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snowman

Contact Patch Vs Grip

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Just thought I'd show a video that illustrates to some point how much grip there is available for acceleration even at pretty severe lean angles. Besides, it's a cool video of bike control :D

 

http://www.youtube.com/watch?v=XCGaQMgkzw8&feature=youtu.be

 

Hi Eirik, you should check out this series of japan police bike videos too ^^

 

 

I personally have tried to mimic the upper body movements to great success in very slow speed / consecutive tight turns ^^

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Hi Steve, good to see you here helping us understand this!

 

I have a habit of opening forum threads and letting them sit for hours or days before I get a chance to reply, so I didn't see your earlier reply before I commented. I just read 'friction topics' and that helped me to get a better idea of the whole thing and started to bring in some real-world applications. If only there was a link at the bottom of the 'friction' page that took me straight to the followup... it should be mandatory to continue with reading that second page.

 

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.

 

Yep, that does help thanks. I think I understand all of that now, I can't argue with the facts, but it's still doing my head in... I keep thinking up different scenarios that just confuse me. Say for example we were playing tug-of-war. Now that's something I'm not recently familiar with, but thinking back on my childhood experiences I'm fairly sure that I'd start having trouble if I tried to stand on one foot. But if I had both feet on the ground my chances would improve greatly. But according to the rules of friction it shouldn't be so?

 

>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.

 

 

What I meant was - sure I learnt about friction, but then what? I'll just go on riding the same as I always do and not give it a second thought... But if there was a direct application to everyday riding, that would be something really great. Maybe just a summary at the end, even if it was just to correct myths (like riders thinking that there's less grip available on the edge of the tyre because of reduced contact patch). And people sometimes just like to skip to the end and find out what it's all about. tongue.gif Anything that helps riders to get a more accurate understanding of motorcycling and increases their confidence... well just about the only thing better than that would be being able to surgically implant riding skill.

 

There's some interesting things up there though, I'm still making my way through the motorcycling pages.

 

Going a bit of topic... but dark-sider?! blink.gif

I'm going to assume that you ride alot of straight roads... right? Is it just because of cost (or to reduce your costs)?

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>Say for example we were playing tug-of-war. Now that's something I'm not recently familiar with, but thinking back on my childhood experiences I'm fairly sure that I'd start having trouble if I tried to stand on one foot. But if I had both feet on the ground my chances would improve greatly. But according to the rules of friction it shouldn't be so?

 

 

That's a really instructive analogy. Yes (if the scientists and engineers are correct) you will have no greater chance of sliding with one foot than with two. But your stability would certainly be impaired with one foot and that leads to the impression, quite correct, that you're better off with two feet. And THAT leads to the impression, quite incorrect (if the scientists and engineers are right) that your traction is greater with two feet rather than one.

 

 

>What I meant was - sure I learnt about friction, but then what? I'll just go on riding the same as I always do and not give it a second thought... But if there was a direct application to everyday riding, that would be something really great. Maybe just a summary at the end, even if it was just to correct myths (like riders thinking that there's less grip available on the edge of the tyre because of reduced contact patch). And people sometimes just like to skip to the end and find out what it's all about. tongue.gif Anything that helps riders to get a more accurate understanding of motorcycling and increases their confidence... well just about the only thing better than that would be being able to surgically implant riding skill.

 

I'll be making some additions in response to this, but it'll probably be some days or even weeks before they appear. Thanks for the push.

 

>There's some interesting things up there though, I'm still making my way through the motorcycling pages.

 

Flattery will get you --- well, I doubt it'll get you anything, but it gets me a warm feeling. Thanks!

 

>Going a bit of topic... but dark-sider?! blink.gif

I'm going to assume that you ride alot of straight roads... right? Is it just because of cost (or to reduce your costs)?

 

No no no! That was a rhetorical device! That page appeared first as an article I published in the Iron Butt Association's magazine, and the "I'm a dark-sider" heading in my imagination was a conversation with another person about the implications of the physical laws I was describing. I have changed it so that it no longer gives the impression that _I_ run auto tires on my motorcycle. I do not; but I'd be mighty interested in riding someone else's motorcycle to see what the handling characteristics are.

