Conference terri::cars_uk

    I remember having this same discussion when I was at college...
    Common sense suggests that a bigger area = more grip, and practical
    experience shews it to be true.
    Then you get told that frictional force has nothing to do with area.
    Not being a Physicist, I wouldn't want to argue, (Dave Hazel, where are
    you ???) but...
    
    The theoretical ideal and an actual road are two totally different
    beasts, perhaps the extra grip has something to do with the increased
    chance of actually getting a bit of rubber in contact with a bit of
    tarmac at any given time ?
    
    Perhaps it has nothing to do with friction at all ! (Just thought of
    this), surely friction is to do with two surfaces sliding across each
    other, which is not the case we are describing. (Any racer will tell
    you that once his big fat tyres let go, they might just as well be
    155's..) So, what we should be talking about is a property that I refer
    to as 'sticktion' (I don't know if that is the proper term for it), and
    that is the way things tend to stick together, perhaps due to surface
    bonding. Ever tried unscrewing a screw that has been left for a while ?
    It takes an enormous force to get it started, and then it 'clicks' and
    starts unscrewing normally. The unscrewing normally bit is friction,
    the sticktion is what requires the enormous force to overcome. 
    What I am suggesting here is that this same phenomenon is what makes
    tyres grip, and therefore this would be proportional to the area
    irrespective of the load on the tyre. This would also neatly explain
    why wide slicks are useless in the wet - the water molecules would
    prevent the 'sticktion' from happening, and all you get is friction
    (which drivers refer to as wheelspin).
    
    Amad�n

    Actually the amount of tyre surface in contact with the road does not
    change much with wider tyres. It is determined largely by the pressure
    of the air in the tyre (approximately you have area=weight/pressure
    - there is some inaccuracy because the pressure is not even due to the
    forces involved in distorting the natural shape of the tyre).
    Wider tyres have a shorter and wider patch in contact with the road
    than narrow tyres (at the same pressure). If you increase the pressure
    in the tyre it becomes more 'round', and the patch in contact with the
    road gets smaller.
    
    Also there are actually 2 different coefficients of friction. One is
    valid for situations where the two objects are not slipping and the
    other for situations where the objects are slipping. The 'slipping'
    coefficient is less than the 'non slipping' one. Naturally you want to
    be driving in the 'non slipping' case.
    
    None of the above answers why wider tyres are used. Wide tyres give you
    2 things
    	1) Low profile
    	   This results in a smaller sidewall, and hence less distortion
    	   when cornering.
    	2) More total tyre area
    	   This may result in less stress and heat build up in the tyre -
    	   the force to accelerate the car is comunicated via the tyre. If
    	   you have a more powerful engine you probably need to spread the
    	   load over a greater amount of rubber to avoid overheating the
    	   tyre. It may also increase the number of miles you get from the
    	   tyre.
    
    Andrew

Sorry Andrew, but I disagree :-)
    
>    Actually the amount of tyre surface in contact with the road does not
>    change much with wider tyres. It is determined largely by the pressure
    
    I am *quite* sure that there is a *big* difference in the amount of
    rubber in contact with the road between a 155 and a 355.
    
    Your point about air pressure only makes sense with similarly sized
    tyres, and even then it tends to move the contact patch from the centre
    to the edges.
    
>    Wider tyres have a shorter and wider patch in contact with the road
>    than narrow tyres (at the same pressure). If you increase the pressure
    
    A tyre will only have a shorter contact patch if it is a smaller
    diameter - nothing to do with the width.
    
>    Also there are actually 2 different coefficients of friction. One is
>    valid for situations where the two objects are not slipping and the
>    other for situations where the objects are slipping. The 'slipping'
>    coefficient is less than the 'non slipping' one. Naturally you want to
>    be driving in the 'non slipping' case.
    
 Basically my friction and 'sticktion' - but I feel that they are actually
    two different things, not just two cases of friction.
        
>    	1) Low profile
>    	   This results in a smaller sidewall, and hence less distortion
>    	   when cornering.
    
    Low profile is caused by building low profile tyres - nothing to do
    with the width - you can just as easilt build a 155/50 as a 255/50.
    
   > 	2) More total tyre area
   > 	   This may result in less stress and heat build up in the tyre -
   > 	   the force to accelerate the car is comunicated via the tyre. If
   > 	   you have a more powerful engine you probably need to spread the
   > 	   load over a greater amount of rubber to avoid overheating the
   > 	   tyre. It may also increase the number of miles you get from the
   > 	   tyre.
    
    True, you will heat up a tyre more by putting more power through it,
    but surely that heat/stress comes from the effect of the interface
    between the tyre and the road ? 
    
    Amad�n

    
    This narrow and wide tyre argument has been explored before.
    
    Mark

    Unfortunaltly friction _is_ proportional to area (Even static
    friction!). This is the study of Tribology (sp?).
    
    I don't profess to know anything more than that - but my Dad lectures
    it so I could ask him for more detail if you want.
    
    	Rik
    
    ``The first run of friction is that there are none.�''
    
    �Well none we understand yet.

Friction of itself is not related to area.  Basically there are two measurements
to worry about -
	1 is the "coefficient of friction" which is the resistance to motion
	between two surfaces.  This has nothing to do with area at all.
	2 is the "total resistance to motion", which is the coefficient of
	friction multiplied by the area (roughly speaking)  This is what
	most people refer to (incorrectly) as 'friction'.
The fomulae for the above are really very simple (O-level stuff in my day, and
long forgotten!).  The application of them in the real world is something else.
If you want lots of "sticktion", simply put, you want a lot of contact area,
ie fat tyres with lots of footprint.  However, as was mentioned, it gets
really complicated by temperature and fluid dynamics ......

