# Why doesn't the road lose its thickness to the tyre?

From this source, on average a rubber tyre loses $$2$$ atoms thickness per revolution due to the friction with the road.

Do these atoms/molecules that were peeled off the tyre get absorbed by the road? Then would the road increase in thickness due to this? If so, why doesn't the road lose it's thickness to the tyre instead? Thus making the tyre increase in thickness over time...

• When you use chalk on a black board does the chalk come away with a black streak on it? Is the chalk permanently bound to the board? I know these are not the same materials, but it does show that just because one material loses molecules to another surface does not mean it happens the other way, nor does absorption have to happen. Dec 8, 2019 at 12:03
• Wear of tires, asphalt and brakes has been identified as a major source of harmful fine dust (up to 70% of what car traffic produces) by the German Environmental Agency: sueddeutsche.de/auto/feinstaub-verkehr-bremsen-reifen-1.4427241 (in German) So yeah, a lot of it ends up as airborne particles
– smcs
Dec 9, 2019 at 9:50
• When I was about 5 years old a common occurrence was that I fell while playing outside and ran home with a bloody knee that didn't appear to have any skin left on it. Even then, through tears in my eyes, I wondered where on earth all the skin had ended up :-). Dec 9, 2019 at 15:47
• Where are you where the roads never wear out? Dec 9, 2019 at 16:25
• If you look at centuries old stone stairs in Europe it is common to see a double indent shape worn by foot traffic. In the stairwell of the Leaning Tower of Pizza the stair indentations have been filled with some protective substance (possibly many times) and are on their way once more to being warn into a double indent. I'd doubt that you'd end up with stair dust coatings on the bottom of your shoes. Dec 10, 2019 at 6:17

Anyone who has lived near a major thoroughfare knows that tire wear creates rubber dust that gets spread around everywhere. In some parts of Los Angeles, the residents have to hose off the sidewalks every day or so in order to prevent that dust from being tracked into their homes. Some of it also gets friction-welded into the surface of the road itself, leaving skid marks that are plainly visible.

Road surfaces themselves also lose material to the passage of tires. On concrete roads, the tire scrubbing action rubs away the mortar between the pebbles and after a while the pebbles become unsupported, and they too become dislodged.

The road material does not end up accumulating on the tire surface because the tire surface is being continually worn away. Any road material that might get embedded in it is thereby shed loose along with the tire dross.

• Don't know how the other answer got accepted. This comes much closer. Fact is, reality is far more complex than the question's author implies. Wear occurs on both the tires and the road. The road may temporarily gain some thickness, but it also loses material from the wear, and the rubber and other particles that settle onto the road are washed/rubbed away over time. Eventually, both tire and rubber wind up in the watershed, and both lose material and thickness. Dec 9, 2019 at 1:47
• Those black patches that form on the road where many heavy cars are braking, are not skid marks, and afaik have nothing to do with the rubber of the tires. It's the asphalt in the road surface that gets moved to the top of the road by the turning force on the stones in the asphalt. Sorry, I cannot cite a source for this right now, but if you look closely at those spots, you can easily see that they are not made of tire rubber. Dec 9, 2019 at 8:02
• @PeterDuniho How do you distinguish that from compactification? Especially in combination with cracks due to non-tyre-related wear? Every road damage analysis I've read attributes ruts to compaction. Do you have some reference that claims ruts are caused by road material being carried away as a result of erosion caused by tyres? In fact, the only damage I've seen mentioned to be caused directly by tyres wearing out the road is polishing - it makes the surface slippery. How often have you seen that on roads? E.g. iosrjournals.org/iosr-jestft/papers/vol9-issue12/Version-3/… Dec 9, 2019 at 9:59
• @Luaan: If you have ever driven on a badly-worn concrete road, you may have noticed that the concrete has been worn down to the point where very large aggregate is exposed. Compaction wouldn't do that, even assuming that concrete (unlike asphalt) even could be significantly compacted. Dec 9, 2019 at 18:11
• @Ardweaden I was not talking about the marbles that show up in F1 racing, I was talking about what niels nielsen called "skid marks", and which are not skid marks at all: Large dark patches on the tarmac where you cannot see a single stone, only plain black asphalt. I don't know the proper name for these, but they typically form where heavy trucks have a habit of braking hard. Like in front of some traffic lights. Those patches are quite dangerous, as they further reduce friction at places where cars already need a lot of friction regularly. Dec 9, 2019 at 20:10

I believe it has to do with the hardness of the tire vs the road. Hardness is a measure of resistance to wear and the material of a paved road is generally harder than the material of a tire. So the tire will wear before the road with a net transfer of material from the tire to the road.

Hope this helps.

