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Contact Patch Vs Grip

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Hey all,

 

I read this article http://www.stevemund...m/friction.html and it kind of turned my understanding about traction and contact patch size around.

 

If I understand correctly contact patch size doesn't matter as far as friction is concerned. All that matters is:

  1. Friction coefficient (something we cannot influence as it depends on track surface and tyre compounds)
  2. Bike + rider weight

Well then where's "contact patch" in the grip equation? If I remember well Keith explained that by rolling on the throttle smoothly while in the corner you actually make use of the larger contact patch of the rear tyre which can carry more load. If this is true it seems these pieces of information contradict with each other.

 

What is your opinion? What's the point of having a larger tyre contact patch?

 

Thanks,

Tony

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Hey all,

 

I read this article http://www.stevemund...m/friction.html and it kind of turned my understanding about traction and contact patch size around.

 

If I understand correctly contact patch size doesn't matter as far as friction is concerned. All that matters is:

  1. Friction coefficient (something we cannot influence as it depends on track surface and tyre compounds)
  2. Bike + rider weight

Well then where's "contact patch" in the grip equation? If I remember well Keith explained that by rolling on the throttle smoothly while in the corner you actually make use of the larger contact patch of the rear tyre which can carry more load. If this is true it seems these pieces of information contradict with each other.

 

What is your opinion? What's the point of having a larger tyre contact patch?

 

Thanks,

Tony

 

Hopefully Steve can answer this in a much better way - but here are my thoughts, for what they are worth:

1) for sure, the real-life situation is not as simple as that equation - there are additional factors like tire flex, tread flex, heat, slip angle, tire deflection, etc. that make the basic static friction formula above not adequate to define the actual traction you experience while riding

2) I don't think the coefficient of friction can be assumed to be constant in this application, because if you had a very small contact patch you could (I think) overload the tire, overheat and melt (or possibly tear) the top layer of rubber, and now you have different coefficient of sliding friction because you are slipping on melted rubber. In other words, the "surface area" component you are looking for may essentially be hidden within that coefficient of friction number.

 

I'm sure there is way more to it than that, but I'm confident that the article above is a vast oversimplification based on static friction and the "ideal conditions" that always became a joke in my dimly-remembered physics classes. (Example: "assume a perfectly spherical body on a perfectly frictionless surface....")

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The friction between two surfaces is proportional to the force pressing them together.

 

 

 

 

TADA! > more force = more friction! I read that a larger force makes a larger contact patch (byproduct) and more frcition!

 

 

imho go ride and learn, you dont learn skills just by reading, you have to practice and try to perfect it.

 

 

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corner speed + tire flex = grip

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I think it's his interpretation and over simplification of these laws that's lacking. He's not taking into account the myriad of variables (as mentioned above) used by motorcycle designers to maximize the use of a given contact patch. You can't apply any more weight (general weight and acceleratory forces) to a 180 tire on a literbike than you can a 600 without loss of traction.

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I was told by a famous road racer that won a lot of races, to get traction, to get feel and to make the tires work, he wanted to flex the front tire enough to make it feel like the rim was going into the pavement. This gives me something to shoot for.

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Hey all,

 

I read this article http://www.stevemund...m/friction.html and it kind of turned my understanding about traction and contact patch size around.

 

If I understand correctly contact patch size doesn't matter as far as friction is concerned. All that matters is:

  1. Friction coefficient (something we cannot influence as it depends on track surface and tyre compounds)
  2. Bike + rider weight

Well then where's "contact patch" in the grip equation? If I remember well Keith explained that by rolling on the throttle smoothly while in the corner you actually make use of the larger contact patch of the rear tyre which can carry more load. If this is true it seems these pieces of information contradict with each other.

 

What is your opinion? What's the point of having a larger tyre contact patch?

 

Thanks,

Tony

 

WOW!

 

You all had great responses and really hit very good points.

