Droppin' Wheels

One of the largest causes of vehicle crashes is when a driver runs off the shoulder of the road and yanks the vehicle back onto the road.

In all my years of instructing, this is one exercise that I continually hear parents ask if it is taught in certain schools. The fact is, even in some of the best driving schools in the country, this situation is not taught. And when I say taught, I mean getting drivers behind the wheel and let them do it the wrong way and then teach them the correct way.

Why is it that so few schools teach this? Well there are several reasons. Space is always an issue. In order to teach this properly and to give the students room to make mistakes, you need a lot of runoff room. Another reason is vehicles and damage. A lot of schools have the students drive their own cars in the school. There are positives and negatives to doing this and for sure one of the negatives is that most people won’t want to start sliding their car around wearing out the tires. Or, driving it off the road where debris may be thrown up and damage the vehicle. Even in schools where a manufacturer provides the vehicles, it is still a concern with regards to damage to the vehicle.  Lastly, to be instructed properly it takes a good instructor, plenty of time and a good course set up to do this correctly. A lot of driving schools these days have to focus more on making money rather than training the driver. Especially when they are supported by a manufacturer. The reality for them is that the manufacturer is not a “driving school”, they are in the business of selling cars. How do they sell cars at a driving school? They put you in the seat of their car and get you to do some really cool fun stuff that makes you feel like their car is the best in the world and doing what you just did and now you HAVE to have that brand of car. Hey, it’s great marketing, but what happens a lot of times is that the school becomes more about “butts in seats” and “throughput” rather than educating drivers. But I digress form that topic for now.

With all of that said, I think it should be a mandatory exercise in any good training course to teach off road recovery. If it is set up properly there is no damage to the vehicle, minimal tire wear and a HUGE educational benefit to the student. 

I know of one school that is teaching this exercise now and it is working good. The program is only a half day school so the students don’t get quite enough time behind the wheel or personal instruction, but what it does do is give them a sense of what its like and let them see that there is a proper way to handle it.

So what normally get reported in these situations? Well, in the news you will typically read something like this:

“…………the driver ran off the shoulder of the road, over corrected and crashed the vehicle.” Sound familiar?

I have a bit of an issue with the above statement because often times it gets used improperly. In a lot of cases it’s not necessarily an “overcorrection” that caused the crash, but rather a lack of “recovery.”

This usually goes down one of two ways. One way is that the driver runs of the edge of the road, panics, turns the steering wheel hard to the left. The vehicle responds by truning hard left. The driver is surprised by the reaction of the vehicle and does nothing else with the wheel so the vehicle ends up in the ditch on the opposite side of the road. 

The driver simply ran off the right side of the road and jerked the wheel so hard that they drove themselves into the ditch on the other side of the road. That's one scenario.

There’s a reason I posted this topic right after the skid control post, they go hand in hand. Again this is another one of those hard to describe in text scenarios, but I will do my best.

      Okay, lets say you are driving down the road and you run off the right shoulder. For simplicity sake we will just say it’s a grassy shoulder with minimal drop off. 

You realize you have run off the road with your right side tires. You panic, turn the steering wheel hard to the left. The front tires are the first ties to have both on the grippy asphalt. However, now you have your right rear still on grass and because you are turning the wheel to the left, the vehicles weight is on the right side of the vehicle. So, good grip up front because both front tires are now on asphalt. Not good grip in the rear because your right rear is on grass, and that’s where a lot of the vehicles weight is at this point. So what happens? The back of the vehicle starts to slide to the right. OVERSTEER!

So you recognize that the rear end is sliding to the right. You remember drivers ed; “steer into the skid”, so you steer to the right. You have managed to stop the rotation, good. And now the rear tires come back onto the grippy asphalt and what does the vehicle do? It HOOKS! Hard to the right. Why? Because you were not taught about eyes in the right place, and you were not taught about RECOVERY. 