 

By the way --and I hope I don't get thrown off the forum for spam for this -- you can read my impressions of my own first and only track experience in a CSS class in New Jersey at www.stevemunden.com/code1.html.

 

 

 

Steve

 

www.stevemunden.com

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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.

 

There are a lot of factors in play with motorcycle racing and especially motorcycle tires. Steve's physics class is well taken and understood. However there are many factors involved beyond just pulling a piece of rubber across a table with a string. To simplify the entire subject of tires down to that one point is not workable and its not the solution.

 

Thanks Steve for your input.

 

Its time to move on and put our attention on things that can really make us faster. things like rider training, checking your tire pressure and having a good basic suspension setup. To put our attention on this subject of contact patch, and place a large amount of emphasis on this exact subject is not the correct placement of our attention. Remember we only have $10 of attention. our attention is much better spent on other more important points.

 

Time to move on.

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>There are a lot of factors in play with motorcycle racing and especially motorcycle tires. Steve's physics class is well taken and understood. However there are many factors involved beyond just pulling a piece of rubber across a table with a string. To simplify the entire subject of tires down to that one point is not workable and its not the solution.

 

It is fortunate that nobody has tried to simplify the entire subject of tires down to the one point of apparent area of contact. Instead, we have examined that one point to see its effect on traction -- specifically adhesion -- and concluded that engineers and scientists have known for centuries that it has no effect on traction. We have tried to make sense of this counterintuitive fact and tried to understand what other factors come into play to explain why so many high-traction applications, like drag cars and bikes, use huge tires.

 

>Time to move on.

 

Is it really? I suppose it is, if nobody has any additional real data to present. If anyone does, I hope that person isn't dissuaded by your proclamation.

 

Steve

www.stevemunden.com

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Thank you all for the interesting discussion. I learned some curious facts - hope you did so too.

 

Now, I'd like to underline what we seem to have agreed upon - rear tyre's larger patch alone does not give it better traction than the front tyre.

 

If this is true I'd like to "challenge" the cornering guru Keith Code to share his opinion on the topic. As far as my memories don't play funny tricks with me I think Keith said in a movie of his that we have to roll on the throttle evenly smoothly and continuously throughout the turn so that we offload the front tyre *AND* make use of rear tyre's larger contact patch.

 

I tried this technique thanks to CSS and it definitely makes miracles. BUT is the second part of the sentence true? Can we really load the rear tyre more than the front because it has a larger patch OR do we just have to offload the front as it needs grip to counteract cornering forces and to also steer the bike vs the rear tyre only having to counteract the cornering forces?

 

I know many people say that you need to ride and ride and ride and not to look so much into theory. However I believe that understanding how things work can actually help you make better decisions when you need to. And maybe be less afraid of myths like "I can't ride faster because I do not have enough contact patch"...

 

Thanks and hang on - spring is knocking on the door :)

-Tony

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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)

 

If you are going to use the smaller front larger rear scenario shouldn't we take into consideration that the larger contact patch on the rear tire is on the sides of the tire. The weight bias is on the front and it might be true that the amount of front tire contacting the pavement under braking is actually larger than the rear tire under acceleration. Stop a motorcycle at a thirty degree lean angle and accelerate at a thirty degree lean angle and see what differences there are.

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I do honestly think contact patch does increase grip, however, so does a higher friction coefficient.

 

If you have a wider tire than you have more surface area supporting the same amount of weight. This means there is less pressure per square inch pushing down on that contact patch. If you have less pressure pushing the rubber into the pavement per square inch of contact patch than you have a lower friction coefficient. Which is excellent for tire wear.... however the contact patch vs. friction coefficient pretty much cancel each other out so you end up with more unsprung weight and no extra grip to show for it.

 

This would make perfect sense looking at the way our sportbike tires are shaped and sized front to back.

-The rear tire has more surface area which reduces the tire wear so it can handle the bikes power without ripping to pieces in no time. Most of the tire wear at the track happens on the side of the tire which also happens to be where the contact patch is the largest while the tire is under heavy load on the side of the tire. Having that extra surface area of the large rear tire is perfect for getting the rear tire to last longer compared to a smaller tire. Of course the extra weight of a big tire drains power from the engine and probably has quite a few other draw backs to the performance of the bike that I'm not really aware of.