 

The simple matter is this. The physics of tires are not as simple as a brick sliding on a table. The rubber moves around, it can push into cracks and rough surfaces, it does this differently at varying temperatures ,PSI settings, construction variations and compounds, thus creating variables not noted in the link. To directly answer your question, there is no direct simple "big is always good and small is always bad" answer.

 

Simply having a bigger contact patch does no ensure better traction. If this was true, then we would all run 5 PSI in our tires and the story would be over. And for that matter, why 5 PSI, why not 4 or 3 PSI?

 

Its about USABLE contact patch. Can you use the contact patch effectively? Tires have widely different constructions, and thus some make better use of a larger contact patch while others may not, and some may not have a larger contact patch but still get the job done.

 

I would not try to place any large emphasis on this "contact patch" theory. The patch is the patch. If one brand says their tire has a bigger patch, that is no guarantee that it WILL BE BETTER, nor is it a guarantee that a smaller contact patch will be less performing. The tire has the grip that is has for MANY MANY reasons, contact patch being one of them, but not the deciding end all factor.

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WOW!

 

You all had great responses and really hit very good points.

 

The simple matter is this. The physics of tires are not as simple as a brick sliding on a table. The rubber moves around, it can push into cracks and rough surfaces, it does this differently at varying temperatures ,PSI settings, construction variations and compounds, thus creating variables not noted in the link. To directly answer your question, there is no direct simple "big is always good and small is always bad" answer.

 

Simply having a bigger contact patch does no ensure better traction. If this was true, then we would all run 5 PSI in our tires and the story would be over. And for that matter, why 5 PSI, why not 4 or 3 PSI?

 

Its about USABLE contact patch. Can you use the contact patch effectively? Tires have widely different constructions, and thus some make better use of a larger contact patch while others may not, and some may not have a larger contact patch but still get the job done.

 

I would not try to place any large emphasis on this "contact patch" theory. The patch is the patch. If one brand says their tire has a bigger patch, that is no guarantee that is WILL BE BETTER, nor is it a guarantee that a smaller contact patch will be less poerforming. The tire has the grip that is has for MANY MANY resons, contact patch being one of them, but not the deciding end all factor.

 

 

Great, great points.

 

 

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I think I'm going to mount a pair of 125GP tires on an S1000RR and see how that grip is...

 

If it's better, I'm going for a pair of 10-speed wheels and tires after that.

 

It that's even better, next I'm putting an S1000 engine in a 10-speed bicycle.

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Greetings, I'm Steve Munden, the author of the web page cited by the original poster, Tony, who mentioned the forum discussion to me with an implicit invitation to participate. I hope it isn't too late to be interesting; I know that these topics have a short lifetime, generally only a day or two at most, but I didn't get time to enlist until Saturday and had to wait until the administrators approved my participation.

 

That web page http://www.stevemund...m/friction.html and its companion http://www.stevemund...tiontopics.html get a lot of traffic and provoke considerable invective, mostly much less courteous than what appeared on this forum. I congratulate you on the civility of the group you've assembled here.

 

I have to ask: What is it about the clear experimental fact that apparent area of contact does not affect friction that causes such denial? Yes, it's counter-intuitive. So what? Is this the first time that the world has confounded your expectations? The world appears to be flat and stationary, with the sun circling it, but only cranks believe those things.

 

If you truly believe that contact area is relevant to friction, one of two conclusions is inescapable. Either you are correct and the scientists and engineers of three centuries don't know what they're talking about; or they do know what they're talking about, and you don't.

 

To find out which, you can do the experiment done every year by high-school students in physics classes throughout the US and, I presume, the world. Cut up a tire (a sawzall is good, those suckers are tough) into strips, and glue them to the bottom of a small piece of plywood. Glue more of them to another piece, and fewer to still another. Weight all of them the same. Take a fishing scale and measure the pull required to start sliding across another surface. Plot the results against the area of the surfaces in contact.

