My understanding is that lowering the tire pressure raises the tire temperature and makes the tire compound sticky, higher coefficient of friction.

An excellent article by a tire engineer said that track and road tires are completely different animals. Track tires have a stiffer carcass esand are designed to run low pressure to keep heat in the tires. He recommended not dropping street tires too low or the tire loses stiffness and can affect handling adversely. I've played with tire pressures on the street and prefer recommended pressures. The only time I've experienced tire slip was on cold tires and cold asphalt, around 40 degrees F.

Ethin, not trying to start a pissin contest bro, if something works for you, keep doing it!

t.Could it be....the now lowered rear tire pressure is acting more like a shock absorber than a fully dedicated rolling tool?

Could be. I softened my rear suspension because it was flying off of bumps. Maybe I should stiffen it back up and stiffen the front as much as it will go. If I can handle that on the street, it would seem to ba about as good as it can be for the track.

sometimes when things work you tend not to care about why and just run with what you know.

on the street I run tyres up hard to protect the rims and you seem to get better wear and good grip

for some reason a performance tyre has always had the link between lower pressures and grip this may well be more of a bad habit than science.

Its all good Kruz, can remember a Physics debate on a forum about a ship at anchor, I was wrong about that one two
silly me thought the anchor held the ship, a Physics Professor in a light hearted discussion explained what was really happening.

a great read just amazing

Let me guess...it's the chain that's holding the ship?(and I swear...I honestly have not Googled for the answer).

* Last updated by: Grn14 on 4/17/2012 @ 11:05 PM *

Grn14, you go to the top of the class, "BUT"

Its more than that , its about the hole anchoring system.

The anchor should only be used to remove the chain from the chain locker, not sure now if he was a Physics or Engineer guy but its about pressure over area, energy, friction and heat and the radius of the chain at the bottom as it goes vertical.

I was off in the land off experience, and what I saw with my eyes, bit like bike tyres, only to realise I was wrong

will find a link if interested

I was wrong about that one two
silly me thought the anchor held the ship, a Physics Professor in a light hearted discussion explained what was really happening.

a great read just amazing

OK, you have my interest piqued...what did the Physics Prof say about the anchor?

I found that article by the Motorcycle Products manager at Avon tires, this is what he had to say about dropping tire pressure too far on street/sport tires when doing track days. He is not talking about race tires here:

But the biggest area for debate has to be track days. If you've ever been to one it's almost certain someone has told you you'll be best off reducing your tyre pressures. You get more grip that way, they tell you. Smith has radically different advice. You should leave them alone, he says. Racing tyres are of a totally different construction and stiffness to road tyres so they need less pressure to maintain the carcass shape. That's where the rumours and bad advice comes from. If you drop the psi in road tyres you will get more movement in the tread pattern. They will heat up too much and that will eat into tyre wear. You'll almost certainly ruin a set in a day without gaining any advantage in grip. " Smith says he's known people to drop their rear tyre to just 22psi when heading for the track. His advice is to leave your tyres alone, saying a good tyre at standard pressures will give more grip than you need on a track day because you almost certainly won't be going as fast or for as long as racers. Track surfaces offer much better grip than the road, too - another reason for leaving your tyre pressures the same for the ride to the track as for the ride around it.

Its all good Kruz, can remember a Physics debate on a forum about a ship at anchor, I was wrong about that one two

Not so sure you're wrong on the tire debate yet Ethin. After more research on the subject I am finding some validty to the contact patch claim as some sources are saying rubber shear accounts for additional tractive forces. That is the idea that to shear or tear away rubber at the road/tire interface requires work and work is force applied through a distance..... or traction. This effect could be dependent upon contact area.

There is also some credible evidence that the coefficient of friction of rubber may be different than other materials in that it varies with pressure. The gradient is negative, i.e. increasing pressure decreases friction coefficient. If true, then an increase in contact patch area would decrease pressure on the patch, and could increase the friction coefficient and deliver more grip. How great this effect is, I don't know.

I am willing to concede that contact patch may play a role in ultimate grip, I'm just not sure how great a role. More research is needed.

Comments from Steve Brubaker at Dunlop on tire partch/traction issue.

Dunlop Answer Man

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 not 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.

Steve Brubaker
Dunlop/Race Tire Service Inc.
US National Distributor for Dunlop racing tires

615-641-3323 work
www.dunlopracing.com
Facebook: http://www.facebook.com/dunlopracing

...and a response from Steve Munden:

Posted 08 January 2012 - 01:50 PM

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

I'm still researching and the debate rages on, however I do have a good observation that is empirical in nature. Not trying to stir more debate here but just trying to get folks thinking outside the box.

We all know that liter bikes get 190 profile tires on the rear and and 600s get 180s. If a 190 has a larger contact patch than a 180 and grip is determined by contact patch, why not stick a 190 on the 600 and corner like a cat on velcro?

On the other hand, if the contact patch is relatively unimportant with respect to traction but very important with respect to wear rates, it makes sense to outfit the machines with higher torque and power output with a larger tire and save weight and rotational inertia on the 600s that don't need it.

Just thinking out loud here.

We all know that liter bikes get 190 profile tires on the rear

Mine has a 200/60... And I run 29psi front and 24psi rear (hot)... Why?... Because that what I was told is best by an expert. Works for me.

