[watashi75]

This has not been discussed in detail in this forum though everyone seems to agree that bald tyres are not slick tyres.

How do slicks work actually? Is it only the rubber that differentiates slick tyres from the bald tyres? Will regular tyres grip better on dry tarmac if we can somehow fill the grooves?

Let us try to keep the discussion scientific!

Slicks are supposed to increase grip by increasing the contact patch. But if the contact patch is increased the pressure decreases (pressure = load / contact patch). So would that not decrease the tyre grip?

[bblost]

one thing that comes to my mind is thickness.

[Live To Jive]

Slicks are specifically for racing. They use a different compound compared to road tyres. Bald tyres behave quite similarly to slicks as long as it is dry.

[predatorwheelz]

Originally Posted by watashi75

How do slicks work actually? Is it only the rubber that differentiates slick tyres from the bald tyres? Will regular tyres grip better on dry tarmac if we can somehow fill the grooves?

Allow me a small technical explanation.

On dry tarmac, it is the rubber in the tyre that provides adhesion. The softer the rubber, the better the grip. This is the principle behind slick tyres, which have soft rubber and no grooves whatsoever to maximize the contact patch with the road.

However, in wet weather and rough road conditions, its the grooves in regular tyres that dispel water/stones and allow the tyres to maintain adhesion on the road. Slicks are useless in these conditions.

Regular tyres will be unuseable in regular road conditions without the grooves, for the same reason!

Originally Posted by watashi75

Slicks are supposed to increase grip by increasing the contact patch. But if the contact patch is increased the pressure decreases (pressure = load / contact patch). So would that not decrease the tyre grip?

P.S. Mods can you please merge these two posts?

You are mistaking a basic concept here. Under horizontal movement, grip does not remain a function of load, but rather of contact patch and downforce. Otherwise why would featherlight F1 cars have such monstrously wide rear tyres?

[nitrous]

Few points from my side:

*Slicks are pure race tyres and the compound rating is called 'R'. Soft enough to grip well and hard enough to last the designated laps.
*They are not street legal.
*Slicks have a different nomenclature for size.
*They have wear-holes at specific intervals to show the amount of rubber depth left.

Basic difference between a slick and a bald tyre is compound. Street can never match a race compound.

[Abbas]

Slicks are very soft compound tyres which are generally used in racing. This is because they provide maximum contact area and hence, maximum cornering grip. Bald tyres are no way comparable since the term bald means its extensively used and the grooves have disappeared. Forget racing, this makes them very hard and unfit for even road use.

PS: watch youtube slow motion videos of US drag racing cars / phunny cars. While the lights goes green, just watch the tyres. Due to heavy torque the rim rotates faster then the tyre itself and you can see the rubber moving in a whirlpool action. It shows how soft the compound in the tyres are. Also, see when the do burnout just before the event to generate heat in the tyres. From the rear view you can see the tyres getting thinner.

[gpa]

Originally Posted by predatorwheelz

Otherwise why would featherlight F1 cars have such monstrously wide rear tyres?

Apologies for the OT, but F1 cars' rear tyres are the same size as the front ones. Tyres companies (Bridgestone at present) are bound by regulations and hence can’t make use of different rubber and wider tyres, because of these regulations.

[predatorwheelz]

Originally Posted by gpa

Apologies for the OT, but F1 cars' rear tyres are the same size as the front ones. Tyres companies (Bridgestone at present) are bound by regulations and hence can’t make use of different rubber and wider tyres, because of these regulations.

Err, when did i say that the rear tyres were wider than the front?. I guess my earlier statement was misleading. I emphasized the rear tyres in particular because (1) they are the powered wheels in F1 cars (2) the rear wing exists to provide downforce to them.

[watashi75]

Originally Posted by predatorwheelz

You are mistaking a basic concept here. Under horizontal movement, grip does not remain a function of load, but rather of contact patch and downforce. Otherwise why would featherlight F1 cars have such monstrously wide rear tyres?

Could you explain this in more detail? The basic premise seems to be that wider tyres = more grip. But I have a different opinion - the contact patch area remains same. Please look at this figure which I had posted in another thread -

http://www.team-bhp.com/forum/techni...ml#post1139159

For the same F1 car, why do slicks provide more grip than grooved ones? They have around the same downforce.

[gpa]

Originally Posted by predatorwheelz

I guess my earlier statement was misleading.

It was misleading indeed. Glad to have clarity on that one. And you are right about the emphasis on the rear tryes. They are very critical to the car's performance.

Cheers,
gpa

[predatorwheelz]

Originally Posted by watashi75

Could you explain this in more detail? The basic premise seems to be that wider tyres = more grip. But I have a different opinion - the contact patch area remains same. Please look at this figure which I had posted in another thread -

http://www.team-bhp.com/forum/technical-stuff/52058-filling-right-pound-per-square-inch-psi-4.html#post1139159

Thank you for the car bibles link Watashi. I spent a good hour trying to understand what the guy is trying to say about size of contact patch.

But I'd still say his argument has 2 big flaws:

1) The size of a contact patch is more like an oval, than a complete square (this he has acknowledged).

2) His theory of contact patch holds perfect as long as the car is steady, or moving in a straight line. But as soon as a car is moving sideways, it is the sidewalls of the oval that take precedence. In such a scenario, an elongated oval (as made by a wider tyre) offers more grip than a shallow, fat oval (as made by a narrower tyre).

