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Tyres - why is wider better for lateral grip?

  1. Nov 5, 2009 #1
    I did some searches and I see a little bit of discussion on this topic in the past but I am still not sure I fully understand why wider tyres give better lateral grip. I have always put wide tyres down to loading requirement ie sports cars with lots of power and cornering ability need the extra rubber so the tyres don't get overworked and stop working. But I hear that you can somehow get more lateral grip from a wider tyre regardless of heat / loading.

    Finally, why does a narrow tyre result in a longer contact patch and a wider tyre result in a shorter contact patch? Surely pressures are altered to get the best contact patch in either case resulting in the length being about the same in both cases?

    Thanks
     
  2. jcsd
  3. Nov 5, 2009 #2

    rcgldr

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    One issue is tire load sensitivity, at higher loads, the coefficient of friction decreases:

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

    The pressures would be adjusted, lower in the wider tire. A larger diameter tire will have a longer contact patch.

    The effective size of the contact patch is affected by the tire diameter, width, and pressure. There's a point of diminishing returns as a larger tire is heavier and requires more volume to place inside the car. The total size of the tire tread surface allows for heat dissipation which is another factor.
     
  4. Nov 6, 2009 #3
    Thanks jeff, that explains it, it is that non linear relationship that was throwing me.
     
  5. Nov 6, 2009 #4

    Ranger Mike

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    We race a Formula car (Reynard) with 6 inch front and 8 inch rear..here is a recap of discussions about going to 8 inch front and 10 inch rear.
    Downside of larger tires
    More rolling resistance.
    Harder to get heat in them.
    More aero drag.
    More unsprung weight.

    Bigger wheels = heavier wheels and more rolling mass = equal or even slower lap times. The wider wheels (tires) have no proven advantage at all.
    1 - The cars will accelerate slightly slower because of the greater MOI of the heavier wheels and tires.

    2 - The cars will be a bit slower in a straight line because of the extra drag.

    3 - Assuming that there is a diameter difference ( I honestly don't know - never looked at the spec comparisons), geometry may be adversely affected - camber curves may need to be VERY different.

    4 - If the car was at max width with the smaller tires, with the wider tires offset inwards to match the max width, scrub radiuses will be adversely affected. Without making major changes to the upright geometry, this will necessitate caster changes that may be out of the range of available adjustment

    5 - If the wider tires are used on a car that wasn't already at max width, the above doesn't apply, assuming that the center of the contact patch can be placed at the same widths, though caster changes may still be necessary for decent steering effort.

    6 - If the tires have to place all their extra width inboard, suspension interference can become an issue, especially front lock availability. Front wing interference might also become an issue on many cars.

    7 - Differences in lateral and vertical tire spring rate will necessitate different springs and shock valving ( already necessary because of the greater unsprung weight). These differences might also necessitate changes to the RC heights.

    There's a lot more, but you should get the gist of what I'm saying : Just throwing on a wider set of tires proves NOTHING one way or the other. It is ONLY when the car gets optimized (which might necessitate cutting and welding) for the wider tires that you can start to get legitimate answers.

    Aero footprint-wise, one of the biggest factors is the width of the contact patch, and therefore the width of the high pressure wedge of air that is being pushed just ahead of it. Wider = more drag (and lift). Some old SAE papers actually show that both ( drag and lift) can be decreased almost linearly with grooves in the tread, but obviously at the expense of contact patch area. Overall width played a larger role than height, but the percentage differences were small. The shape of the shoulder ( how square or round) played a major role.

    The wheel design also will play a major role in aero drag - try a spin coast down test with different designs and you'll be in for a surprise.


    The next big factor will be getting heat in the rubber - generally a bit harder to do with the bigger tires as the unit loading of the contact patch is decreased - but this can be overcome quite a lot with the right shock valving, and gets helped big time when the track temps are sky-high - when the skinny tires are more prone to over-heating unless you go to harder compounds. Fat tires will generally allow the use of much softer compounds, even in the high heat situations. Under these conditions, the fat tires will almost assuredly have a big advantage at tight, twisty tracks.
     
  6. Nov 6, 2009 #5
    Another thing (or just another way to look at it) is that for a given cornering G the wider tyre has a lower slip angle. ie less distortion.
    The tyre starts to slide when the slip angle reaches a threshold so a wider tyre can naturally generate more grip.

    On the tyre load sensitivity issue it works out that putting twice the weight on the same tyre reduces grip by around 15%. And vice versa - halving the weight increases your grip by 15%. That's why true race cars generally weigh about half that of a typical family car - just for that extra 15% on the coefficient of friction.

    formula is:
    grip is proportional to weight^(-0.2)

    likewise fitting tyres that put twice the ammount of rubber on the road gives you roughly 15% more grip.
     
  7. Nov 11, 2009 #6
    Ok cheers guys I think I get it now
    an 'overloaded' tyre can be fixed by making it bigger but this will bring with it other disadvantages that are best avoided by addressing root cause which is vehicle weight
     
  8. Nov 11, 2009 #7
    With regard to tyre contact area patch size, consider a very soft tyre with a load weight mg, a patch area A, and a tyre pressure P

    mg = P A

    Bob S
     
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