Vehicle Stopping Performance

There are actually two stopping distances that apply to your car. One is stopping without locking your wheels into a skid, the other is skidding to a stop with your wheels locked.

Threshold braking will provide the greatest stopping force

When driving on a wet road, for example, applying your brakes to a point that is just prior to wheel lock–up (threshold braking) will provide the most powerful stopping force. Locking your wheels into a brake induced skid on the same surface will produce a far longer stopping distance.

However, whether you lock wheels under braking, or use threshold braking, when on a good clean and dry road surface, there will be practically no difference in the amount of distance to be covered before bringing the vehicle to a stop.

Tyre grip is created through friction

As we have already discussed in part–1 of this series of four articles, the tyres of a car only grip the road because of down force. Down force, for a road–going car, is created by nothing more than its own dead weight (mass), as without gravity, no matter what the road conditions, there would be no tyre grip.

Kinetec Energy, Mass and Velocity

If down force gives us tyre grip you could be forgiven for thinking that if you made the car heavier that would give you better tyre grip. That is actually true, but it won’t give you any better road holding or stopping performance, because that additional weight needs a lot more braking force to stop. It will also need a lot more effort to get the vehicle to steer around a bend.

Any extra grip you create by adding more weight is immediately overcome by the additional Kinetic Energy created by the added weight (mass) in motion (velocity).

The down force present within a normal road car is organic down force. That means it occurs naturally through gravity. You may be more familiar with down force when used as a term connected with a different sort of motoring activity, that of formula one racing perhaps? A formula one racing car has a terrific amount of down force, but unlike the road car, this down force is mainly created artificially and not by gravity. It is what you might call manufactured down force.

Down Force For Formula One Racing Cars

The formula one car is equipped with large wings, both at the front and at the back, but these are not made for flying. It is true the wings for this application do work as aeroplane wings, but instead of generating lift, they work in reverse, pushing the car onto the road surface. This means the faster the car travels the greater the level of down force is generated, but without increasing the vehicle mass. It is even rumoured that when driven at the right speed, a formula one car can actually drive upside down on a ceiling!

The advantage with manufactured down force is the amount of additional ground pressure this creates is disproportionate to the mass of the vehicle, and in fact, as the wings are made of carbon fibre, there is little additional dead weight added to the car. Adding organic weight to the road car, of course, means you are trading the additional tyre grip performance gained on the one hand against the additional effort required to change direction, or brake the additional weight on the other.

It therefore follows that any advantage the adding to the mass of the vehicle provides in terms of added tyre grip it is defeated by the additional kinetic energy produced by increasing vehicle mass. When it comes to stopping performance the greater the mass the more distance will be required, providing the braked wheels are still revolving.

However, once the wheels lock into a skid, as a consequence of excessive braking, the mass of the vehicle becomes irrelevant as different vehicles will skid to a stop over the same distance from the same speed (and on the same road surface).

Mass and velocity cancel each other out and that is why a 3.5–tonne fully laden panel van will skid under braking for the same distance from any given speed to a stop as a small saloon car (providing the comparison test is completed on the same patch of road under the same conditions and from the same speed).

Even a Giant Block of Tyre Rubber Will Skid as Far as a Car

To take this comparison test further, it matters not what the difference in mass of the compared vehicles, nor does it matter if there is a difference is in the number of wheels and tyres fitted, because from skid to stop from the same speed on the same patch of road surface, and under the same test conditions, all will use up the same distance. You could even test this with a giant block of tyre rubber, as that too will cover the same distance from skid to stop as any size of wheeled vehicle.

To put it mathematically, it can all be summed up by one short piece of formula,

How Police Collision Investigators Use Skid Marks

In the course of investigating serious Road Traffic collisions a great deal of effort is put into determining what took place, and much of the evidence that is obtained comes from marks that have been left on the road surface by the vehicles involved.

Tyre marks, for example, provide a wealth of information, whether these are striation marks (the arc–shaped marks from critical speed cornering) or those that have been made when the car has skidded under braking. In order to use the skid marks left at the scene, collision investigators have to first work out a known value, which is then put into the mathematical formula to yield the information required.

