Know-why of your car tyre designs

Preamble:
Considering the fact that your tyres are the only means of your communication with the road you drive on, they can either save your life or kill you – depending upon what care you exercise in choosing AND maintaining them.

With a new car, the first part is taken care of beyond doubt, as other wise it won’t qualify as ‘street-legal’ for sale to public at large. Trouble comes when people mindlessly try to ‘upgrade’ them – leave alone neglecting them by way of properly maintaining their inflation pressures round the year, not to mention their ‘concentricity’, dynamic-balance and alignment.

So let’s first talk about the philosophy behind the design of our stock car tyres as explained to me by a Sr. Exec of a prominent MNC Tyre Co., when I picked his brains with the following questions posed by an enlightened visitor…

Questions
Regarding tyres, I’ve seen on TV and in Books that F1 tyres have 3-5 plain treads around or are completly plain, having full contact to the track. But in real life for our cars, we have lots of gaps, in the form of different tread designs. If these are worn out, i.e. when the tyre becomes bald we change it, which is in contrast with the idea on F1 machines where they run mostly on smooth tyres. Why can’t we use smooth tyres provided we run our cars only on metalled roads and not off-roads?

Answers
Our compliments to you on your observations and conclusions. Regrettably, what’s good for the Goose is seldom so for the Gander as well! Consider the following:

The ‘Road-Grip’ a tyre provides is directly related to the ‘Friction’ generated between road and the tyres. And ‘Friction’ is directly proportional to the ‘Coefficient of Friction’ and the ‘area’ in contact. Worn out tyres thus have more ‘area of contact’ on ‘flat’ roads whereas ‘CoF’ varies with road material (Tar, rubber mix or concrete), its finish (smooth, coarse) and of course tyre tread rubber ‘composition’ AND condition.

Tyres exposed to strong Sun/UV light for long or aged over time makes their rubber hard, leading to reduced CoF. CoF also reduces if it has just rained and again increases if it continues to rain for some time and water gets drained out from the road surface.

Let’s break your query into two now:

Q1. Why do the stock tyres have treads while F1’s don’t, and
Q2. Why is it not recommended to use worn out tyres i.e. that have no tread left.

A1.
F1 tyres for dry tracks do not have any tread for the reasons stated above i.e. max contact area and the fact that F1 tracks are clean - no mud, no dust etc. However, F1 tyres for wet surfaces do have grooves i.e. ‘Ribs’ which run circumferentially on tread, to channel out water so that there’s no water trapped between tread and road. If dry/flat tyres are used on wet tracks, there will be ‘hydro or aquaplaning’, which will result in skidding/loss of control. In reality, public roads are not exactly that flat and free of loose material. There is loose dust, sand, small stones and gravel all over the road.

So in stock tyres, we have to have tread patterns to assist channeling out of water, dirt and air (to reduce ‘rolling noise’) as in reality, we have to drive the same car/tyres in rainy season, on roads with dust particles, surface irregularities, and of course off the road some times. One more thing - to judge a worn out F1 tyre, expert inspection is needed, whereas in real life, any one can see if a treaded-tyre is worn out, tho’ most penny-wise pound-foolish types ignore it and keep on driving until steel wires come out or worse still, till the tube itself starts showing up!

A2.
A worn out tyre has better braking and traction on plain clean roads but it is not recommended to use worn out tyres on Stock as well as F1 vehicles because the roadside tyre-wall thickness reduces considerably. After such a degree of wear, chances of hydroplaning increase manifolds and even if there is no water on road or other obstacles, a tyre can burst due to sheer centrifugal force alone at high speeds.

Further, in Indian road conditions, due to their vulnerability towards cuts and bruises, worn out tyres have more chances of moisture seeping into steel belts, leading to their rusting and consequent loss of adhesion with rubber, causing belt separation and ultimately leading to tyre burst.

To sum-up, for F1 racing, all that matters is ‘grip’ and they’re no bothered about the costs of changing even up to 4-sets during one race that may not last even a few hundred kms. In real life, at average speeds that may not even be 1/10th of the F1, ‘cost per km’ to the owner is the overriding factor and a pax car tyre in India is expected to last at least 40-50 kkm - besides expectation of ‘reasonable’ grip under all possible surfaces that an average motorist is likely to drive on – hence a multitude of ‘tread designs’, each one claiming to be superior than other!