Friction, physics, and racing

dinomite · 05-18-2001, 07:31 PM

Ok, we are doing frictin in my physics class, and we just today went over the fact that friction is independant of surface are, and i believe that. but i also know that putting wider tires on your car (of same type) gives you the ability to launch harder, but if friction is independent of surface area, why is this so? I talked to my physics teacher and he was very confused, cause obviously he believes the law that friction is independent of surface area, but he knows that my statements are valid as he knows that i know alot about racing. he talked to the other physics tachers at my school, one gave some odd conveluted answer, the other simply agreed with physics laws.

This applies in two ways, one in drag racing where your trying to begin movement from a standstill. it also applies to cornering, where the tires keep you from sliding off the road, they are pretty much the same thing. My teacher just said that bigger tires were for heat dissapation, which is part of road racing and they do engineer tires for that in some ways, bt thats not the case, and its definently not applicable in drag racing. All of us here know that putting wider tires on can give you better traction, and thats why we do it, and they limit tire sizes in all types of racing for that reason.

Any thoughts?

dinomite · 05-18-2001, 07:54 PM

Found some stuff:
http://www.physlink.com/ae200.cfm
http://adtsea.iup.edu/adtsea/TheChro...techniques.htm

basically, stuff is different when you come to rubber, with adhesion and heat. I'm still confused, gonna talk with teachers at lunch on monday.

69fastback · 05-19-2001, 12:05 AM

Also on the same side of that people put smaller tires on the front to cause less friction. I am confused here so everthing drag racing has shown does not go with the laws of physics. Ask ur profossors about the front tires being small then too. I am really intreseted on hearing the answers.

jimberg · 05-19-2001, 01:58 AM

Okay, here's what I read and how I understand it. First of all, there's more forces involved in a car than just basic friction, but I can start with the basic friction.

Say you have a block of rubber 10" square. That's 100 square inches. You have a force of 100 pounds pushing it down against the pavement. This force is distributed at 1 pound per square inch. Now, make the block of rubber 20" square. That's 400 square inches and only 1/4 pound of force per square inch. No matter how much you change the contact area, the amount of friction will remain the same as long as the same force is being applied. If, using this example, you would apply 400 pounds of force with the 400 square inch piece of rubber, you would quadruple the friction.

That's all they mean as far as friction not depending on surface area.

How does this work with dragsters? Let's say that a dragster has a 50/50 weight distribution, which wouldn't be good, but let's just say it does. The car weights 3000 pounds. This means that each tire is responsible for holding up 750 lbs. You've got skinnies on the front and big old slicks on the back. Let's assume that tire compounds have the same coefficient of friction. If you locked up all the wheels and tried to drag the car across the pavement, all the tires would be producing the same level of friction. That's what I explained in the previous paragraph.

Now, let's think of the car launching. Force is only being applied to the slicks and not the skinnies. This force is rotational, also. As the rear tires plant, the front end will go up and decrease the amount of force being applied between the front tires and the pavement. Because of this, the force that is being taken from the front is now being distributed to the rear. The wheels and the tires in the front are small to save weight. To save on friction, they can also use a harder compound.

Now, think of a real dragster with true weight distribution and how it applies.

I'm no physics major or anything, but I know the second paragraph is true and the next one is my extrapolation of that fact to the problem on hand.

------------------
351W 89 Mustang GT Convertible

97snakedriver · 05-19-2001, 02:57 AM

Quote:

Originally posted by 69fastback:
Also on the same side of that people put smaller tires on the front to cause less friction. ...I am really intreseted on hearing the answers.

Moment of Inertia

Let me see if I can't find a better example, but:
http://www.howstuffworks.com/roller-coaster5.htm

has some stuff on it.

But one reason why cars can turn or acellerate harder then 1.0 G despite what simple physics says is that tires, 1. deform, and 2. adhere to the road surface, which the basic "law" of friction doesn't take into account.

Its not a simple nothing can have a coefficent of friction greater then 1.0 situation.

Capri306 · 05-19-2001, 03:01 AM

Right on, dude. It's all about the deformation (elasticity) of the rubber compounds. Your freshman physics book usually will not cover this, either. All you're doing with drag slicks is getting more deformation over a larger area.

Other than that, I suggest we stop questioning the drag racing gods and continue to use slicks & skinnies.

------------------
Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch

jimberg · 05-19-2001, 06:44 AM

Howstuffworks is one of my favorite web sites.

