There are many reasons why riders abhor the rain, be it the discomfort of wet clothing (wet socks are the worst), the risk of coming unstuck, or the extra maintenance required to clean up the bike and keep it running smoothly and quietly.

Having spent several years as a year-round commuter, I thought I was familiar with all of the reasons why it was worth staying off the bike on a rainy day, but one recently caught me by surprise. Apparently there is also a bigger risk of getting a puncture.

I won’t argue with the misery of getting a puncture on a wet day so I can understand a certain amount of dread, but it’s not something I’ve ever noticed. If there is any kind of law that applies to punctures then it’s this: a puncture will always occur at the most inconvenient time.

Of course, getting a puncture during a storm will normally count as a pretty major inconvenience, so perhaps rain has some influence on the puncture resistance of a tyre.

Nevertheless, it’s easy to find any number of Internet forums that have considered the issue with the kind of authority that you’d expect for an Internet forum. What surprised me though, is that multiple forums arrived at the same three explanations for the extra risk that rain poses.

First, the rain washes debris onto the road or bike path. Second, the wet has an adhesive effect, such that debris is more likely to stick to tyres. And third, water acts as a lubricant for the debris to reduce the puncture resistance of tyres.

While each of these explanations seems plausible, none of the participants could provide any primary data in support of each mechanism. After an extensive online search, I couldn’t find any academic studies on the matter even though some were certain they had “read about it in a magazine somewhere”.

Thus, we have a phenomenon where the weight of all the available evidence appears to be purely anecdotal, and a trio of mechanisms that have become anchored in popular belief seemingly on the strength of their intuitive appeal.

Where’s the evidence?

All of the major tyre manufacturers devote plenty of web-space to expounding the advantages of their various rubber formulations and puncture-proofing strategies, yet none mention an increased risk of a puncture in the rain. My barrage of enquiring emails was met with silence, leaving me to wonder if the industry’s stance on the issue was a stern “no comment.”

What I did discover is that Schwalbe and Vittoria both suggest latex tubes as an extra measure to reduce the risk of a puncture. Here the reasoning is clear: latex is more flexible than butyl rubber, so a latex tube is more likely to deform than puncture when compared to a butyl rubber tube. I’m not going to argue the suggestion, but I’ve heard more than one experienced cyclist pronounce that latex tubes were more susceptible to punctures.

I couldn’t help my scepticism at this point. After all, I could see a much simpler explanation for the whole phenomenon, namely tyre wear and/or bad luck. Rather than dismiss the issue though, I decided to test the notion with an experiment. It seemed to me that all I needed was a tyre, a tub of water and a knife. My plan was to compare how easy it was to cut the tread, dry versus wet.

I came away from my lab (kitchen) with two observations. First, a rubber tyre is very difficult to cut; and second, water did not make it any easier. My approach was admittedly naïve: I simply tried a variety of knives, concentrating on those that could not cut the tyre when it was dry, then testing them again after immersing the tyre in a tub of water.

A look at automotive tyres

I went back to the literature and widened my search, which lead me to the automotive industry and a wealth of information on the cutting resistance and fracture properties of rubber. Here at last, the data was clear, water reduces the friction of cutting.

Rather than dig up the original articles dating back to the ‘60s and ‘70s, I was lucky to find video footage that demonstrates the principle with elegant ease:

What I like about this experiment is that the relative scale of the tyre compared to the knife approximates that for a bike tyre and some debris. However, it doesn’t provide any data on the magnitude for the effect of water. All it shows is that, once wet, a tyre can become susceptible to a sharp object.

Generating such data shouldn’t be too difficult, so I’m a little surprised that I’ve haven’t been able to find it. Researchers have been able to determine the cutting energy for rubber compounds in other settings, so the same could be determined for any number of bike tyres, dry versus wet, to get a handle on the difference that water makes.

What about the big picture?

Of course, there’s more to getting a puncture than simply making a cut in the rubber tread. The debris must work its way through the full depth of the tread and any puncture-proofing (non-rubber) belts in order to penetrate the casing and the tube beneath.

Having ridden a variety of tyres that have developed cuts in the tread without experiencing a puncture, it’s clear that the former does not automatically predispose the latter. Indeed, I’m sure most cyclists have discovered pieces of glass and other debris embedded in their tyres before a puncture has occurred. Nevertheless, we all know that the risk of a puncture increases (in any weather) as the tyre wears out, so if the rubber tread can be defeated more readily in the wet, then surely that will translate into an increased risk of a puncture?

There are no published studies on this matter, so I can only speculate, and while there’s a certain intuitive appeal to the notion, I’d rather see some evidence. In my experience, such academic vacuums can normally be explained in one of two ways: first, the studies have been carried out and the data was unremarkable; or second, nobody has ever bothered to do the experiment.

