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Technical FAQ: Tire grip in wet conditions

This week, Lennard Zinn provides detailed information about which tires, treads, and pressures are best for wet riding conditions

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Traction in wet weather

Dear Lennard,
I’m wondering if slick tires have any more or less traction than tires with treads in wet weather. I’m mostly curious about racing weight 700c clinchers, so the treaded ones won’t have much tread for starters. I suspect that the size of the contact patch is so small that the road surface will have more of an impact on traction than any tread texture you could apply to a tire and possibly the rubber compound may have the greatest efficacy.

Also, is there any proof that lowering tire pressure will increase wet weather traction (let’s say from the 100-115 psi range down to the 90s, as I think anything higher would be silly and lower would affect handling)? I realize that it might increase contact patch and pliability, allowing the tire to hug the road better, but I don’t imagine it does by much.
— Michael

Dear Michael,
Here are very thorough answers to your question from tire designers at Specialized, Challenge, Continental, and Vittoria.
― Lennard

Answer From Specialized:

I did some research on this topic. Still no data to back up the claims for road tires, but the picture gets clearer. Common perception is that tread has no influence in road bicycle tires. But this is not entirely true. The question has to be how much tread do you need?

Does tread have an influence on traction in road bicycle tires? Yes. Tread does have an influence on traction in road tires. Tread, if it is not so high that it lifts the tread ground off the road (<0.2mm), helps sink tread rubber into rough road surfaces and penetrate lubricants (water) or dirt. Peaks in the tread increase press into surface crevices and add contact points between the road and the tire, and thus increase friction. Just a roughened or scuffed tread surface passes as a tread already. It does not need to be designed shapes.

Note that bicycle tires do not get as warm as automotive tires. In automotive tires, temperature helps to soften the compound and sink in. In extreme cases, like in race car tires, the tires are driven so hot that the leading edge of the tire lays a rubber strip to which the rest of the contact patch sticks. This is why slick tires with high tread stability and gradual wear are preferable in dry racing conditions. In the rain, tread is necessary in car racing to create high load edges to penetrate the water film and to channel water away to prevent float.

However, this is different from cool running bicycle tires.

Does have a lower tire pressure an influence? Yes. With lower air pressure, the contact patch size increases. Also, with less air pressure, the casing and tread can filter more road vibrations. Less disturbances and strain on the links between the tread and road. The tire tracks better.

The compound for sure has the biggest influence on tire traction on the road. In road tires where the influence of tread is relatively smaller than in off-road tires, especially.

So what does the compound do? A rider wants the tread compound to adapt to the road as quickly as possible and link up for safe grip. At the same time, the rider wants it to let go of surface connections without drag and no internal friction that slows down the ride. We achieve this through carefully modeling the tread compound formulation. The compound needs to be as soft as can be for the tread to sink into the road and still give defined road feedback. We want rather high compound hysteresis to filter road vibration and thus enhance tracking stability and rider confidence, but not at the cost of high hysteresis losses and high rolling resistance.

The way is purpose-designed synthetic polymers and process oils, silica fillers and — very important — a force- and temperature-controlled process that blends and connects the materials evenly without destroying them.

The compound can only do its work when in direct connection to the friction partner — the road. That is where the tread comes back in to help increase the contact area and penetrate barriers.
— Wolf Vormwalde
Tire Product Manager
Specialized Bicycle

Answer From Challenge:

When trying to determine the optimal tire performance characteristics for wet weather riding, it is best to study the riders who spend the most time and ride the most aggressively in wet conditions — professional riders who are paid to suffer in these conditions. Pros do not like to crash any more than the rest of us, but two things they have that most of us don’t are the choice of the best tires — regardless of cost — and the most experienced mechanics in the world who are experts at fine-tuning the tires for the riders and conditions.

It is critical to fine-tune every component of the tire before riding aggressively in wet conditions to maximize the surface area gripping the road surface while squeezing water out from under the tires wherever possible. This is why all top level road teams and even experienced pro triathletes will ride tubular tires with soft, supple casing materials, natural rubber tread compounds, and supple latex inner tubes, sized to the rider weight and road condition and adjusted to a minimum pressure to keep the rims from bottoming out on the bumps. If you must ride a clincher, then a use a clincher made with the same materials as our tubulars (we call them Open Tubulars) that when matched with a latex inner tube are the next best option.

Pro Tour team mechanics will carry small charts listing tire size and pressure for each road type — “Grand Tour Perfect” (paved yesterday), “Normal,” “Poor,” “Tour of Flanders Bad,” and “Paris-Roubaix Worst” — and rider (due to weight), for front and rear wheels. Unfortunately, this information is rarely shared due to the strategic advantage it gains the team with the most technical savvy.

The most important issue is to have a soft tire — casing, tread rubber and inner tube — as described above, adjusted to the proper pressure to maximize tire patch size, grab every road imperfection and to deform and absorb bumps. The casing is the primary factor in this fast and grippy tire system. The casing must be able to deform as much as possible to adapt to the surface of the road, having always the maximum contact patch. If the casing is stiff due to material or pressure, then cornering traction will be compromised. This is true in wet and dry conditions.

A hard (due to materials or pressure), bouncing tire will lose contact, allow water to penetrate under the tread and lose traction quickly and without warning. This is why current tubeless technology that requires a stiff sidewall to keep from burping runs counter to optimal road performance.

