Traction question

[N8Dogg98]

I marked the things you said that were wrong in red and the things you said that were right in green.

^^This message brought to you by real life experience, not Sir Isaac Newton.

Sorry Sharad, but the items in red you marked are in fact correct.

Wow, this is like beating a dead horse, what don't you understand about this. The surface area/amount of rubber touching the road is dependent only on the weight applied at the tire AND the pressure within the tire, and has nothing to do with the tire width or height.

Now, real life experiences or not what you are claiming about contact patch being dependent on the sidewall height or section width is simply incorrect. For the last time, contact patch is dependent on 2 things tire pressure and weight applied to the tire.

Traction (friction) and surface area have no correlation with each other...

This is exactly what Sharad is having a difficult time understanding

 

[Sharad]

Just to everyone a quick physics lesson

Traction (friction) and surface area have no correlation with each other...
That being the said only thing that makes wider tires stickier is the material they are made out of, and the sidewall.

Wow, this is like beating a dead horse, what don't you understand about this. The surface area/amount of rubber touching the road is dependent only on the weight applied at the tire AND the pressure within the tire, and has nothing to do with the tire width or height.

Now, real life experiences or not what you are claiming about contact patch being dependent on the sidewall height or section width is simply incorrect. For the last time, contact patch is dependent on 2 things tire pressure and weight applied to the tire.



This is exactly what Sharad is having a difficult time understanding

I'm just having fun with this guys. The horse is dead. He doesn't mind the beating.

You guys are clearly better with the physics than I am, but your physics equations cannot explain why wider tires hook better. Or why the tires hook better with less air in them. The fastest cars run huge tires. F1, NHRA Top Fuel. Huge. WHY ON EARTH would they do that?

 

[N8Dogg98]

Seriously man, no-one is disagreeing with you about wider tires being better for racing, but it has a lot more to do than the size of the contact patch... oh and yes, less air in the tire means more tire on the road, please show me where I said otherwise. There is of course diminishing means of returns, you can get to a point where any wider or any lower with the air pressure and you don't gain any thing at the track and in fact start to lose e/t... in other words, a 315 tire on a 100 hp econo box will be much worse for drag racing than a 195 tire, no? Likewise a 225 road tire with 5 psi will likely perform much worse on the track than a 225 road tire with 30 psi, no?

For the last time, I was simply pointing out to you that contact patch has nothing to do with tire width, and everything to do with tire pressure and weight... however you don't think this is true... and it is.

Also, do you really believe that you'll run measurably better times with a 17x10" wheel with a 315 tire as opposed to an 18x10" wheel with a 315 tire assuming both wheels weigh the same, are using the same tire, and the outer tire diameter was equal? I'd love to see an example of this. There simply isn't enough of a difference in sidewall height to create a measurable difference. Nearly everyone who runs a better 1/4 mile time with a 17x10 wheel as opposed to 18x10 wheel is doing so because of the additional rotational mass of the 18" wheel.

 

[40oz]

I'm just having fun with this guys. The horse is dead. He doesn't mind the beating.

You guys are clearly better with the physics than I am, but your physics equations cannot explain why wider tires hook better. Or why the tires hook better with less air in them. The fastest cars run huge tires. F1, NHRA Top Fuel. Huge. WHY ON EARTH would they do that?

Tire compound is more important than how wide the tire is. You aren't going to win any road races running an all-season radial no matter how wide it is when everyone else is using a racing tire with a better rubber formula.

Tires "hook better" with less air for the reasons that have already been stated - contact patch is a factor of vehicle weight and tire pressure.

If you have a 3200 lb. car and the tires have 32 lbs. per square inch of air pressure, the total contact patch is 100 square inches regardless of the tire spec. Lower the air pressure to 24 lbs. per square inch, and there are more square inches on the ground. Life experience tells me the tire deforms more as pressure drops or weight increases. IOW, contact patch increases as tire pressure drops or weight increases.

