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Old 4th August 2009, 11:01   #16
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Quote:
Originally Posted by GTO View Post
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.
Sorry, I probably should have used the term "ungrooved" instead of "bald". I understand that the tyre compound is an important factor for grip. But given that the tyres are made from the same material, how do the tyre tread or grooves affect grip? If we could fill the grooves in a regular street tyre, then would it provide better grip on dry tarmac?

I understand that slicks provide more grip than grooved tyres because there is less deformity under cornering and better heat dissipation. If that is the case, for the same tyre material, tyres with minimal or no grooves will provide more grip than the tyres with grooves since there would be less deformity.

Tyres with grooves will provide more traction than ungrooved ones only on wet surfaces or surfaces such as sand or mud.
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Old 4th August 2009, 12:57   #17
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The treaded street tyres are made to work in dry as well as wet conditions. Yes, you could get way more dry grip if the manufacturer makes these tyres without the treads but then they wouldn't be street legal.

Even the fastest supercars come with treaded tyres and even if they come with optional slicks, they are only supposed to use them on track.

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Old 4th August 2009, 16:17   #18
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Originally Posted by watashi75 View Post
There is one basic mistake...force of friction does not depend on contact area. The correct formula is -

Friction = Coeffecient of friction * Normal Force

got it, but isn't normal force a function if the total load and the area of contact?
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Old 4th August 2009, 18:01   #19
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Originally Posted by Sprucegoose View Post
but isn't normal force a function if the total load and the area of contact?
Here, normal force is nothing but the weight or load.

F = m * g

Force of friction is independent of the area of contact. However for tyres this formula is not 100% true since area of contact does seem to have some affect on the grip.

I am also trying to find out which tyre characteristics affect longitudinal grip (acceleration and braking) and horizontal grip (cornering). Will post it here once I find out.
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Old 4th August 2009, 18:07   #20
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Its not like they keep gripping and gripping as the downforce increases.
There is a threshold for every tyre. Beyond a certain amount of downforce, they just break away.
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Old 4th August 2009, 19:37   #21
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Quote:
Originally Posted by watashi75 View Post
Here, normal force is nothing but the weight or load.

F = m * g

Force of friction is independent of the area of contact. However for tyres this formula is not 100% true since area of contact does seem to have some affect on the grip.

I am also trying to find out which tyre characteristics affect longitudinal grip (acceleration and braking) and horizontal grip (cornering). Will post it here once I find out.
No dude..
or atlease when it comes to drag, it is directly proportional to the surface area of the surface its acting on.
now isn't drag friction between a solid and a gaseous surface?

I'm sure contact area has alot to do with friction.
for example, if you decrease the air pressure in the tyres, then the contact patch will increase. lets assume that it doubles.

Effectively the load is being distributed over double the area.
so the load per unit area will reduce by half.

Now if we assume that:
1> If friction is independent of the contact, then the friction should reduce.
2> If friction has a direct 1:1 relation with the contact area and the load per unit area, then the friction should not change at all, but
3> If friction has a direct 1:(>1) relation [for a 2 fold increase in surface area there is more than a 2 fold increase in friction], that will explain why with an increase in contact area, the friction increases.

And as i said earlier, just as drag is directly proportional to the square of the surface area its acting on, i think friction will have a similar relationship.

Then again, all of this is conjecture on my part.
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Old 4th August 2009, 20:19   #22
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Traction is not only dependant on contact area but also the tyre compound, tyre temperature, track surface, track temperature, tyre pressue, load, COG, suspension setup etc....

Take the F1 slicks itself, there is a world of differance between the option and prime slicks, even though they look identical (except the green line).

Moreover, the traction levels are never consistant. 10 laps down theres no saying which tyre will show what characteristic. You suddenly see some car with options struggling for grip while someone else is putting in fastest laps with the same tyre.

