[m2not1LE]

Originally Posted by MineralGreyMetallic

The wheel manufacturer I'm working with says there should be no rubbing with the 20" on the G87.

I'm wondering if the different offset, which from what I understand is fairly aggressive on these wheels, avoids the issue.

I'm a bit clueless with this wheel fitment stuff at the moment, as you can tell.

Offsets that significantly differ from those BMW engineers specified substantially compromise suspension system kinematics and dynamics and are therefore something to avoid.

Focus should be on reducing wheel weight by choosing a very good forged wheel, with all other specs being identical to factory.

One of many available related third party articles here:

https://www.motortrend.com/how-to/ht...est-lip-wheel/

Bottom line: Keep these cars production line stock unless you are certain (though loads of research) that the considered mod will indeed improve the vehicle overall.

[m2not1LE]

Originally Posted by MineralGreyMetallic

So, you're saying there is absolutely a negative impact, but what is it?

And if there is a negative impact, when does it come into play? At 50% of the car's limit? 85%? 95%?

I would not take these wheels on a track. If I wanted to track the car I would get OEM spec forged wheels.

That article you listed doesn't really go into any of the specifics you're trying to highlight - like affect of suspension characteristics and handling.

The whole article is based around the assumption that the larger rim will be significantly heavier. Their cons were things like ride comfort, fuel efficiency, and 0-60 decreasing because of the weight.

The wheels I'm going with (20/21 setup) weigh less than the stock wheels so some of those won't apply.

What I took away from the BMW Engineering video is they chose a 20" in the back to increase grip and a 19" in the front to increase comfort. Larger tire in the front should increase understeer, which your linked article also mentions.

I'm a mechanical design engineer and have a very deep understanding of this. Suggest you do more research on this specific matter, including stock front and rear wheel offsets. Thiner sidewalls alone (to maintain stock tire diameter with the plus one front swap will) impact the entire front suspension system, with incorrect wheel offset being much worse.

May be that front/rear factory offset is identical, though the other issues I mentioned (plus fender well clearance, especially if increasing tire size). Also, even if weights are identical, the rational moment of inertia for the the larger diameter set-up would be less favorable per the related mathematical equation.

Related equation here. As you'll see, radius is a second order determinant of rolling inertia, with weight (mass for any given gravity) being only first order:

https://www.texasgateway.org/resourc...0chosen%20axis.

I'm not going to post a dozen or so other links just they can be be mostly ignored.

Also, please post actual weight figures from well respected third party sources to back your claim that the larger diameter wheels will be lighter than OEM.

Most owners somehow *believe* they're more knowledgeable than the combined knowledge of BMW M engineers who designed these cars, which is of course absurd on every level.

[MineralGreyMetallic]

Originally Posted by m2not1LE

I'm a mechanical design engineer and have a very deep understanding of this. Suggest you do more research on this specific matter, including stock front and rear wheel offsets. Thiner sidewalls alone (to maintain stock tire diameter with the plus one front swap will) impact the entire front suspension system, with incorrect wheel offset being much worse.

I'm not going to post a dozen or so other links just they can be be mostly ignored.

Also, please post actual weight figures from well respected third party sources to back your claim that the larger diameter wheels will be lighter than OEM.

Go ahead and swap factory rear 20" wheels onto the fronts of your M2 if you need to learn the hard way.

I read both articles you linked word for word.

The first article doesn't really have anything to do with suspension kinematics, which is what I'm interested in.

The second one does, but it doesn't necessarily assert that a change in offset is a net compromise to the system. It can be a benefit. Depends on the change.

I was hoping you might shed some light on the impact, since you're knowledgeable, but I can also just read up myself now.

My assumption, since the wheel manufacturer I'm going with specifically designs wheels for M cars, was that the offset would not compromise the system.

At least I know what to research now, so I appreciate it. I'll dig more and see what the difference is in offset between stock and the wheels I'm going with. Maybe once I figure that out, we can have a more informed discussion.

These are the wheel and tire specs, btw, from the manufacturer. I don't know of any third party source that has weighed these wheels, sorry.

