Quote:
Originally Posted by Rehaan Pressure = Force / Area
Therefore the larger the contact area, the less the pressure on the road. Surely pressure on the road (think of it like "downforce" per unit area) is somewhat responsible for traction no? |
This is a very imporant point and points to why wider tyres actually do not improve a car's grip, they only change its behaviour.
IF you think about a tyre and imagine its contact patch, you realize that it has a finite area because the tire deforms under the weight of the car. If it did not deform and remained a perfect circle, its contact patch would look like a straight line of very tiny thickness (approaching zero for a perfect circle on a perfect plane). But in real life, the tyre does deform and this contact patch has length from back to front. How much this length is for a given tire of a certain size depends on how deformed from the perfect circle it is by the weight of the car. THe lower the inflation pressure, the greater the deformation, and the greater front to back length of the contact patch. WIth the same width, lower tyre pressure makes for a longer contact patch, and thus a larger contact area.
For the same weight, this reduces the contract pressure on the ground.
Why is this important?
Here is why: Let us say you fit a swift that has 185/65-15 tyres with 215/55-15 size tyres of exactly the same model (no difference in design or materials.) Overall diameter is nearly unchanged. Have you in fact improved grip of the swift?
NO.
This is because the size of contact patch of the 185/65 tyre will be the same size as that of the 215/55 tyre. Only the shape will change. As the contact patch becomes wider, it will shrink in length from front to back. So where the 185/65 has a contact patch closer to a square, the 215/55 has a patch shape closer to a rectangle. This is because the size of the contact patch is largely determined by the pressure in the tyre and load bearing down on the tyre. If you maintain pressure and weight the same, a wider tyre of the same overall diamater will maintain about the same contact area and provide no more grip!!
(its not perfect so since sidewall and treadblock deflection with pressure introduces some non-linearity, but for the purposes of anybody outside an F1 team, its not a difference that matters.)
net result, the 215/55 will produce no more grip than the 185/65, on the same car, using tyres of the same design and materials. So why do people who install wider tyres swear that their car can corner much faster?
Slip angles. A slip angle is the difference between the direction a tyre is pointing, and a tyre is actually travelling. WHen going straight, its 0 (assuming the wheel alignment ha 0 degrees of toe). When you turn the steering wheel, you generate a slip angle at the tyres. every tyre in the world generates a slip angle, no matter how tiny the angle is.
Slip angles have a big impact on how the car feels while cornering. A 55 aspect ratio will generate a smaller slip angle than a 185 because its shorter sidewall will deflect less under the same lateral force. This is felt by the driver as sharper steering. A 55 section tyre generating the same slip angle as a 65 section tyre will be under greater lateral load. Since we judge how hard a car is cornering by a combination of how much we have turned the wheel for a given speed and how much lateral g force we feel, a tyre that generates smaller slip angles for the same steering input, will for the same steering input be generating higher lateral loads. This means the car will be travelling faster. Which means that if you have lower profile tyres, you will find yourself travelling faster through corners with the same amount of steering input. The sensation is of enhanced grip.
But in fact, outright grip is unchanged! If you drove either car to the absolute limit of grip, you would find that they lost grip at about the same level of lateral load!
What does this mean? It means that tyres with larger slip angles will start to make you feel those slip angles sooner, and they will have larger slip angles at breakaway, making them feel, right up to the point of breakway that they are gripping harder. But it also means that they will warn you of impending breakaway with large slip angles well before you reach breakawy. A tyre with smaller grip angles will have much smaller changes of slip angle, and to the driver, it will not feel like the limits of grip are appproaching till they are very much closer, or actually reached!
So long as you don't actually reach the limit, a wider tyre of the same wheelsize and overall diameter will feel like it has more grip.
So you could say that even if the overall grip level is not higher, so long as a tyre feels like it has more grip and allows you to drive harder, wouldnt that mean wider low profile tyres are better than narrow high profile tyres?
THe answer is, it depends. A tyre that generates large slip angles is far more communicative and tells you a lot more about whats going in, including the approach of breakaway, IF you can trust what its telling you. A tyre that generates small slip angles can provide the thrills of the car feeling faster, more agile, more grippy, but doesn't communicate its limits so well and doesn't actually provide more grip (given identical design and materials).
A 215/55 tyre will come at a cost compared to the 185/65. It will be heavier with a detrimental effect on the suspension, acceleration and fuel economy and it will cost more. What tire size a manufacturer chooses for a car depends on the compromise of cost, ride, handling, and fuel economy that it intends for a particular car.
As far as brio vs swift statement in ACI goes, the only thing you can learn from that is that a wider tyre does not make up for difference in chassis design and set up or differences in tyre design and construction.