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      10-07-2015, 03:50 AM   #1
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Post 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?
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      10-07-2015, 07:52 AM   #2
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Quote:
Originally Posted by stranger View Post
Surprised that they are running a 20" rear and a 19" front wheel.

People are always talking about rolling radius on this forum?
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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Quote:
Originally Posted by mkoesel View Post
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.
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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And Mercedes making the 19/20" split available via the C63S. Gotta keep up with the competition!
does this give any performance benefit as I havent seen anyone else go with this stance on aftermarket wheels?
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      10-07-2015, 10:06 AM   #5
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Quote:
Originally Posted by stranger View Post
Surprised that they are running a 20" rear and a 19" front wheel.

People are always talking about rolling radius on this forum?
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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Quote:
Originally Posted by stranger View Post
does this give any performance benefit as I havent seen anyone else go with this stance on aftermarket wheels?
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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Quote:
Originally Posted by mkoesel View Post
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"
It's a complex issue with many variables, and I don't claim to have all the answers. But many manufacturers are doing it, and BMW clearly had good results if they stuck with it. I'm guessing it is a factor in the vastly improved 'Ring time.

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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Quote:
Originally Posted by TahoeM3 View Post
It's a complex issue with many variables, and I don't claim to have all the answers. But many manufacturers are doing it, and BMW clearly had good results if they stuck with it. I'm guessing it is a factor in the vastly improved 'Ring time.
No doubt. There is certainly some benefit to a larger wheel or BMW and others would not go that route.

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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Quote:
Originally Posted by mkoesel View Post
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.
^^ This is my guess.
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      10-10-2015, 06:27 AM   #11
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Just curious, is this the first/only BMW sold with mixed diameter wheels front (19in) and rear (20in) from factory?
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      10-12-2015, 07:19 AM   #12
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Quote:
Originally Posted by Asbjorn View Post
Just curious, is this the first/only BMW sold with mixed diameter wheels front (19in) and rear (20in) from factory?
I think it is.

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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Quote:
Originally Posted by mkoesel View Post
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"
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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Quote:
Originally Posted by nbennettksu View Post
Then why do it?
I don't think it has been answered with certainty yet, but some speculation appears above.

Quote:
Doesn't it just add weight? The 20" wheel has to weigh more than a 19, right?
The wheel weighs more, but the tire weighs less. I would guess that the 20" combination is slightly heavier than an equivalent width 19" combination, but not by much. Not necessarily confident in that guess though.
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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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Quote:
Originally Posted by Sean@PSI View Post
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.
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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Quote:
Originally Posted by dgp View Post
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).
Not really. The electric drive on the 918 is split between the front and rear axles with 129hp and 156hp respectively. Further, the peak power of the electric motor is not necessarily at the exact same RPM as the 608hp from the combustion engine, so the two power number cannot really be added. But even if we do, the 918 has at most 264hp more than the RS at the rear wheels, not 400 .
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      10-23-2015, 12:38 PM   #18
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Quote:
Originally Posted by dgp View Post
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).
Undertired? - More power doesn't necessarily mean a need for wider tires.

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, 12:49 PM   #19
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Quote:
Originally Posted by Sean@PSI View Post
Undertired? - More power doesn't necessarily mean a need for wider tires.
It sure helps
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      10-23-2015, 12:54 PM   #20
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Quote:
Originally Posted by CanAutM3 View Post
Not really. The electric drive on the 918 is split between the front and rear axles with 129hp and 156hp respectively. Further, the peak power of the electric motor is not necessarily at the exact same RPM as the 608hp from the combustion engine, so the two power number cannot really be added. But even if we do, the 918 has at most 264hp more than the RS at the rear wheels, not 400 .
What I meant to say was torque- 338ft lbs peak for RS and over 900 ft lbs for 918 through the same tires. Which one is undertired?
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      12-14-2015, 07:03 PM   #21
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It seems to be the new trend to have bigger diameter wheels at the rear, the C63 coupe also has 19s/20s combo!
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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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