Besides, in that very page he has illustrated why a wider tyre provides more grip than a narrower tyre. Reproduced below for everyone's benefit:

QUOTE

If the contact patch remains the same size and the coefficient of friction and frictional force remain the same, then surely there is no difference in performance between narrow and wide tyres? Well there is but it has a lot to do with heat transfer. With a narrow tyre, the contact patch takes up more of the circumference of the tyre so for any given rotation, the sidewall has to compress more to get the contact patch on to the road. Deforming the tyre creates heat. With a longer contact patch and more sidewall deformation, the tyre spends proportionately less time cooling off than a wider tyre which has a shorter contact patch and less sidewall deformation. Why does this matter? Well 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 in the first place. The harder compound has less mechanical keying and a lower coefficient of friction. The wider tyres are typically made of softer compounds with greater mechanical keying and a higher coefficient of friction. And voila - wider tyres = better grip. But not for the reasons we all thought.

UNQUOTE

Originally Posted by watashi75

For the same F1 car, why do slicks provide more grip than grooved ones? They have around the same downforce.

Yes, but as clarified already, slicks have softer rubber which grips better.

[nitrous]

Originally Posted by watashi

For the same F1 car, why do slicks provide more grip than grooved ones? They have around the same downforce.

Moving to slicks from ribbed tyres,this year, has increased mechanical grip by 10% which is significant.

[Sprucegoose]

Off the top of my head,
Friction is directly proportional to
1> Load [PSI] (L)
2> surface area [contact patch] (A)
3> coefficient of friction [varies depending on the materials]. (u)

If any of L, A, or u increase friction will increase.

The point in question is that when A increases L will decrease.
so will the decrease in L have more of an effect than the increase in A?

Friction is directly proportional to the square of the surface area [i think]
Load [PSI] is inversely proportional the the surface area.
Lets assume that the original values are as follows:
1> Load is at 100 PSI
2> surface area is at 10 sq.in.
3> coefficient will remain constant so we need not worry about it.

Original Friction = 100 x (10 x 10) x u
= 100 x 100 x u
= 10000 x u

When you double the surface area from 10 sq.in. to 20 sq.in., the Load (PSI) is half.

New Friction = 50 x (20 x 20) x u
= 50 x 400 x u
= 20000 x u

The new friction is still higher than the original.


PLEASE NOTE: this is a generalisation.
The formula is a lot more complicated that what i have mentioned.
Mine may even be completely wrong.
This is how i look at it.

[watashi75]

Originally Posted by Sprucegoose

...

There is one basic mistake...force of friction does not depend on contact area. The correct formula is -

Friction = Coeffecient of friction * Normal Force

Originally Posted by predatorwheelz

...

Thanks predatorwheelz. But my point is this.

For the same car, even if the tyres are of the same width and made of the same rubber compound, ungrooved slicks provide better grip than grooved tyres. It cannot just be the size of the contact patch area since this area remains almost the same. Could it be because there is less deformity?

Like nitrous has mentioned, this increase in grip is significant. It could the effect of 2 positive things - as the car goes slightly faster with slicks there is more downforce which provides more grip.

While searching on the net I came across the following article which dispels a couple of myths -

Article

I guess the main reason slicks are grippier is because they have less deformity, so they can be made of softer compounds and can dissipate heat more uniformly.

Now, if deformity is the main reason then bald tyres should also provide more grip than regular tyres, assuming the tyres have not hardened over time.

[GTO]

Bald tyres simply don't have any of the "grippy" rubber left. They may look similar to slicks, but the sticky rubber has worn out completely. A relevant post from another thread:

Originally Posted by GTO

The Goodyear Eagle F1s on my C220 (at about 35,000 kms) & the Bridgestone S322s on my Vtec (in the high 40,000) still had rubber on the tyres when I got them replaced. And no, they didn't have any of the chords and stuff showing either. Butttttttttttttttttttt either car STILL used to slip and slide all around. Here is why:

In a typical Indian driving cycle, tyres will start balding at about 35000 - 45000 kms and / or 3 - 5 years.

1. Tyre rubber hardens with use, you will see the contact patch getting less grippy at this point.

2. Rubber deteriorates with age. As a rule, I'd never use tyres that are over 4 - 5 years in a high speed car.

3. Tyre structurally wears out. You can bet that a tyre that's run 5 years + 40,000 kms is pretty much weak on the insides.

4. Worn out tyres suffer from heat buildup. This is the reason why they are more likely to blow out.

Thus, most likely, by the time that you have bald / wornout tyres on your car, the rubber has hardened, deteriorated, is structurally weak, will run hot and has lost most of the grippy stuff. Further, bald tyres will also puncture more easily.

I am much too concerned about my safety & that of others on the road to be driving around on worn-out / bald tyres. Whether in the wet or in the dry, the above-mentioned points clearly show them as a risky proposition.

Lastly, bald (from wear & tear) road car tyres & the slicks on racing cars are NOT the same. Racing slicks are specifically manufactured using soft (read grippy) rubber and for, most likely, a standard driving condition. They don't even consider the million heat + cool cycles that a road tyre is designed for. One of the few road tyres that I'd compare to racing slicks is the rubber on the BMW M3 CSL. Again, that is a slick tyre, NOT bald.