 

(You can get more precise measurements at the cost of a hair less clarity by tipping the surface on which the test strips are sliding and measuring the angle required to start sliding.)

 

If you find that the engineers and scientists have been wrong about friction for the last 300 years, send gloating email to me as you fly to Stockholm to collect your Nobel prize.

 

With that question settled, however it turns out, we can turn to other matters. There is no doubt that there are many factors that affect traction. The temperature, the presence of lubricants, the presence of sand or gravel or paint, the stability of the tire. Nobody said otherwise. What was said, and only what was said, is that contact area is not one of those factors.

 

At least, contact area is not a direct factor. A larger area will allow use of a stickier rubber for better traction and still obtain adequate wear. A tall skinny tire of a given rubber will have the same traction -- that is, resistance to sliding -- as a short fat tire with the same rubber, but the tall skinny tire might squirm around. In the latter case you would certainly be justified in saying that the traction was worse than it would be with a fatter tire having a greater contact area, but you'd be confusing the issue. It isn't the resistance to sliding which would be different.

 

To reply to some of the specific comments:

>for sure, the real-life situation is not as simple as that equation

(and other comments with the same thrust)

 

This is to misunderstand the equation. What it describes -- friction, specifically adhesion -- is exactly that simple. The equation doesn't describe a lot of things, like the temperature of the tire, the price of gasoline, the skill of the rider, the phase of the moon. But what it describes _is_ that simple.

 

>I don't think the coefficient of friction can be assumed to be constant

 

Correct, it isn't constant. It varies with temperature, tire compound, road surface. It doesn't vary with contact area.

 

>In other words, the "surface area" component you are looking for may essentially be hidden within that coefficient of friction number.

 

It isn't. There is no surface area component.

 

 

>the "ideal conditions" that always became a joke in my dimly-remembered physics classes. (Example: "assume a perfectly spherical body on a perfectly frictionless surface....")

 

I'd find the school which presented your dimly-remembered physics classes and ask for my money back.

 

>The tire has the grip that is has for MANY MANY resons, contact patch being one of them, but not the deciding end all factor.

 

Almost correct. The tire has the grip that it has for many reasons, but contact area is not one of them.

 

Thanks for the opportunity to sound off. I look forward to reading the email as you make your way to Stockholm.

 

Steve

www.stevemunden.com

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Hmmm... an interesting article at the link there. Although I'm not sure if I learnt anything, I do have some questions... mellow.gif

 

Firstly - starting with the microscopic texture of surfaces, even two flat surfaces do not truly have a 100% contact area. There are microscopic peaks and valleys, no problem - I can understand that and it makes sense. But then it goes on to say that if you double the surface area the true contact area of the peaks will not change. Please explain?

Let's say a certain piece of rubber has 100 true 'contact peaks' at the microscopic level. Assuming a certain level of surface uniformity we could reasonably say that another piece of the same rubber, at the same size would also have 100 true 'contact peaks'. Correct? But then if we cut one single piece that equalled the area of the two single pieces we would still only have 100 contact points?? That just seems illogical to me... or am I missing the point or reading this wrong: huh.gif

If you take another pair of mountain ranges/plates of twice the area and pressed them together with the same pressure, the actual contact area of asperities wouldn't change. You'd have the same number of asperities in contact, but further apart, in the larger plates. Only additional pressure would change the true contact area.

 

But the example of the pie graph is really interesting - so we have the exact same amount of available traction at full upright position as well as at full lean. Just that at full lean the majority of available capacity is used in turning. So in theory, if the bike is upright and accelerating hard enough that the tire is not slipping or sliding (let's say 95% of available capacity is used in acceleration), then the rear tyre should be able to support just the same type of force in a turn, as a cornering force rather than an acceleration force? So if I go ahead and apply it literally... thinking of the feeling of the rear tyre digging in and gripping the road when the bike is upright and the throttle is at the stop... then cornering at 95% would feel just the same, but with a lateral load? If I'm understanding it right, that's pretty amazing. ohmy.gif

That could really give someone the confidence and trust to push their tires.