Quote:
And as i said earlier, just as drag is directly proportional to the square of the surface area its acting on, i think friction will have a similar relationship.
Take 2 glass panels, one which is placed vertical and measures 2ft H x 2ft W. Now take another panel which is 6 ft H x 2ft W but place this panel at an angle such that it only measures 2ft when measured vertically.

Im no engineer so guys correct me if im wrong here. Im guessing that both panels when looked at from front should have similar surface areas right? But when tested in a wind tunnel i think the 6 x 2 panel which is placed at an angle will have a lower COD and thus provide less resistance?

So i think it should be possible for 2 cars to have identical frontal surface areas but diff co-efficient of drag.

I made this diagram to show what im talking about...

Image A and B are side views of 2x2 and 6x2 panels, image C is what they would look like when viewed from front (ofcourse the 6x2 will have slight perspective angles in reality).
Attached Thumbnails
Difference between slick tyres and bald tyres-untitled1-copy.jpg  


Last edited by Shan2nu : 4th August 2009 at 20:32.
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Old 5th August 2009, 11:01   #23
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Quote:
Originally Posted by Sprucegoose View Post
I'm sure contact area has alot to do with friction.
Static friction is independent of the contact area.

The formula for static friction is -

Name:  FrictionFormula.jpg
Views: 667
Size:  2.1 KB

Reference: Friction

However for tyres this formula cannot be applied directly since rubber is deformable and hugs the road contours. So a different coefficient is used called coefficient of adhesive friction.

Reference: Google Books -> Advanced Vehicle Technology [page 270-271]

I am quoting a section from that book.

Quote:
It has been found that the frictional grip of a bald tyre tread on a rough dry road surface is as good or even better than that achieved with a new tread. The reason for this unexpected result is due to the greater amount of rubber interaction with the ground surface for a given size of contact patch. It therefore develops a larger reaction force which opposes the movement of the tyre. Under ideal road conditions and the amount of deformable rubber actually in contact with the road maximized for a given contact path area, the retarding force which can be generated between the tyre and ground can equal the vertical load the wheel supports. In other words, the coefficient of adhesive friction can reach a value of 1.0. However, any deterioration in surface roughness due to surface ridges being worn, or chippings becoming submerged in asphalt, or the slightest amount of wetness completely changes the situation. A smooth bald tyre will not be able to grip the contour of the road, whereas the tyre with a good tread pattern will easily cope and maintain a relatively high value of retardant force.
Effect of adhesive friction when road surface condition changes from dry to wet -

Name:  Friction_Regular_Bald.jpg
Views: 878
Size:  34.2 KB

As you can see in the figure, grip condition deteriorates abruptly in case of bald tyres compared to regular treaded tyres.

What this means is that you can use bald tyres on controlled conditions like race tracks to get better grip and a better timing. But what makes bald tyres risky is that once you lose adhesion it would be significantly more difficult to regain grip because of the absense of treads.

Hope this information was useful!
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Old 5th August 2009, 11:41   #24
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Quote:
What this means is that you can use bald tyres on controlled conditions like race tracks to get better grip and a better timing. But what makes bald tyres risky is that once you lose adhesion it would be significantly more difficult to regain grip because of the absense of treads.
Its still dangerous to use bald tyres on track as the structural strength will be nowhere close to when it was brand new.

By using bald tyres, you increase the chances of a tyre failure.

So irrespective of how much grip it provides, avoid using them in the first place.

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Old 5th August 2009, 11:53   #25
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Quote:
Originally Posted by Shan2nu View Post
Its still dangerous to use bald tyres on track as the structural strength will be nowhere close to when it was brand new.
I agree it is dangerous. I just wanted to dispel the myth that

Quote:
bald tyres = less grip
This is similar to the other popular myths -

Quote:
1. wider tyres = more contact patch

2. larger contact patch = more grip
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Old 5th August 2009, 12:23   #26
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Well, its not always true.

Sometimes its possible that even though a bald tyre has a larger contact area, the rubber compound would have bcom so hard that you can't really get much traction from it.

