Why do we use racing slicks if surface area =/= friction?

[Wade3]

I understand how surface area does not come into the equation governing the resistance an object has to sliding. It leads one to believe that all tires would perform equally in dry conditions. Why do people then say that bald tires give better grip?

 

[Bystander]

Why do people then say that bald tires give better grip?

Do they?

 

[rcgldr]

Coeffeicient of friction is affected by the load per unit area. Wiki article:

http://en.wikipedia.org/wiki/Tire_load_sensitivity

The load per unit area also affects the heating of a tire, and more surface area means more cooling. There's a point of diminishing returns when the tire gets too big and aerodynamic drag or tire / wheel weight become an issue.

In the case of racing slicks, there is less deformation due to side loads than with a treaded surface.

 

[SteamKing]

I understand how surface area does not come into the equation governing the resistance an object has to sliding. It leads one to believe that all tires would perform equally in dry conditions. Why do people then say that bald tires give better grip?

All tires, especially competition models, are specially designed pieces of equipment. Tires are more than round pieces of rubber which roll and do little else.

Racing tires differ from road tires in that they are specially designed for competition. They don't need to last 40,000 miles; they aren't going to be driven in snow or ice or mud.

The rubber compound used for racing tires is designed to stick to the road surface, so by eliminating treads which serve to channel away water, the amount of rubber in contact with the surface of the track is increased, which helps the car stay on the track.

http://auto.howstuffworks.com/auto-racing/nascar/nascar-basics/nascar5.htm

 

[cosmik debris]

I understand how surface area does not come into the equation governing the resistance an object has to sliding. It leads one to believe that all tires would perform equally in dry conditions. Why do people then say that bald tires give better grip?

Racing tyres do offer more rolling resistance than street type tyres but they also offer far more grip in both the longitudinal direction, for braking and accelerating and in the lateral direction for cornering. In the end this extra grip is more beneficial than the losses.

I you are referring to bald tyres as in those that are worn out then they do not offer better grip. By the time the tyre has worn down that far the rubber is old and probably hardened and well beyond the design spec for the tyre. If you mean bald as in slick tyres then the increased surface area is important.

Cheers

 

[berkeman]

Racing tyres do offer more rolling resistance than street type tyres

Why do you say that?

 

[cosmik debris]

Why do you say that?

Because they are sticky.

 

[berkeman]

Because they are sticky.

LOL, no they're not. Have you ever handled a slick at a racetrack? If you push them down on the ground and lift them up, they do not stick to the pavement.

 

[OmCheeto]

LOL, no they're not.

Yes they are!

We had a similar discussion a while back: Frictional accelerations greater than one G.
Wiki's quote has since changed, so I'll use the new one:

Friction, Limitations of the Coulomb model
...
When the surfaces are conjoined, Coulomb friction becomes a very poor approximation (for example, adhesive tape resists sliding even when there is no normal force, or a negative normal force). In this case, the frictional force may depend strongly on the area of contact. Some drag racing tires are adhesive for this reason. However, despite the complexity of the fundamental physics behind friction, the relationships are accurate enough to be useful in many applications.

Perhaps racing slicks are the automotive analogy of a semiconductor. They don't follow the normal rules.

Have you ever handled a slick at a racetrack? If you push them down on the ground and lift them up, they do not stick to the pavement.

We might be discussing two different things. I've got top fuel dragsters in my head.
And no, I've never handled one at a racetrack.

Here's a short blip from Popular Mechanics: Anatomy of a Top-Fuel Dragster

Tires
...
The special blend of tread rubber is designed to adhere to the adhesive that's applied to the track. The resulting grip is akin to driving on flypaper.

They list distance and velocity for a then recent record run at 1/2 second intervals.
The fastest acceleration appears to be off the line:

The Race
On March 13, 2010, in Gainesville, Fla., Kalitta driver David Grubnic set a top speed record on the 1000-foot track. Here's a breakdown.

0.50 Sec
10.27 ft — 73.89 mph
...

v = u + at
u = initial velocity = 0
so a = v / t = 73.89 mph / (0.5 sec) * 5280 ft/mile * hr / 3600 sec * g / (32.2 ft/sec^2) = 6.7 g's

I have the equation plugged into my spreadsheet, so if you are curious about the g forces for the rest of the run, let me know, and I'll plug them in.