Thad · 05-20-2001, 09:02 AM

Well from what i understood of physics a drag car is using friction to it's advantage. without friction we would be spinning our tires to the max rpm of the engine without going anywhere. That's where the rubber comes in. The rubber is used to oppose the rotation of the engine. And how much it opposes depends on its friction coefitient. You have to remember that friction is not the only force being used in drag racing. rpm's or rotation, speed, weight, friction, etc..
So if you have a 10" wide super sticky slick you in turn get a bigger area of contact to work against the engine and the u of friction is greater. And if you have a 5" hard tire you have less contact and u of friction. I think i'm starting to lose myself so i'm going to stop here. i hope i didn't confuse you more cause i am. Good luck!

------------------
90 LX 5.0 ,bored 302, MSD 6AL, Twisted Wedge TrickFlo HDs, World class T5, BBK fender cold air induct., 70mm TB,Edelbrock performer Intake, aluminum Drve shaft, 3.55 Gears, Long Tube Headers to Flowmasters.

jimberg · 05-20-2001, 11:09 AM

The area of contact is why this this whole thread was brought up in the first place. The original question was that if friction is independent of surface area, which we all agree it is, then why do wider tires grip better?

I've thought about this a little more and have some more ideas on the subject.

In a lot of physics examples they are done under perfect conditions. The ones I gave above are such examples. e.g. A feather will fall as fast as a lead weight, but only if there is no air resistence. My examples assume that the property of the rubber doesn't change with compression of the rubber. I don't believe that is the case. There are also shearing forces as well as heat to deal with. Using wider rubber allows shearing force to be less. It also allows there to be less compression of the rubber which probably maintains the rubber's coefficient of friction.

------------------
351W 89 Mustang GT Convertible

dinomite · 05-20-2001, 11:34 AM

what i've found is that it has to do with shearing forces on the very rough roadway (and treads on street tires) and the adhesion of rubber as it heats up.

The fact that physics theorys are based on perfect situations doesn't completely screw them up in the real world, though. What we did in class was to take a wood block about 6x8in on one side (area of 48 sq/ins), and only 1x8in on the other (8 sq/ins). We hooked a scale up to the block, and pulled it along the table on the large side, saw that it took X newtons to pull. Then put it on the thin side, and it would still take X newtons to pull.

05-18-2001, 07:31 PM	#1
dinomite The Dude Join Date: Feb 2001 Location: Arlington, VA Posts: 1,262	Friction, physics, and racing Ok, we are doing frictin in my physics class, and we just today went over the fact that friction is independant of surface are, and i believe that. but i also know that putting wider tires on your car (of same type) gives you the ability to launch harder, but if friction is independent of surface area, why is this so? I talked to my physics teacher and he was very confused, cause obviously he believes the law that friction is independent of surface area, but he knows that my statements are valid as he knows that i know alot about racing. he talked to the other physics tachers at my school, one gave some odd conveluted answer, the other simply agreed with physics laws. This applies in two ways, one in drag racing where your trying to begin movement from a standstill. it also applies to cornering, where the tires keep you from sliding off the road, they are pretty much the same thing. My teacher just said that bigger tires were for heat dissapation, which is part of road racing and they do engineer tires for that in some ways, bt thats not the case, and its definently not applicable in drag racing. All of us here know that putting wider tires on can give you better traction, and thats why we do it, and they limit tire sizes in all types of racing for that reason. Any thoughts?

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05-18-2001, 07:54 PM	#2
dinomite The Dude Join Date: Feb 2001 Location: Arlington, VA Posts: 1,262	Found some stuff: http://www.physlink.com/ae200.cfm http://adtsea.iup.edu/adtsea/TheChro...techniques.htm basically, stuff is different when you come to rubber, with adhesion and heat. I'm still confused, gonna talk with teachers at lunch on monday.

05-19-2001, 12:05 AM	#3
69fastback IRAQ VET Join Date: Feb 2000 Location: high desert California Posts: 1,480	Also on the same side of that people put smaller tires on the front to cause less friction. I am confused here so everthing drag racing has shown does not go with the laws of physics. Ask ur profossors about the front tires being small then too. I am really intreseted on hearing the answers.