A shorter sidewall is stiffer often just because it is shorter. But a stiffer sidewall is desired because it reduces the difference between the amount the steering wheel is turned and the direction the tire is rolling. A very stiff sidewall reduces the perceived lag between steering input and car response. But it also transmits more feedback from the road surface, such as cracks, ridges, and potholes.

A wider tire changes the shape of the contact patch. It doesn't change the size. The wider tire is deforming less as it rolls. Deformation generates heat. Heat in a tire is only good up to a point. For a road racing tire, it is possible to have too much tire to generate enough heat to get them to ideal operating temps across the tread. A heavier car will benefit from a wider tire than a lighter car is using to good effect. And when drag racing, alot of weight is transferred to the rear tires. Same concept - too narrow a tire and it will be easily overheated and traction will suffer.

For an extreme example, say you have two tires of the same circumference. One gives a contact patch 4 inches long and and 2 inches wide. The other gives a patch 1 inch long and 8 inches wide. Any given point on the narrower tire spends four times as much time on the road, deforming and/or slipping, generating and absorbing four times as much heat. Any point on the wider tire both generates and absorbs less heat and has more time to cool before meeting the road again.

There is also a whole slew of things that happens to the tire during a drag launch that are not obvious and are hard to study. A narrower tire is better on ice and snow, so clearly something is happening between the dry pavement and the tire on a launch that is both not obvious and different than what happens between a snow covered road and a tire. I know it's a mistake for me to try to come up with a satisfactory answer to "Why is a wider tire better for drag racing" in an afternoon on a forum.

Wider isn't always better, but outside of autocross or road racing, most people wouldn't run into problems from too much tire. Outside of winter driving with a sports car, of course.

But one thing that is clear and provable is that the size of a contact patch is dependent only on vehicle weight and tire pressure.

 

[Sharad]

But one thing that is clear and provable is that the size of a contact patch is dependent only on vehicle weight and tire pressure.

Only if you ignore sidewall flex:

^^negligible sidewall flex on 22s.

^^visibly obvious sidewall flex on 15s & 16s.


Right about now you're saying "but you're comparing slicks to radials". Yep. I am. I only used those pictures to make the difference more visible. Sidewalls on lower profile tires are stiffer. Tires with a fatter sidewall sit like they're a little flat because their sidewalls are softer.


Not only that but all of this physics debate has been centered around a static condition. Put the tire in motion (like weight transfer from the launch) and the softer tire (taller sidewall) will flex and wrap more, putting more rubber on the road.

Now, you might say Sharad doesn't know what the heck he's talking about... and maybe you're right? Maybe I'm not doing such a good job explaining myself.

But I do know that racers go for the wider & taller tires unless the rule makers require smaller tires to slow the cars down (hint hint). I've not met a professional drag racer that was an expert in physics, but I've noticed that the faster cars have wider tires and shorter rims. There's a reason for that.

 

[N8Dogg98]

You're still acting like I am under the impression that wide tires are useless when I haven't said anything of the sort. Are you really trying to drag this out? Just admit you're wrong already. Pressure and weight are the ONLY contributing factors to contact patch area.

That said, a stickier tire (such as a d/r) will tend to launch harder allowing MORE weight to transfer to the rear tires compared to a car with street tires that won't have enough traction for as hard of a launch. The aggressiveness of launch isn't due to contact patch, it is due to the additional friction offered by a D/R as opposed to a street tire. Now, obviously the contact patch will increase after launch with the car using d/r's because there is more weight on the rear tires. Again contact patch has nothing to do with the compound, the width of the tire, or whether or not it is a d/r, contact patch is determined by pressure within the tire and the weight applied to that tire and nothing more. In your pictures above, I have no idea what those cars weigh and what pressure is within the tires. It's impossible to tell from looking at a picture the area of the contact patch. The shape of the contact patch may vary from tire to tire, but again if the pressure is identical and the weight at the tire is the same, the contact patch will be identical no matter what the tire is made of.

