Reference: Power/ Weight/Grip Article Review



imazn

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#1
I read a recent article concerning different size rims and what effect they have on times and what not. Some members were interested in the information too so I thought I just post up a summary of what i read from a January, 2001 issue of Sport Compact Car magazine.

Pretty much they tested Enkei wheels and BFGoodrich Comp T/A ZRs tires on an Eclipse. The tires were three different aspect ratios but the same section width (225/50-16, 225/45-17, 225/40-18). This means they had almost identical rolling diameters which allowed them to evaluate how the difference in rotating weight and sidewall heigh affects performance in a series of tests.

As for wheels, they used Enkei's NT03 in the 17- and 18-inch sizes. For the 16s they used the RF1 which is relatively light as well as the other wheels.

The quarter-mile times by wheel size for the 2000 Mitsubishi Eclipse GT are as follows:
16-inch 15.5 sec. @ 89.4 mph
17-inch 15.5 sec. @ 89.0 mph
18-inch 15.6 sec. @ 87.9 mph

The 16-inch wheel and tire package weighed 44.5 lbs per corner, while the 17-inch package weighed 47 lbs per corner. The oddball was the 18-inch wheel and tire, which was actually slightly lighter than th 17-inchers, at 46 lbs. How did that work? First, the 18-inch Enkei NT03 weighs only 1 lb. more than the 17-inch wheel, but the 18-inch tire is 2 lbs. lighter than the 17-inch tire. The shorter sidewall on the 225/40ZR18 shaves significant weight.

As you can see, surprisingly, the 16-inch and 17-inch setups returned 15.5-second quarter-mile times, but the 17-inch trap speed was slightly slower at 89.0 mph vs. the 89.4 of the 16-inch wheel.

What does it mean?
The fact the 18-inch wheel was slower despite being slightly lighter than the 17 suggests that rotational inertia is, indeed, a significant factor in acceleration. As weight is moved outward, rotational inertia increases with the square of the distance from the center of the wheel. Not only was the wheel rim a half-inch farther from the center (which means a 12% increase in rotational inertia), but the 18-inch wheel was 8 inches wide while the others were 7, making the rim a larger percentage of the wheel's weight.

Now what if you start adding weight to the Eclipse then what would happen (just like having another passenger in the car with you). This test is done with the 16-inch rim.

The curb weight without Ballast for the Eclipse is 3,050 LBS
No ballast 15.5 sec. @ 89.4 mph
50-lbs ballast 15.5 sec. @ 89.4 mph
150-lbs ballast 15.6 sec. @ 88.6 mph
250-lbs ballast 15.8 sec. @ 87.1 mph

Enough of this straight-line stuff. Big wheels and low-profile tires aren't designed for straight-line acceleration, the lower-profile tires are supposed to benefit handling by providing more immediate steering response and less deformation of the contact patch. For this test they used a 2000 Mitsubishi Lancer (AWD)

16-inch 1:03.7 96.1 mph straight speed
17-inch 1:03.2 95.5 mph straight speed
18-inch 1:03.9 95.0 mph straight speed

With the 17-inch wheel, there was more grip and better stability under braking, but more understeer all around. The 16-inch wheel felt tall and had mushy sidewalls. Taller side-walls should feel squishy. The smallest, lightest wheels went faster at the end of the straight, even though their soft sidewalls cost lap time elsewhere on the course. The 18-inch wheels gave the most balanced handling but their acceleration penalty made them slowest. It was the 17-inch wheels that offered the best compromise, between handling and acceleration.

hope this helped...
 


knockoff

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#3
Re: Power, Weight, Grip! ~from SCC January 2001

[align=center]----->STAR<-----

well, it's orange, but you get the idea!!!!![/align]
 


cpforyou

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#9
Exactly why I stick w/ 205/50 on 15" wheels. I like the straight line performance. As far as stiff sidewalls, Falken Azenis is stiff enough as it is.
 




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