Kruz...you don't want to be running lower pressures on the street.The dynamics of the road(s)make trying to get your street tires to behave like a track tire counterproductive and unsafe.Lowering pressure in a street tire to mimic the behaviour of a hot track tire is not gonna work.TWO different animals.YOUR street tires are designed for the weight and characteristics of your street bike.If you want really good grip...go with the 003 Bridgestones.They're street tires but they have a good amount of track-like composition in em.They will grip like a Gorilla.And you run them at street psi...not track psi.I had really good results using those...as per Rook.They don't last however if you are rolling at triple digits...I think I got 2600 miles from my last one.But they held excellently.Leaning in was never a sketchy feeling.That was on my 07.

* Last updated by: Grn14 on 4/18/2012 @ 12:25 PM *

I run 38 front and 42 rear on my Q2's and have never had traction issue's.

The whole hanging off debate is a mute point. Regardless of what the math shows it works, I know from experiencing it first hand. As far as lean angles those are dependant on the tire your running.

There's a reason you don't see the normal street and trackday tires on moto gp and ama bikes.

My .02 LOL

* Last updated by: heathun on 4/18/2012 @ 2:28 PM *

that's a good read, when you think about it the path the tyre takes around the track is a constant variable always changing, your grip would also be constantly changing

track surface, texture changes then load on the tyre changes ,such as going through a dip the contact patch would also be forever changing add lean angles, power input changes, rider position ,braking and so on.

Its a fascinating thing a mixture of beauty and science

Anchors
By Patrick Lockerby

Love to here his view on bike tyres

http://www.science20.com/chatter_box/why_anchors_dont_work

From earlier

I don't follow the last bit

Now imagine a couple of mountain ranges on plates. Turn one over and put it onto the other. The actual contacts will be parts of the various peaks of the two plates onto whatever part of the other plate happens to match the peak. Those peaks — asperities, the tribologists call em — are where the friction occurs, not, obviously, in the valleys where there is no contact. Press em harder and you'd get more actual contact as the asperities bend under the pressure.

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.

the actual contact area of asperities wouldn't change

Wouldn't you have twice as many asperities if the plates where twice as big friction being the same
also if the number of asperities were the same and the plates were twice as big it would be more pressure over same area

Know doubt the way I read it is funky, just odd

* Last updated by: ethin14 on 4/18/2012 @ 4:11 PM *

That's what thought but I guess not. I'm sure se asperities are bigger than others. Doble the surface area and you doublr the largest asperities.. The small ones wouldn't be able to touch because they were all held away by the larger ones.

"Wouldn't you have twice as many asperities if the plates where twice as big friction being the same
also if the number of asperities were the same and the plates were twice as big it would be more pressure over same area"

No...you wouldn't have twice as many.It said...DOUBLE the SIZE...not the amount of asperities.Your mountain peaks are twice as big...your asperities are twice as big...but not more numerous. 'Double the size'only makes the plates BIGGER...it does NOT add asperities.

You now need to push HARDER to get the same results as you did at the smaller size.The plates being twice as stout now.(I think that's what he's meaning?)

"plates of twice the area"....he's saying the same plates..only twice as big(wide,long)....it does seem like he's saying expand the total area bigger...to include MORE peaks and all...but that's not what he's meaning.He kinda dropped the ball on that explanation.It's not clear to most what he's saying.Or should I say...to a more critical mind...your guy's way of interpreting it would be totally accurate.He did say"twice the surface area"...which is misleading.

Now...how does this apply to tires at the track or street...and Heathun's right...Straight stock tire pressures MAY have a bit of 'tolerance' in lowering and such...may work better depending.But not to track tire pressures...if they're running 22 or something.

* Last updated by: Grn14 on 4/18/2012 @ 11:02 PM *

I'm sure smee asperities are bigger than others. Double the surface area and you double the largest asperities..

How many aspertames could you fit on the tip of an asperite? How many asteroids could you stuff into a aperature? All this stuff is making my head hurt, I think I'm just going to go ride my bike...lol!

I was pronouncing that word, A-`spare-i-tee like, austerity.
Were you saying, `a-sper-ite?

The whole hanging off debate is a mute point. Regardless of what the math shows it works

The math shows it works also, the only point was that the effect is not as great as most would believe. As Grn observed, hang off your bike going down the road and correct with your steering, that's all your going to get, a couple degrees at most. It's not magic.

How many asteroids could you stuff into a aperature?

Speaking of asteroids, my Uncle Buck had those real bad. He was a truck driver and said they'd swell up and itch like the dickins. Sorry, this has nothing to do with asperites...lol!

Grn said:

Kruz...you don't want to be running lower pressures on the street.The dynamics of the road(s)make trying to get your street tires to behave like a track tire counterproductive and unsafe.Lowering pressure in a street tire to mimic the behaviour of a hot track tire is not gonna work.TWO different animals.

Kruz said in an earlier post:

An excellent article by a tire engineer said that track and road tires are completely different animals. Track tires have a stiffer carcass and are designed to run low pressure to keep heat in the tires. He recommended not dropping street tires too low or the tire loses stiffness and can affect handling adversely. I've played with tire pressures on the street and prefer recommended pressures. The only time I've experienced tire slip was on cold tires and cold asphalt, around 40 degrees F.

Grn, correct me if I'm wrong but isn't that exactly what I said a few posts before?

You know....I was just shootin from the hip with my beliefs about tires there...I read yer posts..but it really didn't like sink in when I did...so...apparently...ya...you did..or he did...somebody did .Certainly wasn't tryin to paraphrase or jack what you said...not even.Guess if we BOTH agree on that deal...then there must be something to it

My brain donesn't work so good sometimes.I can read something...or whatever,and completely 'forget' I even read it.Especially if I'm kinda skimming the statements maybe 'looking' for something else in the topic.I can be reading one thing,and thinking totally something different while I'm doing that...sometimes.

* Last updated by: Grn14 on 4/19/2012 @ 12:03 PM *