[nearwater4me]

The looks have grown on me enough that I might consider one for my next car. The thing that I still find loathsome is the 18/19” or 19/20” wheel diameter. It sure was irritating on the F80 CS, and BMW decided to use the same setup on the new M3/4.

American sports cars like the Corvette and Viper got away with using different diameter wheels by having bigger wheel housings on the rear with matching tire proportions, which made them look okay. For the M vehicles, however, 19” fronts with fat tires mixed with 20” rears and skinny tires look absolutely stupid IMO.

I know it’s an easy fix, but it still costs $3-5k for an aftermarket wheels set. What gives? What’s the reason behind this trend?

Sorry, rant over.

[UpstateCP]

I had been wondering the same about the F95/96 with mixed 21"/22" setup, could never get a clear answer.

[solstice]

It was mentioned by a BMW representative in one of the launch videos as improving turn in. I.e. one of the parameters that contributes to the reported much improved front axle. Porsche is doing the same with the 992.

[BBoyM3]

I found this on the Lamborghini of Palm Beach website. It discusses the rear wheel diameter:
https://www.lamborghinipalmbeach.com...o%20the%20rear.



The UK product manager explains the front wheel diameter for the M3/M4 at 2:43:

[CanAutM3]

Most likely to reduce understeer. In general terms, a larger sidewall tire provides greater ultimate grip at the expense of handling response.

The same thing was done on the F8X M3/4CS and M4GTS.

[Ilyam5]

I call BS on that.
Smaller - lighter wheels with lighter rubber will always gain more performance. The rule should be - as long as it clears your brakes - that is the diameter wheel you should get. Find the best weight to strength option; get the widest and stickiest rubber that fits. Get a square set up to reduce understeer that's build into every car now. Reducing Rotational unsprung weight is the biggest performance gain.


large diameter wheels and low profile tires are a product of vanity . Looks are dictating the wheel diameters. It has practically no advantages. You get

- heavier wheels,
- heavier tires;
- Worse ride;
- expensive wheels,
- expensive tires.
- No ability to rotate 4 corners.

Different size front and rear is the product of that vanity. 20s with wide tires do not fit on the front so they go 19s while leaving the bling on the rear.

First thing I change in all modern Ms is wheels and tires. Look at track set ups on most well prepared cars - it will answer your question.

[solstice]

Increased diameter come with increased contact area. I.e. you can improve stability without increasing width. Porsche mention this as their reason for larger diameter rears. However larger diameter come with unwanted steering baggage for the fronts. Overall it sums up to a rather complex equation but I’m pretty sure looks ain’t part of it and if after testing and tuning they found the staggered diameter being the most beneficial that’s what we end up with.

[heavyD^2]

Originally Posted by Ilyam5

First thing I change in all modern Ms is wheels and tires. Look at track set ups on most well prepared cars - it will answer your question.

Most track set up vehicles arrive at the track on a flatbed. Automakers have much more considerations including how the vehicles will handle and ride on the drive home or on a twisty mountain road or perform emergency maneuvers, etc. I tend to believe that automotive engineers of companies like BMW and Porsche typically know what they are doing and prioritize performance over vanity.

[Ilyam5]

Originally Posted by solstice

Increased diameter come with increased contact area. I.e. you can improve stability without increasing width. Porsche mention this as their reason for larger diameter rears. However larger diameter come with unwanted steering baggage for the fronts. Overall it sums up to a rather complex equation but I’m pretty sure looks ain’t part of it and if after testing and tuning they found the staggered diameter being the most beneficial that’s what we end up with.

If you look at the contact patch geometry - width is the most contributing factor. Diameter of the tire and thus growing it lengthwise is miniscule.
275/40/18 tire is within single percent of 275/30/22 but weight gains are huge

and narrower 255/30/22 will have less rubber on the road and less stability and grip vs 275/40/18

I think you underestimate the marketing and sales pressure of the bigger looking wheels. Small wheels do not sell. Look at what people do to their cars first - drop them and buy huge wheels to fit the wheel gap tp achieve cartoonish stance. It has nothing to do with performance and most of the time deteriorates it for the looks.

[Transfer]

To me the question is not why 19" up front but why 20" rear? Does an M3 benefit from a 20" wheel? If it's for looks then put 20" up front too. The car is big enough that looks benefit from 20" wheels. If performance is the goal then make it 19" front and rear (or 18" for that matter but looks bad). I don't understand this mismatched diameter issue either.

[Ilyam5]

Originally Posted by heavyD^2

Most track set up vehicles arrive at the track on a flatbed. Automakers have much more considerations including how the vehicles will handle and ride on the drive home or on a twisty mountain road or perform emergency maneuvers, etc. I tend to believe that automotive engineers of companies like BMW and Porsche typically know what they are doing and prioritize performance over vanity.