The physics of tyres is highly complex and I do not profess to be an expert by
any means.  I would however like to give my observations on wide vs narrow 
tyres since I have driven on both types on the same vehicle.

o The amount of traction you get from a tyre depends on the frictional force
  generated by the tyre and the road.  This is determined by a combination of
  factors.  Tyre width, inflation, tread design, and hardness/softness of the 
  rubber are the major factors influencing friction from the tyre's perspective.
  I will not discuss the road since we have no control over that.  For maximum
  traction going straight, the goal is to hit some optimal window of stress 
  (pounds per square inch) on the road. I do not know what this stress window 
  is.  Too little stress and the tyres become airborne as speed increases, too
  much and the tyres start spinning.

o Obviously a car must do more than go in a straight line so you can't have
  giant marshmallow tyres on a tiny car.  To corner properly, the tyres must
  again apply an optimal amount of stress to the road.  Cornering is 
  complicated in that the tyre tends to roll sideways due to lateral stress 
  being applied.  Rolling removes some of the area in contact with the road,
  the stress dynamically increases, and the tyre loses traction.  Suspension
  geometry and sidewall height/stiffness determine how much roll the tyre will
  have and hence how much traction the tyre will have on turns.  A suspension
  and tyres appropriate for maximum performance on curves will give a stiff,
  harsh ride so tradeoffs must be made.

o As you go faster, the amount of stress applied to the roads should increase
  to keep the car from wandering in the straights and sliding on the turns.
  Formula-one cars accomplish this by the use of wings.  A front and rear wing
  applies more downward force on the tyres the faster the car goes.

o To go well in rain and snow means that the tyres must channel the 
  precipitation away from the contact area so the tyres can see that optimal 
  stress level with the road.  Optimal traction on wet roads requires a much 
  higher stress window than on dry roads.  Narrow tyres are preferred over 
  wide ones for wet surfaces.

o A street tyre must be designed for all types of road surfaces, weather
  conditions, speeds, and turns. It is a compromise in every area in that it
  does nothing great but everything well.

Jim
  

Very interesting, but I'll still stay off Michelins on wet roads :^)

Lets face it, how many of us actually care very much about a 1% difference in
sticking force, really?  Derek is certainly the only one who has a critical
need for real grip uinder all sorts of different conditions.  The rest of us
drive on compromises - fairly good in the dry and on corners, but suboptimised
to attain some form of grip in the wet.  As I said, I've never had a Michelin
that I would trust on wet roads (I've had X's and MXV's) so it boils down to
my driving requirements and my driving techniques and abilities.  I've had to
slow down recently (55 limit and all that) so my current requirements aren't
as stringent as they would be in the UK, perhaps.

The Alfa is shod with Firestones - we use it predominantly in the dry, the tyres
are great on dry roads, and fair in the wet, and they were cheap!  They are
185/70 x 14.  The last set did almost 50,000 miles before wearing out.
The Audi has Bridgestones, fat ones!  Don't know the size off hand, but it must
be something like 205/70 x 14  They're fine in the dry and I haven't had a 
chance to check in the wet yet.
The Ford has it's OE Yokohamas.  Thin things for a small car.  They slide
very easily in the dry, and I don't know what they're like in the wet either.
These tyres will probably be the first to be changed!
The best tyre/car combo I ever had was my Fiat 124 on Semperits.  What grip!
The E-type was shod with Dunlops, tyres I also normally avoid but at 200 quid
a time, they stayed on!  That car was totally impossible in the wet, but it
did have over 300 horses pumping away .....

Oh yes, the plane is shod one side with a McCreary Air Hawk and the other with
a Goodyear.  They are 6.00 x 6, 6 ply.  The nose wheel is 4.00 x 5 Goodyear.
With luck, and good maintenance, they should last about 10 years .....

    	We had the same question come up awhile back in US carbuffs.  I
    answered it there and didn't get too many negative comments so I'll try
    again.  The first thing to understand is that even when the car is
    moving forward, the contact area that touches the road (unless the tire
    is spinning) is stationary to the road.  
    	Now, if you look very close at that contact area, you will find two
    irregular surfaces that are "bonded" to each other.  Someone mentioned
    the viscous (or fluid like) tire sinking into the solid irregular road.
    This is how you get traction.  If both surfaces were smooth, there
    would be no traction, such as on ice.  The larger the bonding area, the
    stronger the bond and the more power you need to break the bond. 
    Hence, a larger tire of the same compound will have more traction than
    a small one.
    	Consider two boards and a handful of nails.  If I drive in one nail
    and apply a strong enough lateral force, the nail will shear and the
    boards fall apart.  Drive in twenty nails and it will require a much
    greater force.  This is effectly increasing the contact area.
    	Back to the tire, instead of nails, you have all the little pieces
    of rubber grabbing the road.  The larger the contact area, the larger
    the bond.
    	As far a classic physics goes, all the formulae work.  But the
    surfaces are smooth and not bonded together.  In the lab we used blocks
    of wood with weights on top.  And as far as we could measure, the
    surface area did not make a difference.
    	One more point.  When the drag racers spin their tires, they are
    actually melting them and the thin layer of molten rubber is a better
    bond than a cold tire.  However, even with the extra traction, if too
    much power is applied, all the bonding is broke and the wheel spins
    freely.
    
    
    Regards, John
    

    Dunno about drag racers but heavy haulage railway locos are now built
    with "creep control". more tractive force can be achieved if the wheels
    are allowed to slip ever so slightly. This has got nothing to do with
    bonding as it is steel on steel. Clearly uncontrolled wheelspin is a
    bad thing.
    My physics is not up to explaining this.
    
    Andrew