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Why do we use racing slicks if surface area =/= friction?

  1. Aug 23, 2015 #1
    I understand how surface area does not come into the equation governing the resistance an object has to sliding. It leads one to believe that all tires would perform equally in dry conditions. Why do people then say that bald tires give better grip?
     
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  3. Aug 23, 2015 #2

    Bystander

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    Do they?
     
  4. Aug 23, 2015 #3

    rcgldr

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    Coeffeicient of friction is affected by the load per unit area. Wiki article:

    http://en.wikipedia.org/wiki/Tire_load_sensitivity

    The load per unit area also affects the heating of a tire, and more surface area means more cooling. There's a point of diminishing returns when the tire gets too big and aerodynamic drag or tire / wheel weight become an issue.

    In the case of racing slicks, there is less deformation due to side loads than with a treaded surface.
     
  5. Aug 23, 2015 #4

    SteamKing

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    All tires, especially competition models, are specially designed pieces of equipment. Tires are more than round pieces of rubber which roll and do little else.

    Racing tires differ from road tires in that they are specially designed for competition. They don't need to last 40,000 miles; they aren't going to be driven in snow or ice or mud.

    The rubber compound used for racing tires is designed to stick to the road surface, so by eliminating treads which serve to channel away water, the amount of rubber in contact with the surface of the track is increased, which helps the car stay on the track.

    http://auto.howstuffworks.com/auto-racing/nascar/nascar-basics/nascar5.htm
     
  6. Aug 24, 2015 #5
    Racing tyres do offer more rolling resistance than street type tyres but they also offer far more grip in both the longitudinal direction, for braking and accelerating and in the lateral direction for cornering. In the end this extra grip is more beneficial than the losses.

    I you are referring to bald tyres as in those that are worn out then they do not offer better grip. By the time the tyre has worn down that far the rubber is old and probably hardened and well beyond the design spec for the tyre. If you mean bald as in slick tyres then the increased surface area is important.

    Cheers
     
  7. Aug 24, 2015 #6

    berkeman

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    Why do you say that?
     
  8. Aug 25, 2015 #7
    Because they are sticky.
     
  9. Aug 25, 2015 #8

    berkeman

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    LOL, no they're not. Have you ever handled a slick at a racetrack? If you push them down on the ground and lift them up, they do not stick to the pavement. :smile:
     
  10. Aug 25, 2015 #9

    OmCheeto

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    Yes they are! :biggrin:

    We had a similar discussion a while back: Frictional accelerations greater than one G.
    Wiki's quote has since changed, so I'll use the new one:
    Perhaps racing slicks are the automotive analogy of a semiconductor. They don't follow the normal rules.

    We might be discussing two different things. I've got top fuel dragsters in my head.
    And no, I've never handled one at a racetrack.

    Here's a short blip from Popular Mechanics: Anatomy of a Top-Fuel Dragster

    They list distance and velocity for a then recent record run at 1/2 second intervals.
    The fastest acceleration appears to be off the line:

    v = u + at
    u = initial velocity = 0
    so a = v / t = 73.89 mph / (0.5 sec) * 5280 ft/mile * hr / 3600 sec * g / (32.2 ft/sec^2) = 6.7 g's

    I have the equation plugged into my spreadsheet, so if you are curious about the g forces for the rest of the run, let me know, and I'll plug them in.
     
  11. Aug 25, 2015 #10

    rcgldr

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    Hot racing slicks will pick up rubber marbles and gritty stuff off a race track.

    The rubber sheets on some table tennis paddles have enough "stick" to lift up a table tennis ball for a brief moment after pushing the paddle down onto the ball and the pulling the paddle upwards.
     
  12. Aug 25, 2015 #11
    I was thinking about these responses, and it does make sense that stickiness might behave differently than a coefficient of friction, since a piece of paper coated with glue would have increased resistance as it increases in size because there is more glue... okay.

    It is still odd though. I do notice that since I switched from bald tires to good treaded ones, I have been doing unintentional burnouts all over the place.
     
  13. Aug 25, 2015 #12

    DaveC426913

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    The tires are sticky when hot. They are designed this way.

    @berkeman : next time you're at the track, please pick up a fresh tire - right off the track, hot enough to boil water - and let us know if if it's sticky. :smile:
     
    Last edited: Aug 25, 2015
  14. Aug 26, 2015 #13

    russ_watters

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    Setting aside some of the peculiarities of certain sticky racing tires, in general the issue with the simple model is deformation. A harder object(s) will behave closer to the simple model than a soft one because the soft one deform and fills in the roughness of the other surface.
     
  15. Aug 26, 2015 #14

    Nidum

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    Racing slicks road grip is enhanced considerably by suction effect .

    Air is continually squeezed out of contact zone . Atmospheric pressure acting on net difference of area top and bottom of tyre produces a down load which enhances grip .
     
  16. Aug 26, 2015 #15

    OmCheeto

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    That sounds like a macroscopic version of the Casimir effect. Scientifically plausible, but dubious realistically, IMHO.
    Do you have a reference?
     
  17. Aug 26, 2015 #16

    OmCheeto

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    I was thinking along the same line, and was going to make some bad doodles to illustrate the effect, but decided it was too much effort.

    sophiecentaur made a similar analogy in the thread I mentioned earlier:
    This is just a bit rubberyer(sp?)
     
  18. Aug 26, 2015 #17
    Well, I do a fair bit of racing, I drag race two cars, circuit race one car and circuit race a motorbike and I think that when the slick tyres are hot they are pretty sticky. If I roll my car or bike after some hot laps over some not very good tar-seal, it will pull chunks out of it. That's sticky in my book.
     
  19. Aug 27, 2015 #18
    yes, the little bit of width and more touching is not enough of a loss vs the traction benefits.

    Rubber compound is much different. However even with "bald" tires you will get better grip in dry conditions granted the tires are not aged and drying out. An old tire that is dry'ed out and has tread is horrible for traction due to the rubber getting hard. Either way even a bald time has more contact patch and less tread flex, which allows more grab. also larger tread blocks on tires allow better grip due to less flex while cornering. Many small tread blocks allow for better traction on ice and in cold conditions as the cold makes tires hard and more "slippery".

    Odd fact that is fun to share. A newer tire wears out faster than say it will with 30k on it. Why? the taller tread moves more, causing faster wear. As a tire wears out it takes longer to wear out. Just imagine a tall building. it sways more due to the leverage of the height.. same concept with tires. A tire with 10/32 will wear faster to get to 8/32 than a tire would take to get to 8/32 to 6/32.. :)
     
  20. Aug 27, 2015 #19
    The equation is what is called "first order." It gives a basic result, ignoring complications. In physics, there are often many complications. These are "second order." When something gets so refined that second order is all optimized, then there are "third order" effects. And so on.

    About a decade ago I heard about special Formula One racing tires called "gumballs." These were very sticky rubber tires that were would last only for the five laps of qualifying! There was some talk about regulating them because their use was so expensive.
     
  21. Aug 27, 2015 #20

    berkeman

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    Sticky sure, but do you think it increases rolling resistance (which was the poster's claim)?
     
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