What is the purpose of a tread pattern on city/road tyres?

Question

What is the purpose of tread patterns on city/road tyres?

I think I've understood (perhaps wrongly) from this site that they're unnecessary on paved roads:

• Grip (friction) is more-or-less contant, or perhaps a question of the material
• Racing tyres for example are "slicks" made without grooves
• Grooves are needed on other tyres (e.g. for cars) to prevent aquaplaning; but that bike tyres are too narrow and too slow to ever aquaplane

Schwalbe say:

When is a tire worn out?

In bicycle tires the tread is far less important than for instance in car tires. So using a tire with a worn out tread is less of a problem that is of course with the exception of MTB tires.

When the puncture protection belt or the carcass threads can be seen through the tread the tire has reached its wear limit and must be replaced. As puncture resistance also depends on the thickness of the tread layer, it may be useful to replace the tire sooner.

The sidewalls of tires often fail before the tread is worn out. In most cases, this premature wear is due to prolonged use of the tire with insufficient pressure. Checking and re-pumping the inflation pressure at least once a month with a pressure gauge is most important.

Is it only a matter of puncture-resistence?

The bike shop called mine "dead", saying I should change, because the tread is worn (like the photo in this answer).

Answer 1

Schwalbe has another page on this - https://schwalbetires.com/technology-faq/tread/#:~:text=On%20a%20normal%2C%20smooth%20road%2C%20the%20tread%20has,smaller%20and%20the%20contact%20pressure%20is%20much%20higher.

On smooth road, slick tires perform best in all conditions (unless you doing over 200km/h), however in rough roads tread provides predictability, which is far more important than absolute grip on a bicycle. Choice of tire you use should be based on the types of road you ride (and in some countries, tread is a legal requirement). Keep in mind that few of us are in the luxurious position of riding perfectly smooth roads all the time.

This is different to what you have, which is a worn out tire. The tire shop has to advise you based on lots of factors - but the liability and reputation precludes them advising a worn out tire is safe to ride. If the country you requires tires to have tread, they would probably be legally bound to advise you to replace the tire.

Given all this, IMHO they are being professional and have advised you correctly.

You still have a choice, the liability of your decision not to replace the tire lies with you. The real risk of running worn tires (as long as you keep an eye on them) is they are more vulnerable to punctures. Riding once the chords show though can lead to catastrophic tire failure, which can cause crashes, but you really need to ride well past the chords just showing in one spot. The loss of predictability of a worn tire in adverse conditions can be mitigated with riding style.

Answer 2

In my experience, smooth or treadless tyres are perfectly good in the dry, and on smooth/good roads.

Smooth tyres are adequate on wet smooth roads.

Wet AND oily roads, or wet AND gritty roads is where tread starts to becomes useful. When the road surface is not firm and cohesive, tread allows spaces for those particles to be such that they're not a ball-bearing.

Have you seen occasional islands or moraines of fine dirt or sand or stones that accumulate where cars don't drive? In the corners of intersections and between lanes.

^{Example island of loose gravel, in thew middle of the green cycle lane.
Fine to roll over, but if you were hard-braking for the red light then it might be different.}

These are okay to ride straight over, but exacerbates slidiness when turning or manoevering or braking. Tread may reduce the "ball bearing" effect of turning while on loose rolly gravel. If its hard-packed like cement then its just bumpy and not relevant here.

Ice is another matter completely - tread helps not-at-all when turning on hard ice. In this case you want spiked tyres.

Snow - I can't speak for riding in snow etc, because there's not enough of it here for me to have experience.

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The two main reasons for changing a worn tyre are:

1. if there's a sudden increase in the frequency of punctures (this means the rubber is too thin for your riding conditions)

2. there's damage where you can

   • see the tube inside at any time, or
   • see a lot of threads, or
   • if the tyre is bulging in weird ways.
     In these cases, you're asking for a blowout which is never wise. Booting may help, but only to get you home.

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Personally I'd ride either pictured tyre until the frequency of punctures increases or significant damage happens to the tyre.

Answer 3

Few users of bicycles will be riding utterly exclusively on good tarmac.

Crossing verges, cutting a corner across dirt, taking avoiding action off the tarmac, riding through a region where the tarmac has been temporarily removed or where the road surface has degraded to little better than dirt, crossing a bit of garden or gravel to a shed at the back, and even short distances across parkland are all very common for riders who use their bicycles in the city or for commuting. A tyre with a bit of tread is much better at handling these circumstances than a slick tyre particularly in damp conditions, while a tyre with tread is not much worse at riding on normal roads than a slick tyre.

Since the majority of riders aren't going near the mechanical limits of their bikes, the advantages of extra flexibility of a treaded tyre easily outweigh the performance advantages of a slick tyre.

