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Discussion Starter #1
I gotta tell you I'm a little apprehensive about modifying my future M6 at all, except the chip for the speed limiter, because it seems like such a delicately crafted and highly-tuned machine that any meddling with its physics is off-putting.

However, on the M5 forum, it's being talked about where some of their cars are being lowered, changing to 21' tires and Hartage rims with no ill effects.

Since the lowering seems like a do-able operation, I'm emboldened to consider different rims too instead of the specially designed M6's. To do this, however, it would be nice to know the weight of the exisiting M6 rims to compare them to whichever ones I consider. I'd hate to choose a rim so overweight that it puts such undue strain on steering joints as to effect its handling, ride characteristics and mechanics in the long run.

Any input would be appreciated.
CJ
 

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CobraJet said:
I gotta tell you I'm a little apprehensive about modifying my future M6 at all, except the chip for the speed limiter, because it seems like such a delicately crafted and highly-tuned machine that any meddling with its physics is off-putting.

However, on the M5 forum, it's being talked about where some of their cars are being lowered, changing to 21' tires and Hartage rims with no ill effects.

Since the lowering seems like a do-able operation, I'm emboldened to consider different rims too instead of the specially designed M6's. To do this, however, it would be nice to know the weight of the exisiting M6 rims to compare them to whichever ones I consider. I'd hate to choose a rim so overweight that it puts such undue strain on steering joints as to effect its handling, ride characteristics and mechanics in the long run.

Any input would be appreciated.
CJ
They say the M6 forged wheels are 4 lbs lighter than the M5 wheels. I would guess the front and rear M5 cast wheels are around 28 and 30 lbs respectively. That would make the M6 wheels 24 and 26 lbs - pretty light for 19's. I have BBS RS-GT's that are the same size - 2 piece forged and weigh 22 and 24 lbs. You can get lighter, but you may sacrifice strength. If you get top-quality forged wheels, you could probably go to 20's without any weight penalty. However, you're changing the suspension dynamics somewhat with the larger wheels and stiffer sidewalls.

I'm not going to change the wheel size on my M5 because I don't want to ruin the incredible job BMW has done tuning the suspension. I will go with my lighter wheels (about 6 lbs per corner), but the same size.
 

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Discussion Starter #3
Thanks, Wolverine.

Wolverine said:
They say the M6 forged wheels are 4 lbs lighter than the M5 wheels. I would guess the front and rear M5 cast wheels are around 28 and 30 lbs respectively. That would make the M6 wheels 24 and 26 lbs - pretty light for 19's. I have BBS RS-GT's that are the same size - 2 piece forged and weigh 22 and 24 lbs. You can get lighter, but you may sacrifice strength. If you get top-quality forged wheels, you could probably go to 20's without any weight penalty. However, you're changing the suspension dynamics somewhat with the larger wheels and stiffer sidewalls.

I'm not going to change the wheel size on my M5 because I don't want to ruin the incredible job BMW has done tuning the suspension. I will go with my lighter wheels (about 6 lbs per corner), but the same size.
That's the kind of input I was looking for. Something to think about.
 

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Wolverine said:
They say the M6 forged wheels are 4 lbs lighter than the M5 wheels.
For a given weight, two rims can have two very different rotational inertias (depending on the mass distribution). That would affect the car acceleration and determine the effects of gyroscopic precession. Lighter wheels are better for the handling but they *may* have higher rotational inertia and therefore, have a negative effect on acceleration!

Unfortunately, manufacturers communicate the rim weight, but they seldom give information on inertia. In the past, some car mags made measurements.
 

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AnthonySoete said:
For a given weight, two rims can have two very different rotational inertias (depending on the mass distribution). That would affect the car acceleration and determine the effects of gyroscopic precession. Lighter wheels are better for the handling but they *may* have higher rotational inertia and therefore, have a negative effect on acceleration!

Unfortunately, manufacturers communicate the rim weight, but they seldom give information on inertia. In the past, some car mags made measurements.
True. If the wheels are the same diameter, the rotational inertia difference will be small, and won't significantly affect performance. However, if you are talking two different diameters, then there can be a big difference in inertia - enough to have a slight effect on performance. Say if you have an 19" wheel vs a 21" wheel of the same weights, the rotational inertia of the larger wheel is 22% more because of the increase in diameter.

Basically you won't notice a difference of just a few lbs per wheel for wheels of the same size. Once you get over 5 lbs or so, you can start noticing. If you go to a larger wheel, get the lightest and strongest wheel you can. Otherwise you might adversly affect your performance.

I replaced my 18" OEM 545 wheels and runflat tires with the 19" BBS RS-GT's and Goodyear F1's. I saved 14 lbs per corner! The difference is tremendous - better ride, response, braking and significantly better acceleration. Wheels and tires affect performance more than any other modification. Make sure you know what you're doing before you make a change.
 

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Wolverine said:
I replaced my 18" OEM 545 wheels and runflat tires with the 19" BBS RS-GT's and Goodyear F1's. I saved 14 lbs per corner!
FYI, on a 19" rim the Michelin PS2's are about 4 lbs lighter than the Goodyears.
 

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GaryRudolph said:
FYI, on a 19" rim the Michelin PS2's are about 4 lbs lighter than the Goodyears.
WoW 4lbs is alot.... I wonder what is the lightest tire?
 

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artur323 said:
WoW 4lbs is alot.... I wonder what is the lightest tire?
I used www.tirerack.com using a 645 with BBS RS-GT rims (245/40-19, 275/35-19):
Michelin PS2: 24 lbs, 27 lbs
Pirelli PZero: 25 lbs, 28 lbs
Bridgestone RE050A: 30 lbs, 30 lbs
GoodYear F1: 32 lbs, 28 lbs

All US lbs front, back.

Not as significant on the rear, but very signficant on the fronts.
 

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GaryRudolph said:
I used www.tirerack.com using a 645 with BBS RS-GT rims (245/40-19, 275/35-19):
Michelin PS2: 24 lbs, 27 lbs
Pirelli PZero: 25 lbs, 28 lbs
Bridgestone RE050A: 30 lbs, 30 lbs
GoodYear F1: 32 lbs, 28 lbs

All US lbs front, back.

Not as significant on the rear, but very signficant on the fronts.
The 32 lbs for the Goodyear F1 is incorrect. I called Tirerack, and the guy told me that was the number they got from Goodyear. I told him that I weighed my wheels and tires when I got them from the Tirerack, and the number was wrong. He almost bet me, but called Goodyear back, and yes, the number is incorrect (I could've had a free lunch). The new weight Goodyear gave them for the 245/40 is 27 lbs.

Now, I actually weighed the tires on a physician's scale, and I recorded 28.4 lbs for the 275/30 ZR19, and 25.0 lbs for the 245/35 ZR19 Goodyear F1 GS-D3's. My numbers I can guarantee are within 1/4 lb.
 
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