I rem when the Bridgestone 175 S322s on the VTEC had bcom bald, I couldn't get away in 1st without spinning the wheels and even while braking, i had to be extra careful not to let the wheels lock up. That is how bad those tyres were (inspite of having a larger contact area than when they were new).

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Old 5th August 2009, 12:39   #27
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Hey watashi75,

This is a very informative thread technically and you have researched well on the said topic. However, i feel it is pure common sense in undertsanding which is better was what use.
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Old 5th August 2009, 13:01   #28
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Quote:
Originally Posted by Shan2nu View Post
I rem when the Bridgestone 175 S322s on the VTEC had bcom bald, I couldn't get away in 1st without spinning the wheels and even while braking, i had to be extra careful not to let the wheels lock up. That is how bad those tyres were (inspite of having a larger contact area than when they were new).
Could it be due to aging of the tyres and not strictly due to balding? With age the tyre compound becomes hard which would reduce available grip. But if it was a new tyre and then the treads were cut out, the grip would be more.

Of course, most of the bald tyres we see on the road also happen to be old tyres which might have been out there for 3 years or more.

Apparently Burt Munro ("The World's Fastest Indian" fame) used to cut off the treads of his tyres to increase speed.
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Old 5th August 2009, 13:23   #29
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Quote:
Could it be due to aging of the tyres and not strictly due to balding? With age the tyre compound becomes hard which would reduce available grip. But if it was a new tyre and then the treads were cut out, the grip would be more.

Of course, most of the bald tyres we see on the road also happen to be old tyres which might have been out there for 3 years or more.
Just goes to show that "Bald tyres = less grip" is a fact. Under certain circumstances, it is very much possible.

So it can work either ways...

Quote:
Apparently Burt Munro ("The World's Fastest Indian" fame) used to cut off the treads of his tyres to increase speed.
The grooves on these tyres were not designed to handle speeds of 320kmph, creating unwanted turbulance and instability. To overcome this, he used to cut the grooves and smoothen out the tyre surface so that the air would flow more efficiently around it, allowing him to reach higher speeds.

Shan2nu

Last edited by Shan2nu : 5th August 2009 at 13:25.
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Old 5th August 2009, 13:53   #30
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@shan2nu: agreed, but by having a different Angle Of Attack (AOA) for each slab of glass, the aerodynamic properties change.

i was saying that if you have 2 slabs of glass with exactly the same properties except for one being double the size of the other, the drag of the larger glass will not be double, but 4 times greater.

"Traction is not only dependant on contact area but also the tyre compound, tyre temperature, track surface, track temperature, tyre pressue, load, COG, suspension setup etc.... "

I know man....
I was considering test conditions, where everything except for the contact area between the tyre and the test surface changes.

@watashi:
"It has been found that the frictional grip of a bald tyre tread on a rough dry road surface is as good or even better than that achieved with a new tread. The reason for this unexpected result is due to the greater amount of rubber interaction with the ground surface for a given size of contact patch. It therefore develops a larger reaction force which opposes the movement of the tyre. Under ideal road conditions and the amount of deformable rubber actually in contact with the road maximized for a given contact path area, the retarding force which can be generated between the tyre and ground can equal the vertical load the wheel supports. In other words, the coefficient of adhesive friction can reach a value of 1.0. However, any deterioration in surface roughness due to surface ridges being worn, or chippings becoming submerged in asphalt, or the slightest amount of wetness completely changes the situation. A smooth bald tyre will not be able to grip the contour of the road, whereas the tyre with a good tread pattern will easily cope and maintain a relatively high value of retardant force."

Dude the bold part essentially says that there is more rubber in contact with the road, hence more friction.

now the contact patch of a treaded tyre and of a slick might remain of the same size, but effectively the slick will have more rubber in contact with the road, so its effective contact patch is bigger than that of a treaded tyre.

Thats exaclty what I'm saying. more tyre in contact with the road, more grip.
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