# Coefficient of Friction, car tires and road

Is there a point where car tire rubber becomes so soft that it becomes “adhesive” and will allow a pull of more than a G of acceleration? I’m not talking about smoking the tires first before the take-off or rubbing the tires down with stickem’, i’m talking theoretically, like measuring the coefficients of the road and tire and determining if this coefficient is more than 1. If so, then a car can pull more than a G off the line without any help from stickem’ on the track or tires… correct?

How’s this for info?

That’s a helpful article. It says that even the best conditions of racing slicks and raod is still under 1, which is what I always hear. But, please, hear me out on this:

My theory is that the grains on the road protruding up is cheating this rule of thumb. Whenever I analyze things, I always consider extremes, in order to to come to a conclusion. So, let’s look at an Indy car with slicks set on top of spikes about half an inch apart and made of hard steel, but not so sharp that they puncture the tire. Now… What would you say the coefficient is now? It has to be way bigger than 1, right? Well, if this is true, then why can’t rubber grab onto the grains in the road just like this and produce at least a small amount over one? I guess this matters if the spikes or grains actually puncture the outer layer of the rubber, so let’s say it doesn’t puncture the outer layer because the tire is pure rubber, no air inside. But the grains dig in deep.

Remember, some of the rubber is acting on the spikes or grains in a non-horizontal manner. This surely opens the door for more available road counter-force, correct?

I can’t back this up with a lot of on-the-web links, but if you’re interested in this and related topics, you might look for a copy of How to Make Your Car Handle, by Fred Puhn, which I actually found pretty interesting, even though I’m not at all a car hobbyist.

It’s my understanding, from this and from other sources, that the old “coefficient of friction” model that is taught in high-school physics class is an oversimplification that works pretty well for fairly smooth, rigid objects on fairly smooth, rigid surfaces, i.e., for the sorts of experiments you’re likely to do in high-school physics class. The model doesn’t work so well when the surfaces are very rough (like asphalt or concrete) or very soft (like rubber).

It is certainly the case that the very fastest drag racers routinely accelerate at considerably more than 1G - even right off the line, where their actual speed is fairly low, so they have little or no assistance from areodynamically-boosted normal force. The high-school physics model says that can’t happen.

In short, I think you’re right - there’s no reason the coefficient of friction can’t go over 1 in the case of a rubber tire on rough pavement. It might be better to scrap the notion of a coefficient of friction anyway, though - I’ve seen some claims that rubber doesn’t even have a linear relationship between normal and friction forces, so this style of analysis may lead you astray again and again.

Exactly my point.

The old friction physics rules of thumb fall apart laughingly in the face of modern racing, isn’t this obvious or am I being too frivilous? Maybe you won’t see some vettes pulling 2 G’s off the line without stick’em, but theoretically they could pull say1.2 G’s, right?

By the way, the drag cars you speak of pull much more than a G because of adhesive stickem’ on the tires and road. They pull like 2.5-3 G’s without any help from downforce off the line, if I’m not mistaken. But this is another sub-topic, I am talking about purely rubber meets the road friction.

Nobody wants to say whether the coefficients are worthless or not? Doesn’t the protrusion of the grains in the road being hugged by a soft tire bring a whole new element into the picture, rather than a dinner plate on a glass table? This seems so obvious to me, but I have had no backup with this from anyone!?

Don’t know if this is what you are talking about at all, but according to Road & Track, there are numerous vehicles that can pull better than 1 G laterally on a skidpad. Porsche 911 GT1 (1.07G) and GT2 (1.02), Dodge Viper SRT-10 (1.04), and the Corvette C5-R (1.44). I seem to recall reading (I think R&T) that Formula 1 cars can pull something like 4G laterally, and well over 1G (probably over 2, but I don’t remember) in both acceleration and braking, but they have crazy aerodynamics

Yes, this is almost what I’m talking about. If i am not mistaken a skidpad is sprinkled with water too? Assuming the cars weren’t going fast enough to produce significant downforce, then Mu>1!!!

Anybody have other numbers like this? I will search the internet more for skidpad results… didn’t think of that word.

Well, according to the physics definition of friction, tires and road have coefficient of less than one always. But we’ve also seen that the tires and road have more than 1 in actuality. Ok. Now what? Re-write the books? It’s all stupidity…

Listen, i realize that physisists tried to simplify things with this equation but it’s still wrong nonetheless.

no passenger car with normal tires will ever even approach 1g.

it requires a ‘race car’ and good competition tires.

maybe that was the point?

The simple model of friction works quite well in a whole lot of situations, even though everybody knows it’s only an approximation.

