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Old 06-18-2001, 03:28 PM   #1
rao
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Lightbulb Interesting Tire Size/Contact Patch Size/Shape Info - aka why I don't have wide tires

Check this article out:

http://www.autospeed.com/A_0996/page1.html

I have been saying this for years, but it usually ends up in a big argument , but at least now someone agrees with me.

Have fun.
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Last edited by rao; 06-18-2001 at 04:38 PM.
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Old 06-18-2001, 04:12 PM   #2
Zahnster
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Default Did you read the same artical I did?

Perhaps I missed something here, but this article seem to be saying the larger and wider the wheel the better. The larger wheel will have lower sidewalls. The wider wheel will have less heat generation.

So, looking at our cars(or the WRX specifically), 18" wheels seems to be the biggest wheel size people are putting on the car. 7.5" wide seems to be the key dimension(funny how some of the best wheels out there,Volks, come in 18x7.5 for our car). Then you want to put 225 or perhaps 235 tires on them with a 35 sidewall(if your planning on your odo being close to the same reading).
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Old 06-18-2001, 04:16 PM   #3
jjameson
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Good article. No big argument, but to quote the article:

"On the issue of wheel size (the diameter, not the width), it is therefore clear that increasing the wheel/tyre diameter combination is beneficial. The reason for this is that the tyre will not have to deform so much to get the required contact patch length, and the percentage of the tyre tread in contact with the road will be less than for a smaller diameter combination."

So I don't quite understand how this supports "why I don't have 18" wheels?"
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Old 06-18-2001, 05:55 PM   #4
rao
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Ok, so I should have said that is why I don't favor wider tires.
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Old 06-18-2001, 06:00 PM   #5
rao
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Ok, so I should have said that is why I don't favor wider tires.
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Old 06-19-2001, 06:03 AM   #6
Chaincat
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I don't know if I entirely agree with that article. First, my physics professor at college and a couple of books on different forms of racing have stated that wider tires are better. We all know about the friction formula (Ff=Cf*Fv) with the friction coefficient being constant so it doesn't make sense initially.

The reason those guys think that a wider tire is better is that it interfaces with the pavement better. Pavement is generally an irregular surface and in theory the tire's tread/surface has more oppurtunity to react to the pavement resulting in a higher friction coef than just the coef of the material itself. Think of it this way. It's generally accepted that thinner tires have less rolling resistance (which means less friction losses for better efficiency). Which is why high gas mileage cars have thinner tires than most cars. (Think of those solar cars with tires that are an inch wide, granted aero is probably a more signifigant reason for thin tires for those cars. I'm sure low cost is also a factor, you don't see many ultra fuel efficient Lexus' these days) Anyways, if a thinner tire has less rolling resistance than a wide one I'd be willing to wager that the degree of interaction with the pavement (with wider tires theoretically having more) is a signifigant part of that.

As far as the guy saying that wider tires can be made of softer rubber because they heat less I'm not sure I agree there either. For instance, according to the book "Inside Racing Technology" it has charts (p68, 3rd Edition) that shows temps for dry and rain tires vs lateral force in Gs. It shows that the optimal operating temp for dry slick cornering is about 210-220 degrees F. Rain tires generate peak forces at about a 170 F operating temp. The tires in question are for pro racing (stockcar, CART, ect) and they are obviously wider than any tire that we're going to put on our cars. But our cars have much narrower tires and if the physics were like the author of the article stated then I don't know why we're not all melting the tread off our tires. (Granted race tires have much more load on them more of the time but even when I take my DMS SP8000 equipped Scoob to the track the tires only get warm to the touch after hard lapping on a short technical course) Anyways, I think tires with soft rubber will operate at higher temps than tires of hard rubber. More interaction = more friction = more heat. I haven't put slicks on my car but I'd bet someone out there has and I would bet that they will testify that slicks run much hotter even though they are only increasing the cornering forces marginally. (.1 or .2 for probably a 80 or 90 degree temp increase. If someone has some tire temps for slicks at a track that would be a good thing to post)

Also, I don't think wider tires are made of softer rubber because they heat less. The aforementioned race tires run at boiling temps yet you can make an impression on them with your thumb nail. I think the reason they are made of softer rubber is because that fulfils the requirements of the tire. Softer rubber has a higher Cf than hard rubber. It's more maleable and conforms to the irregularities better (see a pattern here?) but the problem is it wears a lot faster too. Why do most people want wide tires? For better performance. The trade off is that the tires wear faster and have to be replaced more often, usually at more expense. I'd be willing to bet that a 25 buck tire on a geo metro is going to last longer than a bigger and wider 250 buck high performance tire that is 'running cooler' according to our author.

Anyways, if I had that animated BS flag I'd be raising it right now. To make it even more annoying the author totally brushes over the point that wider tires generally have a lower aspect ratio that effects the overall performance of the tire which is theoretically the point. I admit, all of this is theory to me. I don't optimize tires for Formula 1 teams but I do think that a lot of what this guy is saying doesn't jive with a lot of smart people.

-Brian
Sorry for the long post.

P.S. I'm not saying that the guy is wrong about wider tires running cooler I guess. My point is that writing an article about wide tires running cooler is totally missing the point of wide tires: to help cars corner better. I also think a lot of his points are very flawed.

Last edited by Chaincat; 06-19-2001 at 06:07 AM.
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Old 06-19-2001, 04:08 PM   #7
inpreza kid
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i went with a set of 17" rims over 18" mostly because i don't want to dent up my rims because of the rough roads. with 18" rims your going to be running atleast a 35 series tire. might be good for track but on rough roads is where it isn't going to be so good.
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