Modern technology has provided more scientific ways of gaining the data for the necessary calculations, but a description of an older method is easier to visualise and will help illustrate the point concerning the distances travelled by vehicles with locked wheels under braking.

Police road collision investigators will produce a great deal of evidence
from performing skid tests at the scene

Typically, a police patrol car would be used to perform skid tests at the scene, but one that had been modified to allow the ABS to be turned off. Fitting a special chalk gun to a body panel of the car, and connecting the electronic trigger switch to the police car brake pedal, provides a marking apparatus with which to conduct the skid tests.

The chalk gun is like a rifle and fires a pellet of blackboard chalk by means of an explosive charge. With the gun mounted so that it is pointing vertically at the road surface a reference mark will be made when it is fired.

With the car so equipped, the collision investigator would drive over the same section of road as that upon which the collision took place. Whilst driving at a known speed, usually 40mph, the patrol car brakes are applied so hard, and so suddenly, as to lock the wheels into a skid.

A known value gained from skid tests
will be factored into the skid marks found at the scene

As the chalk gun is triggered by the application of the brakes, a measurement can be taken from the pellet mark to the point where the test vehicle came to rest, thus giving a known value that represents a vehicle performing a skid to stop from 40mph.

This skid to stop test would be carried out three times and the results added to each other and then divided by three to get an average. By using this method the speed at which the test car was travelling when it made the skid marks is a known quantity. This data is then factored in to the data provided by the marks as made by the accident vehicle, from which the speed at which the accident vehicle was travelling at the point when it started to slide can be worked out.

How Skid Marks on The Road Don’t Tell The Whole Story

Whilst this method will tell you the speed at which the collision vehicle was travelling when the driver locked the wheels into a skid, it will not tell you how much speed was lost prior to wheel lock–up. Therefore, the speed calculated from skid marks at the scene will always be less than the actual speed of the collision vehicle. Therefore, the evidence is presented at court will show the speed calculated minimum speed.

In case you are wondering what happens when the collision vehicle hits another object, and how that is factored into the calculations, there is some quite specific data available concerning the amount of intrusion and crush damage caused when vehicles collide at certain speeds. Therefore, by taking measurements of bodywork distortion and collapse, it is possible to find out what collision speed will have been necessary to create such damage.

Crash testing in controlled conditions has produce a lot of useful data

This sort of data comes from the crash testing as carried out by establishments like the Transport and Road Research Laboritory and the Thatcham Research Centre. Anyway, this is beside the point, so let’s get back to skidding vehicles.

Once all the data from the accident scene has been gathered, and the calculations made, the whole package is sent away to a forensic scientist for verification.

The above described method of road collision investigation is widely used the world over and is a proven method of reliably calculating the speed of a vehicle from information taken from skid marks left at the scene. The formula that is used is always the same.

The point of telling you all this is to show that all types of vehicles, from the smallest car to the biggest truck, different sized vehicles with different numbers of wheels, when skidding under braking will slide for the same distance as each other. It doesn’t matter that the test vehicle is a different make, model or size.

Do Wider Tyres Improve Brake Performance?

The answer to that is no. Just because you have put more rubber onto the road doesn’t give you a greater ability to stop. In fact, for some circumstances, wider tyres can actually make things worse.

It is true that the wider the tyre the more rubber you have in contact with the road surface, which of course makes a larger tyre footprint. However, you are now spreading the weight of the car over a larger surface area, which means there is less ground pressure being applied per square centimetre of tyre contact area.

To try and put that into perspective, imagine finding yourself walking across an area of soft mud. If you take a piece of board, place it on the soft ground, and then stand upon it, you will find you can so do without sinking. Conversely, if you step off your board you will find you will sink. In both circumstances your weight has remained the same, and yet in one case you sink, but in the other you don’t.

Standing on the board spreads your body weight over a larger surface area, so that at any given point across the surface of that board there is less pounds per square inch ground pressure.