 This means as the rear end was sliding to the right, and you steered to the right, you were also LOOKING to the right and when the rear tires regained grip you didn’t take out the corrected steering input, so the front tires were pointing to the right. So guess what? The vehicle went to the right! And somehow we will blame this situation on the vehicle. It will sound something like;  “I don’t know what happened. The car just took off to the right and I ended up in the ditch.” NOOOO, What happened was, the car did exactly what you told it to. you told the car to go to the ditch, so it did, in a big way. Lol

Now I just described a scenario that takes about a second to happen on real life. So there is no way for you, in the real world, to “think” about this as it’s happening. You don’t have time to think about it. Your reactions to these types of situations have to be instinctive. The only way for that to be the case is to train so that you have some muscle memory.

So, without being able to put you in a car and show you here’s how I will put in to text how to handle the Off Road Recovery situation.

Should you run off the road, first and foremost, DON’T PANIC. Panic will NOT help. It will cause you to tense up and over react with the steering wheel.

Next, don’t look off the shoulder of the road. You know you have run off the road, you don’t need to visually confirm it. Look up and ahead for your intended path.

Retain a firm grip on the wheel. Don’t choke the wheel, but hold it firm. If the shoulder is rough you don’t want the wheel jerked out of your hands.

Ease your foot off the accelerator. And GRADUALLY steer the vehicle back onto the asphalt. As you are bringing the vehicle back onto the asphalt you can expect some jerking of the vehicle. Do not over react to this either. Remember; keep your eyes out ahead of you. This will help you ensure that the vehicle is pointed in your intended direction.

Should the rear of the vehicle start to oversteer, you will need to do C.P.R. (see previous post)

As long as you don’t panic, this situation can be a non-issue. But if you panic and start doing excessive or random things with the steering wheel, you can turn a non-issue into a life-threatening situation.

Now let’s say that it was s pretty significant drop off, like maybe a four or five inch drop. 

You can still handle it as I described above. Police, Secret Service and many other agencies teach this technique for hopping on and off of curbs and sidewalks. It is a bit more aggressive and tricky, but it is most certainly manageable. With that said, what I would recommend for the average driver, if you drop off a large shoulder, if there is room, stay off the shoulder and bring the vehicle to a slow roll and bring it back up then. Again, this means you have to make a quick assessment of the situation without panicking and decide if there is enough room to bring the vehicle to a slow roll or stop while it is off the shoulder.

And that’s pretty much it. I have done this on curbs, sidewalks, gravel edges, grass edges etc. and no matter what the situation, they are all manageable and can be minimized as long as they are dealt with properly.

You will notice recurring comment in a majority of my driving posts, know what it is?  KEEP YOUR EYES ON YOUR INTENDED PATH!

The reason it is a recurring statement is because it is a VITAL key to your success. You should always be training you path of vision while driving. Continually think to yourself, am I looking far enough ahead? Did I look through that turn before I turned the steering wheel? If you constantly pay attention to these things they will become natural. Again, in a lot of situations you don’t have time in the moment to think through it. It has to come naturally.

JM

Skid Control

Now we get to talk about something I REALLY enjoy, sliding a vehicle. For most people however, when a car slides it's a scary thing. And the reality is, in the real world, out on the public roads, it's not something you want to have to deal with, not even me. But this does not mean that you shouldn't KNOW how to deal with it.

Most of us will drive in some sort of bad weather climate at some point in our driving careers, rain, snow, ice, sleet, dirt road whatever it may be, these are all conditions that can easily result in dealing with a sliding vehicle. Can it happen on a dry paved road?, sure it can and it does, just not as frequently as it happens when there is inclement weather.

So first we will discuss a couple of different and typical types of slides, why they happen and what you as a driver can do to control them as well as possibly prevent getting into the situation to begin with.

The first type of slide we will talk about is UNDERSTEER

Understeer, when relating it to a skid, is when the front tires of your vehicle lose grip and start to slide, typically as you are entering a corner. (see image1)

In the image above you see a car that is attempting to make a left hand turn. The driver has steered the car to the left, but the vehicle is not turning at the rate the driver would like it to, it's taking a straighter path which leads to the edge of the road.

What are the typical reasons for this situation to occur? Well, the number one reason is speed. The driver has typically gone into a corner faster than the front tires ability to maintain grip and make the car change directions at the angle requested.

