Why 19" front wheels, but 20" rears?

[mkoesel]

Originally Posted by Savory

I really can't see a reason that there is a 20 on the rear. Can't be for caliper clearance. Rake?

Tire diameter determines rake, not wheel diameter. GTS has less rake than the standard M4 if my calculated numbers above are correct.

[BRBM]

Originally Posted by mkoesel

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"

Originally Posted by TahoeM3

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.

Thank you both for the lesson!! Always wondered what the staggering of wheel diameters was all about........'Vette's being the obvious and (possibly) most common example. Interesting.

[Lojs]

Originally Posted by TahoeM3

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.

The tiny delta in the diameter in this case has a negligible effect on the contact patch.
Playing around with tire pressure has much more impact.

[colin_e90]

Curious as to why the gts has the double staggered setup when the zcp doesn't

[JD1100]

Originally Posted by TahoeM3

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.

So what is the advantage of a 19" front rim? What is described above would accomplish the exact same thing with a 20" front rim. It makes no sense.

[YKIGI]

Originally Posted by JD1100

So what is the advantage of a 19" front rim? What is described above would accomplish the exact same thing with a 20" front rim. It makes no sense.

The 20 666m with the 265/30 fronts suck. I tossed those as fast as possible.

[fiveohwblow]

My .02? HATE the look. Hardest of passes.

[JD1100]

Originally Posted by YKIGI

The 20 666m with the 265/30 fronts suck. I tossed those as fast as possible.

Thank you for the reply. ..But is there a technical reason why BMW and others do this on the GTS and CS ? Is there any advantage other than looks?

[YKIGI]

Originally Posted by JD1100

Thank you for the reply. ..But is there a technical reason why BMW and others do this on the GTS and CS ? Is there any advantage other than looks?

Having a 19 in the front gives you more compliance and a much better ride over all (short answer)
Having a 20 or a 21 (think Porsche) in the rear gives you access to wider tires without a massive sidewall increase.

Reason 3 is many cars these days with wider rears just look better with larger wheels in the arches versus the front.

Again. There are way more answers and tech specs. These are just the basic answers.

To me, the staggered tires are the way to go

[4play]

Originally Posted by JD1100

Thank you for the reply. ..But is there a technical reason why BMW and others do this on the GTS and CS ? Is there any advantage other than looks?

Weight and packaging are two of the reasons also

[JD1100]

ok. Those answers makes sense.

thanks guys !

I can immagine 19" with a 35 sidewall vs a 20" with a 30 sidewall would ride much better.


I'm considering selling my 666Ms and getting Richland GT4s ... was thinking 20" rear and 19" front possibly.

[CanAutM3]

Originally Posted by JD1100

So what is the advantage of a 19" front rim? What is described above would accomplish the exact same thing with a 20" front rim. It makes no sense.

Originally Posted by JD1100

Thank you for the reply. ..But is there a technical reason why BMW and others do this on the GTS and CS ? Is there any advantage other than looks?

Reduced understeer and more stability from the rear.

Higher sidewalls generally produce more ultimate grip while shorter sidewalls improve handling response. Comfort was definitely not the primary objective on the CS and GTS...

[CanAutM3]

Originally Posted by YKIGI

Having a 19 in the front gives you more compliance and a much better ride over all (short answer)
Having a 20 or a 21 (think Porsche) in the rear gives you access to wider tires without a massive sidewall increase.

Reason 3 is many cars these days with wider rears just look better with larger wheels in the arches versus the front.

Again. There are way more answers and tech specs. These are just the basic answers.

To me, the staggered tires are the way to go

The reason Porsche went for staggered wheel diameters is different than BMW. When Porsche increased the rear wheel rim diameter, they maintained the same sidewall height, thus increasing the rolling radius. This increases contact patch size without having to increase tire width.