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
2021 Aprilia RSV4
2020 BMW S1000RR
2016 ZX-10R KRT
2016 959 Panigale Red
2015 CBR1000RR SP Repsol
2011 ZX-10R Ebony
2009 ZX-6R Lime Green
2006 ZX-14 Red
2004 VTX 1300C Candy Red
"For we walk by faith and not by sight" II Corinthians 5:7