I think its the opposite - smaller wheels are even more beneficial for the street - Not only they will handle and ride on the drive home or on a twisty mountain road or perform emergency maneuvers much better vs 22 inch monsters

- you will have much better tire availability - tons in 18-19 inch sizes and none in 22
- Tire and wheel prices half or more lower vs big wheels (id rather spend 200 dollars on a similar performance 18 inch tire vs 600 for the same 22 inch)
- Ride quality is dramatically better on 18 vs 22

[Bimmr7]

Originally Posted by Transfer

To me the question is not why 19" up front but why 20" rear? Does an M3 benefit from a 20" wheel? If it's for looks then put 20" up front too. The car is big enough that looks benefit from 20" wheels. If performance is the goal then make it 19" front and rear (or 18" for that matter but looks bad). I don't understand this mismatched diameter issue either.

Did you even read what was posted above or click on the link? BMW and other car manufacturers like Porsche researched their platform and decided it (staggered set up) improved handling ie... Turn in.

[Transfer]

Originally Posted by Bimmr7

Did you even read what was posted above or click on the link? BMW and other car manufacturers like Porsche researched their platform and decided it (staggered set up) improved handling ie... Turn in.

Yes I did and I agree with that. Like I said in my post so then why a 20" rear?

[solstice]

Originally Posted by Transfer

Yes I did and I agree with that. Like I said in my post so then why a 20" rear?

From the link above:

“Advantages of larger rear wheels in an RWD car?

Having a larger diameter rear wheel increases the amount of rubber that contacts the road along the axis of the vehicle which increases your acceleration potential. When you accelerate, much of your vehicle’s weight shifts to the rear. By comparison, the front wheels don’t need as large a contact patch to take care of steering and braking.”

In summary more rubber to the road.

[solstice]

Originally Posted by Ilyam5

I think you underestimate the marketing and sales pressure of the bigger looking wheels. Small wheels do not sell.

Sorry but this make little sense as a reason for staggered wheels. Following your logic BMW would have put 20s or even 21s front and rear. The staggered diameter is most definitely a result of testing for stability, balance, traction and turn in. That the staggered setup is offered across the wheel size options further validates this.

[CanAutM3]

Originally Posted by Ilyam5

I call BS on that.
Smaller - lighter wheels with lighter rubber will always gain more performance. The rule should be - as long as it clears your brakes - that is the diameter wheel you should get. Find the best weight to strength option; get the widest and stickiest rubber that fits. Get a square set up to reduce understeer that's build into every car now. Reducing Rotational unsprung weight is the biggest performance gain.


large diameter wheels and low profile tires are a product of vanity . Looks are dictating the wheel diameters. It has practically no advantages. You get

- heavier wheels,
- heavier tires;
- Worse ride;
- expensive wheels,
- expensive tires.
- No ability to rotate 4 corners.

Different size front and rear is the product of that vanity. 20s with wide tires do not fit on the front so they go 19s while leaving the bling on the rear.

First thing I change in all modern Ms is wheels and tires. Look at track set ups on most well prepared cars - it will answer your question.

That's uninformed BS in your post...

BMW could have easily fitted the same diameter rim front and rear if they chose to. It has nothing to do with clearance since the rim diameter is offset by larger profile sidewalls on the tires.

[CanAutM3]

Originally Posted by solstice

Sorry but this make little sense as a reason for staggered wheels. Following your logic BMW would have put 20s or even 21s front and rear. The staggered diameter is most definitely a result of testing for stability, balance, traction and turn in. That the staggered setup is offered across the wheel size options further validates this.

The M-Performance offering is indeed 20"/21" with 285/30R20 and 295/25R21 tires

[CanAutM3]

Originally Posted by solstice

From the link above:

“Advantages of larger rear wheels in an RWD car?

Having a larger diameter rear wheel increases the amount of rubber that contacts the road along the axis of the vehicle which increases your acceleration potential. When you accelerate, much of your vehicle’s weight shifts to the rear. By comparison, the front wheels don’t need as large a contact patch to take care of steering and braking.”

In summary more rubber to the road.

You'll have to explain that one to me: how does a larger wheel diameter for a given tire diameter puts more rubber on the road ?

[heavyD^2]

Originally Posted by CanAutM3

You'll have to explain that one to me: how does a larger wheel diameter for a given tire diameter puts more rubber on the road ?

That's pretty easy. Increasing wheel diameter for a given tire OD necessitates a lower tire profile and we already know that the lower the profile tire, the more contact area.

[CanAutM3]

Originally Posted by heavyD^2

Increasing wheel diameter for a given tire OD necessitates a lower tire profile and we already know that the lower the profile tire, the more contact area.

Says who?

[heavyD^2]

Originally Posted by CanAutM3

Says who?

Says physics and reality. Are you searching for something that's not there?

[CanAutM3]

Originally Posted by heavyD^2

Says physics and reality.

OK, explain the physics to me.