If you have ever driven in snow or ice this is something you have likely dealt with, however, it can happen on any surface.

Another reason this may have happened is a surface change. Possibly the driver was going at a speed that was perfectly acceptable until they got to the portion of the road that had sand on it, or wet leaves or some sort of fluid. This change in the surface changes the available grip level of the front tires.

So what would a typical driver do in this situation? Well let's see, first, panic. This would be the OMG moment where you either take a deep breath and hold it, or you don't breath at all. This is typically followed by the desire to turn the steering wheel MORE to the left. Why? well, because we are trying to make a left hand turn, and if we have turned the steering wheel left and the car isn't turning the way we want it to, we must not have turned the wheel enough, right? All the while, we are staring at the edge of the road looking at the ditch we are about to end up in, thinking to ourselves, no! no! no! no!  If you've ever been in this situation in the real world, you know what I am saying is true, and it doesn't fell good.

What SHOULD we do about this situation? Well, the good thing id that understeer is the easiest type of skid to deal with. In fact, all vehicles are engineered and designed to understeer as their first type of loss of traction because its easier to control than an oversteer (we'll get to that) and typically a less significant crash should you not be able to fix it.

So what do we do for understeer. the obvious first thing would be to say, plan further ahead. Get your eyes up and look further into the corner so that you have a better idea how sharp the corner is, this way you will slow the car down to the appropriate speed for the corner. I have talked about eyes and how important vision is with respects to driving and skid control is no exception to the EYES UP rule. One of the most common places that you can see where people have understeered is on off ramps. Typically a driver will exit the freeway where they have been traveling 70mph +, then they approach their exit. As most drivers, they are not looking far enough ahead and don't realize that the off ramp is a decreasing radius corner (meaning the corner gets sharper the further you drive into it) so they don't slow down enough and by the time they do realize they are going too fast, it's too late. This would be why you see a lot of the scratches on the concrete barrier at your local off ramp. If you have never noticed it, take a glance the next time your on your local ramp.

Now let's say that you are in the situation, now what? first, keep your eyes on your intended path, through the corner. Remember, your eyes tell your hands where you want to go, so if you are looking at the ditch, you significantly increase your odds of getting there. So eyes on your intended path.

Next, release the throttle. By getting off the accelerator you get two benefits from one reaction. The first benefit is that you slow the car down. Speed was likely the cause of the problem, so let's not complicate things by adding more. The second benefit is weight transfer. When you lift off the accelerator it will cause a weight shift to the front of the vehicle. This weight shift puts more down-force on the front tires therefore giving them more grip, this is a good thing.

After the above, the next part is the hardest, don NOT turn the wheel more! You will instinctively want to turn the wheel more, its just the way we are wired. Turning the wheel more can actually make the situation last longer and be significantly worse.

Think of it like this; if at X speed, you turn the wheel say 5˚ and you are going too fast and the car starts to understeer. At the same X speed, if you turn the wheel MORE, let's say to 15˚, the tires are just going to slide more. See, no matter what the angle, if you are in an understeer, you are already trying to make the car turn at an angle sharper than it can. So with the steering wheel you either, do nothing with it and just hold the angle that you already have and wait for the vehicle to slow down to a speed that the tires can regain traction and make the car change direction or, if there is enough road space available, you could try to reduce the amount of steering you have requested. On the public roads, the latter option is not very common. Public roads a pretty narrow and don't often allow room to reduce steering angle.

Once you have done these steps;

1-eyes on intended path

2-off the gas

3-don't turn the wheel more, reduce steering input if possible

The rest is kind of a waiting game. You have to wait for the car to slow down to a point at which the front tires regain grip. Another option that you have, but it has to be done with some delicacy, is braking. You can use the brakes to help this situation, but not in a panic, stomp the heck out of it kind of way. Remember, you have already lost grip with the front tires due to steering angle, so also asking the front tires to slow the car down can complicate things and make the situation worse, much like adding more steering. However, if you can "lightly" apply the the brake, you can accentuate the effects of getting off the throttle. You can slow the car down more and you can also create a larger transfer of weight. Both of these will help the situation.

