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Old 01-06-2006, 03:05 PM   #17
ripvw
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Member#: 27790
Join Date: Oct 2002
Chapter/Region: E. Canada
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contact patch area is determined by pressure, wheel rim diameter, tread width and section height for a given load. the static contact patch area is (virtually) the same for the following sizes:

205/55x16
225/45x17
225/40x18

what changes as you lower the profiile of the tire, reducing it's section height and increasing it's tread width, is that you increase the width of the contact patch and shorten it's length. A shorter, wider contact patch will provide better dry cornering capability, while a longer, narrower contact patch can be better for resistance to aquaplaning and dealing with snow.

the contact patch shape becomes wider - the size (area) remains the same

a shorter, wider contact patch at the same slip angle begins to slip at the same distance from the tire's leading edge as a tire with a longer, narrower contact patch. However, the wider contact patch has more of its length in contact with the road than the narrower contact patch and therefore a larger portion of the overall contact patch area is creating grip. For the same load and slip angle, a wider contact patch generates more grip in dry road conditions.

of course, when you are actually using the tire as intended, the dynamic contact patch of the tire constantly changes shape depending upon the load forces acting on the tire. once you choose your tire the section height, tread width, and tire diameter become fixed. increasing the width of the wheel by .5" will increase the tire's section width - and by implication tread width - by .2" resulting in a wider contact patch. the remaining variable - tire pressure - becomes crucial in determing contact patch shape and therefore the performance of the tire.

for the track, research done by Jim Hall of Chapparal Can-Am racing fame along with Firestone led to a doubling in width of race tires in the mid-60's. Every time Firestone sent Hall a wider tire, he and his team tested them on consecutively wider wheels, going from 15x7 to 15x9. Each time the increase in tire width and wheel width led to greater cornering forces during skidpad testing. Then Firestone would create a new tire designed for the 9" wheel and Hall would increase wheel width again with the same results. The difference in grip was enough to force Hall to redesign his transmission to handle the greater loads. the end result was an increase in Can-Am race tire width from 6" to 12" in less than 18 months and the acceptance of the fact that a wider contact patch translates into better dry cornering and braking.

as far as street testing goes, in January of '04 Consumer Reports of all people tested plus-sized tire and wheel combinations and found that the wider wheels and tires improved wet/dry handling, emergency handling, wet/dry braking and amazingly enough ice handling! comfort, noise, snow and hydroplaning resistance all decreased in performance. in typical Consumer Reports fashion, the specific tires and the raw test data is left up to the imagination of the reader. The differences were not great in terms of percentage - less than 10% improvement - all we have to go on thanks to their testing methods. However, had they broken out dry handling and dry braking results from the wet the performance differences may have been more dramatic. the fact that the car will handle better in emergency situations should be enough reason for street applications. isn't that the real reason we buy better tires in the first place - to save our lives when the unexpected happens?

Want a meaningful comparison between performance tires and all season, wide vs. narrow, newer Subaru technology vs. older? According to Road & Track's testing, my '02 WRX wagon on 16x6.5 OEM alloys and 205/55x16 RE92's takes 70 feet longer to stop from 80mph in the dry when compared to an '05 STI running 225/45x17 RE070's on 17x8 BBS alloys

you can pack a lot of obstacles to hit in 70 feet, and from 80mph it only takes one hit to ruin your day...
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