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The answers to this question strongly advocate for smooth tires or maybe tires with just a very shallow profile. What are mountain bike tires with deep profile good for, do they have their own important strong sides? Why are new bicycles sold with them at all if smooth tires are better nearly everywhere?

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  • 13
    they're good for mountain biking :)
    – MaplePanda
    2 days ago
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The question you linked to was specifically asked for roads. Smooth tires are indeed better on hard and smooth surfaces like roads. The shallow profile helps evacuate the water that can be between the tire and road.

By contrast, MTBs are designed to be used on loose surfaces (earth, mud, sand, stones, ...). The knobs are better in those environments for several reasons:

  • on "surfaces" like stones/roots, you can only have limited contact points (the edges of the rocks/roots): the knobs can approach edges "perpendicularly" instead of "tangentially", and not rely on friction in the same way.
  • they allow gripping on hard features that are under the loose surface (for example, a root under mud, a stone under sand) — that's also why they need a certain distance between them: if they are too close together, it's like if it were a continuous surface).
  • on wet surfaces, friction coefficients are much lower, and smooth tires just slip...
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  • 3
    The main benefit of knobblies is predictability. When cornering hard on slicks, you get around the corner, or wipe out. With knobblies, the 'bite' allows you to both feel, and respond, to changing levels of traction. Keep in mind many riders choose tires for the trail conditions, some only for winter/summer, some do it on the day, to balance the traction and control with rolling resistance.
    – mattnz
    2 days ago
  • On a very short sharp incline (10m@5%) over a bridge, my CX knobblies tend to slip on wet tarmac whereas slicks don't.
    – Carel
    yesterday
9

Renaud's excellent answer can be elaborated upon regarding soft surfaces.

Since it is obvious that bicycle tire rubber is not hard enough to penetrate asphalt and the rider's weight is not a large enough force to do so anyways, tire tread will not appreciably dig into a paved road [citation needed]. That's why a micro-tread pattern like seen in the following image is all you need. This tread will help engage with asphalt's existing surface roughness:

enter image description here

However, when riding off-road, the soil is often soft enough that tire rubber will dig in. This creates a paddlewheel effect, which gives extra traction. For reference, here is a tire made for muddy conditions with suitably exaggerated tread:

enter image description here

You can see how the pronounced tread will dig into the mud. As Renaud mentions, you transition from purely "tangential" friction to now having the perpendicular shear area of the soil/mud trapped between the knobs giving your input forces something to push against.

On slightly harder surfaces (eg. hardpacked dirt), the knobs increase the pressure placed on the surface (less area for the same force), so there is a higher chance they will dig in versus a slick tire.

Some other factors:

  • Having tread can somewhat protect the tire against punctures on hard surfaces as the tire is raised off the ground when the knobs aren't fully submersed.
  • Knobby tires are arguably part of the look of a mountain bike, and riders might not like the association between slick tires and roadies.
  • The knobs visibly wear much faster than a smooth tread, so tire manufacturers can sell more tires :D
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    Also, consider that the GP5000 tread will be flat in the middle within a few hundred km anyway. The shoulders will retain some tread much longer, but the main road contact strip will flatten off quickly leaving only a natural touch strip.
    – Criggie
    2 days ago
  • 2
    Those microtread things are completely pointless FYI, they are there for marketing/sales reasons, rather than technical ones. 2 days ago
  • 2
    @Michael The minutely stippled/dimpled surface of the entire tread, not the fancy trapezoids.
    – MaplePanda
    yesterday
  • 2
    The trapezoid things on the GP4 and 5ks, which may be called sipes or siping, actually are thought to improve aerodynamics, and may not be about grip at all. Reason may be something about tripping boundary layers, which I don't understand. People have actually measured this in the wind tunnel, though. Note in the link how the GP4s and 5s have marginally lower aerodynamic resistance than the GP TTs. aero-coach.co.uk/gp-5000-tubeless-data
    – Weiwen Ng
    yesterday
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    If you need a citation that for road tires, tread patterns are not thought to improve grip, this statement may originate with Jobst Brandt. Jan Heine has a quote at this blog post, which he only attributes to "some experts", but commenters have repeatedly referenced Brandt. renehersecycles.com/…
    – Weiwen Ng
    yesterday
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Why are new bicycles sold with them at all if smooth tires are better nearly everywhere?

Because:

  • slick tyres are terrible on anything except hard, smooth surfaces like asphalt
  • most hybrid/mtb/commuter bikes will at least sometimes be ridden on gravel/dirt/mud

New road bikes are indeed sold with slick tyres.

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  • Actually, have you tried riding on dirt roads on slick tires, especially wider ones like 28-35mm, before? It’s perfectly fine. It’s true that with much more loose gravel, you would want knobs to dig in to it. renehersecycles.com/why-we-dont-make-gravel-tires
    – Weiwen Ng
    19 hours ago
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    I was going to make the same comment as @WeiwenNg: road tires work just fine on most off-road trails, most off road trails are hard packed dirt. I've put a lot of trail miles on my crosscheck with 25mm GP4000s. 12 hours ago
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I think a big reason for knobby tyres on hybrids and similar bikes is psychology.

People like to be prepared for everything (same reason why people buy 4WD cars, or huge cars in general, just to commute or buy groceries 99% of the time). Even if they basically never ride off-road or in snow.

I think many people don’t realize how little knobs help on fine gravel, which is probably the non-tarmac surface people are most likely to encounter (if they ever leave tarmac at all). Wide tyres with low pressure help much more on gravel than the presence of knobs.

People think they need knobs or treads, which is probably also the reason why manufacturers keep adding these negative treads which do absolutely nothing for grip (maybe they help to reduce weight?):

enter image description here

Edit: I should point out that for “real” off-road usage on rough gravel, tree roots, grass, mud or snow knobby tyres definitely help a lot. But I think most “off-road” tyres never see such terrain.

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  • Concur - when the motor has 70 horsepower (lile a very small car) then differences in tyres can be felt. When your motor is several-hundred horsepower, the extra weight/resistance is nothing. We tiny cyclists have a quarter of a horsepower at best, so every little bit is important.
    – Criggie
    yesterday
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    Negative threading seems to be an easy solution to a thick layer of rubber, meant to increase the durability of the tire. The example that you show is a typical commuter tire, and I think commuters are valuing longevity more than other cyclists. The most obvious ways to increase durability are: a less tender rubber (at the cost of grip) or a ticker layer of rubber (that can become to rigid if it is continuous). Negative threading seems a nice compromise: it allows to use a more tender rubber, while mitigating against the rigidity of a thick layer of rubber. It may improve water evacuation.
    – Renaud
    yesterday
  • @Renaud: Maybe it’s there to improve flexibility and reduce weight while still allowing for a thick, protective layer of rubber on the shoulders. I think on road bike tyres they just reduce the rubber thickness on the shoulders.
    – Michael
    20 hours ago
  • @Michael I didn't think about protection (seems to me in the Marathons that it's done by a dedicated layer in the structure), that's indeed another good reason to have thick rubber.
    – Renaud
    19 hours ago

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