10-07-2015, 03:50 AM | #1 |
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Why 19" front wheels, but 20" rears?
Surprised that they are running a 20" rear and a 19" front wheel.
People are always talking about rolling radius on this forum? |
10-07-2015, 07:52 AM | #2 |
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Rolling radius is determined by the wheel radius plus sidewall height. So, for the 20" wheel, you just use a tire with less sidewall than the 19" wheel so that it has roughly the same rolling radius front to back.
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10-07-2015, 08:21 AM | #3 |
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And Mercedes making the 19/20" split available via the C63S. Gotta keep up with the competition!
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10-07-2015, 09:04 AM | #4 |
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10-07-2015, 10:06 AM | #5 |
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A lot of manufacturers do this. Corvettes have had 19 fronts and 20 rears since last generation. Several Ferrari models have it. The reason is that increasing the length of the contact patch helps with straight line performance, but the only way to do this without significantly increasing sidewall is to go to a larger wheel diameter.
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10-07-2015, 10:07 AM | #6 |
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See above.
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10-07-2015, 11:34 AM | #7 |
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No real change in contact patch length.
Standard M4 rolling diameter: F: 255/35 R19 = 26" R: 275/35 R19 = 26.6" M4 GTS rolling diameter: F: 265/35 R19 = 26.3" R: 285/30 R20 = 26.7" |
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10-07-2015, 10:07 PM | #8 | |
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I'd imagine that the larger wheel diameter itself changes the length of the contact patch by adjusting where the weight rests. For example, if you rest a wheel on the ground (standing up as it would be when mounted) and then draw a line around the area where its weight rests (essentially a rectangle), you can see how the contact patch of a larger diameter wheel would be different than that of a smaller diameter one.
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10-08-2015, 07:10 AM | #9 | |
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I'm not necessarily convinced its about contact patch length when overall tire diameter stays effectively the same. For example, they could have went with a taller tire on the rear as well but they chose not to. Whatever benefit the 20" wheel itself may have on the length of the contact patch, increasing the tire diameter would have even more so. Clearance issues start to come into play, sure, but some of that can be mitigated with changes in the suspension or perhaps the fender lining. It could be that the 20" wheel is there to decrease sidewall height which should reduce flex when cornering. If it helped shave even a second or two off of the Nurburgring time then that would be justified. As you note, they clearly targeted ring performance as part of their business case and marketing of the car. |
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10-08-2015, 07:44 AM | #10 | |
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10-12-2015, 07:19 AM | #12 | |
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991 GT3 RS also have different rim sizes (F20, R21). There have to be some benefits, but on the other hand less sidewall usually means less comfort, too.
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10-13-2015, 10:35 AM | #13 |
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Then why do it? Doesn't it just add weight? The 20" wheel has to way more than a 19, right?
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10-13-2015, 11:56 AM | #14 | |
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I don't think it has been answered with certainty yet, but some speculation appears above.
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10-22-2015, 10:15 PM | #15 |
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Sidewall definitely plays a part on the rear wheel specifically.
I do think that the GT3 RS shares its rear wheels and rear hubs with the 918, which is a good thing. |
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10-23-2015, 12:19 PM | #16 |
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Why? It just means that 918 is undertired (almost 400hp more than RS) - Dodge Viper has wider rear wheels/tires and so do many other cars... RS' performance comes from similar tire setup as 918- if it had the same wheels/tires as base GT3 it wouldn't be much faster than a GT3(certainly not 5 seconds quicker at the ring).
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10-23-2015, 12:29 PM | #17 | |
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10-23-2015, 12:38 PM | #18 | |
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Porsche built the 918 from the ground up and I am sure (in their eyes) the width is perfect, which is why its a great rear setup for the RS. |
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10-23-2015, 12:54 PM | #20 | |
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12-17-2015, 09:36 AM | #22 |
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I really can't see a reason that there is a 20 on the rear. Can't be for caliper clearance. Rake?
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