The road surface is the counterpart to the tire, which are in the business of generating grip. Therefore, with the exception of understanding the tire’s side in the equation, one must strive to understand the way in which road surfaces vary in the grip levels they give, in the tire matching to it, and to the driving style of it. Another important aspect of the road surface for us the deal with is the forming of it into a pattern which we call “road” or “track”.
The Coefficient of Friction
Road + Tire = Friction. Take given friction and add roll motion, plus vertical force, plus heat, and you got yourself a type of friction that is called “static” friction. And that, ladies and gentlemen, is what you call “grip” or maybe “road holding”. Add changes to the variant of the roll motion (be it a change in speed or direction) and you have “adhesion” that generates either acceleration, deceleration, or cornering force. Changing the amount of friction changes the overall amount of adhesion it can generate, and this means how much a tire can accelerate, slow you down or turn you aside, and combinations of the above. If a driver goes over the limit, he gets his car to slide.
If “grip” is static friction, “sliding” is represented by “dynamic” friction. In fact, the contact between the road and the tire is never perfect. Even the smoothest of tarmac surfaces is slightly bumpy at a microscopic level, and even the best tire is never “perfect”. Hence, static and dynamic friction (grip and slip) will always have to coexist. When you are at “full grip”, you actually have, say, some 99.90% static friction (grip), but also a 0.10% of slip. Even a tire that is fully sliding on a skidpad, will still have that 0.20% of grip and slide only with 99.80% of it’s overall fricative potential.
Every two objects in contact in this world would experience friction between them. Just how much friction can be generated by the relationship of a tire and road is defined by the following equation: F=µN. “F” Stands for “Friction”. “µ” (The greek letter Mue) stands for the coefficient of friction and “N” stands for the normal force, which is the vertical load that presses the tire down on the surface. So, this complex math stuff, if you are not analytical in mind, is that the maximal amount of grip a tire can generate is the coefficient of friction, multiplied by the force that presses the the road and tire together.
So, what is the coefficient of friction? It is simply how “grippy” the surface and the tire are. This is where the quality of the road surface comes into play. The “normal” contact is tire (rubber) and road (dry tarmac). They include a shared chemical agent: tar. The connection between the two is generated by the distortion of the rubber fibers. The distortions allows the fibers to squeeze into the little imperfections of the road surface and then the tire rotates, which moves you forward and continue to grip another patch of tarmac further ahead (or/and to side in a corner).
The connection between a good tire and good tarmac, can increase the coefficient of friction. Formula-1 can now reach over 4.0Gs on some smooth circuits. Reduce the quality of tarmac into crumbling and bumpy tarmac, and you can reduce that number significantly. Levels of grip in the area of 0.8-1.0Gs can be reached in a road car on good road tires and on standard road tarmac. Bitumen, tile-roads, bright or scrubbed tarmac all offer slightly less than that level. In general, look for solid, rugged and dark tarmac and, where possible, avoid bright, crumbling, dusty patches of tarmac, and “black snakes” which are in fact repairs made of fresh tar.
Soak the tarmac in water and you got a different story at hand. Tire rubber is not meant to mangle with water, period. It’s only hope, is to run through the layer of water and into the tarmac below. That is the ONLY reason why road tires are not slick-smooth. The tread formation reduces the amount of rubber that contact the dry pavement, but allows to drain water when it’s wet. Tarmac, either dry or wet, has a single disadvantage: on tarmac you have lots of grip, until you no longer have grip. The transition to sliding occurs sharply, unnoticeably and at speed. This is part of the charm of track driving: keeping the car just shy of breaking away
Obviously, there would be dependencies on water depth. Deeper water is obviously more slippery. A flooded road can render a tire almost completely segregated from the tarmac, and this is known as aquaplaning or hydroplaning. The grip levels of a practically floating tire, are lower than those encountered on smooth ice! Yet, the grip levels of a normal wet road with a normal road tire are really not so problematic, and are approximately 0.6-0.5Gs. Some wet roads carry similar levels of grip to some bumpy dry roads!
One factor can change this comparison dramatically and that is how dry and how busy was the road before it got wet. Some of you readers may live in more sunny and drier parts of the world, where some bit of rain is expected on, say, one or two months of the year. The driving public in these areas tends to be less aware of how to drive on slick surfaces. Let’s leave the subject of bumpy and dry roads, sand and other slippery things, and even the subject of bad, worn or under-inflated tires and maybe faulty dampers too, the fact is that the more dry is the road for more time, the more dangerous it is when it gets wet.
