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How tyres work in the rain

  1. Apr 11, 2006 #1

    Hootenanny

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    I was just curious to how tyres work in the rain. How does the tread shift the standing water so the tyre can have traction? Any information would be appreciated.
     
  2. jcsd
  3. Apr 11, 2006 #2
    The varied geometric shapes of a tires tread serves to deflect the water into raised "channels" between these geometric shapes.
    The challenge is that these shapes must have sufficient surface area, else they would wear down quickly.
    So, there is a trade-off. Yes, you could design a tire that is extremely optimal in rain, but in so doing, it might have very poor tread life.
    I suppose they strike a happy medium.
     
  4. Apr 11, 2006 #3

    brewnog

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    The first most significant feature is a tread. You can have the stickiest compound in the world, but if you have no tread, wet weather performance will be rubbish. As you know, tread patterns break up the layer between the rubber and the tarmac so that aquaplaning is discouraged.

    As a tyre rolls over wet tarmac, a 'wedge' of water forms in front of the tyre. If the tyre rises up on this wedge, contact between the tarmac and rubber is broken, and aquaplaning occurs. By breaking up the surface of the tyre such that there are channels for this water wedge to infiltrate, the tyre is less prone to rising up on the wedge. For a similar reason, (like-for-like) narrow tyres perform better in very wet conditions, - the tyre is less prone to 'floating' up onto the wedge of water.


    More detail: Circumferential grooves (going 'around' the tyre) act as channels to drain the wedge, and to some extent, store this water. Lateral grooves, - going side-to-side across the width of the tyre - drain this water from the circumferential 'grooves' to the tyre edges. At 100km/h, an average tyre in average rain needs to shift something like 6 litres of water per second to maintain contact. New wet tyres have massive grooves in the centres which can displace huge amounts of water, - something like 10 litres per second, but these wear very quickly in the dry. Circumferential 'ribs' provide contact zones, particularly for cornering. Small, shallow dimples around the shoulders of the tyre increase surface area to allow for more efficient cooling. Sometimes a 'void ratio' is specified, which quantifies the amount of non contact/ contact area across the width of the tyre, - for a slick tyre this ratio will be 0%, for an off-road mud or snow tyre this will be much higher, perhaps 40%?


    More detail still: Tread patterns vary for a number of reasons. Simple block-shaped treads fundamentally work well in muddy and wet conditions, but are prone to rapid wear of the front and back of the block on dry tarmac. Regularly spaced grooves are avoided because they set up harmonics which can cause hefty vibrations.


    Finally, asymmetrical tyres will have more large blocks on the outside for cornering stability, and smaller blocks on the inside for heat dissipation and water displacement.
     
  5. Apr 11, 2006 #4

    Hootenanny

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    Thank-you both of you. That was excellent. :biggrin:
     
  6. Apr 11, 2006 #5

    DaveC426913

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    Sunufagun. Lots of interesting info there, more than I ever knew about tires, but surely, this is by far the most interesting bit:

    "...Regularly spaced grooves are avoided because they set up harmonics which can cause hefty vibrations..."

    which explains one of the most curious aspects of tire treads - the complexity of the geometrical shapes.
     
  7. Apr 11, 2006 #6

    brewnog

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    It's not the only explanation. Looks are a major selling point, and it's quite a hard job to design a tyre which performs well, and also looks the part.


    Interestingly, we're currently experimenting with radiator fans with unequally spaced blades, - it's looking like noise can be drastically reduced if we can address the balancing.
     
  8. Apr 11, 2006 #7

    Hootenanny

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    Well, good look with that. I've never thought about harmonics with tyres, fans and such like before. Puts a different perspective on things :smile:
     
  9. Apr 11, 2006 #8

    Danger

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    Hmmm.... I saw a car fan once that had five blades, not equally spaced. Maybe that's what it was all about.
     
  10. Apr 12, 2006 #9

    FredGarvin

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    Balancing and performance I would also imagine. I would tend to think that the blade profiles would not be the same. Interesting....
     
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