blueovalz Posted October 2, 2001 Share Posted October 2, 2001 I've worked all night, so I thought I'd throw a question out to all you rocket scientists out there. A tire is traveling down the track on a car at, say, 200 mph. Through space, what forces are working on the tire? Being the patch of rubber that is in effect "at rest" while in contact with the asphalt, is then quickly accelerated forward to about twice the speed of the axle at the top of the tire (400 mph), and then back to rest at the bottom again. Does this mean the tire growth due to centripetal force is greatest over the top of a tire on an axle moving forward vs an axle at rest? Does this also mean I need much sleep when I get off work this morning and quit asking absurd questions? Quote Link to comment Share on other sites More sharing options...
Guest Aaron Posted October 2, 2001 Share Posted October 2, 2001 You are thinking of the velocity of the edge of the tire parallel to the ground. Centripetal force is directed out from the center of a rotating object and is a function of the rpm and diameter of the object. I would have to find my physics book to give you the exact equation for calculating it. Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted October 2, 2001 Share Posted October 2, 2001 Hmmm, 400 mph, Your not putting a jet engine in your car are you? Regards, Lone PS: Sorry, couldn't resist.... Quote Link to comment Share on other sites More sharing options...
johnc Posted October 2, 2001 Share Posted October 2, 2001 You need more sleep. Your point of refernce for the term "rest" is wrong. A rotating tire is never at "rest." The tire is rotating around an axle at a certain rpm and contact with the road is incidental to that rotation. No part of the tire is ever at rest. In fact, the greatest stress point on a tire is the contact patch. Read Chapter 2, Tire Behavior in "Race Car Vehicle Dynamics" by the Millikens. Incindentally, this book will help you sleep... Quote Link to comment Share on other sites More sharing options...
blueovalz Posted October 2, 2001 Author Share Posted October 2, 2001 Well, after sleeping on it, I decided that since the tire patch in contact with the ground is "at rest" or not moving, but the axle is still moving in relation to it, much as a satellite or moon revolves around a moving planet, so in relation to each other, centripetal force may not change after all. But this does not address the accelertion/deceleraton that the mass or point on the tire tread goes through in a single rotation (at rest, double the axle speed, then at rest with each rotation) in a viewpoint of space (assuming the earth itself is not moving through space itself). Much like the carnival rides do (or a picture of a tire at high speed with no panning). I wonder what kind of shear forces or cycles the compound goes through at high speed (high repetition rate) Quote Link to comment Share on other sites More sharing options...
blueovalz Posted October 2, 2001 Author Share Posted October 2, 2001 let me play the advocate here. If the tire is in contact with the ground and not sliding , and the ground is not moving, the the tire patch in contact with the ground would not be moving. Yes, the axle is moving forward, but at that instant the tire is in contact with the ground, that patch is not moving . This is why a turned tire makes a car go in circles. Each part of the rotating tire bites or sticks as the next or following parts advances the car around the curve and pull the car sideways more. No grip, no turn, as the compound flexes each time. I'm not sure of the term used for the process, but a steel tire would not turn a car because it cannot have that minute flexing needed as the tire "advances" the car around a corner. Quote Link to comment Share on other sites More sharing options...
pparaska Posted October 2, 2001 Share Posted October 2, 2001 Isn't the real issue with stress in the tire the fact that sidewall and tread belts/rubber are constantly (and quickly flexing? Quote Link to comment Share on other sites More sharing options...
DavyZ Posted October 2, 2001 Share Posted October 2, 2001 Ummm, yep, you are right--see? I told you I was no engineer Davy Quote Link to comment Share on other sites More sharing options...
Drax240z Posted October 2, 2001 Share Posted October 2, 2001 Time for my Kevin Shasteen impression. Terry, your theory is sound. What occurs at the point of contact is called an 'instantaneos center of zero velocity'. Basically the rest of the tire is basically pivoting around the I.C. at any given time. (of course, the I.C. is constantly moving as the tire rotates as well) Now the velocity of the center of the tire is for our discussion, the same as the velocity of the vehicle, in this case 200mph. w=v/r : rotational velocity=linear velocity/radius. w is constant everywhere on the non-accelerating wheel, while v is different depending on how far from the I.C. the point in question is. Just for $#!+$ and giggles, if we assume a 25" tall tire, the tire sees a rotational velocity of 290 rad/s. (forgive me for not showing my work) Or 46 rev/s. Thats a cool 2760rpm. Back to Einstein. The stationary observer will see you and your car moving at 200mph. They will see the top of the wheel (directly across from the I.C.) travelling at 400mph. You (in the car) will see the top of the wheel moving at 200mph, and the stationary observer moving at 200mph. As far as centripital acceleration goes, its dependant on the distance from the center of rotation. This is because its not instantaneous, but sustained. (remember the last example is only for in INSTANTANEOUS center, and that center has a new position around the circumference of the tire for every instant) In this case, you must consider that the wheel is rotating around its center. The wheel will grow uniformly in all directions, even at the bottom. (granted the weight of the car will slightly affect the shape and growth) Hope that helps visualize. I do highly recommend the book Johnc mentioned. I don't know what he means about it being boring though! (engineers are funny that way) Quote Link to comment Share on other sites More sharing options...