• I guess the followup question would be "why don't roads leading into intersections get coated in rubber all the time"... Dec 8, 2019 at 12:15
• @John Dvorak I edited that out since it wasn’t relevant Dec 8, 2019 at 12:16
• @John Dvorak maybe they would if it were not for eroding factors such as rain. But are you saying tire material is not deposited on the road surface. Just trying to understand your point Dec 8, 2019 at 12:23
• @John Dvorak it ends up on the road surface as well as probably particles blown about in the air. You can most readily see evidence of it ending up on the white surfaces of older concrete roads such as those used in the US interstate highway system. There are photos on the internet Dec 8, 2019 at 12:33
• One rather dramatic example I'd give is the touchdown area at the end of an airport runway - it rather nicely illustrates the difference in hardness between the tire and the surface, courtesy of the black marks left there by landing aircraft. Dec 9, 2019 at 11:34

Others have noted that roads wear - an example of softer materials wearing harder one is liable to be useful.

If you look at centuries old stone stairs in Europe it is common to see a double indent shape worn by foot traffic. In the stairwell of the Leaning Tower of Pisa the stair indentations made by foot traffic have been filled with some protective substance (possibly many times) and are on their way once more to being worn into a double indent.

While both stairs and shoes wear, the rate of wear of the stone is no doubt such that the amount worn off in a climb is minimal - I'd doubt that you'd end up with a visible amount of stair dust coatings on the bottom of your shoes.

I took this picture in the 'Leaning Tower' in July 2003. You can see both the wear patterns in the stone and the added protective material:

Detailed crop. Larger image

• "clicking the larger photo gives a better overall impression" - Impression, heh. Nothing clickable though. Dec 10, 2019 at 23:40
• @JohnDvorak Larger image link added. They expire after a while. Dec 14, 2019 at 8:24

Both the tire and the pavement wear from the contact between the two. The road will take significantly more damage if the tire has rocks stuck in the treads, but even in a "pure system" of perfectly clean rubber tires, the road suffers erosion just like everything else.

The folks who commented on the hardness or density of the pavement are right, but they left out the part where silk can be used to sand hardwood. It may take a really long time to make a visible dent in the harder surface, but your inability to perceive something doesn't make it nonexistent.

• Do you have a reference for silk can be used to sand hardwood? I wouldn't be too surprised to see that silk can be used as an abrasive (it's rather strong, and it could certainly be hard enough to sand mere wood), but I couldn't find any examples. We did easily cut wood with both cotton and silk threads, but that's not exactly surprising either - both are harder than wood. Not that I'm saying the hard part doesn't suffer damage from the soft part; after all, you still need to sharpen your knives even if you're only ever cutting carrots or whatever :) Dec 9, 2019 at 9:19
• This. So much this. The assumption that the tires don't do anything to the road is simply false. See for example this photo. Our climate makes the problem much worse, for during the icy months (mid-November to mid-April) motorists are required, by law, to have seasonal high traction tires on the cars. More than half of us opt for studded tires, which are the only way to have a safe amount traction on ice (on snow a less drastic tire surface will do). Dec 9, 2019 at 12:11
• @Luaan Shortly after I posted this I did websearches for "silk to sand hardwood," since I figured I'd be asked for references. There aren't any on the internet, or at least there aren't any I could find. I don't know where I came across the information, but at some point in history silk was used as an ultra-fine sandpaper for hardwood weapons and furniture. It's comparable to an 8000 or 12000 grit whetstone used to finish the cutting edge on a straight razor. Sidenote that whetstones commonly dish out from use and have to be re-flattened. Dec 9, 2019 at 16:10
• I remember old roads made from stones - where you could see the stones surface getting polished - much like railways' rails getting polished and derusted from having trains roll over them frequently (up to a point where you can measure the amount of traffic that uses them Dec 9, 2019 at 16:10

Wheels sweep the road and the car creates enough wind to move most sizes of particles to the sides of the road. The rubber and road aren't comparatively adhesive enough to build up.

Sun and frost ( freeze thaw weathering ) are a lot more forceful than tyre contact.

Chemistry formulas may demonstrate that if the road grit's atomic bonds are a thousand times stronger than rubber, they will only wear out from impact with stones caught in tyres rather than the rubber itself.

Sharp bends in the road recieve a lot more physical force than straights and can wear multiple times faster.

• hhttps://www.alamy.com/stock-photo/worn-stone-steps.html more examples. What sprang to my mind immediately was the entrance to the London pub "The Old Bank of England" but I couldn't find a photo (of what ~400 years of foot traffic does to a step). Dec 10, 2019 at 16:03

It can be the other way around too, it is dependent on the modulus of elasticity of the materials, if the road has higher modulus of rigidity then it will not break easily and a large amount of shear stress needs to be applied in order to do so , equal stress can cause different effects on different materials, According to IS:456 the modulus of concrete is 5000√(fck), MPa , where fck is the characteristic compressive strength of concrete whereas rubber has modulus of elasticity 0.0003GPa so when the two surfaces interact rubber loses more.

In regards to the other question, yes the thickness of road will increase but in a non uniform manner. Thanks and regards