So that's understeer and it's the EASY one, now let's discuss oversteer.

Oversteer, this is likely the most difficult skid for people to control or deal with. Oversteer is when the rear end of the car is sliding around. 

Basically, the rear of the car wants to pass the front of the car. 

A term you’ll hear a lot, especially in snowy climates is “fishtail”. You may hear someone say something like; “ I was doing ok and then all of a sudden the car started fishtailing down the road.” A “fishtail” is basically a series of oversteers that have not been dealt with correctly.

So, what causes oversteer? Well, several things. Oversteer can actually be a result of fixing an understeer situation. When you lift off the throttle to slow down the vehicle due to an understeer, you transfer weight to the front of the vehicle, which is what you wanted to do. However, this weight transfer has lightened the load on the rear of the car therefore reducing the grip of the rear tires. In this situation it is easier for the rear tires to start sliding.

Another way to induce oversteer is by accelerating too aggressively, especially if you have a high horsepower vehicle or you are driving on a slippery surface. Let’s say for example you are coming out of a corner and the road is wet. You step on the accelerator too hard and start to spin the rear tires. (rear wheel drive vehicle of course) Well, the spinning tires mean the tire is not getting good grip with the road and it will now slide.

You could also find yourself oversteering due to locked rear wheels. Maybe you are driving a manual transmition and you were downshifting in the middle of a corner (something you shouldn’t be doing unless absolutely necessary, but that’s another topic) and during the downshift you don’t rev-match the engine speed to wheel speed (also another topic) there is a good chance you will get a “chirp” from the rear tires. This chirp is the tires locking for just a second. This brief lock up can put you in oversteer.

Basically, any time you spin or skid the rear tires, you can place the vehicle in oversteer. So how do you deal with it?

Most of us have heard it said that when the rear of a vehicle slides, “steer into the skid”, and this is true. However, over the years I have found that many people have no idea what this means. In a lot of cases they can't even tell you which way the vehicle is sliding and by the time they figure it out (if they do) it’s too late to fix it.

But let’s say for the sake of argument that the driver does recognize the direction of the slide and they do “steer into the skid”, then what? ………………………. exactly, crickets. If you ask an untrained driver to tell you what to do AFTER they have steered into the skid they don’t know what to tell you. Better yet, even before that question, simply ask them “how much should you steer into the skid?” Again, you’ll get some guesses, but they really don’t know.

Why is it that people don’t know how to deal with this? Because they haven’t been properly educated. Sure, maybe they have seen a diagram in a manual or maybe even seen a video, but as I’ve said before, you cannot learn how to drive from a book. The driver’s education system in this country doesn’t really have the means to train drivers how to deal with this and a lot of people will not invest in their own driver training. Most think "it will never happen to me." Well, this mentality and lack of training is what has gotten us to the point of about 6000 teens a year being killed in motor vehicle accidents. 

So, what is the proper way to deal with an oversteering vehicle?

In most driving schools you will hear a term CPR, this stands for Correct, Pause, Recover. This is a great, easy method of remembering how to deal with oversteer. I will step you through the process of CPR.

Correction

For whatever the reason, the back of the vehicle is starting to slide. There are a few things that need to happen almost simultaneously as part of the "correction". 

first, keep your eyes on your intended path! this is HUGE. Remember from earlier posts, your eyes tell your hands what to do. If you are looking at the ditch your vehicle is headed for due to oversteer, you are going to significantly increase your odds of getting to that ditch. Be sure to give yourself GOOD information, keep your eyes up and on your intended path. Also, keeping your eyes on your intended path will help calm you down a bit. If you are looking at a ditch or a tree or whatever, as your vehicle is sliding toward it, nothing in your path of vision is a solution. Its all negative information and you panic. If you keep your eyes on your intended path, you can see a clear path and a means of escape or a solution. This alone can help keep you more calm. 