Why? Because the road suffers from the cars going over it. Dirt and grease fall off of cars, rubber debris is scrubbed off of tires, oils and glass are spilled over it from crashed or faulty automobiles and bikes. Dust, dirt and maybe leaves, tend to reach the road from the shoulders also. Foliage and paint are massed on the road surface, particularly in urban districts. The road itself becomes dry and hot. When exposed to water, the road reacts to this whole mixture. Especially during brief showers because the dust, oil and foliage are not washed away. The grime may be so bad you can actually see the a bright layer of oil and dirt floating on top of the water. As a Las Vegas resident, I can attest to the numerous accidents that occur every time we have a rain shower. Accidents everywhere!
The grip levels encountered by first rain are surprisingly low: nearly 0.3Gs! When it does starts raining for the first time after a long dry period, drivers are yet to readjust their driving habits to the low levels of grip. First rain feels even more slippery. You can expect to see this situation on popular race tracks, especially on the racing line and on the edges of the track (and on busy urban intersections of public roads). Freshly painted parts of road are especially hazardous when in contact with water.
Another problem is that on a long ride, some of the road might offer high grip however wet, while other parts have half that grip due to the existence of oil and contaminants. The trick is to look out for ANY rainfall that arrives within about a week of dry driving, and also look for slippery surfaces in areas where more cars drive more slowly (intersections, roundabouts and bus stops for example). Also be aware when the road is at an incline in which case grease will be stored at the edge of the downhill slope. It’s also beneficial to look out for the edges of the road and for areas deemed slippery due to constant existence of acrylic paint, leaves and sap from trees and dust from the dirty shoulder of the road. Slippery roads would appear glimmering or seem to be covered with murky water, sometimes with visible traces of oil, mud or leaves.
Water is not the only thing that makes roads slippery. Not nearly so! First rain, to remind you, is rendered so slippery because dirt, sand, leaves, oil and alike, tend to mix with it. Likewise, each of these by itself, is usually quite slippery. Many people think of skidding as a situation that occurs merely when the driver is taken by surprise by a patch of ice or a stain of oil, and are taken by surprise when their car suddenly breaks away violently on gravel or mud. These elements can prove more slippery than a wet road, and maybe even than the fearful first rain. First, all sorts of foliage, oils and dirt are being piled up on the road surface. Nightly dew can create water puddles simulating the aforementioned “first rain”.
In short, rain is more slippery where:
- The road is already slippery: bad tarmac, bitumen, dust, tar “snakes”, tile roads, etc. all become more slippery when wet.
- Where traffic congestion occurs or cars move slower in general: intersections, junctions, bus stops, or the racing line on the track.
- Inclines and edges of the roadway: However, in many cases, these are also the areas which dry out faster. The dry racing line becomes extremely slippery on wet tracks, but it’s the first part that gets dry again. However, the water might set oil and mud traces afloat so that parts of the seemingly dry road are still slippery.
Aside from the whole discussion on tarmac, we have “soft” surfaces that are described as “soil”, which includes dirt, sand, gravel, and mud. When roads are covered by one of (or a combination) these surfaces, the grip levels are reduced but, with similarity to a wet road, the tire can dig through to find the tarmac below. When off-road, the equation is different and depends on various factors. First, we must distinguish between dirt, sand and gravel, to surfaces like clay. Dirt and sand are typically described as “cohesive”, because friction tends to build up not only between the tire and the sand, but also between the particles of the sand. Dune-conquerers are well aware of the need for BIG tires and maybe for lowering the tire pressure so that the tire contacts more sand, which than contacts more sand below, etc. Furthermore, it’s important that such a tire would not be too studded, so that it would not dig itself into the sand, which creates a “step” for the tire to climb on. Under-inflation carries an additional gain in actually giving the tire a concave shape that tends to “trap” some sand inside it.
Clay, for one, is not cohesive and is therefore more like tarmac. It requires tires that are known to be more “aggressive”: they have a tread and studs that latch into a lump of earth and push forward, generating movement. Rocks are also very slippery, but heavily treaded tires can latch onto them through cracks and slops. Soil becomes more grippy when it’s compacted, but compacted soil is like tarmac in the sense that once grip is lost, it happens sharply, with little warning and at speed.
Mud is a different story. Sand and gravel can, with the right car and tires, give the driver a sufficiently large amount of grip. Certain types of sand mix with water without creating a large change in grip levels. However, rich soil abundant in rural districts (or loess sand), mixed with animal droppings and a sufficient amount of water can create a layer of mud, on the road or off of it, and this is rather similar to snow or quicksand, as anyone who had an embarrassing tumble over some mud would know. This mud is one of the most common and dangerous causes of slides. The amount of grip it offers, goes not only below those of wet roads, and even first rain, but also below those of some oils! It is in fact almost as slippery as snow, a sum of about 0.25Gs!