9.5 kg | 10.5x20 ET14 for 285/30 R20
10.3 kg | 11x21 ET12 for 305/25 R21

[m2not1LE]

Originally Posted by MineralGreyMetallic

I read both articles you linked word for word.

The first article doesn't really have anything to do with suspension kinematics, which is what I'm interested in.

The second one does, but it doesn't necessarily assert that a change in offset is a net compromise to the system. It can be a benefit. Depends on the change.

I was hoping you might shed some light on the impact, since you're knowledgeable, but I can also just read up myself now.

My assumption, since the wheel manufacturer I'm going with specifically designs wheels for M cars, was that the offset would not compromise the system.

At least I know what to research now, so I appreciate it. I'll dig more and see what the difference is in offset between stock and the wheels I'm going with. Maybe once I figure that out, we can have a more informed discussion.

These are the wheel and tire specs, btw, from the manufacturer. I don't know of any third party source that has weighed these wheels, sorry.

9.5 kg | 10.5x20 ET14 for 285/30 R20
10.3 kg | 11x21 ET12 for 305/25 R21

Again, I'd need to see actual weights of both the factory and the aftermarket wheels.

Even if they're identical, the rotational moment of inertia must be considered, wherein wheel radius is second order while mass is only first order.

This is just how real engineering works.

EDIT: Would also need tire weights (OEM vs. aftermarket)...

Total larger diameter wheel/tire rational inertia is almost certainly larger than factory.

Also need to see wheel offset (in mm) - OEM vs. aftermarket with front and rear broken down individually.

[m2not1LE]

Originally Posted by MineralGreyMetallic

I read both articles you linked word for word.

The first article doesn't really have anything to do with suspension kinematics, which is what I'm interested in.

The second one does, but it doesn't necessarily assert that a change in offset is a net compromise to the system. It can be a benefit. Depends on the change.

I was hoping you might shed some light on the impact, since you're knowledgeable, but I can also just read up myself now.

My assumption, since the wheel manufacturer I'm going with specifically designs wheels for M cars, was that the offset would not compromise the system.

At least I know what to research now, so I appreciate it. I'll dig more and see what the difference is in offset between stock and the wheels I'm going with. Maybe once I figure that out, we can have a more informed discussion.

These are the wheel and tire specs, btw, from the manufacturer. I don't know of any third party source that has weighed these wheels, sorry.

9.5 kg | 10.5x20 ET14 for 285/30 R20
10.3 kg | 11x21 ET12 for 305/25 R21

Good rule of thumb from race car engineers:

Specify the SMALLEST diameter wheels that will (barely) clear the brake calipers - both front and rear, with rotational inertia being the primary factor in that. Then select tire aspect ratio to maintain stock rolling diameter (at least with ABS equipped street cars).

Pretty much the exact opposite from what what actual cars owners seeking "the look" are doing.

[MineralGreyMetallic]

This guy seems to be weighing the 930M wheels in this post.

https://g87.bimmerpost.com/forums/sh....php?t=2066465

Front ~12kg
Rear ~14kg

[MineralGreyMetallic]

Wheel offset comparison between OEM and the wheels I posted

OEM
9.5x19 ET20 for 275/35 R19
10.5x20 ET20 for 285/30 R20

Aftermarket
10.5x20 ET14 for 285/30 R20
11x21 ET12 for 305/25 R21

[m2not1LE]

Originally Posted by MineralGreyMetallic

So, you're saying there is absolutely a negative impact, but what is it?

And if there is a negative impact, when does it come into play? At 50% of the car's limit? 85%? 95%?

I would not take these wheels on a track. If I wanted to track the car I would get OEM spec forged wheels.

That article you listed doesn't really go into any of the specifics you're trying to highlight - like affect of suspension characteristics and handling.

The whole article is based around the assumption that the larger rim will be significantly heavier. Their cons were things like ride comfort, fuel efficiency, and 0-60 decreasing because of the weight.

The wheels I'm going with (20/21 setup) weigh less than the stock wheels so some of those won't apply.

What I took away from the BMW Engineering video is they chose a 20" in the back to increase grip and a 19" in the front to increase comfort. Larger tire in the front should increase understeer, which your linked article also mentions.

