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Steve Munden

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Everything posted by Steve Munden

  1. >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
  2. >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. 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?! 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
  3. 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.
  4. 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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