 

fossilfuel, that visualisation of the rim going into the pavement seems really extreme! But judging from my own experience (I'm still surprising myself with how much speed I can carry into turns and how quick I can turn in, yet the front tire still feels completely solid), and if I have the correct understanding of what I've written above then it seems like your example of pushing the rim into the pavement may not be so extreme after all!

 

If the purpose of the linked article was to increase riders confidence in the ability and performance of tires then there may be something to it, although I wish it was continued through to that conclusion because as it is now it just seems like it's unfinished...

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Nothing corners faster than the 125 GP bikes. These are very light and have narrow tyres. Large racing machines have wider tyres, have more power and more weight. And do not corner as fast. So from this perspective, you would think that the rules stated in the article could well be true.

 

Bikes also seems to be able to accelerate and stop equally hard, despite the often huge difference in tyre size. Again, support for the article.

 

Furthermore, grip seems similar between front and rear tyres during cornering. Another support for the article.

 

My uneducated guess is that if the rear tyre is too narrow it will quickly overheat lokally. This will shred rubber fast, which again will induce a slide. So by spreading the load over a greater surface, the tyre will run cooler on the surface and grip will be more consistent.

 

There is of course every chance I am wrong.

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Bikes also seems to be able to accelerate and stop equally hard, despite the often huge difference in tyre size.

 

 

Motorcycles don't really accelerate equally. A 125 can't accelerate near as quickly as a 1,000 cc bike. That's more a power issue. But, a 1,000 cc bike with a 125 tire couldn't accelerate near as hard as a 1,000 with a 190.

 

When they put a 190 tire on a literbike, it's considered the optimum tire for that size engine for what they want the bike to do. The 1,000 cc bike can't have a smaller tire, because that would cause more of a traction problem. Bigger tire, cornering is compromised. Longer swingarm, cornering changes as well.

 

Drag racing bikes don't have such a wide contact patch because they use all the rubber in a drag. It's so they can apply more power for quicker acceleration in relation to the larger contact patch.

 

There's also tire flexion, which the author doesn't take into account.

 

Nothing corners faster than the 125 GP bikes. These are very light and have narrow tyres. Large racing machines have wider tyres, have more power and more weight. And do not corner as fast.

 

As you stated, they are very light and have less power. That's why the 125's only require the tire size they have. Weight application for a given contact patch size is optimum. If you have that small a contact patch with 2 times the weight, the traction would break.

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By accelerating and stopping equally fast, I meant that both seems to be limited to just over 1 G - provided you have the power to do it, of course. I could be wrong again. That's the problem with discussing things when not fully qualified. OTOH, it's sometimes the innocent comment from the imbecile that bring stuff forward :lol:

 

What would be interesting to know is whether the grip goes away because the contact patch is too small, or if the small patch wears too soon and as such cause slip that cause more wear and more slip. Not sure if I manage to explain it properly, and frankly I'm not sure there really is a difference. But let's try it this way:

 

If you took one motorcycle with a 100 mm wide tyre and one with two 100 mm tyres, all being identical and both bikes being identical as well. Let them sit stationary with locked wheels (front wheel in the air) and measure the effort required to get them moving. If my thinking is correct, it would take the same effort to get them moving, but less effort to keep the one with one tyre going because the small contact will heat up and shred rubber more easily.

 

Sorry if I make no sense. But should my thinking have anything going for it, this will also lend support to what Steve says - that there are more ways to create grip than contact patch size. If you then have a smaller patch that you somehow manage to keep cooler because it can grip longer before it starts to slip, it may work well?

 

 

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What the guy writing the article is misinterpreting is size and force. The law of friction is in regards to two objects causing friction, not their respective size and force applied (as is taken into account when talking about motorcycles and tire size). He's overcomplicating basic rules: the rules of friction. These are related to friction only. It's not force/weight/material/elasticity/angle/etc that these rules are talking about. Once you start talking about force applied to contact size of said objects, you redirect to other laws, or add these laws, regarding physics of friction and application of pressure.