Please post actual wheel/tire weights of OEM vs. the 20/21 set-up your considering using well respected third party sources.

Even if weights are identical, rotational inertia on the larger sizes will be larger due to this:

https://phys.libretexts.org/Courses/...ss_and_Impulse

With the larger wheel, the (heavy) "hoop" section moves further away from the center of the wheel. Now look at the page and formulas above. You'll see that mass (weight for the intents of this thread) is but a first order determinant of rotational inertia, while radius is a second order (squared) determinant. As such, radius has a much greater influence.

To some small degree, that will impact braking and acceleration.

Also, the front suspension was engineered around the stock sidewall height. Engineers may well have specified different bushings, for example had they selected 20" front wheels for the new M2.

[m2not1LE]

Originally Posted by MineralGreyMetallic

Wheel offset comparison between OEM and the wheels I posted

OEM
9.5x19 ET20 for 275/35 R19
10.5x20 ET20 for 285/30 R20

Aftermarket
10.5x20 ET14 for 285/30 R20
11x21 ET12 for 305/25 R21

Much more to it than fender clearance, though, with the resulting change in scrub radius being the most significant.

https://www.miata.net/garage/offset.htm


Edit: Another link here and the information is accurate:

https://en.wikipedia.org/wiki/Scrub_...0be%20affected.

Proper approach in choosing aftermarket wheels: Ensure that factory scrub radius is maintained, select a wheel/tire combo that will result in no interference throughout the suspension's full articulation, ensure the new wheel/tire package won't increase rotational inertia and unsprung weight and THEN focus on aesthetics of the wheel itself.

Pretty much nobody does this of course because they're unaware of the involved physics.

[Ronin76]

Originally Posted by MineralGreyMetallic

Wheel offset comparison between OEM and the wheels I posted

OEM
9.5x19 ET20 for 275/35 R19
10.5x20 ET20 for 285/30 R20

Aftermarket
10.5x20 ET14 for 285/30 R20
11x21 ET12 for 305/25 R21

Notice no answers….just more articles to read lol.

[Mr. Hankey]

Originally Posted by MineralGreyMetallic

So, you're saying there is absolutely a negative impact, but what is it?

The theoretical masturbatory flagellations going on here, while interesting, are accurate when considered within their own vacuum of isolation from everything else, but entirely unhelpful and misleading and you're completely fine to mess with your offsets with little worry about upsetting the car.

Basically BMW has a line of G-series M-cars and has a number of rims that all have the SAME offsets because they use the same suspension components across the line, for the most part. This increases compatibility and decreases production costs for BMW. The cars are not dialed in to some 'synergistic nirvana' that is and always will be world without end, amen...they are rather dialed into an average use case that also meets regulations across worldwide markets so BMW doesn't need 2-3 variations of a half shaft or hub and can just build 1 car for the world - hence we all get orange reflectors up front (thanks a lot Belgium - actually I think its the US )

It is important to note that the components are all over engineered and can handle wider offsets. Adding millimeters to 1/2 an inch of offset is not going to cause the world to spin off its axis. Yes more torque to the hub, but minuscule amounts and well within tolerances for average expected lifespan. Sure the increased lever arm will slightly change what the springs/hubs/bearings see, but the wider wheels will absorb the changes seen at the hub because you have more tire which is also increasing your damping force. Everything is a trade off. Heck the suspension the car comes with is a series of trade-offs.

Honestly I'm putting on the MP-HAS which we all know has higher spring rates than the stock G87 spring rates, so a wider track, (ie smaller offsets positioning wheels out more) will help with that.

While rotational inertia is interesting, tires in general are HEAVIER than rims (except, perhaps, these 930Ms that are clearly a cost reducer for BMW but their weight will be advantageous in the winter). Therefore going up to a 20" forged rim in general will make the wheel/tire combo LIGHTER and DECREASE rotational inertia in addition to less unsprung mass compared to a 19" or 18" rim. Also because the tire is so dense, the heaver component at the furthest distance from the axis of rotation is reduced and that gives you more acceleration over an equivalent 19" rim and equal diameter tire. Sure back in the 60s when rims were heavy steel they would run a 15" wheel and a high sidewall tire because the tires were relatively lighter than steel. Aluminum has changed that ratio. Rims are lighter than tires.