All these surfaces, and snow also, suffer from another interesting characteristic. Being of a soft nature, a wheel tends to dig into them, especially once it has begun sliding away, either when locked or when sliding sideways. The tire then slows down, builds up a high layer of the sand in front of it, which gets it to slow down and grip again faster. Also, it can reach other layers of the surface, be it more grippy (say, on gravel or on a road covered in sand or mud) or maybe less grippy (like black ice under snow). This is also why ABS and threshold braking work poorly in such conditions and is part of the reason why rally drivers tend to adopt a more aggressive driving style. With these surfaces, it becomes more important to get the car to slide a bit more than on the dry and the wet.
If the dirt or sand are compacted, the grip levels become higher and the car acts more naturally for tarmac drivers. The advantage here is that once the car has less grip, it will reach the limit more quickly and slide at a slower speed. Soft surfaces that have been compacted therefore offer a progressive car (rather than one that suddenly breaks away when not expected and at speed) on good-grip tarmac.
Another important subject that comes into mind here is split grip. This is a known and very hazardous situation where cars tends to slip towards the gravel or grass shoulder of a rural two-lane roadway. A common cause is exhaustion on long straight roads in the early morning by younger drivers, but it can also be the result of an un-calculated evasive maneuver. So, one side of the car is on the tarmac. Another side is on gravel or grass, and also usually about an inch or two lower than the road. This creates instability that hurts not only grip, but also the vehicle’s stability. A panic attempt to make a recovery back onto the road or an attempt of braking in panic and hesitation is common. The approach would commonly be slowing down controllably. Agressive adjustments make the car spin and swerve, or even veer sharply at interstate speed towards oncoming traffic.
Snow, Ice and Oil
Now we have to deal with the more slippery stuff such as oils and snow/ice on the road. In countries that do not suffer from snow and ice, the most fearful road condition, is for the driver to find the road to be covered in some sort of oil: motor oil, diesel, bearing grease, etc. Oils, like water, do not like tire rubber. However, they are more sticky and hence harder for the tire to clear out of it’s path as it tries to grip the road surface. However, not all oil-based agents are similarly sticky. Diesel and frying oils, for instance, are relatively thin, hence being relatively grippy in comparison to other oils and even to mud. It is more similar to first rain than it is to stickier oils like motor oil (and transmission lubricants even more so), which is extremely slippery indeed.
One must add that this depends on the amount of oil, it’s temperature and mixture with agents like water or sand. Sand tends to soak oil, and is therefore the common tool for covering oil spills from cars, especially bigger rigs. Water tends to create a colloid when coming in contact with oil. Sadly many oils are even hydrophilic, hence drawing water almost instantly. Water decreases grip levels of all oils to such similar of those of ice.
When dealing with with the other trouble that water causes, (snow and ice) we have a slightly different story to tell. Snow suffers from the same characteristics of mud. It is very slippery, requires similar types of tires, is encountered on relatively similar conditions, be it on the road or in the wheel-tracks on off-road tracks. In times, mud and snow mix together. Snow tends to offer slightly lower grip levels, again depending on how cold, dense and fresh it is. Fresh snow tends to be relatively grippy because the wheels can dig through to the tarmac more easily. When the snow becomes more dense and cold, it becomes harder for the wheel to dig into, there is less scrub when it is sliding, and there also tends to be frost below. Frost by itself, and likewise hail or melting snow, is more grippy than the snow (0.3G and even 0.4Gs!), and yet is more slippery than tarmac.
At a certain point, the frost turns into real ice, but the grip levels are yet to suffer from a dramatic change. This change only occurs when the road actually becomes frozen. This is especially dangerous when the ice is thin and watery enough, for it to be nearly fully transparent but, unlike with frost, not accompanied by white coating. This is known as patches of “black ice”, because the driver sees black tarmac, which is actually covered by thin ice, especially on parts of the road which are shaded. This also carries the risk of split-grip: of one side of the car or even one wheel alone, boarding the ice, hence making the car unstable rather than merely very slippery.