Along with keeping your eyes on your intended path, release the accelerator. Once again, there is a good chance that the accelerator was part of the problem to begin with, so lets get our foot away from that pedal. There are SOME instances in which MILD throttle application can reduce or help fix oversteer, but these are not the norm for an everyday driver. However, as an example of when adding throttle may help, let's say you are driving a rear engine vehicle and you get whats called a TTO (Trailing Throttle Oversteer) this means you lifted off the accelerator in a corner and it caused the rear end to start sliding. A mild reapplication of the throttle can help plant the rear end again. Its simply a matter of putting the weight back on the rear tires. Old Porsche drivers are well aware of this phenomena.

Finaly, as part of this combined sequence that is called the Correction, turn the steering wheel in the direction that the back of the vehicle is sliding. Now, I say it like that because saying "steer in the direction of the skid" tends to leave room for an interpretation. See, I can show you a site where a professional driver made the statement "To fix oversteer reduce speed and turn the steering wheel in the opposite direction of the skid." see how it can get convoluted. I know what the instructor that wrote this article means, but it can make thing confusing. When dealing with oversteer DO NOT OVER THINK THINGS. Whatever direction you feel the BACK of the car sliding, that is the direction you steer. If the back of the car, as in our diagram above, is sliding to the right, then you steer to the right.

Now the question becomes "How much do I steer in that direction?" Well I cant give you an exact answer on that. In fact, no one can. You steer as much as it takes to stop the rotation of the vehicle. I don't mean you stop the skid, I mean the rotation of the vehicle, two different things. It may only take a small steering input to stop the rotation or it may require steering all the way to the end of the steering rack. It all depends. When you feel that the vehicle has stopped rotating around, you can stop giving steering input. If when you stop it feels like its coming around more, get another handful of steering. 

So all of the above was the C part of CPR. If you have done all of the above correctly you will now be at P. Pause. 

Pause

The Pause can be VERY short lived or it can last for quite some time. How long the Pause lasts is dependent on several factors but the reality is, you are waiting for one thing. GRIP. If you have done everything else properly up to this point, then you have nothing else to do at this time but to wait for your rear tires to start to regain adhesion with the surface.

Once the tires regain their adhesion, the most important part of the sequence comes into play, RECOVER. This is also unfortunately the least educated portion of oversteer skid control. See, in driver's Ed you are lucky if you get taught to "steer in the direction of the skid". But even if you do get that information, that's pretty much where it ends. Well as you can see, that's only about a third of the required information needed to fix the problem. Only steering into the skid is what will have you "fishtailing" up the road. A fishtail is just a series of "steering into the skid."  Back and forth, steer to the right, whoa!, steer to the left. I got it, I got it. Whoa! Steer to the right again. I got it, I got it, Whoa! ........ And so forth and so on. As in the words of Metallica, You know it's Sad but True!

Recover

The Recovery means you have to take out all of the steering you put in to stop the rotation. If you do not take out this corrective steering, hold on! your going for a ride. 

Look at the oversteer example above. If the rear tires regain grip, where is the vehicle going to go? To the right, and in a BIG way. And where is your intended path? To the left. So definitely not a good scenario. 

Taking the Corrected steering out typically is going to require you to move the steering wheel faster than you did when you put the correction in. This is NOT always the case, but it certainly can be. 

So when you feel the rear end of the vehicle start to regain grip you need to start taking out the corrected steering input. This is also the reason you need to keep your eyes on your intended path. If you are not looking toward your intended path, your hands don't know where to steer to. 

So, if done correctly, in the diagram above, you would get off the accelerator, steer to the right, wait for grip, take out the amount of steering you put in. All the while keeping your eyes on your intended path. 

As I said many time before, this stuff cannot be taught in a book, or blog as the case may be. I can help give you a head knowledge of what you are “supposed to do. But unless you try this and practice it you will likely not fully understand it. I would strongly encourage everyone to attend some sort of driving school that teaches car control so that you can feel what these situations are like. If you would like to attend a school and are not really sure where to go, contact me and I’ll be more than glad to point you in the right direction. 

JM 

Feel your car!

What the heck am I talking about? I don't mean rubbing your hands down the side or caressing the dashboard. I'm talking about what the car is telling you through your sense of feel as you drive. Feel would be the second most important sense you utilize as a driver. Well, the third if you include common sense, which of course should be number one. So this being the case, the order would be; Common sense, Eyes and then feel.