Notice how d/r's tend to pick up basically everything that isn't attached to the road (rocks, sand, glass, etc..). Do you think they do this because they are wide or because the material in the tires is sticky? It's that sticky property of d/r's that allows them to launch hard, which results in an increase in weight transfer, effectively increasing the contact patch. The contact patch however is only increased as a result of the hard launch that can be acheived due to the sticky rubber in d/r's.

 

[40oz]

...

Now, you might say Sharad doesn't know what the heck he's talking about... and maybe you're right? Maybe I'm not doing such a good job explaining myself.

...

The pictures you showed prove the point. The pressure in the tires on the Mustang on dubs is higher than the pressure in the slicks on the two drag cars you showed. You would know that if you had been drag racing for 20 years.

And I covered why wider tires are better on a racetrack. It has zero to do with the size of the contact patch. It has to do with the shape of it.

Just stop. It's like you are trying to say the moon orbits the earth because of the earth's magnetic field instead of because of gravity. Part of what you are saying might be true, but the reasons are wrong.

 

[Sharad]

Okay guys. It looks like you're both from Minnesota. Friends, I assume. (maybe not, but who cares?)

In my first post I said more sidewall - more traction. I'm willing to put my theory to the test. My car has 60-series tires on it. Would you like to set up a race? (friendly run, no e-thuggery involved)

 

[N8Dogg98]

First off, I've never met 40oz... secondly you didn't say more sidewall - more traction you said the following in your first post:

the difference with the sizes you specified is pretty minimal, but the sidewall on the 17 will sag a little more than the 18 with equal tire pressures, so the contact patch on the 17 will be slightly larger.

This statement is incorrect and again you said nothing about traction.

For the last time NONE OF US ARE DISAGREEING THAT A 60 SERIES D/R WILL BE BETTER FOR DRAG RACING THAN A 30 SERIES ROAD TIRE. What we are disagreeing about is your inability to admit that you are wrong about contact patch area being dependent on sidewall height and tire width.

 

[Mattstang04]

Thanks for the info everyone. Im leaning toward a 17x10.5 with a big nasty 315 tire.

 

[FastDriver]

I'm just having fun with this guys. The horse is dead. He doesn't mind the beating.

You guys are clearly better with the physics than I am, but your physics equations cannot explain why wider tires hook better. Or why the tires hook better with less air in them. The fastest cars run huge tires. F1, NHRA Top Fuel. Huge. WHY ON EARTH would they do that?

I saw your comment in my thread about arguing physics in these forums and it made me curious to find the thread you were referencing. So here I am.

Since they only made it through high-school and entry-level college physics, these guys are amatuer physicists that actually believe that tractive forces in real life, away from a textbook, are only relative to the normal force and the coefficient of friction. Unfortunately for them, they didn't go far enough to learn about a term called hysteresis, which is a significant contributer to maximum tractive force, and it does not change the coefficient of friction ("bilinear friction") between 2 given materials. When you introduce an asphalt surface with normal (perpendicular) deformations, and an elastomeric material that allows it to apply that force directly into those deformations, the elementary formula for traction no longer works. However, when you consider this effect, it quickly becomes evident why lowering tire pressure, hence increasing contact patch, improves traction. So, they're wrong to think that this increase in maximum tractive force can be explained away simply due to the "sidewall flex" of the tire. Though sidewall flex can alleviate the shock in the initial launch, and it can delay the application of some of the force from the launch, it cannot actually increase the maximum tractive force that can be applied by the tire. Due to hysteresis, though, increasing the contact patch actually will increase the maximum tractive force that can be applied through the tire.

