DRIVING SKILLS NUMBER THREE: Negotiating Medium and High-Speed Turns

Today we are going to look at something that causes a lot more headaches, and a lot more accidents, than most people realize: turning a vehicle at medium or higher speeds. At first glance this is no more difficult or should take no more thought than it does starting the car: take keys out, put key in to ignition, turn, drive. But there is much more to it, as we will go over now.

There is a reason why there is a Formula One car from the 1960s at the top of this blog. Formula One cars, then, as now, operate on a knife's edge of adhesion. What that means is that it only takes an unsmooth motion or jerking of the wheel to make the car lose control. And it is exactly this jerkiness that causes most of the accidents that take place while turning.

Accidents that occur while turning in either a medium or high speed corner can be attributed to one of two things. The first is a mechanical failure (tire blow out, suspension failure, hitting a pothole which causes damage to the car). And the second is the driver aggressively operating the steering wheel while the car is being turned. Apart from making sure you keep up on maintenance of the car, not much can be done about the first kind of accident. Therefore, all of our focus today will be on preventing the second attribute to corner accidents: aggressive operation of the car while turning.

I can imagine a potential question right away: Why would a car that weighs around one and a half to two tons (as most modern cars do) have an issue with being turned aggressively?  The mass of the car should be able to resist that, shouldn't it? Not really, actually.

What is important to remember about a car is that all cars run on four tires. This you already know, what you may not know about is the contact patch of those tires. Whether you are talking about a donut spare (that little rubber thing you have in your trunk that resembles a black Frisbee, yet is supposed to be a spare tire!) or mammoth off road tires fitted to a one-ton truck, all tires have a contact patch which is not much larger than the palm of your hand. This contact patch is that part of the tire that is in contact with the road while the car is being driven. But that doesn't actually tell the whole story. Remember, tires have tread, that means that the contact patch isn't a solid oval, it is an oval that is not connected at all points because of the spaces between the tread blocks. These spaces are necessary because they allow water to be removed from the road surface by the tire going over the water. (but that is a subject for another post.) What's worse, that contact patch can move, because unless your tires are bald (and in that case you have bigger issues for sure) those tread blocks will move side to side depending on how aggressive you steer the car. So a car that might be fine at 60 going at a straight line (where the contact patch, and the tread blocks, are being pushed in the direction the manufacturer intended them to be pushed, in line with the direction the car is travelling) will be totally different when the car is turned at 60 miles per hour. At that point, not only does the contact patch shift  because of what we already discussed with the tread blocks, but the outer tires have more weight on them because of the g-forces of the turn. This causes the tread blocks to be both pushed down, and out. If too much force is applied, and applied too quickly, it can cause the tread blocks to be pushed further than they can go and still maintain grip with the road surface, and in a worst case scenario, those tread blocks can slide along the surface of the road altogether, meaning that there is no longer any control of the car. Here is a picture of the tread of a modern tire.

In this picture, you can see the individual tread blocks, and the large open spaces between the tread blocks. Remember that contact patch that was about the size of your hand? That patch is made up of what you see in the picture above. That is why driving smoothly, or unaggresively, is so important. By operating the car in a smooth manner, you allow the car to maintain the level of grip that it was designed to have, which results in a safe trip.

So, how does a driver operate a car smoothly in a turn? A driver does this by first off being prepared  to steer in to a fast turn, and then knowing how to turn in smoothly, and safely. 

In my experience, most accidents involving medium to high speed turns are the result of surprise. The driver was either daydreaming or focusing on a song or their phone, or just wasn't paying the attention that they needed to the road. Other factors are drivers not believing turn speed limits, and drivers being overconfident in their abilities to drive a car. Regardless of the reason, most drivers who have accidents while turning were not ready mentally for the turn they were entering. A driver gets ready by first off always being prepared for a turn. At no time should a driver enter a turn of any kind without knowing in the back of his or her mind that at any time in the turn, the car could lose traction if the driver operates the car aggressively in the turn. A good driver is prepared for this, and whether they are entering a 20 mile per hour turn, or a 60 mile per hour turn, they drive with the same focus and caution, knowing that it is most likely going to be their fault if an accident happens due to poor driving. So the first step in turning safely is being ready and prepared for the turn, and knowing that the turn could go bad if the driver is too aggressive.

The next way that a driver negotiates a medium or high-speed turn safely, is in how the driver turns in to that turn. All turns should be done smoothly, and deliberately. There should be no sawing of the wheel (the rapid back and forth movement of the steering wheel in a turn that is sometimes seen in auto racing), nor should there be sudden movements. A driver should enter the turn, see where they need to move the steering wheel, and once the driver gets to that point, they move the steering wheel in a smooth, but deliberate way, in the direction of the turn. Now don't let that word deliberate confuse you. What that means is that the wheel is turned not in a weak or indecisive way, but in a solid way. Not in a way that is jerky or abusive, but in a confident way in the direction that the car needs to go. 