In some extreme cases concerning rubber-and-asphalt, the underlying assumptions break down, and the model doesn’t work so well.

Big deal. I mean, seriously, what’s the problem here? When the simple model works, you use it to save yourself work. When it doesn’t, you’ve got to find something better. And better (read: more accurate) models for friction no doubt exist - they just aren’t taught in high school physics.

Saying “the equation is wrong” and talking about re-writing texts is pretty silly, IMO.

It sounds like you’re mistakenly assuming that “physics” requires that the coefficient of friction (“mu”) always be less than 1. This is not true. There’s nothing magical about mu>1; this is only saying that the frictional force can be greater than the normal force, which indeed is sometimes the case. (For example, see this table, where various grades of rubber on steel, glass, and aluminum have mu > 1.)

It is true that the whole idea of a “coefficient of friction” is only an approximation, and in some cases a poor approximation, to the real physics. Rough surfaces in contact (such as tires and pavement) have behaviors that are different from a simple linear frictional-force model governed by mu. But of course you can say that about just about any equation in a first-year physics textbook. Physics is not about teaching Ultimate Truths, but about understanding models about how things work. A useful model, such as the linear frictional-force model or Newtonian mechanics, makes valid predictions in some regime, but outside of this regime it might not be “correct.” So what? It’s still useful, and that’s why it’s taught.

I guess I missed that day in freshmay physics when they said that the coefficient of friction couldn’t be greater than 1. What’s the reason they gave for that? I just took a rubber band out of my desk, placed it on a pad of paper, and tilted it to an angle of around 70 degrees before it slipped off. I think that’s what I would have done in college had my professor told me that the max for mu is 1.

However, he did tell me that glass flows, like in several decades, and I believed that.

Listen, all i know is what i was taught in HS and what my brother is tellingme at this moment he is being taught in HS: Two surfaces’ coefficient cannot be more than 1. All the books say this from what i know.

Was that supposed to be a clever observation? This is the third time you have come into one of my posts with some short, trite comment that doesn’t mean anything. What does it matter if it’s a race car or not, we are talking about surfaces having greater than 1 coefficient… sorry, i am just getting the feeling like i am getting attacked, sorry. Just try and offer better advice please.

I said that my bro and i were taught in school that two surfaces cannot have more than Mu 1 (we went to same school, 4 years apart). Now i see cars pulling more than a G. All i am asking is why are the books wrong. I offered that maybe the reason is for simplification purposes, but why isn’t this taught as a simplification? It’s all a joke i guess. High school physics is a joke. Whatever, it doesn’t matter anymore!

Your brother and high-school teacher are wrong. There’s no physical reason that a coefficient of friction can’t be greater than 1. In the link I posted above there are several examples given of material pairs (like some kinds of rubber on steel) with coefficients of about 1.6.

man i dont know what your problem is. im not attacking you. i dont know why you think im doing so, or the three other "

you asked, i suggested something. would you like me to type it out like a scientist? sorry… “IANAS”

the fact is what you were told is wrong when you look at the whole scope of things. it is probably right when you consider normal cars and normal tires. that however, is limiting your focus. It is probably simplification just for the fact that no one else cares enough to want to know the truth. For instance credit card companies sell card insurance to people for say \$10 a month so that incase your credit card is stolen, you arent liable for damages. guess what, legally you are only liable for \$50 of a stolen credit card ‘bill’ i guess you could say. so that insurance is a rip off… not many people know that because not many people care enough to go look it up. is that example good enough for you?

the way i see it, you stumbled across information, that, based on what you were taught, was wrong. it confused you. no big deal. its not my fault you didnt know this simple automobile knowledge until you looked it up two days ago.

oh, and when it comes to drag racing, most of the time its a rubber-rubber connection, not a rubber-cement, or rubber-asphalt. “stickem” also know as VHT is not always used, nor needed.

if you want to know what kind of G forces are created during a launch you should try smokemup’s 60ft / launch G-force calculator. my car routinely pulled 1.90 60ft times with the old crappy street tires, which is over 1.0g. With slicks it should be able to pull 1.69 or so 60fts.

and… yes, if youve ever touched a slick or a roadracing tire you would notice how soft they are. dirt track tires are the same way, very very soft compared to normal street tires.

[/clever observation]

I saw no distinction between soft and sticky tires, just saying that you didnt want to know about smoky VHT aided burnouts.

That is why i assumed you didnt know. You asked if there was a point where car tire rubber becomes soft enough to be adhesive, to which the answer is yes. If you have slicks on and drive through an old parking lot with loose gravel, you would notice they are what most people call “rock slingers” as in they pick up rocks off the ground and sling them in your wheel wells. but, you already knew this.