At this point, I can’t say which scenario is more likely but after trading an email with Wolf VormWalde at Specialized, it seems as if it might be the first. According to Wolf, “We kept a tire in water for some time and punctured it, following our standard procedure with a nail and a knife. The wet tire did not show reduced puncture forces compared to a dry tire.”

While this statement is not enough to put the point to rest, it sits well with my experience. Which leads me to ponder the other mechanisms that may be at play, namely an increase in debris, both on the road and sticking to the tyres.

Opportunity over susceptibility?

I have no doubt that the rain washes debris onto the road, and I don’t expect there would be many that would dispute this fact. After all, one ride along any road in the aftermath of a heavy shower is enough to prove the point, where the amount of debris depends entirely on the immediate environment.

Thus, roads without guttering show streaks of debris containing everything from leaves and sticks to sand and stones. Some stand out against the bitumen while others are less obvious, so I think it’s fair to conclude that rain can increase the opportunity for a puncture.

However, it all depends on what exactly is washed onto the road. Living on the west coast of Australia, there are no stones or flint in the soil, just a lot of sand. This is in contrast to some parts of Europe, where I’m told heavy showers can wash sharp flint onto the road.

What about the stickiness of the tyres in the wet? This is not something I’m going to dispute because I’ve seen it happen every time I’ve veered off a paved road with wet tyres. Everything from sand to leaf litter can stick to the tyres, but I can’t see any reason to worry about the debris that sticks onto the tyres.

Nevertheless, both mechanisms remain plausible, though I would welcome any data on either to better judge the relative risk of each.

Lessons from the peloton

During the final stages of my research for this post, I had a chance to chat with Morgan Nicol at Challenge tyres. As the product manager for the company, he has spent many years working with a variety of professional teams in Europe, and he was able to assure me that, yes, the rain poses a very real risk to cyclists, but it all depends on what kind of tyres are being used.

For those cyclists that are using vulcanised clinchers (ie. the majority of clincher tyres on the market), there is no extra risk of a puncture in the rain. In contrast, those that choose racing tubulars or open tubulars, where the tread is glued on by hand, the risk of a puncture increases significantly in the wet.

“The biggest problem is with competition tyres that have soft rubber,” said Morgan. “The rubber catches stones very easily and they can get pushed through the casing to puncture the tube. The latex tube can deform to resist a puncture, but the difference is only a few percent.”

The extra susceptibility of the racing tyres is why Challenge normally sends vulcanised clinchers to its team riders for training purposes. However, there was one year where a team received open tubulars instead. “We had a lot of complaints from the riders about too many punctures when they were training,” explained Morgan. “Once we realised the mistake in tyres, we understood what was going on.”

Morgan doesn’t have any quantitative lab data in support his observations, however years of first-hand experience combined with real world testing with a large number of riders count for a lot. And it was clear from our conversation that punctures are something that Morgan Nicol (and Challenge) takes very seriously.

“Water doesn’t do anything other than bring debris onto the road,” said Morgan, “and it is worse on country roads where there is no guttering. The time of year is also important. In Belgium, they have the ‘season of the little stones’ when it is very hard to avoid a puncture, and we see it in other countries too, like Italy and France.”

“The other important factor is contamination of the tyres with oil and gasoline from the road. Once the tyres get contaminated, they get very sticky and the stones can’t be shed so they start cutting into the tyres.”

It is this second factor that accounts for a curious home remedy that Morgan recommends to team mechanics: white vinegar. According to Morgan, the vinegar works well to remove contaminants from the tyres, so are less likely to pick up small stones and other debris. “The younger, less experienced mechanics don’t know about the vinegar, and we always see more punctures until we tell them about it.”

When our roving tech reporter, Dave Everett, asked some of the mechanics at this year’s edition of the Tour de France about vinegar, he got a slightly different view. According to them, the practise has become outdated as the quality of the racing tubulars has improved in recent years. Nevertheless, it was something that older mechanics still used.

Summary and final thoughts

I think it’s fair to say that the quality of bike tyres has generally improved to the point where they appear to be more puncture-resistant than ever. That doesn’t mean that they have become puncture-proof, however ideas about the threat of rain seem to be a hold-over from earlier times when the rain was perhaps more of a threat because tyres were more susceptible.

I’m not about to dismiss the threat of rain though, since the run-off that it creates can bring debris onto the road. I don’t think any experienced road cyclist would ever choose to ride in the dirt or sand on the verge of a road because of the risk of a puncture, so it’s equally prudent to stay out of the gutter and away from areas of run-off when it starts to rain.

For those riders living in areas where there is a rainy season, I think it’s also wise to install a fresh set of vulcanised clinchers on the bike before the rain starts. And if those tyres are a little heavier with extra puncture-proofing material, or it’s a tubeless system with extra sealant, then even better.