-The front tire has no power going to it what so ever. It simply needs to slow the bike down and corner. Its also less likely to slide or slip compared to the rear tire. So it doesn't need to be as durable as the rear tire which makes it easy to get away with a smaller tire size. The smaller tire size reduces the unsprung weight (the worst kind of weight a race machine can have) and makes the steering more responsive. Since the contact patch vs. friction coefficient cancel each other out the small tire size has plenty of grip to support heavy braking/cornering forces.

 

I'm definitely no expert when it comes to this stuff so I can't say for sure but personally I'd think there would be a point of efficiency that the people making these race tires have already found. A larger contact patch on a hot race tire probably does give you slightly more grip but the extra weight would probably hinder your bikes performance more than the very slight amount of traction you gained with a bigger contact patch.

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If this is true I'd like to "challenge" the cornering guru Keith Code to share his opinion on the topic. As far as my memories don't play funny tricks with me I think Keith said in a movie of his that we have to roll on the throttle evenly smoothly and continuously throughout the turn so that we offload the front tyre *AND* make use of rear tyre's larger contact patch.

 

I tried this technique thanks to CSS and it definitely makes miracles. BUT is the second part of the sentence true? Can we really load the rear tyre more than the front because it has a larger patch OR do we just have to offload the front as it needs grip to counteract cornering forces and to also steer the bike vs the rear tyre only having to counteract the cornering forces?

 

Yes, I think that's still accurate to say that rolling on the throttle causes a transition that makes use of the rear tyre's larger contact patch. If you don't roll on, you won't have as much weight on the rear tyre, therefore it won't be as 'squashed' and won't have as large a contact patch on the ground. Right?

 

As discussed, there are a lot of factors that determine how a tyre will work. It's not just about the technical aspect that we've touched on here...

 

Have a think about this: how many times have you seen a motorcycle and rider (racing, or maybe just showing off) exiting a turn and they are still leaned over and the front wheel is off the ground? Compare that with the number of times you've seen someone braking into a corner and the rear wheel is off the ground... while the bike is leaning in? wink.gif

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Fascinating discussion here and one that is hard to wrap ones head around as it seems counterintuitive. I remember the same experiment in a college physics class and it always amazed me that surface area had no affect on the outcome. I think someone might have hit the nail squarely on the head when they suggested that a skinny tire and a fat tire with the same loading would work equally well initially but the skinny tire would soon experience melt down changing it's chemical properties along with it's coefficient of friction.

 

It would be interesting (and entertaining) to see a top fuel dragster equipped with both types of tires and data log the first few miliseconds of acceleration. It might turn out that skinny delivers the exact same acceleration characteristics for a split second then goes into early melt down mode and starts spinning wildly leading to further destruction. The fatter tire will absorb the huge power loading and live, hopefully to the end of the quarter mile. In the end, the fat tire has an advantage at the end of the track but not at the beginning. Only a sensitive gmeter could record such an event as it would occur over the briefest period of time.

 

So as I see it, the physics don't lie, there are just mitigating factors in the real world that come into play.

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Fascinating discussion here and one that is hard to wrap ones head around as it seems counterintuitive. I remember the same experiment in a college physics class and it always amazed me that surface area had no affect on the outcome. I think someone might have hit the nail squarely on the head when they suggested that a skinny tire and a fat tire with the same loading would work equally well initially but the skinny tire would soon experience melt down changing it's chemical properties along with it's coefficient of friction.

 

It would be interesting (and entertaining) to see a top fuel dragster equipped with both types of tires and data log the first few miliseconds of acceleration. It might turn out that skinny delivers the exact same acceleration characteristics for a split second then goes into early melt down mode and starts spinning wildly leading to further destruction. The fatter tire will absorb the huge power loading and live, hopefully to the end of the quarter mile. In the end, the fat tire has an advantage at the end of the track but not at the beginning. Only a sensitive gmeter could record such an event as it would occur over the briefest period of time.

 

So as I see it, the physics don't lie, there are just mitigating factors in the real world that come into play.

