Mountain biking is often associated with a relatively small ecological footprint. No exhaust fumes, no engine noise, no paved roads. But even mountain biking isn’t entirely without its impact: every turn wears down the tires, every brake application, every pedal stroke. The tires wear down, and the abrasion remains where the bike is ridden: on forest floors, along stream banks, in protected areas. Not on asphalt, not in wastewater treatment plants. Directly in nature, which is what makes the sport possible in the first place.

How much abrasion occurs on natural trails has hardly been studied to date. Reliable measurement data for mountain bike tires on off-road terrain was lacking. In April 2025, researchers from the University of Bayreuth published the first field study on this topic in the journal Science of the Total Environment. The results are more uncomfortable than expected.

Less than expected

For the study, the team led by doctoral student Fabian Sommer equipped nine riders with new Schwalbe Wicked Will tires (29 × 2.4 inches). The riders rode their usual routes, tracking every kilometer via GPS. At regular intervals, the researchers weighed the tires in the lab. Result: 3.62 grams of wear per 100 kilometers. Depending on annual mileage, this amounts to 59 to 88 grams of tire material per rider per year. The rear wheel accounts for 64 percent of the wear.
New tires wear down more quickly than broken-in ones. In the first 500 kilometers, the rate is nearly twice as high because production residues and sharp tread edges break off first. Those who replace their tires early leave behind more per kilometer than someone who rides their tires until they’re worn out.

On a national scale, the result initially seems small. Cycling in Germany accounts for less than 1 percent of total microplastic emissions. Compared to cars and trucks, the contribution of mountain biking is thus significantly smaller. Comparisons with shoe soles are only partially helpful because the material and entry path are different.

More toxic than expected 

So far, this sounds like a all-clear. But grams alone are the wrong unit of measurement. What matters is the composition of the rubber compound and the location where the abrasion is deposited.

An MTB tire isn’t made solely from natural rubber from the rubber tree. It’s a laboratory product: synthetic rubber, carbon black, plasticizers, and dozens of chemical additives that blend together. One of these is called 6PPD; it protects the tire from ozone. Without 6PPD, tires would age significantly faster and become cracked. According to the U.S. Tire Manufacturers Association (USTMA), there is currently no equivalent substitute worldwide.

The problem arises when 6PPD reacts with ozone in the air. This produces 6PPD-quinone. U.S. researchers described this compound in more detail in 2020; the findings are serious. The lethal threshold for silver salmon is 790 nanograms per liter of water. That is less than one-thousandth of a milligram. At this concentration, the fish’s cardiovascular and respiratory systems fail within hours.

Whether mountain bike traffic alone reaches this threshold in a mountain stream has not been measured. What research on tire abrasion generally shows is that particles and degradation products can enter water bodies. For mountain bike trails, however, it remains unclear how significant this local input actually is. A mountain stream dilutes far less than a river. This is precisely why springs, streams, and wetlands along heavily used trails are particularly vulnerable.

What the soil absorbs

Laboratory and soil studies suggest that tire particles in forest soil can alter the microbiome. Certain bacteria, particularly those of the genus Pseudomonas, proliferate disproportionately. This disrupts the nitrogen cycle: the soil releases nitrogen as a gas instead of keeping it available for plants. Local greenhouse gas emissions rise, and plant growth suffers from nitrogen deficiency. Earthworms and nematodes, the foundation of every food chain in the forest soil, show significant damage in laboratory tests at certain concentrations. Whether these concentrations are reached on real trails has not been conclusively proven.

What the industry is doing

The industry is responding in different ways: Continental measures abrasion using vacuum technology, according to its own statements, while Schwalbe recycles rubber via pyrolysis. The Norwegian company reTyre goes further: Tires made of thermoplastic elastomers, without 6PPD, without vulcanization, and, according to the company, with a lower carbon footprint in production. The technology is initially aimed at the road sector. Whether it will ever meet MTB requirements remains to be seen.

What remains

Compared to motorized traffic, the impact of mountain biking appears small as things stand today. However, this does not relieve the sport of its impact everywhere equally. Those who ride in sensitive areas, near springs, streams, and nature reserves, take on a specific risk. The location and the chemistry in the rubber are decisive. The forest floor is not a road; there is no wastewater treatment plant behind it. That makes the difference: not the quantity, but where it goes and what it brings with it.