 

[rcgldr]

Have you ever handled a slick at a racetrack? If you push them down on the ground and lift them up, they do not stick to the pavement.

Hot racing slicks will pick up rubber marbles and gritty stuff off a race track.

The rubber sheets on some table tennis paddles have enough "stick" to lift up a table tennis ball for a brief moment after pushing the paddle down onto the ball and the pulling the paddle upwards.

 

[Wade3]

I was thinking about these responses, and it does make sense that stickiness might behave differently than a coefficient of friction, since a piece of paper coated with glue would have increased resistance as it increases in size because there is more glue... okay.

It is still odd though. I do notice that since I switched from bald tires to good treaded ones, I have been doing unintentional burnouts all over the place.

 

[DaveC426913]

The tires are sticky when hot. They are designed this way.

@berkeman : next time you're at the track, please pick up a fresh tire - right off the track, hot enough to boil water - and let us know if if it's sticky.

 

[russ_watters]

I was thinking about these responses, and it does make sense that stickiness might behave differently than a coefficient of friction, since a piece of paper coated with glue would have increased resistance as it increases in size because there is more glue... okay.

It is still odd though. I do notice that since I switched from bald tires to good treaded ones, I have been doing unintentional burnouts all over the place.

Setting aside some of the peculiarities of certain sticky racing tires, in general the issue with the simple model is deformation. A harder object(s) will behave closer to the simple model than a soft one because the soft one deform and fills in the roughness of the other surface.

 

[Nidum]

Racing slicks road grip is enhanced considerably by suction effect .

Air is continually squeezed out of contact zone . Atmospheric pressure acting on net difference of area top and bottom of tyre produces a down load which enhances grip .

 

[OmCheeto]

Racing slicks road grip is enhanced considerably by suction effect .

Air is continually squeezed out of contact zone . Atmospheric pressure acting on net difference of area top and bottom of tyre produces a down load which enhances grip .

That sounds like a macroscopic version of the Casimir effect. Scientifically plausible, but dubious realistically, IMHO.
Do you have a reference?

 

[OmCheeto]

... A harder object(s) will behave closer to the simple model than a soft one because the soft one deform and fills in the roughness of the other surface.

I was thinking along the same line, and was going to make some bad doodles to illustrate the effect, but decided it was too much effort.

sophiecentaur made a similar analogy in the thread I mentioned earlier:

After all, if you had splines cut across the track and had gear teeth on your wheels, you could get pretty much any forward force you wanted.

This is just a bit rubberyer(sp?)

 

[cosmik debris]

LOL, no they're not. Have you ever handled a slick at a racetrack? If you push them down on the ground and lift them up, they do not stick to the pavement.

Well, I do a fair bit of racing, I drag race two cars, circuit race one car and circuit race a motorbike and I think that when the slick tyres are hot they are pretty sticky. If I roll my car or bike after some hot laps over some not very good tar-seal, it will pull chunks out of it. That's sticky in my book.

 

[Berettaspeed]

yes, the little bit of width and more touching is not enough of a loss vs the traction benefits.

Rubber compound is much different. However even with "bald" tires you will get better grip in dry conditions granted the tires are not aged and drying out. An old tire that is dry'ed out and has tread is horrible for traction due to the rubber getting hard. Either way even a bald time has more contact patch and less tread flex, which allows more grab. also larger tread blocks on tires allow better grip due to less flex while cornering. Many small tread blocks allow for better traction on ice and in cold conditions as the cold makes tires hard and more "slippery".

Odd fact that is fun to share. A newer tire wears out faster than say it will with 30k on it. Why? the taller tread moves more, causing faster wear. As a tire wears out it takes longer to wear out. Just imagine a tall building. it sways more due to the leverage of the height.. same concept with tires. A tire with 10/32 will wear faster to get to 8/32 than a tire would take to get to 8/32 to 6/32.. :)

 

[Hornbein]

I understand how surface area does not come into the equation governing the resistance an object has to sliding. It leads one to believe that all tires would perform equally in dry conditions. Why do people then say that bald tires give better grip?

The equation is what is called "first order." It gives a basic result, ignoring complications. In physics, there are often many complications. These are "second order." When something gets so refined that second order is all optimized, then there are "third order" effects. And so on.