 

 

 

 

and IMHO the occasional tar snake/white line , a bigger contact patch on half the tar snake/line IMHO is going to be so much safer than said small tire where the whole grip surface can go onto said low friction coeffecieont surface and result in something nasty...

 

 

 

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It would be interesting (and entertaining) to see a top fuel dragster equipped with both types of tires and data log the first few miliseconds of acceleration. It might turn out that skinny delivers the exact same acceleration characteristics for a split second then goes into early melt down mode and starts spinning wildly leading to further destruction. The fatter tire will absorb the huge power loading and live, hopefully to the end of the quarter mile. In the end, the fat tire has an advantage at the end of the track but not at the beginning. Only a sensitive gmeter could record such an event as it would occur over the briefest period of time.

 

So as I see it, the physics don't lie, there are just mitigating factors in the real world that come into play.

 

Since you mentioned that it got me thinking about an aspect that the original article doesn't cover. Dragsters use those wide tires because they spray the drag strip with what ends up being an adhesive. The more surface area you have with an adhesive the better the surfaces stick to each other without a doubt.

 

So what about a hot race tire that's up to its full track temperature? The rubber pretty much turns into a soft adhesive that sticks to whatever it touches (like all of the rocks that stick to your tire while riding back to the pits or your finger if you press your thumb into it). If that is the case than a larger contact patch on a hot tire should give you more grip because its adhering to the track surface for the moment that its touching.

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It would be interesting (and entertaining) to see a top fuel dragster equipped with both types of tires...

 

Not quite in the same league, but you might find this interesting. A reasonably powerful car (480bhp) which usually has 255 section tyres, running on 125 section space savers.

 

 

edited to add: yes the tyres did eventually delaminate, check here for more discussion about that (skip ahead to around 2:40):

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Ok, I know this is an old thread but I only read it yesterday and I've been mulling it over in my (slow) brain.

Modern sportsbike tyres, especially soft track tyres, develop adhesive properties when hot......think of coming off a track session and running over some gravel. The gravel will get stuck to your tyre......The formula "Force (sliding) = Coefficient of friction x Force (normal)" doesn't apply......does it? You can have a zero or even a negative normal force and an object can remain stuck to something sticky....

I think contact patch is still king....the bigger the better.

I'm not a mathematician so please be gentle if I am wrong....hehe

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I remember this thread from long ago about friction. I've been going into tire grip and the elements affecting it for some time and had this recently pointed out to me: The laws of friction were cited with regard to tires. But there just one problem: those laws only apply to RIGID objects. Because a tire is not rigid, the law would not hold true.

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I've been trying to develop a better understanding of traction over the last couple of months as well. As such I've read a number of articles and technical papers about friction, and specifically, friction as it applies to the interaction between rubber and hard/asphalt surfaces.  

As I understand it, there are four types of friction: static, sliding, rolling and liquid. I may be wrong but I think the Amontons-Coulombs three laws of friction do apply to tires/rubber as it relates to "static" friction - tires may not be rigid but they are solids.  However as soon as you move to sliding and rolling friction (which are the only applications we really care about) the three laws need to be modified for rubber specific properties and what happens between rubber tires and asphalt friction as heat and tire deformation change. 

Attached below are a couple of articles that help explain rubber-hard surface dynamics that cause rubber related friction to act differently. 

After reading these articles (and several others) I am now thinking about the "coefficient of friction" between tire rubber and asphalt, at any given moment in time, as being determined by the following ...

  • For road surface … its material, texture, condition (including extent of surface oils/moisture present), temperature, gradient/camber.
  • For tires … their material, construction, condition (also including extent of oils present), energy dissipation and resulting temperature (a hugely significant factor), stiffness, extent of flex/deformation and how well the rubber fills asphalt surface gaps and creates microasperity contact points (so yes, contact patch size matters), and finally tire pressure (which has direct impact on extent and rate of tire temperature changes and flex/deformation).  
  • Pressure or load between the tires and road surface … from both vehicle weight and riding forces (ergo cornering forces)

The list may be missing a few things, but I think it hits the major elements. 

And since cornering force has such high impact, I think about the extent of "cornering force" being driven by a number of factors as well, including ...

