Mark Collett
12-10-1998, 04:37 PM
Okay. Where were we? Here's a copy of your last reply...<p><br>>It was my understanding that the tire compound had mostly to do with speed ratings. So, heat >is actually the main factor? The role of the belts are to keep the tire from flexing. Therefor, >generating less heat...<br>><br>>I'm still missing something. I want my tires hot. I want them sticky, to grip the road. I also >want them somewhat flexible, where more tire is in contact with the road during a high-speed >turn. I seems to me, with the role of the belts as you explained, are defeating the pourpose of >what I want my sport tires to do??? <p>Okay... first I must admit that I don't know everything. I sure wish I did, but it's impossible to learn everything after having been with Michelin for only 6 months. Here is my limited understanding... <p> You're right, belts have to do all of that, and more. It's complicated! There are belts to help the tire maintain it's shape at high speed (consistent contact patch) plus there are belts to support the tire during high loads (cornering speed) plus more that I'm just unsure about.<br> Here's where I'm making an educated guess based on what I've learned. To keep the tires sticky, there must be heat - but too much heat will grease a tire. A greasy tire is one that has so much heat applied that the rubber compound goes from a grippy substance to a slippery one. I would equate this to a reverse curing process - from semi-solid to semi-liquid instead of the the opposite. <br> So how do we keep heat in the tire, but not too much? It's a relationship between the belt construction and compounding. If a tire flexes too much, too much heat builds up and greases the compound. To keep the tire "as it should" the belts try to limit the flex to only what is needed for a good contact patch - or a consistent one, throughout acceleration and lateral forces. <br> Now we've got a tire that hypothetically rolls as it should. Now it's the compound's responsibility to warm up quickly and stay at optimum temperature. That's what the chemists do. An added note, if you have belts that help keep a consistent contact patch, you can use "harder" compounds for extra life while still maintaining high levels of traction. <br> So... here's an example... the tires are cold on your car, you get it out on the track and "warm them up" with a quick lap. During the lap the tire has built up heat through it's own "hysteresis" (internal friction). Now I believe, once you get out there and start racing, the belts try to dissipate heat and the rubber tries to stay at optimum temperature. <br> Here's where my knowledge runs out. I know that one of the reasons the belts are there are to maintain tire shape and therefore reduce internal friction and heat buildup. But the rubber needs heat to work, but not too much - so it's made to get to a certain temperature and then stay there. I guess if the belts do not do their job well enough, the rubber is heat cycled too high and therefore greases the tire. I've seen this alot on motorcycle tires that couldn't take it.<br>(Motorcycles are where I get most of this info, but I'm sure it's the same for cars). There's more problems too, like we want the tire to absorb bumps, but still transmit major forces (cornering) to the suspension system. So we not only want damping but a spring force as well. I think I'm going crazy just thinking about it!!! <br> It's all a really complicated relationship between steel and nylon cords, plus probably dozens of rubber compounds in the tire. There's not only the tread rubber, but sidewall, bead, internal, etc. etc. <br> I hope this helps, and I hope I know more someday!<p>Mark<p> <p><br>