Cornering

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When cornering the car is taken on its chosen path by the varied directional influences of each individual tyre. The two rear tyres are for the most part trying to make the car go straight on unless steering geometry has been built into the rear suspension. The two front tyres are the ones that enable the car to change direction. This may seem obvious and not worthy of mention but it is an important point as for the most part the rear tyres are resisting the change of direction that the front tyres are making.

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The direction of the car is therefore a compromise between the influences of the front tyres and rear tyres and the car does not actually go in the direction that we point the front wheels.

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The tyres across the front or the rear have their highest total grip (by which we mean ability to influence direction change by inducing a force) when the weight supported by each of the pair is equal. As soon as there is any weight transfer the maximum possible grip decreases.

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This would not be the case if we were dealing with the grip or friction exerted by solid objects on a smooth solid surface, like bricks on a table for instance. If we had two piles of bricks with two bricks in each pile then if we move a brick from one pile to the other, the total frictional force will remain the same.

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With tyres however, what happens as we push sideways, from the rim of the wheel to the road surface (or vice versa), is that the tyre sidewall distorts.  The time taken and magnitude of direction change is dependent on the flexibility of the sidewall. This means that there is not an instant direct relationship  between the steering wheel and direction change of the tyre tread.

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It is the assumption that tyres behave in the same way as bricks (exerting what is known as coulombic friction) that has lead to lots of misinformation about cornering in books and on the net. While the figures are dependent to a degree on tyre construction it may be that a weight increase of 50% leads to a 10% decrease in grip.

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Although any tyre with less weight would have its grip increase (as its sidewall distortion is lessened) this would not compensate for the decrease in grip of the other tyre.

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If two tyres are able to generate 100 units of side force each at the road surface, we could write that as;

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Without weight transfer

Left 100 units of force x 100% force transfer = 100    Right  100 x 100% = 100    100+100 = 200 units of cornering force

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But with weight transfer

Left  150 units of force x 90% force transfer = 135        Right  50 x 110% = 55    135 + 55 = 190 units of cornering force

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Note that weight transfer to the front from the rear also decreases the tyre grip or vice versa as it does not matter where the increased weight comes from.

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This explains why fitting low profile tyres increases grip. The sidewalls are not able to distort so much and therefore transfer the cornering force better.

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What also confuses matters is that when braking in a straight line the resisting force is directly opposite to the applied force. The tyre now tends more toward the normally understood rules of friction as there is less sidewall deflection with grip increasing in direct proportion to weight and so weight transfer plays no part in the total available stopping grip. The tyres behave more like the bricks described earlier. Any reduction in braking ability is then down to the capabilities of the front brakes themselves.

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What all of this means is that as weight is transferred, the tyres’ cornering grip decreases, particularly when there is forward weight transfer when decelerating into a corner. This is contrary to what is written everywhere else. Most explanations say that as weight is transferred forwards front grip is increased, while that at the rear is reduced.

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Experiential evidence also seems to bear out what has been written before as being correct, a classic example being lift off oversteer.  We lift off into a corner and the back steps out. So, in the face of such strong evidence how can we possibly believe that what has been written almost everywhere before is wrong and that this article is correct?

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Simple.

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In spite of the amount of grip at the front being reduced, its effects are smaller in comparison to the fact that as a car slows down (due to lifting off) it doesn’t need as much steering to make it follow a curved path.  It is the slowing down that improves the cornering. The weight transfer to add more grip is a red herring.

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Many of the articles written on the subject are simply re hashed versions of earlier works written by people who didn’t understand the physics of what was occurring. The original authors can be justified in their lack of understanding as there wasn’t the information available to them and many may not have any understanding of physics. However articles written since the days of the internet should have shown better understanding as the information is out there.

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Anyway, to prevent lift off oversteer all you need to do is to correctly reduce the amount of steering at precisely the same instant as you lift off, or don’t lift off so suddenly

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It’s as easy as that.

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Or rather it isn’t, because just like easing brake pressure if a wheel locks up, steering away from the way you want to go, even a little, is not something that is intuitive and is something that we’ll come to elsewhere.

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In the same way that weight transfer to the front will increase understeer, weight transfer to the rear will decrease it. This means that if you accelerate as you are cornering, as long as the front tires don’t pass their optimum slip angle for grip, the radius of turn will decrease.

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The speeds and turn radii vary due to road and car factors. It is a driver’s skill that will enable them to decide what is the most effective acceleration in any particular situation.

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The best way to make use of the effect of weight transfer,  to help cornering speed is to accelerate just as the steering wheel angle increases, once the deceleration has finished. By using the two separate forces together at the same time we can increase the instability of the rear of car more than we can if using them one after the other. The increase will cause the car to turn more for a given steering input.

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Some advanced driving instructors advocate being on the gas before steering. This will have a similar effect if the two forces are used in tandem.

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The effect is found in front, four and rear wheel drive cars. It is likely to be greatest in high powered rear wheel drive cars but may be countered in any by sophisticated suspension geometry or electronics in order to provide a more intuitively predictable driving response. This is to keep drivers with average car control skills safer.

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If you are going to practice this skill remember to improve a little bit at a time. It is best done alongside an instructor experienced in these matters.

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If you are habitually still slowing down using the brakes while entering a corner first break this habit by checking that your foot is off the brake pedal before you turn in.  A good way to do this is to say ‘Off’ as your foot leaves the pedal. It focuses the mind on the reality of whether you are off the brakes or just think you are. Remember of course to come off the brakes correctly (see here if you do not know how). Initially this will feel wrong and you are likely to go slower. Don’t worry. That is all part of the relearning process.

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Once you are doing that automatically, get gently on the gas using your normal steering. You should normally notice that the car will turn in better and you may have to wind off a tiny bit of steering in order to compensate. When you are happy with that skill and you are aware of how much less steering you need simply apply this lesser amount of steering while getting on the gas in the same way that you have been. Continue this exercise, gradually increasing the gas and decreasing the steering input.

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If you are able to continue accelerating without the car understeering once you have finished steering this means that your corner entry speed could be higher or your acceleration firmer while initiating the turn. In order to stay within the limits of both you and the car rather than entering the corner more quickly try slightly firmer acceleration. Work up to the maximum gradually and once you feel comfortable with that speed then is the time to practice entering the corner very slightly faster. No more than 1 mph at a time.

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Average drivers often find that they can safely increase cornering speeds by 10% and sometimes even more. By using this increased skill it is not necessary to brake or accelerate so much, thus saving wear and tear on the brakes and reducing fuel consumption while maintaining a higher speed. A win, win, win situation!

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