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I have tires similar to this one: enter image description here

My point is that the tire is quite "flat", with large knobs on the edges, unlike most MTB tires, which are round.

The tires feel quite unstable on any hard surface when cornering. What is the design reason behind making an MTB tire non-round?

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That design, in (much) less severe form, is seen even on many road tires. The idea is to not engage the inefficient heavily-lugged edges unless one is cornering, which is a situation where extra traction is needed. –  Daniel R Hicks Jul 2 '13 at 11:16
    
@DanielRHicks, don't get me wrong. You are probably referring to semi-slicks, while I am asking about fully off-road tires, with deep thread all around, but event deeper on the edges. –  Vorac Jul 2 '13 at 11:21
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Well, the specific tires in the picture are clearly designed to provide traction in dirt/mud when headed straight ahead, but the additional traction is still needed for cornering. (But of course a substantial consideration in tire tread design -- especially for off-road -- is "sex", and the above design may also simply be judged "sexier" than alternatives by those calling the shots.) –  Daniel R Hicks Jul 2 '13 at 15:07
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2 Answers 2

up vote 7 down vote accepted

These are downhill MTB specific tyres. The extended side knobs will fully engage when cornering hard. By hard I mean:

The large gap between the center and the side knobs is mainly to reduce weight and to improve the mud management of the tyre (to not allow mud to be blocked and carried between the knobs).

A review of the specific tyre you posted can be found at http://www.pinkbike.com/news/Maxxis-Minion-High-Roller-2-review-2011.html and for the side (cornering) knobs it says:

The cornering knobs were altered with this in mind, angling them slightly
for more bite and adding a vertical sipe, again for added knob flex to 
allow them to conform to the ground better than a solid knob. Changes 
were also made to have the cornering lugs bite sooner than on the 
previous design.
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I wouldn't consider it to be too downhill specific. Typical all-round MTB tires such as Schwalbe's Nobby Nic are built like this as well. –  Benedikt Bauer Jul 2 '13 at 11:04
    
I was talking about the tyres in the photo (High Roller II) which are definitely built for more aggressive riding than Nobby Nic. –  cherouvim Jul 2 '13 at 11:06
    
I didn't doubt that. But the OP wrote "I have tires similar to this one:" which may fit on a wide variety of tires. Also what you write is not wrong, I just wanted to add that this feature isn't a feature which is unique to downhill tires. –  Benedikt Bauer Jul 2 '13 at 11:11
    
Yes you are right. I just commented on the photo I saw and on similar tyres which have "extreme" cornering knobs to deal with extreme situations. –  cherouvim Jul 2 '13 at 11:14
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More grip than on good quality asphalt should be impossible with probably any tyre. Adequate grip on dirt is only possible with good tyres and good rider technique. –  cherouvim Jul 2 '13 at 11:29
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What feels unstable when cornering on hard and smooth surfaces will turn out opposite on soft or structured ground. In this case the pronounced edge of the thread will bite into the soft ground or hook into the structure of hard ground which will give you a better side control than a thread that is just following the roundness of the tire.

You can compare it a bit with the shape of the skid of an ice skate: when gliding you want to have a smooth surface that has the lowest possible friction while you want to have a sharp edge on your skid for good control in corners and when actively skating.

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Then why make round mtb tires? A quick google search shows almost only round tires. Or maybe this is a different question? –  Vorac Jul 2 '13 at 12:03
    
The impression, whether a tire is looking either "edgy" or round may depend on the arrangement of the knobs: If the outer knobs are aligned more or less in a line that is clearly separated from the central thread, it will give a much edgier look than a thread where the different rows of knobs are more interlaced. I wouldn't tell that one design has in general a significant advantage over the other. It's more about either the best design for a certain terrain or preferably good all-round properties. –  Benedikt Bauer Jul 2 '13 at 12:19
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