I’m going to use Audi 17″ rims and 205/40/17 tyres.
Does having bigger tyres mean I will have more or less grip?
Having owned a chipped 300ZX for a couple of years, I think there will be 2 answers:
- Yes in the dry
- Definitely NO in the wet
The answer isn’t quite what I thought it would be – I was close but no cigar.
I’m afraid the answer is going to involve sums again. Sorry, but in order to rule out the answer that most people would give, I needed to clear a few things up.
Bigger tyres must have a bigger contact patch??!!? Right?? Wrong!
- The physics bit……….
Pressure = weight ÷ area
Whilst stationary, or traveling in a straight line over a smooth road, this equation is a little over-simplified but not too far out.
Let say for the point of this example:
- Your tyres are inflated to 2.11Kg/cm² (30psi)
- Standard tyres – 185/65R14
- Uprated tyres – 205/40/17
- A Locost or Haynes Roadster weighs 700Kgs (1543lbs).
- The front to rear weight distribution is 50/50%
The weight (Vertical Load) on each wheel would be 175Kgs / 386lbs
Pressure = weight ÷ area = 175 ÷ 2.11 = 82.9cm²
For 185/65/14 tyre:
Roughly speaking a 185 tyre has a contact patch 185mm (18.5cm) wide.
Contact patch = 82.9 ÷ 18.5 = 4.48cm long
For a 175Kg wheel weight, a 185/65/14 tyre would have a contact area of 18.5cm x 4.48 cm (82.9cm²)
- Diameter of wheel in mm = 14 x 25.4 = 355.6mm
- Section height = 65% of 185mm = 120.25mm
- Outer diameter of the unladen tyre = (wheel diameter) + 2(section height) = 355.6mm + 2 x 120.25mm = 596.1mm
- Approximate rolling circumference is 0.96 x OD x π = 0.96 x 596.1mm x π = 1797.79mm
For 205/40/17 tyre:
Roughly speaking a 205 tyre has a contact patch 205mm (20.5cm) wide.
Contact patch = 82.9 ÷ 20.5 = 4.04cm long
For a 175Kg wheel weight, a 205/40/17 tyre would have a contact area of 20.5cm x 4.04 cm (82.9cm²)
- Diameter of wheel in mm = 17*25.4 = 431.8mm
- Section height = 40% of 205mm = 82mm
- Outer diameter of the unladen tyre = (wheel diameter) + 2(section height) = 431.8mm + 2 x 82mm = 595.8mm
- Approximate rolling circumference is 0.96 x OD x π = 0.96 x 595.8mm x π = 1796.89mm
You can see that the 205/40/17 tyre has a wider yet narrower (front to back) contact patch. In reality the contact patch would not be rectangular, but more of an oval.
We can see that the overall area of the contact patch has remained the same, with just the shape of the contact area changing. We all know that wider tyres can give more grip under the right conditions;but why?
The things that do affect grip are:
- Coefficient of friction of the rubber compound (The stickyness)
- Load on the tyre
- Slip or distortion of the tyres sidewalls
Friction (F) = coefficient of friction (u) x Normal Force (N)
In the case of a tyre, the Normal Force (N) basically stays the same – mass of the car multiplied by gravity.
The coefficient of friction (u) is in part determined by the rubber compound’s ability to ‘key’ with the road surface at a microscopic level.
Why do wider tyres grip better in the dry?
Well it has a lot to do with friction and distortion of the side walls. With a narrow tyre (left in above picture), the side walls are distorting more to give the same contact area. The side walls are taller and a greater proportion of the wheels diameter. As the side wall deforms, heat is generated, this in turn creates friction / heat. The contact patch is longer and a greater proportion of the circumference. Therefore, a given area of the tyre spends less time in the cool air as the wheel rotates. Because the narrower tyre has less capacity for cooling off, it needs to be made of a harder rubber compound in order to better resist heating. Unlike race tyres, a lot of heat is unwanted as this equals wear. A harder tyre has less ability to deform to the texture of the road surface. A wider tyre can get away with a softer compound rubber and therefore grips better.
Another reason is ‘Slip‘. Slip is where the surface of the tyre is at a slightly different angle to the wheel due to cornering forces. The twist occurs in the sidewall. The angle of the wheels is being controlled by the steering wheel, but the tyres’ tread is being controlled by the grip it has on the road surface. During cornering, the lateral forces increase on your tyres. At some point, the lateral force is going to overcome the tyres’ mechanical grip and that point is defined by the ‘peak slip angle’. A wider tyre; has less deformation and a shallower peak slip angle but will reach its’ peak slip angle at a higher speed.
Wider tyres perform better when cornering under the right conditions. The softer rubber compound conforms to the road surface better, with a higher friction coefficient, plus they have lower profile sidewalls. They have greater resistant to deformation under lateral load. All this results in a more predictable and stable contact patch. In short, you can reach a higher lateral load before reaching the peak slip angle. As a vehicle corners, the weight transfer will cause each wheel to distort differently. This uneven distortion can produce ‘interesting’ handling. The more a tyres’ distortion is limited, the more predictable handling will become.
Why don’t wide tyres grip so well in the wet?
In wet weather, when oils leach from the road surface and water fills all the tiny pits and holes, traction for wide tyres becomes a nightmare. The softer compound rubber cannot get to a working temperature and the whole surface of the tyre is being cooled by spray. The narrower tyres have a harder compound that can get to a working temperature more easily, dissipating the water sideways through the tread. On a wider tyre the water is cooler and has further to travel, which in turn cools the tyre even more. Because the contact patch is shorter, the water has less time to disperse. When the load on the tyres is insufficient to disperse the water, the often sudden deformation of the tyre and change in vehicle loading can cause unpredictable handling. In wet weather, you’ll be better of with a higher profile, narrower wheel as water will disperse more easily, the rubber will be closer to a working temperature and often; most importantly; handling will be more predictable, with a much more gentle transition from grip to no-grip or visa-versa.
In the 70’s the craze with some cars, super cars included (The Countach) was to have super wide tyres. Now, as tyre technology has move on, manufacturers use taller tyres. The reason being, the contact patch is wider as well as longer, plus the contact patch spends longer in the cool air. However, the cars’ weight has been distributed over a larger contact patch, so ever cm2 you’d think would have less pressure upon it. You’re right, but today a modern car weighs more than its’ historical counterpart. Modern cars are getting fat, so wide tall tyres are not a problem. If I use those Audi RS4 19×8 rims, my Locost will grip very well in the dry but simply float on water at the sign of a grey cloud. Plus without power steering, I’ll need arms like Arnie to turn the steering wheel. When the original 7 was designed only luxury cars had power assistance. Now, even a tiny Smart car comes with it as standard.
However………….I’m still no closer to finding out how to calculate how much negative camber I need, just that the answer is potentially ‘not a lot’ and ‘more complex than I hoped’.
I now understand, if my corner weights are uneven, I will have some degree of understeer; so would an anti-roll bar help?
- Do I need front and rear anti-roll bars?
- How big?
- How do they stop understeer
Currently, all I know is, the front ones on Caterhams are tiny, so watch this space. Now where are my books???