AskPablo: Where do car tires go?

Kristina recently needed new car tires so she asked me where the old ones go. She wasn’t referring to the big tire pile seen in the Simpsons’ hometown of Springfield but where the tread on the tire goes. In order to answer that question we first need to learn a little bit more about car tires…

Early car tires were little different from wagon wheels, and provided a jarring ride. Then tires were made out of natural latex from the rubber tree. In addition to a finite supply this had other limitations, including durability (although it was biodegradable). In 1839 Charles Goodyear mixed it with sulfur and invented a process called vulcanization which drastically improves rubber’s properties. In order to strengthen it further carbon black is added, along with several chemicals that protect the material against ozone and UV damage. Basically tires will not biodegrade anytime soon and the chemical additives also inhibit any other degrading. For a more in-depth discussion of car tires please see p.130 of Alan Weisman’s The World Without Us. Old tires can be used for playground padding and can be burned for energy but, because of the vulcanization process, they can’t be melted down and made into new tires (they are made from a thermoset, rather than a thermoplastic material).
If tires are so indestructible why do we ever need to replace them? Well, the rough surface of our roads acts like sandpaper and slowly grinds them away. Aggressive turning, burnouts, hard braking, and turning the wheel while the car isn’t moving all help this process. Eventually, when enough tread is worn away, you will need to get new tires. In the US the replacement rate for tires is about 1 per person per year. This amounts to over 300,000,000 tires a year in the US alone!
Let’s assume that the average US car tire is 50 cm high and 20 cm wide when new. Let’s also assume that the total diameter decreases by 2 cm before the tires need to be replaced. By subtracting the volume of two cylinders we can figure out how much material goes missing. The original tire has a volume of 0.0393 m^3 and the old tire has a volume of 0.0362 m^3. The difference between the two is 0.0031 m^3, or 3,100 cm^3 (imagine a cube of rubber that is 14.6 cm, or 5.75 inches) on each side. Since tires are not completely solid we need to subtract a small amount to take the tread into account, let’s say 25%. So, each year 2,325 cm^3 of car tire per person disappears. The density of car tire material is roughly 600 kg per m^3, so 1.4 kg of car tire material go unaccounted for per person, per year. This adds up to 420,000 tons!
So where does it all go? Well, some of it ends up in the air as PM10 (particulate matter that is 10 microns in diameter) which we might breath in. Some of it ends up as that dark grime on your rims (although some of that is brake pad dust) and windshield. The rest waits by the side of the road for the next rain storm to wash it away. Once in the storm drain the little rubber bits probably make their way to the ocean where they happily float around. Unfortunately for the little critters in the ocean these little bits look a lot like food. In fact in some areas of the oceans man-made particles outnumber plankton and other microscopic critters by an order of magnitude! When these bits make their way into an animals belly they don’t break down and can’t be digested. This means that they accumulate, leading to eventual starvation. If a larger fish eats it first the rubber may than accumulate in its belly until it is eaten by a seabird or mammal, and so on…
The picture isn’t pretty but no one seems to be talking about it. And what can we do? Our dependence on personal vehicles rather than public transport leads to many such dis-economies of scale in terms of costs externalized to society and the environment. Time to start the dialog…
Pablo Päster
Sustainability Engineer

5 responses

  1. Yes, we must begin to study the TOTAL COST OF OWNERSHIP of all of our “stuff”. But, I highly doubt Americans have any desire to do any of this. Most of us are happy to live 2-5 times beyond our means so what makes any of us think we give a damn about the environment?

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