There are several key areas where a driver gathers, "feel" information from the vehicle. If you utilize the seating position talked about in an earlier post, you will be in a better position to acquire the information from these key points. Lets break down these key areas.

First, HANDS. What information are you gathering through your hands? If you said steering wheel you are close, but that's a little over simplified. Think a little deeper. What ultimately is the steering wheel connected to that you could gather information from? The front wheels.

Lets say for example that you are rounding a corner and you drive over something slippery with the front tires. This will create a bit of understeer (front tire slip). The steering wheel will, depending on the severity of the slip, feel lighter than it originally did. It will become easier to turn. This is because of reduced friction, which means less force, which translates to a lighter feel of the wheel in your hands.

Or maybe you hit a pothole with your right front tire. This will be communicated primarily through the steering wheel. If the impact creates a flat tire, you should notice a change in the way the steering wheel feels in your hands. Likely you would feel a bit of vibration as well as a pull of the wheel to the right.

In order to gather this information through the wheel you should not be squeezing the steering wheel aggressively. Hold the wheel firmly, but don’t squeeze it. Also, having your arm extended straight out to the 12 O’clock position on the wheel with your wrist draped over the wheel is not going to give you adequate information or control.

Next point for information would be the “SEAT OF YOUR PANTS”. It’s the information that translates through the seat. Where does this information start? Suspension is a common answer to this question. Although this is not horribly wrong, it again is not specific enough. If the steering wheel is telling you what’s going on with the front tires, then the seat of your pants and your lower back area is telling you what is going on with the rear tires. In my opinion this feel point is the most important, especially when it comes to car control or performance driving. There is so much information that you get from the seat of a car. Acceleration forces, wheel spin, rear sideslip and suspension to name a few. With the proper seat position you are better postured to gather this information as well as give appropriate feedback.

Your feet can also tell you some information about the car that you may not think of. When I ask the question “What kind of information does a driver get through his/her feet?” a common response is “acceleration”. Now as true as this may be, when I ask for another option, most tend to draw a blank for a while. “BRAKING!” I’ll say. You can feel braking. Its funny the looks that I get when I say that, It’s as if people don’t think about the brakes pedal. BECAUSE THEY DON’T! When you use the brakes you can feel if you have activated ABS (Anti-lock Braking System) or if you don’t have ABS on your vehicle you can feel possible wheel lock. The more sensitively tuned in to the brake pedal you are, the smoother you can learn to utilize it. Or the more aggressive, whichever the situation may dictate. So yes, through your feet you can feel acceleration and braking.

So these are the key points of gathering feel information from your vehicle. It starts with proper seating and then you fine-tune your senses to the car through experience. Otherwise known as “seat time”. You see, you need the seat time to learn WHAT the car is telling you. You may have the proper seating position and your car may be giving you the information and you may be feeling it. But if you don’t understand or know WHAT it is that you are feeling, the information is useless to you. If you have never locked up the brakes on a car, how can you expect to know what it should feel like? Or if you have never had the front or rear tires break loose and slide, how will you recognize that’s what’s happening when it happens? I have literally been in the right seat teaching a student on a skid pad, the car spins out, and the student has no idea weather the back of the car slid or the front of the car. Well if that’s the case, how do you know what the proper reaction from you, as a driver should be? You don’t! That’s why a lot of people do nothing. They just freeze up behind the wheel, and that’s not good.

So as always, seat time, experience and education is king. There are several ways to get the experience of what its like to slide a car, lock up the brakes, utilize ABS and drive at the limit. The legal way is to attend a school. There are several driving schools throughout the country. Most of them work on all of these types of things and have instructors that will coach you along the way. Some of these schools provide cars for you and some you may bring your own. The schools can range in price from as low as a few hundred dollars up into the thousands of dollars. Find one that is within your budget and give it a try. Remember, even if the cost of the school may seem expensive, it’s typically cheaper than the cost of one accident. And that’s provided no one in the accident gets hurt. When you factor in human injury, well, there’s really not a price that should be placed on that is there? Especially when there are schools out there willing to teach you how to reduce or prevent these situations to begin with.  Ya feel me?