  • Weight
  • Vehicle balance (shifting of weight, pitch and angle)
  • Kinetic energy (speed)
  • Corner radius and extent / rate of radius change (e.g. closing versus opening radius corner)
  • Centrifugal force (directional inertia)
  • Centripetal force (force acting on a moving body at an angle to the direction of motion, tending to make the body follow a curved path)
  • Rolling resistance (from braking and accelerating forces)

 I'm still learning on all this (traction / friction) so feel free to correct me where I am off base. 

Dave 

Guidance-Rubber-Friction.pdf Rubber_Friction_and_Tire_Dynamics_A_Comparison_of_Theory_with_Experimental_Data.pdf

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Your collection of data and research shows you are barking up the right trees. Here's some more data regarding tires:

Per the Dunlop engineers tires grip in 4 ways:

1) Adhesion--the temporary chemical bond between the tire and surface.

2) Keying--the tire deforming and filling in all the nooks and crannies of the asphalt or squishing into the depressions.

3) Abrasion--the tire tearing from itself or wearing away.

4) Hysteresis--the energy storage and return by the rubber and partial conversion to heat.

The first two can be looked at as static properties and the last two dynamic properties in my opinion. I'm still learning on all this stuff and when talking to the tire engineers, they don't have all the answers either. Heck, aviation engineers still can't all agree on exactly how a plane flies through the air!

  • Thanks 3

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If this has already been stated I apologize -- but I want to go back to the 'tug of war' example. Let's make it a bit more of a "water ski" example where you're being drug along on a rope and resisting by putting your heels into the ground. In isolation it shouldn't matter if one or two heels are dug in, however, if something in the scenario happens where the grip of one heel is jostled loose then you immediately face plant. With both feet any scenario on one can have the 'load' taken up by the other during recovery. In a similar way, a larger contact patch allows the friction to be applied in more places should there be any disruption in another?

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13 hours ago, Dylan Code said:

Per the Dunlop engineers tires grip in 4 ways:

1) Adhesion--the temporary chemical bond between the tire and surface.

2) Keying--the tire deforming and filling in all the nooks and crannies of the asphalt or squishing into the depressions.

3) Abrasion--the tire tearing from itself or wearing away.

4) Hysteresis--the energy storage and return by the rubber and partial conversion to heat.

Thanks, Dylan. I've read about these dynamics in various articles but have never seen them summarized in such a succinct manner.  Very helpful indeed.  I would add I think 'keying' may also be a dynamic property (in addition to static) given the extent to which keying is constantly changing as tire energy dissipation, surface and interior heat, abrasion / shearing stress, and load change.

For those who may be interested in the engineering details, one of the websites I found helpful is multiscaleconsulting.com. In their publications section are various papers on rubber friction and contact mechanics. A warning - their papers are filled with lengthy algebraic formulas supporting their analysis. I try to just focus on their key points and not get lost in all the math.  

I think the next evolution of this for me will be to learn how to read a tire so I understand what is happening to it and my traction/friction equation.

Dave 

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9 hours ago, yakaru said:

In a similar way, a larger contact patch allows the friction to be applied in more places should there be any disruption in another?

yakaru, I'm not sure it is as much a question of friction being applied in more places (unless more 'keying' is happening) as it is the extent to which the contact patch 'coefficient of friction', at that particular moment in time (so factoring in the portion of the contact patch that has been disrupted) has not reached the point of 'slip'.  In some of the articles I've read the point of slip is expressed algebraically and can be calculated (theoretically) if all the other variables in the equation are known. 

Dave

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On 2/12/2020 at 7:06 AM, CoffeeFirst said:

yakaru, I'm not sure it is as much a question of friction being applied in more places (unless more 'keying' is happening) as it is the extent to which the contact patch 'coefficient of friction', at that particular moment in time (so factoring in the portion of the contact patch that has been disrupted) has not reached the point of 'slip'.  In some of the articles I've read the point of slip is expressed algebraically and can be calculated (theoretically) if all the other variables in the equation are known. 

Dave

Sorry that's sort of what I meant -- if I'm on the 'edge of a penny' for contact patch then to lose traction I only need that edge of a penny to hit something without friction (and the keying/adhesion etc); with a larger contact patch then you have more of an area which would need to be disrupted in order for the maximum grip to be reduced.

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