05-19-2001, 01:58 AM	#4
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	Okay, here's what I read and how I understand it. First of all, there's more forces involved in a car than just basic friction, but I can start with the basic friction. Say you have a block of rubber 10" square. That's 100 square inches. You have a force of 100 pounds pushing it down against the pavement. This force is distributed at 1 pound per square inch. Now, make the block of rubber 20" square. That's 400 square inches and only 1/4 pound of force per square inch. No matter how much you change the contact area, the amount of friction will remain the same as long as the same force is being applied. If, using this example, you would apply 400 pounds of force with the 400 square inch piece of rubber, you would quadruple the friction. That's all they mean as far as friction not depending on surface area. How does this work with dragsters? Let's say that a dragster has a 50/50 weight distribution, which wouldn't be good, but let's just say it does. The car weights 3000 pounds. This means that each tire is responsible for holding up 750 lbs. You've got skinnies on the front and big old slicks on the back. Let's assume that tire compounds have the same coefficient of friction. If you locked up all the wheels and tried to drag the car across the pavement, all the tires would be producing the same level of friction. That's what I explained in the previous paragraph. Now, let's think of the car launching. Force is only being applied to the slicks and not the skinnies. This force is rotational, also. As the rear tires plant, the front end will go up and decrease the amount of force being applied between the front tires and the pavement. Because of this, the force that is being taken from the front is now being distributed to the rear. The wheels and the tires in the front are small to save weight. To save on friction, they can also use a harder compound. Now, think of a real dragster with true weight distribution and how it applies. I'm no physics major or anything, but I know the second paragraph is true and the next one is my extrapolation of that fact to the problem on hand. ------------------ 351W 89 Mustang GT Convertible

05-19-2001, 03:01 AM	#6
Capri306 Moderator Join Date: Sep 1998 Location: Grand Rapids, Michigan, USA Posts: 1,001	Right on, dude. It's all about the deformation (elasticity) of the rubber compounds. Your freshman physics book usually will not cover this, either. All you're doing with drag slicks is getting more deformation over a larger area. Other than that, I suggest we stop questioning the drag racing gods and continue to use slicks & skinnies. ------------------ Capri306, Moderator, The Mustang Works Online 1979 Mercury Capri, 5.0L -- C4 -- 2.73 1987 Mustang LX Notch

05-19-2001, 06:44 AM	#7
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	Howstuffworks is one of my favorite web sites.

05-20-2001, 09:02 AM	#8
Thad Registered Member Join Date: Oct 1999 Location: Stuart, FL US Posts: 192	Well from what i understood of physics a drag car is using friction to it's advantage. without friction we would be spinning our tires to the max rpm of the engine without going anywhere. That's where the rubber comes in. The rubber is used to oppose the rotation of the engine. And how much it opposes depends on its friction coefitient. You have to remember that friction is not the only force being used in drag racing. rpm's or rotation, speed, weight, friction, etc.. So if you have a 10" wide super sticky slick you in turn get a bigger area of contact to work against the engine and the u of friction is greater. And if you have a 5" hard tire you have less contact and u of friction. I think i'm starting to lose myself so i'm going to stop here. i hope i didn't confuse you more cause i am. Good luck! ------------------ 90 LX 5.0 ,bored 302, MSD 6AL, Twisted Wedge TrickFlo HDs, World class T5, BBK fender cold air induct., 70mm TB,Edelbrock performer Intake, aluminum Drve shaft, 3.55 Gears, Long Tube Headers to Flowmasters.

05-20-2001, 11:09 AM	#9
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	The area of contact is why this this whole thread was brought up in the first place. The original question was that if friction is independent of surface area, which we all agree it is, then why do wider tires grip better? I've thought about this a little more and have some more ideas on the subject. In a lot of physics examples they are done under perfect conditions. The ones I gave above are such examples. e.g. A feather will fall as fast as a lead weight, but only if there is no air resistence. My examples assume that the property of the rubber doesn't change with compression of the rubber. I don't believe that is the case. There are also shearing forces as well as heat to deal with. Using wider rubber allows shearing force to be less. It also allows there to be less compression of the rubber which probably maintains the rubber's coefficient of friction. ------------------ 351W 89 Mustang GT Convertible

05-20-2001, 11:34 AM	#10
dinomite The Dude Join Date: Feb 2001 Location: Arlington, VA Posts: 1,262	what i've found is that it has to do with shearing forces on the very rough roadway (and treads on street tires) and the adhesion of rubber as it heats up. The fact that physics theorys are based on perfect situations doesn't completely screw them up in the real world, though. What we did in class was to take a wood block about 6x8in on one side (area of 48 sq/ins), and only 1x8in on the other (8 sq/ins). We hooked a scale up to the block, and pulled it along the table on the large side, saw that it took X newtons to pull. Then put it on the thin side, and it would still take X newtons to pull.