Wider Tyres Give a Bigger Contact Area With The Road

With the wide tyre, whilst the amount of rubber touching the road is greater, there is less down force per block of tread pushing onto the road surface. This means each individual tread block is providing less grip, but as there are more of them, collectively they provide the same amount of overall grip as a narrower tyre on the same vehicle.

Whilst those two factors are cancelling each other out all the way up and down the scale, the kinetic energy pushing forward, and which you are trying to overcome, remains the same, as does the mass of the vehicle. Remember, this is a linear grip we are dealing with and not a lateral grip. Lateral grip is involved with cornering and not braking.

Wider tyres are more likely to aquaplane

Wider tyres can be a handicap as they are more susceptible to a condition known as aquaplaning when the roads are wet. Aquaplaning, which is covered in greater detail within another chapter in this series, occurs where a vehicle is being driven in wet conditions and the tyres are not able to disperse the water from the road surface quickly enough.

What you get is a cushion of water collecting under the tyre foot print, therefore separating contact between tyre and road surface. The wider the tyre the less effective it is at dispersing surface water. Think about the surfer on a surf board, or how water skis work. You wouldn’t be able to ski on water using ice skates would you?

Formula one racing cars have wide tyres
and their ability to stop is phenomenal

We’re back to the difference between organic mass and manufactured mass. Remember that the formula one car gets it downforce from the wings on its body, which means it can increase downforce without adding to its mass. This also means there is very little additional kinetic energy created, as to increase downforce by increasing mass for the road car increases kinetic energy

What is interesting with the formula one car is what happens as the racing driver applies the brakes. At high speed the stopping power is immense. However, as the car slows the amount of downforce steadily deteriorates, as the wings have less airflow over their surface, and so tyre grip (and therefore stopping power) is reduced.

The Work of The Tyre Tread

The tread pattern on a tyre is put there to disperse water so as to allow the tread–blocks to make good contact with the road surface when driving in wet conditions. Different tyre manufacturers have completed a great deal of research and development in producing what they believe is the optimum tread pattern to deal with that task.

Modern tyres have all but reached the end of development where their ability to disperse water is concerned, and although manufacturers are experimenting with different types of tyre material, and within the limits of legislation, the linear force exerted when braking means that there is no appreciable difference in a tyre made by one manufacturer than by another.

On a dry road a bald tyre is capable
of gripping the road better than one with tread

This is supported by the fact that a road tyre worn completely bald, albeit illegal, will perform just as well in stopping your car on a dry road as a brand new one. It’s when driving in wet condition the bald tyre will have you skidding all over the place.

Once a tyre is locked into a brake induced skid, the contact point between itself and the road surface will become very hot. Thererfore, when driving on a road made up of a bitumen compound dressed with stone chippings, that heat will very often be sufficient to soften the bitumen base, bringing it to the surface through the layer of stone chippings. In other words, the tar that is bonding the chippings together begins to melt and shows itself above the surface.

The driver will always have to work within
the bounds set by the laws of physics

However, if the road is wet, the skidding tyre will be sliding on a film of water, which not only has a cooling effect on the contact area, but also lubricates and reduces friction. This is why a skidding vehicle is less likely to leave black lines (skid marks) on the road than when skidding on a dry road. Tyres on soft road tar can easily slide, as will tyres on a film of water, regardless as to who makes them, or how many X’s or Z’s make up the names embossed on the tyre walls.

The moral of the article is, you can have the most expensive and the fanciest road tyres money can buy, and you can have the most powerful brakes in the world, but when it comes to stopping a moving vehicle, the limiting factor will always be set by the laws of physics — and there is no getting away from it.

Julian Smith
Ride Drive Limited

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Part–1 Effective Braking Techniques
Part–2 How a Car Braking System Works
Part–3 Vehicle Stopping Performance
Part–4 Effective Braking Techniques

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This page was last updated
Tuesday, 15-Feb-2011

Vehicle Stopping Performance