While they are, for all practical concern given similar tires, correct in their assertion that the contact patch is the same given a normal force and a tire pressure, I'm also 100% certain you are also right that the deformation of the sidewall, and of the surface of the tire will cause a difference in the contact patch area. To make a more appropriate comparison than your pictures, they would be well served to imagine run-flat tires at 16 psi vs. slicks at the same pressure. A run-flat tire is nothing more than a tire with an extremely stiff sidewall, stiff enough to hold the weight of the vehicle without any air pressure. Even at 0psi it will have less contact patch than the slicks would at 16 psi, even though their model apparently predicts otherwise. They just seem to refuse to accept the fact that sidewall stiffness actually does factor into the contact patch area.

Hopefully, after reading the above you will feel a bit vindicated and they will feel a little more open-minded to what you have to say. I'd love it if your PhD friend would care to validate my statements.

Chris

 

[FastDriver]

BTW, even though you couldn't fully explain it, I love the fact that your experience is more accurate than their intelligence.

 

[N8Dogg98]

I saw your comment in my thread about arguing physics in these forums and it made me curious to find the thread you were referencing. So here I am.

Since they only made it through high-school and entry-level college physics, these guys are amatuer physicists that actually believe that tractive forces in real life, away from a textbook, are only relative to the normal force and the coefficient of friction. Unfortunately for them, they didn't go far enough to learn about a term called hysteresis, which is a significant contributer to maximum tractive force, and it does not change the coefficient of friction ("bilinear friction") between 2 given materials. When you introduce an asphalt surface with normal (perpendicular) deformations, and an elastomeric material that allows it to apply that force directly into those deformations, the elementary formula for traction no longer works. However, when you consider this effect, it quickly becomes evident why lowering tire pressure, hence increasing contact patch, improves traction. So, they're wrong to think that this increase in maximum tractive force can be explained away simply due to the "sidewall flex" of the tire. Though sidewall flex can alleviate the shock in the initial launch, and it can delay the application of some of the force from the launch, it cannot actually increase the maximum tractive force that can be applied by the tire. Due to hysteresis, though, increasing the contact patch actually will increase the maximum tractive force that can be applied through the tire.

While they are, for all practical concern given similar tires, correct in their assertion that the contact patch is the same given a normal force and a tire pressure, I'm also 100% certain you are also right that the deformation of the sidewall, and of the surface of the tire will cause a difference in the contact patch area. To make a more appropriate comparison than your pictures, they would be well served to imagine run-flat tires at 16 psi vs. slicks at the same pressure. A run-flat tire is nothing more than a tire with an extremely stiff sidewall, stiff enough to hold the weight of the vehicle without any air pressure. Even at 0psi it will have less contact patch than the slicks would at 16 psi, even though their model apparently predicts otherwise. They just seem to refuse to accept the fact that sidewall stiffness actually does factor into the contact patch area.

Hopefully, after reading the above you will feel a bit vindicated and they will feel a little more open-minded to what you have to say. I'd love it if your PhD friend would care to validate my statements.

Chris

BTW, even though you couldn't fully explain it, I love the fact that your experience is more accurate than their intelligence.

Wow, both you and Sharad obviously cannot read. None of us claimed that a d/r with substantive sidewall height would lead to the same traction as a 18" road tire. This entire discussion started because Sharad incorrectly stated that an 18" wheel/tire combo would have less contact patch area than a 17"
wheel/tire combo and related it to sidewall height. Do you actually think this is true?

Given the same tire and air pressure, the only reason a 17x10 wheel tire combo would perform better than a 18x10 wheel tire combo in the 1/4 mile has to do with the rotating mass (18's tend to be heavier than 17's). The difference in sidewall height is only 5%, and as I said before even if the wheel/tire combos weighed the same there is no way you could find a driver consistent enough to measure whether or not that additional 1/2" of sidewall height would lead to an increase in 1/4 mile time. In a perfect world perhaps the time would be improved by 0.01 second, but that's still splitting hairs.

 

[FastDriver]

I don't think I need repeat what I believe is true because I already explained it clearly, not to mention my post was addressing Sharad, not you. Since you chose to quote me directly, and are harping on him for his explanations, let me go back and review some of the things you've said that are wrong. That way you'll know that I obviously can read:

contact patch is determined by pressure within the tire and the weight applied to that tire and nothing more.