3, 2, 1 CONTACT…….................”patch” that is.

I have already talked a bit about tires and how important they are and how they should be monitored. Now let’s dig a little deeper into the tire and talk specifically about that palm sized “contact patch”, You know, where the “rubber meets the road”. In order to discuss the contact patch, we will also be talking about weight transfer, how the vehicles weight moves around as we drive.

Tires are way more complex than most people give them credit for, but we’re not going to get into the construction of the tire but more the function of the tire on the surface of the road, as well as how you as a driver effect that function.

An average tire, as I have said a few times, has a contact patch about the size of the palm of your hand. Another way to consider the contact patch size is to take a standard size sheet of paper and fold it in half and then half again. The rectangle shape you now have is about the size of the average contact patch. So as you are driving down the road, the only thing keeping you connected to the surface of the road, is a patch of rubber that totals to be about the size of a sheet of paper! Yea, keep thinking about that for too long and it just might scare you away from driving all together. Now let’s add to the fact of this relatively small contact patch, that as we brake, accelerate and turn the steering wheel left and right, we are constantly changing the distribution of the contact patch on the road surface. 

Cars move on three basic axis, Vertical, transverse and longitudinal.  (Fig. 1)

The vertical axis is the axis the vehicle rotates on if its spinning, also called the “yaw” axis. The transverse axis comes in to play when braking and/or accelerating. And lastly, the longitudinal axis is what the car moves on when cornering. 

                                                                               Fig. 1

 

And let's say we are starting with a vehicle that has equal weight distribution across both axles. This is to say that half of the vehicles weight is supported by the front axle, and the other half by the rear axle. This set up provides four equal contact patches on the road.  (Fig. 2)

           

                                                                           Fig.2

 

As long as the vehicle is either sitting still or if it’s driving in a straight line at a constant speed, the four contact patches will remain equal. But lets say that we press the brake pedal. When we do this the weight of the vehicle shifts to the front. You know, when the smelly little pine tree hanging from the rear-view mirror swings toward the windshield. This shift of weight to the front applies more weight to the front axle therefore making the front contact patches a little bigger. Much in the same way that if you had a beach ball sitting on the ground it would have a certain portion of the ball touching the ground, but if you sat on it, more of the ball would be touching the ground. Same concept. 

But, because a vehicle moves the way that it does on these axis, the weight that was added to the front, came from the rear. This means the rear axle now has less weight on it therefore the rear contact patches become a bit smaller.  (Fig. 3)

                                                                          Fig. 3

  

Obviously the above figure is an exaduration, but I think you get the point. So with that said, then obviously when we accelerate in a vehicle just the opposite happens. Weight moves to the rear axel and increases the rear contact patch and the weight came from the front and reduced the contact patch size on the front tires.

The same happens when you turn the steering wheel except it’s a left to right thing rather than a front to rear.

So this is how the “vehicle dynamics" effects the contact patch, and I’ll talk more about the importance of contact patch "management" later. Probably in a post about skid control. For now I’m going to talk a bit about the contact patch itself. 

There is a way of looking at the contact patch called the “friction circle”. It’s a simple way of looking at a tires grip on the surface.

I’m all about simplicity, so I will explain this as simply as I know how. Well, simple is actually the ONLY way I know how to explain it, so its more about my ability than yours.

The circle below will represent the “limit” of the tires contact patch on a dry road. (Fig. 4) This means that if you go outside of that black circle, the tire starts to slip at a high rate. So if accelerating, it means wheel spin. If cornering it means the tire is sliding sideways. And if braking it means you have locked the tires and it’s skidding.

                                                                                         Fig. 4

 

 

Now,  If its raining and the road is wet, the “limit” becomes smaller because the surface is more slippery. (the blue line Fig. 5)

                                                                                           Fig. 5

 

And of course if you were driving on snow or ice, that circle would be even smaller yet.