In general, the danger of hazards like a stain of oil, a patch of ice or snow, a colis of mud or “marbles” of gravel, is that instead of the slide being initiated by a bad driver’s input, it is being in part caused by a sharp change in grip levels. This change, if drastic enough can make any driver’s input harsh enough to create a slide. This type of slide is usually the worst kind, and very hard to control, perhaps even impossible at times. One problem with a stain of oil, patch of ice and even a puddle of water, is that the front wheels tends to enter the puddle earlier and slip (understeer). Then, the rear wheels enter and start to slip. Depending on the size of the puddle, the front wheels might exit the puddle and maybe dry-out the water/oil when the rear wheels are still in it or soaked, resulting in the rear wheels sliding and probably braking away.
Bad tire maintenance, especially if the driver did not place sets of similar tires on each side of the car, or if the rear tires are very worn, can contribute to this. One must also bear in mind the amount of understeer a car is set to generate and the inputs of the driver in the given moment. It is very common for a puddle of water or oil to be uneven in depth on each side, so that the car tends to drift to one side, an attempt of steering correction back on the right track, which is often not required because the deviation from straight is little, results in a terminal slide.
This is perhaps the main reason for the installation of ESP systems, along with other precautions. Today, cars and bikes are equipped with shielded oil tanks, and paint used for signs like road-crossings, are either mixed with the tar before it is set unto the road, or made out of none-oil-based color that is being mixed with a rugged mixture like sand, salt or powder of crushed glass, that tends to increase grip levels. Salt and dozers are also used to clear out snow-packed roads. These counter-measures carry a downside, because people have become unaware of what such a hazard looks like, therefore being incompetent in detecting the glare of ice or oil and having detected it too late, are unaware of how to deal with such hazards.
As ice becomes thicker, grip levels are reduced. While black ice is normally most hazardous due to the explained facts, “colder” ice tends to be quite more slippery and not matched even by the levels of grip offered by oils. However, the reduction in grip levels is not perfectly parallel to this graph. The ice actually becomes most slippery, when it is only cold enough, so that there is a fine layer of solid ice, covered directly with a thin layer of clear water, and not by frost or snow. Here, the grip levels are lowest, and can only be compared to those of a skidpad (which is based on epoxy-paint soaked with water) or by a hydroplaning vehicle.
One other important note about frozen water (in the form of snow and ice) is it’s surprising rigidity. Packed snowbanks can be quite very solid to hit. Likewise, frozen lakes in cold parts of the world tend to build up to the depth of 1-3 feet and, within that depth, can take loads of dozens of tons!
But, hazards posed by the road surface don’t merely sum up to slippery surfaces. They also include cracks and potholes, bumps and unwanted inclines, shards of glass or tin, rocks, etc. If the loss of grip caused by a sudden change in the road surface is hazardous, a skid caused by the car bouncing is in fact worse.
Roads and tracks are designed and built under calculations of safety, dynamics and economy. The road itself, along with some of it’s basic characteristics, is actually based on the prototype of important Roman roads. Roads that are solid, smooth, long and in times go through obstacles like hills and over streams and rivers. Roads are divided into lanes and are designed with a slight incline to drain water and mud.
A straight road is the best in many aspects, and many roads in the open areas of the US are like that: fast, narrow, long and straight. The obvious advantages are the safety increase by the lack of corners, plus decreased travel times, less traffic, less gas consumption and pollution, etc. The normal drawback is that such a road becomes very expected and boring which not only makes drivers less focused on hazards in long drives, it also shapes a driving culture that tends to revolve mainly around the driver’s comfort in the car.
European roads mark a different type due to the geographical nature of the land. Winding mountain roads with the scenery of trees and mountain tops. This leads to a more interesting ride, and creates curiosity towards dynamic aspects of cars. The downside is the overall larger amount of tarmac used, the more means to be applied to build the road on mountain sides, over and through boulders, across gorges in the form of bridges, building more guardrails, etc.
Roads require more regular maintenance depending on the amount of traffic and the changes in weather. In tropical countries with volatile weather, changing often from cold to hot, from wet to dry, roads deteriorate very quickly. There are several means for “fixing” such a road, depending on the severity and spread of the damage. Often, little pieces of tarmac are being crumbled, normally by a combination of water draining and a collision that made the bottom of a car scratch some part of the road surface. The solution here is to pour fresh tar into those holes. Depending on the quality of the tar, it would either assimilate quickly into the road surface or alternatively form a line or “serpentine” of black tar. This serpentine generates a small change in grip levels, which can be hazardous for bikes and cars are also advised to try and go around a big bunch of such obstacles if possible.