So on that, since someone above is providing advice and asking you to go find information that actually proves some of what was said as incorrect here's a spreadsheet of common tire sizes and specs I compiled:




Here are the weights of the G80 Rims:
https://g80.bimmerpost.com/forums/sh....php?t=1852984

So find the weight of the rim and add the weight of the tire you are planning on using and bingo - guaranteed lighter than what the car comes with stock.

And if you want to go nuts here is a comparo of Michelin PS4S tires relative to rim width. Green boxes are in the expected range of 'ideal' performance due to optimized support of the sidewall from the width of the rim:



Hopefully you find some of this helpful. The easiest performance improvement on these cars is clearly rims and tires based on unsprung mass and rotational inertia alone. Next is battery and exhaust, although that will remove some weight over the rear axle slightly affecting traction as RWD car, negatively impact F-R weight balance (there's a reason why BMW puts the battery back there, and part of why I love them), and heighten your center of mass.

The video you sent says what I've suspected researching all of this. the 20" in the rear will get power down with less tire flex than a 19", and the 19" up front increases damping/comfort. The effect on tire temp will be negligible, and the effect of a bigger wheel will cool your brakes better to--you guessed it again--a negligible amount.

Offsets - don't worry about it, but the rubbing of the liner is real. Many have had these cars over the last decade. Many have changed the offsets/added spacers...etc. and it's not a problem. Ran spacers on my F80 and it drove better, wider track. Trade-off is it theoretically wouldn't turn as well, but not noticeable.

I've looked very closely at all the aftermarket wheels for the G87 and their offsets are modest. Don't let some make you overthink it. It's an entirely interdependent system, not an isolated process you're dealing with.

[Ronin76]

Originally Posted by Mr. Hankey

The theoretical masturbatory flagellations going on here, while interesting, are accurate when considered within their own vacuum of isolation from everything else, but entirely unhelpful and misleading and you're completely fine to mess with your offsets with little worry about upsetting the car.

Basically BMW has a line of G-series M-cars and has a number of rims that all have the SAME offsets because they use the same suspension components across the line, for the most part. This increases compatibility and decreases production costs for BMW. The cars are not dialed in to some 'synergistic nirvana' that is and always will be world without end, amen...they are rather dialed into an average use case that also meets regulations across worldwide markets so BMW doesn't need 2-3 variations of a half shaft or hub and can just build 1 car for the world - hence we all get orange reflectors up front.

It is important to note that the components are all over engineered and can handle wider offsets. Adding millimeters to 1/2 an inch of offset is not going to cause the world to spin off its axis. Yes more torque to the hub, but minuscule amounts and well within tolerances for average expected lifespan. Sure the increased lever arm will slightly change what the springs/hubs/bearings see, but the wider wheels will absorb the changes seen at the hub because you have more tire which is also increasing your dampening force. Everything is a trade off.

While rotational inertia is interesting, tires in general are HEAVIER than rims (except, perhaps, these POS 930Ms that are clearly [...]

Now get ready for the response.

[Mr. Hankey]

Originally Posted by Ronin76

Now get ready for the response.

I'm ready.

But always happy to learn something new.

[Tag]

BMW offers OEM wheels, Style 1000M for the G80/2/7 that are 20" Front and 21" Rear.

[Ronin76]

Originally Posted by Tag

BMW offers OEM wheels, Style 1000M for the G80/2/7 that are 20" Front and 21" Rear.

Such beautiful wheels too.

[Mr. Hankey]

Originally Posted by Ronin76

Such beautiful wheels too.

They are...but brake dust...yet another compromise BMW made...their brakes are so bad ass. I'll take the dust. M-compound brakes saved me and my family from a huge digger. I'm grateful for their engineering on that topic!

Here in New England the real engineering problem with a 21" is how many tires I'd be replacing with some of the potholes!

[MineralGreyMetallic]

I'm open to give "experts with deep knowledge" their fair space, but that guy just kept waving his hands after I provided a bunch of data for comparison.