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Eirik, I think I know what you're getting at when you say that both types of bikes seem to be limited to just over 1G. But don't forget that a 125GP bike cornering at 1G is exerting a very different force to a 1000cc bike at 1G. Let's say the 125GP bike weighs 100kg, therefore at 1G it is supporting 100kg + rider. At 2G it would be supporting 200kg (plus the equivalent of two riders!)

 

If we say a 1000cc sportbike weighs 200kg, then at 1G it is supporting a force of 200kg + rider. The 1000cc bike at 2G would be supporting 400kg (plus the equivalent of two riders again).

 

So when we're talking about the cornering forces of 1G, it's all relative to the type of bike (and rider).

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The point that Steve is making does make sense on its own. But it's the fact that one doubts that a dragster with much smaller contact patches will grip the same as with very large tires. In other words there is truth in the laws he's presenting--they are laws actually, no way around them.

 

I think an engineer's look at the other important factors, or other laws that also relate to this subject is needed.

 

I'm not an engineer so not of much use here, but I'm wondering if the lateral load on the contact patch vs downward gravitational load vs size of contact patch vs friction all interact in a way that would make more sense to the layman.

 

I'd like to learn something here but I'm not all the way there. Imparting truth is one thing, getting someone to personally reconcile (shed incorrect fixed ideas) and understand it is another.

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To rephrase what I think Eirik was saying:

 

A given motorcycle (say a liter bike) can stop with equal or greater force than it can accelerate. This stopping relies on the narrower tire than the acceleration does. If the bike is stopping with only the front wheel on the ground, and accelerating with only the rear wheel on the ground, each example has the same downforce and it would imply that the amount of traction is not directly related to the contact patch size.

 

-Sean

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To rephrase what I think Eirik was saying:

 

A given motorcycle (say a liter bike) can stop with equal or greater force than it can accelerate. This stopping relies on the narrower tire than the acceleration does. If the bike is stopping with only the front wheel on the ground, and accelerating with only the rear wheel on the ground, each example has the same downforce and it would imply that the amount of traction is not directly related to the contact patch size.

 

-Sean

 

It's so nice when somebody can put things in simple terms B)

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Eirik, I think I know what you're getting at when you say that both types of bikes seem to be limited to just over 1G. But don't forget that a 125GP bike cornering at 1G is exerting a very different force to a 1000cc bike at 1G. Let's say the 125GP bike weighs 100kg, therefore at 1G it is supporting 100kg + rider. At 2G it would be supporting 200kg (plus the equivalent of two riders!)

 

If we say a 1000cc sportbike weighs 200kg, then at 1G it is supporting a force of 200kg + rider. The 1000cc bike at 2G would be supporting 400kg (plus the equivalent of two riders again).

 

So when we're talking about the cornering forces of 1G, it's all relative to the type of bike (and rider).

 

Now here is an amazing thing; bikes corner with roughly 2G :blink: So how do they do that, when grip limit them to a little over 1G during stopping :huh:

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Oh, didn't you know that you can corner with 2G? I do it all the time, you ought to see it!

 

tongue.giflaugh.gif I just picked a number for the sake of illustrating a point. But I think I misunderstood what you're getting at. Now I'm just starting to get confused... huh.giflaugh.gif

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60 degree banking angle = 2 G, I am told. 1 G = 45 degrees, which is about the limit for most semi-fast riders, especially on the road.

 

But nobody have been able to stop a motorcycle with 2 G of redardation that I'm aware of. Tyre grip may not be the ultimate limitation as the bike will flip over before running out of traction, but 2 G?

 

Acceleration, however, with a wide slick and wheeliebars, can get as high as 4.2 G on average over a 1/4 miles in top fuellers (4.4 second from a standing start :blink: ) and F1 cars can reach peak cornering forces of more than 5 G - although they "cheat" by pushing harder down on the tyres than what they actually weigh.

