it. Next, yank the paper immediately. It will come out from under the bowl leaving the bowl unmoved, or barely moved. Same bowl, same paper, same table. What was different? The acceleration of the forces applied. In your car, the tires are the paper.

Ease the car smoothly into a corner, and the tire will have a high level of traction. Jerk the steering wheel too quickly, and the tire will not maintain grip with the road. Same car, same tires, same road. The difference is the acceleration of the forces, or the smoothness with which cornering, acceleration, and braking forces are applied. Smoother is grippier.

The principle of driving smoothly is paramount to every factor of improving a car's handling performance. All the hardware in the world will not fix a car with a driver using "jerk and stab" braking, accelerating, and turning control behavior. Inexperienced drivers frequently blame the lack of the greatest hardware in their car for performance problems which are actually caused by their driving style. There's enough stories to suggest even a few pros have this habit. Be honest and analyse your driving, or get an experienced instructor to analyse it for you.

One of the common faults and gripes of new drivers is front end push (understeer) when entering a corner. "Man, my car slides something horrible going through the first half of that turn." There is significant probability that the car is fine, but the driver is braking too late, too hard, and is rushing the turn-in with a sudden steering movement. Brake sooner, let up sooner (and more gradually), and ease into the corner with a larger and smoother radius. This will likely cure the understeering problem, and will most certainly reduce it if suspension setup is an issue.

Smooth driving maximizes tire traction. Maximized tire traction is what leads to fast driving. We repeat -- smoother is grippier.

Mechanical and Aerodynamic Downforce

Another factor which affects tire traction, but one that is not likely to be factor in the weekend racing of your street car is vertical loading -- the combination of mechanical and aerodynamic downforce. Whether applied by mechanical forces (which is essentially gravity), or aerodynamic forces, the total amount of downward push on the tire affects the available traction. To demonstrate, lightly drag a pencil eraser across a table. It slides easily. Now push down on it and drag it. There is much higher friction. Same goes for tires.

This simple demonstration might lead you to believe you should add weight to your car to gain more traction. However, practical experience tells us that lighter cars generally handle better than heavier ones. It turns out that while greater weight on the tires will increase traction potential, it also increases the amount of work they have to do. When cornering, the tire must keep a heavier car on the track, and therefore there are greater lateral forces. If you graph the increase of traction vs. the increase in work based on the increase in weight, the work load increases faster than the traction improvement. So mechanical downforce is not necessarily the way to increase tire traction.

This is where aerodynamics have played such an important role in racing the past couple of decades. Aerodynamic downforce provides that increased push down on the car to increase traction, but that push is not translated into lateral load that the tires have to deal with when cornering. There is increased traction without increased work.

Summary

All handling modifications and adjustments come down to improving the traction of the four tire patches on the road.

Tires are actually their grippiest when there is about 5% slippage involved.

Driver smoothness is a major factor in the car's overall grip. All the fancy hardware in the world won't cure the loss of grip created by a jerky driver (and we don't mean personality).

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Tags: Race Tires, Performance Tires, High Performance Suspension, Lowering Springs, Coil Over Shocks, Sway Bar, Tire Temperature

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