Answer 4

On tarmac/roads the tread has virtually no purpose. Two small uses it has are:

1. The extra thickness of rubber provides more puncture protection
2. Possibly a little extra grip in the tiny beads of gravel often found around junctions.

In my experience the bigger issue with extremely worn tyres is they tend to get very squared off and then have very strange handling (and probably less grip) when you lean the bike over in corners.

In the case of a 'normal' tyre, i'd usually replace it if it was in the condition of the one you linked. However in the case of a touring tyre with an extra rubber layer under the tread there's probably no need.

Edit: Here are some links from tyre manufacturers expressing they believe that on a road tyre the rubber compound is what counts.

https://www.michelin.co.uk/bicycle/tips-and-advice/tips-and-advice-bicycle/what-is-grip

https://www.schwalbe.com/en/profil

Answer 5

Tyre tread won’t improve grip on hard surfaces, even when wet. On soft surfaces (like grass, mud or snow) the treads of city bike tyres are pretty much ineffective. You’d really need knobs with proper spacing.

So why do city bike tyres often have treads like these:

(photos show Schwalbe Marathon and Continental GP4Seasons tyres)

I can see several possible reasons:

1. Marketing and customer expectations. Tread just looks … grippier. I’m pretty sure that’s the reason why even road bike tyres often have tiny, useless treads on the rubber surface. Doesn’t increase costs and doesn’t have any other disadvantages, so they just do it.
2. For thick rubber having those “cuts” (negative tread) probably allows the rubber to flex without cracking and maybe reduces rolling resistance.
3. Maybe some weight savings without sacrificing too much puncture protection.

Answer 6

(Assuming that by tread you refer to a patterned tread; in some other contexts tread is the area of the tire that gets worn whether patterned or not.)

They're there because most bicycle buyers are familiar with car tires and are aware that car tires absolutely do require a patterned tread or else they'll hydroplane.

If you attempt to sell tires without patterned tread, you will go out of business because you will lose N% of potential buyers. Even if the tires are intended for bikes, because bike tire buyers are not aware that the danger of hydroplaning doesn't apply to bike tires.

For this reason, even tires such as Continental GP 5000 that are supposed to have the absolute lowest rolling resistance, and therefore from an engineering point of view should have completely slick tread with no patterns, do have a tread pattern.

The fear of bicycle tires hydroplaning won't happen. To hydroplane, you need:

• A tread that is flat, not curved
• A tread that is wide, not narrow
• A tire that is filled to low pressure, not high pressure
• So much speed that bicycles don't regularly attain such speeds

Even if you manage to ride 80 km/h on a downhill, bicycle tires don't hydroplane.

More about hydroplaning and slick tires on bikes.

As for worn tires, the time to replace a tire is when you see the first casing cords below the worn out tread.

Answer 7

EDIT: Upon further discussion with Andy P and referring to some manufacturer publications, I have found that the macroscopic tread pattern is likely insignificant regarding increasing grip. What matters more for road tires is a good rubber compound that can better interlock with the road on a microscopic level. Hence, we can find very grippy road tires on the market without any sort of tread pattern whatsoever. I'll leave up the answer as bathroom reading if you'd like still.

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I'll discuss the "micro-tread" sometimes found on high-performance road tires in this answer. In the comments on Juhist's answer, Andy P cites aerodynamics as one reason for these tires to have a micro-tread pattern. While I do not have a background in aerodynamics, I could imagine the tread pattern acting like the dimples on a golf ball, which serve to create a boundary layer of air around the ball. That helps reduce the wake generated by the moving ball and ultimately reduces drag. [Citation] At high speed, this could have a meaningful effect.

Another reason for these tread patterns (which I understand better) is the mechanical friction aspect. Commonly, friction is just modeled as the product of the normal force and the coefficient of friction, but this is a simplification that doesn't always work too well in real life. A good analogy for how micro-tread interacts with the road surface is a group of interlocked Lego blocks:

The peaks and troughs (known as asperities) interlock to provide a mechanical bonding effect. Since the surface of even a very smooth asphalt road has lots of little nooks and crannies, it actually makes sense to have a micro-tread pattern which can interlock with these imperfections. Of course, the "micro" prefix inherently means these tread patterns don't last very long as the tire wears down.

Some examples of these tread patterns can be seen on the Continental GP5000, Vittoria Corsa, and Bontrager R4, shown in order:

Answer 8

A minor improvement I noticed when replacing a worn tyre with one with more tread: you get less spray when riding in the wet, as the tread clears more of the water to the sides near the bottom, rather than having water stick to a smooth tyre and end up spraying vertically onto you. Not such a big deal if you have decent mudguards though.