About a decade ago I heard about special Formula One racing tires called "gumballs." These were very sticky rubber tires that were would last only for the five laps of qualifying! There was some talk about regulating them because their use was so expensive.

 

[berkeman]

Well, I do a fair bit of racing, I drag race two cars, circuit race one car and circuit race a motorbike and I think that when the slick tyres are hot they are pretty sticky. If I roll my car or bike after some hot laps over some not very good tar-seal, it will pull chunks out of it. That's sticky in my book.

Sticky sure, but do you think it increases rolling resistance (which was the poster's claim)?

 

[cjl]

Sticky sure, but do you think it increases rolling resistance (which was the poster's claim)?

I'd be very surprised if it didn't - the stickiness means that it takes more force to pull the tread off of the pavement, and I don't see any way that wouldn't result in increased force required to roll.

 

[berkeman]

I'd be very surprised if it didn't - the stickiness means that it takes more force to pull the tread off of the pavement, and I don't see any way that wouldn't result in increased force required to roll.

Hmm, well maybe it's true. But I used to run the stickiest tires I could (that were still street legal) on my old F4 sportbike, and I never noticed any change in rolling resistance on the street or on the racetrack. Maybe if it's a small enough change it's hard to notice.

 

[cjl]

I doubt it would be very noticeable, true. Maybe it would be noticed with true drag racing slicks on a dragstrip (which is incredibly tacky - the amount of traction they can get is astonishing), but with ordinary road racing tires, I doubt it's significant enough to be noticed (though it's probably still there).

(I run pretty sticky tires too sometimes - I take my Porsche to the track fairly frequently, and I've also been part of an extremely low-budget endurance race in a Subaru, and I agree, I've never noticed any sensation of the tires sticking or dragging)

 

[OmCheeto]

Hmm, well maybe it's true. But I used to run the stickiest tires I could (that were still street legal) on my old F4 sportbike, and I never noticed any change in rolling resistance on the street or on the racetrack. Maybe if it's a small enough change it's hard to notice.

Having never been a drag racer, nor a big fan, I spent several hours researching top fuel dragsters on Wednesday.
One article claimed the motors generated between 8000 and 10,000 horsepower, and the superchargers alone consumed 800 hp.
I can't imagine the tire's stickiness would consume anything like that.
hmmm...
Is it possible to guesstimate the power consumed by the tires due to adhesion, if you can develop 6 g's?

Weird problem, IMHO.

 

[russ_watters]

Recognize that at the moment of launch, the power delivered to the ground is zero. Any power developed at the crankshaft is absorbed by the clutch and deformation of the tires. Torque delivered to the ground is what determines the acceleration. Then as the car accelerates, the power output of the engine increases linearly for as long as that initial acceleration is maintained.

 

[rcgldr]

Recognize that at the moment of launch, the power delivered to the ground is zero. Any power developed at the crankshaft is absorbed by the clutch and deformation of the tires.

Mostly the clutch, converting the excess power into heat. The driver pretty much pegs the throttle at launch and only lifts if there's wheel spin (which will probably cost the driver the race). Electronic traction control isn't allowed, so the clutches on top fuel dragsters and funny cars use pneumatic ("air over oil") based timers to delay the pressure applied to the clutch plates by fingers driven by centrifugal force via the throw out bearing. The clutches usually don't fully engage until somewhere around the 1/8th mile mark, where the cars have reached 75% to 85% of their top speed. The mechanics adjust the clutch parameters based on their estimate of the amount of traction that will be available for the next run.

update - Found a video that I used the last time top fuel clutches were asked about:

 

[cjl]

Then as the car accelerates, the power output of the engine increases linearly for as long as that initial acceleration is maintained.

The power output of the engine on a top fuel dragster nearly peaks at launch, decreases slightly as it goes down the strip, then increases again after the clutch has fully locked, peaking again near the end of the run. You're right that the power delivered to the ground increases continuously through the run though - most of the immense amount of power that the engine is making at the beginning of the run is absorbed in the clutch and turned into heat (and it's amazing that the clutch can handle it).

 

[cosmik debris]

Sticky sure, but do you think it increases rolling resistance (which was the poster's claim)?

Well, I have to say yes I would.