The aggressiveness of launch isn't due to contact patch

if the pressure is identical and the weight at the tire is the same, the contact patch will be identical no matter what the tire is made of.

The contact patch however is only increased as a result of the hard launch that can be acheived due to the sticky rubber in d/r's.

Contact patch is the same regardless of the tire size (assuming of course the weight placed on each tire and the pressure within each tire is identical).

This quote, if by "tire size" you're including the height of the sidewall without increasing thickness of the material in the sidewall. The difference may or may not be negligible, but there will be a difference.

It still has nothing to do with the size of the contact patch as the amount of rubber on the road is the EXACT same regardless of tire size (assuming equal tire pressures).

To this quote from another guy:

Traction (friction) and surface area have no correlation with each other...

you said:

This is exactly what Sharad is having a difficult time understanding

He's having a hard time understanding it for good reason.... it isn't correct.

You got stuck on the 17" tire vs. the 18" tire. If they're the same brand, and so the same material thickness in the sidewall then the extra 1/2" would allow for more flex if only in the tiniest degree. So, his contention is correct. There should be an increase in contact patch, even if only slight. Will there be a practical difference in the tire's maximum tractive force? Even if it's the cause of only 1 thousandth of a second difference, that's measurable, and who are you to say it wouldn't matter to anyone? You used .01 seconds as an example. I think there are thousands of drag racers that would not hesitate to make a switch for 1 hundredth of a second, which is absolutely measurable in the most consistent cars.

Furthermore, it's a slippery slope. If you argue that there's no measureable difference in performance between an 18" tire and a 17" tire even with a corresponding increase in sidewall, then is there one between a 17 and a 16" tire? A 16" and a 15"? If no to all, then it logically follows that there is no difference between the 18" tire and the 15" tire. Do you really believe that to be true?

All of the posts quoted are wrong because of the absolute nature of the statements. If you need further explanation, refer to my first post so I do not have to retype it.

 

[N8Dogg98]

You got stuck on the 17" tire vs. the 18" tire. If they're the same brand, and so the same material thickness in the sidewall then the extra 1/2" would allow for more flex if only in the tiniest degree. So, his contention is correct. There should be an increase in contact patch, even if only slight.

Again the contact patch area would be identical in this example.

Furthermore, it's a slippery slope. If you argue that there's no measureable difference in performance between an 18" tire and a 17" tire even with a corresponding increase in sidewall, then is there one between a 17 and a 16" tire? A 16" and a 15"? If no to all, then it logically follows that there is no difference between the 18" tire and the 15" tire. Do you really believe that to be true?

Going from say a 285/35/18 tire to a 285/60/15 tire you're effectively doubling the sidewall height. Meanwhile, going from a 285/35/18 to a 285/40/18 the sidewall height difference is only 5%. The point here is that the performance difference at a 1/4 mile track will have less to do with sidewall height and more to do with the additional rotational mass of 18" wheels. Now, the OP asked about traction difference between 18 and 17" wheels and most of us agree that you're splitting hairs here and that a good quality tire should net good results on the track on both an 18" wheel and a 17" wheel.

 

[FastDriver]

Again the contact patch area would be identical in this example.

Whether it's negligible/insignificant is another argument. Identical, it is not. Didn't you understand the slick vs. run-flat comparison? By the same token an increase in sidewall height without a corresponding increase in material thickness will lead to more flex and a corresponding increase, however slight, in contact patch.

Meanwhile, going from a 285/35/18 to a 285/40/18 the sidewall height difference is only 5%.

Try again. 40/35 = a 14% increase.

 

[FastDriver]

From HowStuffWorks "How do 30 pounds of air in your tires hold up 2 tons of car?"