One question that might come up is how do I know when to start steering in to the turn, and when to start steering out of the turn. That can be judged by the curve itself. Take a look at the turn below.

This is what a typical high speed corner looks like.

The above corner is a corner that has a suggested speed of 45 miles per hour. This is quite typical of many of the high-speed corners you will come across in the United States. So pretend for a minute that you are approaching this corner in your car. How will you get through it safely? Well first, slow down to the recommended speed for the corner before you enter it. Yes, I know that most people can safely get through a corner like this by going ten or sometimes fifteen miles an hour over the recommended speed, depending on their car, but they risk more than just a quick accident at those speeds. Stay with the recommended speeds. Even if you can get through the corner ten miles an hour faster, you will save maybe five seconds off your trip. The danger is not worth that short of a time gain.

Ok, back to the corner. You see that there is a 45 mile per hour corner coming up. Let's say you are doing 55 miles per hour. The way you begin the corner is by braking for the corner and being at the speed you need to be at for the corner BEFORE you enter the corner. This is a piece of advice that came straight from Sir Jackie Stewart, a British Formula One driver who earned three Formula One World Championships and survived Formula One at a time when it was common for three or more drivers to die every year. He knows what he is taking about, and if you take a second to look at what he is saying, the logic is sound. By being at the speed that you need to be at before you enter the corner, the forces working on the car will only be going in one direction, the direction the car is turning (if you brake in to the corner, then not only are cornering forces working on the car, pushing it sideways, but braking forces are working on the car, pushing it forwards. This can lead to a loss of traction much easier than if braking is completed before you enter the turn.) It also frees the driver up to do nothing but focus on steering the car through the turn as smoothly as possible, instead of having to juggle braking and releasing the brakes, and turning the car.

The next thing is when to turn. My rule of thumb is that for a medium or high speed turn on a street or highway, I start to turn in to the corner just before the road begins to change from straight to curving. I do this because the corner was made to be taken at the speed the yellow sign indicates. If I am already at that speed, I should be able to safely turn in to the corner as soon as the corner begins, if I am doing the speed that is recommended for the corner. Of course allowances might have to be made if your vehicle has a high center of gravity (like a delivery truck), or if there is an issue with the car, but those are things that you should be mindful of before you even start driving the car.

Next comes the final part: turning out of the corner, and returning the steering wheel to straight ahead. This action should be done as if it were the undoing of going in to the corner. If you are at or very close to the recommended speed for the corner, you should start turning the wheel back to straight as you see that the corner is ending and the road is becoming straight again. Just like with the braking issue going in to the corner, you shouldn't accelerate past the suggested speed of the curve until the car is all the way out of the curve. This will again give the forces acting on the tire the opportunity to only push the tire in one direction, and will allow you to focus on one thing at a time, which can be important if you approach the corner on a busy traffic day or if you are a new or older driver.

The diagram below gives a good overview of the forces acting upon a tire (and a car) in all situations:

When the car accelerates in a straight line, the forces acting on the tire are in a straight line, and forward. When the car brakes, the forces acting on the tires are in a straight line, but backwards. When the car is turned either to the left, or to the right, G-forces are applied to the tire, which is what causes the tire to slide if the G-forces have a strong enough effect upon the car.

Slip Angle is the amount that the tires slide when a sideways force is applied to them.

The above diagram  gives a good impression of what happens to the tread blocks of a tire when the tire has a sideways load, or G-force applied to it. As already said, these forces will cause the tread blocks to slide, or slip, and the Slip Angle is the amount of slip the tread blocks endure. This is why it is so important to always keep your tires in mind as you turn, because any corner taken to aggressively could lead to a loss of control.

Lastly, let's look at an example of a car loosing traction. In this case, the car is a Ford Escort MK. 1 Rally Car. Look at how his aggressive driving causes the rear end to oversteer, or slide:

As you can see in the video, the driver enters the corner aggressively, and as soon as he gets past the middle of the corner, the rear tires loose traction, and the car slides. On a racetrack, this is no big deal, as cars are often designed to slide around corners because it is the fastest way to negotiate a turn, if the slide can be controlled. But in a motor car, at night, by yourself, this is the last thing anyone would want to experience.

The way to negotiate a turn safely is to first off recognize the importance of the tires and the forces acting on them, and how they could be made to lose traction if a driver is too aggressive. Then it's important to take corners at the speed that is suggested for them, brake fully before entering the corner, turn in to the corner smoothly, but deliberately, and don't accelerate until you are through the turn and your tires are straight. Hopefully these tips will help you negotiate any corner you come across in your driving. I can tell you from personal experience as a test driver that sliding a car on a racetrack or closed off proving grounds is incredibly fun, but it is only fun because there are no other cars or people out where the cars are being tested, and the people testing them are trained professionals. The street is not like that at all, so keep the sliding to the pros, and negotiate turns in such a way as to get you, and your car, home in one piece. Until next time...