Modern Motorsports Ltd Posted October 2, 2001 Share Posted October 2, 2001 quote: Originally posted by Drax240z: Time for my Kevin Shasteen impression. <snip of coffee and toast discussion> I do highly recommend the book Johnc mentioned. I don't know what he means about it being boring though! (engineers are funny that way) BWAHAHAA, ROTFLMAO, sorry, couldn't help it..struck a funny bone here, another list I'm on has the AEOTY award.... Quote Link to comment Share on other sites More sharing options...
pparaska Posted October 2, 2001 Share Posted October 2, 2001 quote: Originally posted by Ross C: BWAHAHAA, ROTFLMAO, sorry, couldn't help it..struck a funny bone here, another list I'm on has the AEOTY award.... Hey, I resemble that remark! Nice work Einstein, oops, I mean Drax . BTW, isn't centripital force a misnomer? What would happen if the observer were a fly travelling along with outer radius as it rolled? I guess it might see the tire as stationary, and then the road come up and smack it flat What about those 9 or ten rolled up dimensions? J/K. You guys crack me up. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 2, 2001 Share Posted October 2, 2001 > If the tire is in contact with the ground > and not sliding , and the ground is not > moving, the the tire patch in contact with > the ground would not be moving. Yes, the > axle is moving forward, but at that > instant the tire is in contact with the > ground, that patch is not moving. I think you're trying to compare apples and oranges. Is your point of refence the tire/road interface (called the contact patch or footprint) or the whole tire? I'm assuming you're talking about the contact patch. The tire is rotating around the axle shaft at a set rpm. The tread of the tire is always moving at a velocity determined by rpm and tire diameter. Given that: 1. The contact patch as a whole is also constantly moving because the tread surface and the sidewalls are constantly flexing. 2. 99% of the elastic forces (stresses) are built up and released through the contact patch area. 3. Longitudinal shear forces are built up in the contact patch due the radius reduction when the tread enters the contact patch area. 4. There is a reduction (compared to the tire tread velocity outside of the contact patch area) in the longitudinal velocity of the tread at the beginning of contact patch, but there is also an increase (compared to the tire tread velocity outside of the contact patch area) in tread velocity toward the end of the contact patch. In essence, the rear of the contact patch is always sliding. > This is why a turned tire makes a car go > in circles. Each part of the rotating tire > bites or sticks as the next or following > parts advances the car around the curve > and pull the car sideways more. No grip, > no turn, as the compound flexes each time. > I'm not sure of the term used for the > process, but a steel tire would not turn a > car because it cannot have that minute > flexing needed as the tire "advances" the > car around a corner. What you are describing are slip angles and yes, they are a significant reason why a tire can turn a vehicle. But they are not the only reason. Steel wheels work fine on asphalt/concrete at their lower traction limits. Steam rollers and guys out-running cops on just the rims are two examples. Solid rubber tires are another (remember your little red wagon?) Tires are very complex things. Much more complex than shocks, springs, engines, etc. There is no such thing as a general theory of tire behavior. You have lots of independent theories about one aspect (lateral force, longitudinal force, camber affects, aligning torque, slip angles, pressure, temperature, footprint, etc.) but no all encompassing algorithm. Quote Link to comment Share on other sites More sharing options...
blueovalz Posted October 3, 2001 Author Share Posted October 3, 2001 Thanks to all for the invigorating scholastic banter for which only this group of folks would either become involved in or care. Tonight, my employer will no doubt be happy that my mind is on my work and not cars (again). Quote Link to comment Share on other sites More sharing options...
DavyZ Posted October 3, 2001 Share Posted October 3, 2001 I drove myself battyfor thinking about this a few years ago. First you are correct in saying that relative to the ground, the tire patch is "at rest" while the top of the tire is travelling 400 mph relative to the ground. Incidentally, it goes to follow that the sides of the tire are travelling only 200 mph relative to the ground. Now, the other point you mentioned is mutually exclusive (not inclusive) with respect to the centripital (sp?) forces put on the tire (i.e. the outward forces due to rotation and diameter). In the real world, there is friction on the street, which creates heat, which can break down the rubber, which can heat the air inside the tires and raise tire pressure, etc. BTW, what is the point of all this? Are you trying to make me lose sleep thinking about this??? I'm no engineer! Aaaack! Good luck, Terry, and be sure to pop a couple of Sominex next time... Davy Quote Link to comment Share on other sites More sharing options...
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