Hopefully a better more understandable explanation of what I have been trying to say:

If you go outside and measure the size of the contact patch, you will probably find that it is actually even bigger than this. You can measure the width of the tread pattern anywhere on the tire to get a pretty good idea of the width of the contact patch. To get the length, take two sheets of paper and slide them under the front and back of a tire until they won't go any farther. Now measure the distance between the two pieces of paper.

The reason that the contact patch is even bigger than this calculation suggests is that, at the back and front edge of the contact patch, the pressure exerted on the ground is not very high. At the point where the tire is just barely touching the ground, almost no weight is supported. As you move toward the center of the contact patch, more and more weight is supported.

Now let's say you dropped the pressure in your tires to 7.5 psi, a quarter of what it was. You would find that your contact patch did not get four times bigger. This is where the stiffness of the sidewall of the tire comes in. When the pressure is this low the structure of the tire starts to bear some of the weight of the car.

Low-profile tires like sports car tires have short, stiff sidewalls, so these tires will tend to squish less than the big tires on SUVs and pickup trucks. In fact there are some tires that are so stiff they can run with no air pressure in them. These are called run flat tires.

 

[N8Dogg98]

Whether it's negligible/insignificant is another argument. Identical, it is not. Didn't you understand the slick vs. run-flat comparison? By the same token an increase in sidewall height without a corresponding increase in material thickness will lead to more flex and a corresponding increase, however slight, in contact patch.

I absolutely understand that. You're comparing apples to oranges though. Slicks, road tires, and run flats are each difficult to compare because they behave differently. In fact, if you're measuring actual rubber touching the road, the slick already has more material because there is not as much tread pattern (sometimese no tread pattern). The area of the patch may be the same, but the actual amount of material contacting the road is different.

Perhaps I should have clarified earlier when I said contact patch is dependent on weight and pressure within the tire, I was making the assumption that both tires would be measured at a reasonable 30 psi. Obviously a run flat tire will have a smaller contact patch at 0 psi than a d/r will at 5 psi, simply because now the air pressure is not supporting the run flat tire.

 

[FastDriver]

I just used the run-flat to illustrate a point that may not be so obvious in other tires but is nevertheless true. The point is that sidewall stiffness does affect contact patch even in an apples-apples comparison.

 

[Sharad]

an increase in sidewall height without a corresponding increase in material thickness will lead to more flex and a corresponding increase, however slight, in contact patch.

FastDriver is far more intelligent and eloquent than I am. ^^ This concept is what I was trying to explain.



I was a decent student, but I don't believe that the pursuit of knowledge ends at graduation. I'm confident of my traction assessment regarding 17/18" tires, but I mentioned this topic to a friend of mine who is a Physicist working for the USAF. Here is his response:

"Hey. I gave up trying to educate people on the internet long ago...once I realized that the average IQ is defined as 100. That means half the people you know are barely able to not drool on their keyboards.

As for physics 101, that's cute, but there's a lot of assumptions that go into that like, oh, I don't know, RIGID bodies that DON'T DEFORM. What you get when you apply an insane amount of torque to a stationary, rigid body (approximated by a zero-height sidewall) is that you instantly go from static-friction to kinetic-friction (ie, you do a massive burnout), but if you deform, the tire can stay in the static-friction zone longer. The force of static friction peaks out higher than that of kinetic friction, then when it breaks loose and slides, the force is smaller, meaning smaller acceleration. Basically if you look at a frictional-force (as seen at the tires) plot as a function of time, you want to maximize the integral from t = 0 to t = 60' time.

....besides, anecdotally, how many low-profile tire funny cars have you ever seen? These guys race for real money and if it were as easy as that, don't you think some ass-clown would have slapped some donks on a funny car and started breaking some records? f#ck, people are stupid."

Pardon the language. But I thought his explanation was interesting. Particularly the part about calculations that apply to a rigid body (zero-height sidewall) and how the deformation of the tire affects the frictional force.

I was gonna say "there's my two cents", but I think I'm approaching a dime's worth of bloviation at this point.