 

So what does this tell us in the original context? I haven't got a friggen idea :unsure:

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Greetings again, and please pardon my tardy replies. I happen to have a lot going on after my regular job this week, but I am very interested in the comments so far.

 

> Firstly - starting with the microscopic texture of surfaces, even two flat surfaces do not truly have a 100% contact area. There are microscopic peaks and valleys, no problem - I can understand that and it makes sense. But then it goes on to say that if you double the surface area the true contact area of the peaks will not change. Please explain?

 

 

The image used by the engineers and scientists is that doubling the apparent contact area might change _which_ peaks and valleys are actually in contact, but the true area of contact will not change. That is, if a new peak is higher than a neighboring peak which was formerly the highest peak, the new peak will replace the prior peak, with no net change in area. If there's a new peak which is lower than a prior peak in contact, the prior peak will prevent the new peak from making contact at all. I'm a mathematician by training, and mathematicians like extreme examples to test the limits of a proposed theorem. Here's my extreme example for this: Consider two surfaces, each of which looks like __________/\/\/\/\/\/\/\__________. If you turn one over and press it onto the other, clearly the only areas of contact will be in the middle /\/\/\ sections. If you now add to those two surfaces, but the additional area is all ______, the actual area of contact will remain the same middle portion.

 

Did that help? I could try again, or invite someone else with better visual-to-verbal skills to step in.

 

>If the purpose of the linked article was to increase riders confidence in the ability and performance of tires then there may be something to it, although I wish it was continued through to that conclusion because as it is now it just seems like it's unfinished...

 

I'll think about that. I'm not certain I understand your point but I think I do, and if pursuing it would result in a better article I'm all for it.

 

 

>If you took one motorcycle with a 100 mm wide tyre and one with two 100 mm tyres, all being identical and both bikes being identical as well. Let them sit stationary with locked wheels (front wheel in the air) and measure the effort required to get them moving. If my thinking is correct, it would take the same effort to get them moving, but less effort to keep the one with one tyre going because the small contact will heat up and shred rubber more easily.

 

 

This is my understanding as well. As long as every other factor is the same -- temperature, mechanical strength of the two surfaces, etc -- varying the contact area will not vary the friction. But varying the contact area WILL affect other factors, such as temperature most obviously, which with then result in a change in friction. The change will be more friction if a cold tire warms up, and less friction if a warm tire overheats.

 

 

>Now here is an amazing thing; bikes corner with roughly 2G So how do they do that, when grip limit them to a little over 1G during stopping

 

I would be very interested to know the source of the statement that bikes corner with 2G of force. If you mean that the radial acceleration, the inward acceleration, is 2G, I am skeptical. In http://www.stevemund.../leanangle.html I derive (re-derive -- none of this stuff will be news to an engineer, or even a student of a 1st-semester college physics course) the equation v^2 = ugr, where v is velocity, r is turn radius, g is gravitational acceleration, and u is the coefficient of friction. It says that the speed v around a turn of radius r is limited by the gravitational acceleration and the coefficient of friction; and the greatest value of u I've seen for street tires is about 1.2. If we're talking motogp qualifying tires, which are trash after a couple of laps, I could be convinced that they have a value of u of 2, so maybe that makes sense; but for street tires, I believe they are limited to cornering about about 1.2 g, corresponding to u=1.2. I'd be interested in any data supporting or denying my conclusions.

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A 60 degree lean angle (of the CG of rider and bike), should net a 2G force compressing the suspension and a lateral force of 1.7Gs. A two G lateral force from tire grip alone, (instead of banking, train tracks, etc) would be pretty impressive for a motorcycle.

 

And yes Eirik, I think the braking and acceleration force is limited by the high CG which causes the tendency to wheelie or endo, while when cornering the horizontal force is held in check by the downward force against the counterbalanced mass.

 

-Sean

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