Unfortunate.

Thanks to the others that put my mind at ease. That guy had me convinced for a moment that my suspension was going to catastrophically fail on my new wheels.

[m2not1LE]

Originally Posted by MineralGreyMetallic

I'm open to give "experts with deep knowledge" their fair space, but that guy just kept waving his hands after I provided a bunch of data for comparison.

Unfortunate.

Thanks to the others that put my mind at ease. That guy had me convinced for a moment that my suspension was going to catastrophically fail on my new wheels.

You provided no data about wheel/tire combination weights or how the changes in offset will impact scrub radius and kingpin offset.

You also ignored the relationship between wheel diameter and its effects on rolling inertia, given that radius (primarily rim "hoop" to wheel centerline) is a second order determinant of rolling inertia.

More here:

https://www.theautopian.com/our-susp...wheel-spacers/

Trust me that the BMW engineers who design these suspension are well aware of these variables and how each effects the others. And even more to it in the form of damper/spring/bushing rates. No doubt advanced computer modeling is involved in all of - including factory specified tires (e.g, selected compounds).

All of these individual components are engineered to optimally work with the others. Basing wheel choice on appearance and fender clearance just isn't nearly enough in the grander scheme.

But obviously you're going to move ahead with the new wheels and tires anyway because you made up your mind prior to considering any of this.

[m2not1LE]

Originally Posted by Ronin76

Now get ready for the response.

https://www.theautopian.com/our-susp...wheel-spacers/

[Ronin76]

Originally Posted by MineralGreyMetallic

I'm open to give "experts with deep knowledge" their fair space, but that guy just kept waving his hands after I provided a bunch of data for comparison.

Unfortunate.

Thanks to the others that put my mind at ease. That guy had me convinced for a moment that my suspension was going to catastrophically fail on my new wheels.

Bud, I hear you…I have never ever done any mods to my previous cars but when I get my M2, I am definitely putting aftermarket wheels on the car that have better offsets than the +20 that the OEMs have. Think about…look at all the people, not just BMW owners in this forum but all “sports car” owners, whi have purchased aftermarket wheels. I have never EVER seen some say “i screwup my car because I bought aftermarket wheels”. There is a whole industry dedicated to it. HRE would not be able to sell $15k sets if their wheels would ruin the very product (cars) their wheels would be going on…no way.

[Ronin76]

Originally Posted by m2not1LE

You provided no data about wheel/tire combination weights or how the changes in offset will impact scrub radius and kingpin offset.

Seems to me you're going to ignore technical article I post because you've already made up your mind on aftermarket wheels - regardless of that information:

More here:

https://www.theautopian.com/our-susp...wheel-spacers/

Great….another article.

[m2not1LE]

Originally Posted by Ronin76

Bud, I hear you…I have never ever done any mods to my previous cars but when I get my M2, I am definitely putting aftermarket wheels on the car that have better offsets than the +20 that the OEMs have. Think about…look at all the people, not just BMW owners in this forum but all “sports car” owners, whi have purchased aftermarket wheels. I have never EVER seen some say “i screwup my car because I bought aftermarket wheels”. There is a whole industry dedicated to it. HRE would not be able to sell $15k sets if their wheels would ruin the very product (cars) their wheels would be going on…no way.

How will the aftermarket wheels offset be "better" than those of the stock wheels when BMW suspension engineers specifically designed suspension geometry around the stock offset (and vice versa)?

Suggested reading here and you might suspect, it's all about engineering (kinematics in particular). Changing wheel offset changes scrub radius and kingpin offset, which in turn adversely impacts handling, braking and acceleration.

Yes those changes will be "very small" in most cases, but they're all for the worse.

Larger diameters wheels are also typically a real issue as the adversely impact wheel/tire rotational interim - even in cases where new wheel/tire weight is identical to OEM.

When changing wheels, offset should be retained and the combined wheels/tires should be lighter in weight if wheel diameter is increased.

Many don't care about these things and just want "the look." Their money and their choice, though more times than not significant compromises in overall vehicle dynamics are the result.

I'm a bit clueless with this wheel fitment stuff at the moment, as you can tell.