Another choice altogether is to pave a road of concrete. Concrete is far more durable and can last 2-3x longer than that of a tarmac road before it needs to be repaved. The downside is difficult and costly repairs, as well as less grip. Modern tarmac-paved roads, especially open roads, follow very high quality standards, which require a more expensive installation, yet pay off due to less need for repairs and more grip. New roads include various layers including rugged materials like gravel and sand. Still, busy roads in tropical locations can benefit from concrete paving due to more durability and less pollution.
For larger patches being repaired, tarmac is often used creating near-equal grip as the rest of the road, or a slightly less grippy patch of concrete may be used which has turned very popular in some tracks. Concrete is less grippy, and should be avoided where possible, but it still gives you good grip, and when you drive over a patch of it, it tends to be pretty smooth, with the exception of the edges, which are quite rough. Concrete patches have turned into a common phenomenon on race tracks and have in times turned into build-in parts of the track, where they either prove an extra challenge or are used as reference points. The Nurburgring’s “Carousel” holds several such patches over it, which are beneficial to drive on when it’s dry.
If an entire road section requires mending, the upper portions of the tarmac will be scrubbed into bumpy and relatively un-grippy tarmac, followed by the melting of new tarmac over it. In times, the fresh new tarmac would appear at first bright, until it absorbs dirt, liquids and mainly tire scrub from the cars going over it (which improves grip levels). This is particularly relevant to interstates where the tarmac quality is higher than in urban districts. As a rule, black and seemingly rugged tarmac is most grippy.
Another subject is the painting of the road. Unfortunately, most paints are oil-products and are hence very slippery. Left-overs from fresh repainting on junctions is hence the most hazardous place on the road in the winter, particularly for those on two wheels. Still, there are massive painting of signs on the road surface, plus colorful roads in some parts of the world and yet they do not seem to cause too much trouble, right? That’s because today, regulations state that the paint must be none-slippery and mangled into the tar before it is paved. Even when repainting, European standards state that the paint must again not be oil-paint, and should be mangled with a substance that makes it more rugged, which increases tire grip on slippery surfaces. The normal material used is thinly-crushed glass or silicon.
In high-performance driving, painting takes the form of either skidpad construction or track layout. Most traditional skidpads are circular areas of tarmac or bitumen with lines marking the radius at each given point. Getting closer to the center decreases the turning radius and at a certain point, water is being add to the mixture on the inside circles. The innermost circle is covered with a seemingly bright-white color which, with the contact of water, proves extraordinarily slippery to drive on. This is a paint mixture, epoxy-based, that when mixed with water reaches an ridiculously low coefficient of friction. Almost as slippery as glare ice!
A reversed form of such use for paint can be viewed on the shoulders of some European tracks (The likes of the famous Paul Ricard), where “sand traps” have been replaced for painted portions of a high-friction material that is supposed to allow a car to re-grip when crossed and slow it down. Like a skidpad, the change in the materials’ friction is gradual. First there are some feet of a seemingly blue painted pavement, followed an obviously red-painted terrain which tends to cause much damage to tires. The advantage of this installation over sand traps are numerous. There is little need for a rescue vehicle to dig out the trapped vehicle, there is little damage from gravel that pops up under the car’s under tray, nor is there the phenomenon where a car enters the sand sideways with rolling wheels, thereby causing one wheel to hit dirt first, “hook” into it, which aggravates the spin and can roll the car over. Also, the dirt tends to invade the sides of the road, especially when a spun driver makes a rushed recovery, and cover it with “marbles”. This becomes even more hazardous when the road gets wet.
Another safety proportion is anchor guardrails, used alongside the road and between the lanes of traffic from both sides. This fence usually takes a rounded-shape, which gives it extra rigidity and more shock distribution over the metal, which then bends back to flat. Guardrails are almost never broken through and absorb much of the of the hit. The only downside of this fence is that it is much less friendly to bikers, where the rugged surface of the steel and the “W-shape” tend to inflict serious injury to the rider. This is part of the reason why the edges of the rail are not supported and are lowered towards the ground, so that they will not pierce drivers and bikers.
This also refers to the existence of a shoulder. Wide shoulders provide a good run-off area in an emergency avoidance situation or a skid, and allow a driver to stop in an emergency. However, roads with wide and open shoulders have, over time, turned into places of stop and rest for reasons which can hardly be defined an emergency. This has caused many very severe accidents. The shoulder-line is extremely dangerous and should not be used unless there is no choice, and even a flat tire is not necessarily a good reason to stop there. Most modern roads therefore have special docks for a safe stop in an emergency, and these are equipped with phones that enable dialing 9-1-1 quickly from any road section. STAY SAFE!
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