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Bald tires do not mean they are more "like" slicks except maybe in appearance.
Bald tires mean worn tires that have a lot of heat cycles on them or are old. Used, bald tires that presumably have multiple heat cycles on them and are old (age) result in a tire that is harder than the new tire was therefore making it incapable of providing the same traction than the new tire did despite their drag slick-like appearance. Further, the lack of tread creates a dangerous tire that will not be suited for inclement weather or, especially, wet roads.
This hardening is also why some tires get noiser as they wear; because they are getting harder. There are also recommendations for replacing tires after they attain a certain age, regardless if they have ever been installed/used or not and despite remaining tread level, in order to minimize the adverse affect this phenomena has on the overall traction of a vehicle. How rapidly this change occurs is dependent on the type of rubber that is used in the manufacturing process and will therefore vary from tire to tire (manufacturer/type).
If they aren't R compound tires, slicks, etc., tires with the tread worn off are simply giving traction up, not helping it at all.
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim
8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
Last edited by josiahg52; 27th May 2012 at 09:57 AM.