In the wet, lowering the pressure will give the casing even more flexibility so the tire will be able to deform and adapt to terrain and weight transfers — to lean, brake, climb, and descend comfortably. The tire is the only suspension on the bike and suspension on a bike is critical! Frames must be stiff for the reasons we all know, but tires have to compromise stiffness with suppleness. The result should be a less nervous bike, smooth rolling to avoid any loss of contact. The tire will also give you constant info on what is going on and where the limit is. It will allow you to adapt, correct lines, and resolve most situations, while hard, stiff tires will go from grip to no grip with no notice and no time to react. Again, casing and pressure are fundamental.

In theory, every riding condition requires a specific tread compound, but this is not possible, at least in bike racing. Car and motorcycle racers have test days to choose the right tire, and if conditions change during a race they change the tires. In bike racing this does not work, so the compound has to be a compromise between good traction in wet, and durability, puncture protection, and strength. A soft compound increases grip in wet but lowers wear and puncture resistance. It must be able to keep its properties during all riding conditions. This is the difficult part.

Tread design on a road tire is like the cherry on the icing on the cake. A slick tread will function on any dry road surface, but once it gets wet, a herringbone tread pattern like on our Forte, Strada, or Paris-Roubaix tires will help channel water outward while slightly deforming and again, maximizing tread contact. Traditional patterns like the herringbone are the most effective and do help in most conditions. The small grooves of a herringbone help the compound to drain the water, and the small rubber wings that come up can flex and deform to optimize grip. A Forte pattern with the deep fine “S” on the side has proven to be a very good tire in wet dirty conditions when used by Bretagne-Séché and Team3M.

In summary, in wet weather a soft, supple tire and inner tube at a moderate pressure is most critical. Reducing the tire pressure does help further increase the contact patch and maintain traction if you have that soft, supple tire and tube. The right tread pattern is the final element to help ride safe at speed in wet conditions.
— Alex Brauns
President, Challenge Handmade Tires

Answer From Continental:

Wet grip vs. tread pattern and inflation pressure. The fundamental premise of pneumatics is that of an air spring, and the tire/tube must be at a pressure that allows it to deflect (approximately 15 percent) beneath the weight of bike and rider against ultimately uneven riding surfaces. Unlike treaded car tires that support much more weight with larger contact patches at higher speeds, high-pressure clinchers are not capable of actually hydroplaning unless traveling at speeds unattainable by human power. On the other hand, slipping or losing traction on wet surfaces can certainly be reduced by lowering inflation pressure, thus increasing the contact area and improving the compliance of the pneumatic system. Tread patterns are largely aesthetic, and although they provide texture that can aid traction both wet and dry, they do not provide channeling like car tires that are actually effective in reducing hydroplaning.

So what is the magic formula for improved wet grip? It’s not a magic formula at all, and a little bit of trial and error dependent on the rider, conditions, and equipment: reduced pressure depending upon bike/rider weight, tread compound, and tire width all help. Choose at least a 25mm with Continental Black Chili Compound and start by reducing pressure around 10 percent from normal riding pressure.
— Brett Hahn
Brand Manager
Continental Bicycle Tires North America

From Vittoria:

This is a very broad question that will have many points to consider and answer. While each brand may have its own philosophy on design, Vittoria believes that a treaded pattern, ultimately, will deliver the best overall performance in wet conditions under the varying road surface conditions that are offered up to riders. So, generally speaking, a treaded design will deliver a more consistent contact to road surfaces due to the points below:

— Tread patterns may decrease a contact patch by creating deformation of the tire due to road pressure under load from the rider and the rubber/casing bulges and deforms to fill the tread voids, but a well thought-out tread design increases its contact area when turning, accelerating, and braking (increased load due to force will flex the diamonds and grooves) and will also provide micro interlinking in between tread pattern edges and road surface grooves. This high flexible tread surface supplies the rider a better idea of contact loss before the point of even return. This provides more safety, but the maximum grip force is not increased

— Lowering the tire pressure can increase contact patch, which in turn creates better traction in wet conditions over recommended tire pressures, < 20-25 psi from the recommended average pressure.

— A higher thread count can also attribute to a tire’s contact patch and greater security in a variety of conditions, such as an all-cotton casing providing better absorption of vibration and lowering loss of contact with the road surface, including wet road conditions.

— Inner tube material, such as latex, will increase a tire’s flexibility and contact patch.

— Unlike a car tire, bicycle tires do not trap the water under the large “planes” created by automotive tires and can better maintain contact and cohesion with road surfaces.

— Compounds come into play and have the most influence on traction (wet and dry conditions); recently developed compounds such as Vittoria’s ISOgrip provide a much broader, stable compound in lower temperatures, this also a key component to a tire’s overall grip in all conditions.
— John McKone
Road Marketing
Vittoria Industries North America

Regarding Road Tubeless Tire Failure

Dear Lennard,
I had the same problem on my American Classic Argent wheels. I used Maxxis Padrone tubeless tires. After about 70 kilometers into our ride, the rear tire popped out while descending. Luckily I was able to stop the bike. A novice in road tubeless wheels and tires, I don’t know why that happened, if it’s improper installation, compatibility issues with tire and wheels, or wheels not being a true tubeless wheel since it’s technically a converted wheelset.

What are thoughts on this? Should I continue using tubeless tires on the Argent wheel?
— BJ

Dear BJ,
When using road tubeless tires, I recommend using tubeless-specific rims. For the lower pressure of mountain bike tires, I think, other than a bigger burping issue on corners due to the lack of the bead-locking “hump” on the medial edge of each bead shelf, that tubeless conversion with most rims is fine.
― Lennard

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