So lets say that we are driving on a dry asphalt surface and we end up in a moment where we have to break pretty hard. Maybe we didn’t have our “eyes up” and we were caught by surprise by a stopped vehicle in front of us. So we apply the brakes hard. (Fig. 6)

                                      Fig. 6

 

 

As you can see I added the directional forces that are applied to the contact patch. Yes, I do know my right from my left. Again I am showing forces applied. So for example, when you turn the wheel to the right, the force on the contact patch pushes left.

You can see that in figure six we are braking close to the limit of the tires ability, but still inside of the tires capability. All is good. 

Now let’s say that we realize that we are closing in on the car in front of us too fast and we are not going to be able to stop before making impact. This means we are going to have to swerve to avoid. We find an opening to the left and we turn the wheel. (Fig. 7)

                                 Fig. 7

 

 

Again, you can see that the red line shows that the forces to the left are still within the limits of the tires grip., therefore we successfully avoid the incident without any loss of traction right? …………..WRONG!

When you vector the two lines, they meet OUTSIDE of the limit of grip. This is because you are utilizing both forces on the tire at the same time. (Fig. 8)

 

                                            Fig. 8

 

So, as i said, you see that when you vector the lines, the two points meet OUTSIDE of the tires limit of grip and now you have a tire that has lost traction. A tire can only do 100% of one thing. This means you can use the brakes all the way to the limit or you can accelerate to the limit and you can corner to the limit. But of you are braking at the limit and then you try to corner at the limit, the tire is not going to be able to handle both of those inputs at 100% and it will lose traction.

Does this mean that we were not able to avoid the incident? Not necessarily, it simply means that the tire is sliding and now you have to "manage" it. You have to get the tire to function back within its limits. You can do this by reducing your braking or reducing the steering input or both. And, provided you have the time to do this, you could still avoid the incident.

Also, these days we now have vehicles with systems that can recognize when you have gotten yourself beyond the tires limit and they can assist you in getting the vehicle on your intended path. Some of these systems are known as; traction control, stability control and ABS. I will do a separate post on how these systems work. In the situation in this example, the systems would have come into play.     (Fig. 9)

                                     Fig. 9

 

The yellow line shows that the systems would operate to keep you near the limit to try and maximize both of the requests you have made, within the tires ability. All you really need to do is stay on the brake and steer the vehicle, the systems will do the rest.

So that’s a quick and dirty way of showing you how inputs to the vehicles control systems affect the contact patch. In a more complicated but realistic picture you would see that a real contact patch is not a circle, it’s more of an oval type shape. (Fig. 10) below is a picture taken by a camera mounted under a glass plate as a vehicle drove over the plate. This shows an actual contact patch. The patch and its size will also vary dependent on tire pressure. I talked a little about that in the post about preparing to drive.

                                                                             Fig. 10

The reality is, the example I have discussed, no matter how detailed (or simplified) is only for one specific moment in time. Its what the tire is capable of at that moment, at that time, on that specific piece of asphalt, at that speed, with that steering angle. As the vehicle moves, the tire (contact patch) is constantly changing the surface that it is in contact with. Maybe it started out on a perfectly clean dry spot of asphalt, but then transitioned to a spot that had a little sand or oil on it. Whatever the case may be, its an ever changing scenario. 

A lot of race cars have data acquisition on them and its really cool to look at data and look at the plot points from the G-meter. They use these meters to show where the car can be faster and where it had more or less grip. It can also show the driver where he can gain time/speed by utilizing the vehicle a bit more to its limits in some areas. And where he/she may be losing time/speed due to going beyond the tires grip level in some places. It’s a cool science, and although you may not be into racing and you feel you may never take a tire to it’s "limit of grip". The reality is, the tire has no idea if it has gotten to its limit or gone beyond its limit because its on a race track and the driver is trying to get the most out of his/her contact patch in an effort to go fast, or if its on a highway trying to avoid a head on collision with another vehicle. All the tire knows is that it has been taken to or beyond it’s limit and it will react accordingly. It’s now up to you as a driver to manage the rest. 

What?......................you mean they didn’t explain that to you in Driver’s Ed?..................... I’m shocked!

Now give you're sixteen year old the keys and send him/her on there way. Scary isn't it?  It's no wonder the leading cause of death for teens is auto accidents.

JM