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Discussion Starter #1 (Edited)
So... first to recall the Brief History of Wheels on my '16:

New set of Forgestars:
https://www.ssforums.com/forum/wheels-tires-suspension/90650-ss-sedan-wheel-directory-2-0-a-35.html#post3470581

Homegrown custom-size full spare combo:
https://www.ssforums.com/forum/wheels-tires-suspension/171714-cheapest-spare-wheel-2.html#post3471359

Recent developments:
The bad news: About two weeks ago my right side rear wheel broke while driving at about 30 mph on a 2 lane regular road. It sounded like I hit something (I was in a line of cars and could not see what it was) however the same side front wheel was/is intact! The busted wheel does not look bent at the inner or outer rims, and I can see no tell-tale marks on the tire. See pics.

The good news (silver lining): I pulled to the shoulder and installed my homegrown spare wheel using my homegrown wheel replacement kit, and it all works perfectly. The car is as smooth as it was before the incident which also confirms that the front wheel is intact!

I communicated the situation and pics to Forgestar and their dealer (GetMyWheels.com) and both say this is the result of an impact with an object and not caused or exacerbated by a manufacturing or design flaw. I ended up ordering the replacement Forgestar wheel (GertMyWheels.com did provide a small discount). It will take 4-6 weeks to get as each wheel is machined and powder coated to order. I put back the inflator kit in the trunk until I receive the replacement...

To be honest I don't know what moral there may be to the story so I thought I'd just share and let y'all form your own views...


Cheers.:WTF:
 

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I gotta agree.
 

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Discussion Starter #4
I suspect so too, although no visual damage. In addition there may also be damage to the shoulders due to driving on the flat tire while slowing down and pulling over.
My trusty Discount Tire store is sure to look at the tire very closely - after all they have considerable motivation (stick and carrot).
 

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Replace the tire.

Period.
 

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Looking at the OP's wheel/tire config makes me question a couple things.
1) Isn't the stock rear wheel width 9"? The OP appears to have changed them to 9.5" but kept the OEM tire size.
2) The rear tires now on the car are Michelin Pilot Sport A/S 3+ having a size/load rating of 275/35ZR19 96Y. The stock Bridgestone rear tires carry a size/load rating of 275/35R19 100Y XL. Even the stock fronts carry a higher rating with a 98Y XL.

I'm wondering if it's possible having a lower load rated tire on a wider wheel could cause this type of impact wheel failure to happen more easily? The OP didn't say what tire pressure he was running with, but if it was below 35 psi, I would imagine it wouldn't take much of a sized object to bottom out a 35 sized side wall tire at any speed.

Here's Tire Rack's rating explanation:
Load Index

The "96" represents the tire's load index. The load index basically shows how much weight a tire can support. A load index of 96 gives the information that the tire can handle a load of 1,565 lbs. Passenger cars and light trucks normally have tires with a load index between 70 and 110.


Speed Rating

The "Y" at the end of the tire information tells the speed rating, or the tire's maximum speed capability. These ratings are initially calculated in kilometers per hour, then converted to miles per hour. A "Y" goes on a tire with top speed capabilities up to 300 kilometers per hour, or 186 miles per hour. These tires are not designed for autos like a family sedan, but for a vehicle with very high speed capabilities, such as an exotic, high-performance car.



Considerations

The load index and speed rating are for tires that are working adequately. The load capacity designation is for a tire that is inflated at its maximum air pressure. According to information on the Tire Rack website, the speed rating only applies to tires that "have not been damaged, altered, under-inflated or overloaded."
 

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Looking at the OP's wheel/tire config makes me question a couple things.
1) Isn't the stock rear wheel width 9"? The OP appears to have changed them to 9.5" but kept the OEM tire size.
2) The rear tires now on the car are Michelin Pilot Sport A/S 3+ having a size/load rating of 275/35ZR19 96Y. The stock Bridgestone rear tires carry a size/load rating of 275/35R19 100Y XL. Even the stock fronts carry a higher rating with a 98Y XL.

I'm wondering if it's possible having a lower load rated tire on a wider wheel could cause this type of impact wheel failure to happen more easily? The OP didn't say what tire pressure he was running with, but if it was below 35 psi, I would imagine it wouldn't take much of a sized object to bottom out a 35 sized side wall tire at any speed.

Here's Tire Rack's rating explanation:
Load Index

The "96" represents the tire's load index. The load index basically shows how much weight a tire can support. A load index of 96 gives the information that the tire can handle a load of 1,565 lbs. Passenger cars and light trucks normally have tires with a load index between 70 and 110.


Speed Rating

The "Y" at the end of the tire information tells the speed rating, or the tire's maximum speed capability. These ratings are initially calculated in kilometers per hour, then converted to miles per hour. A "Y" goes on a tire with top speed capabilities up to 300 kilometers per hour, or 186 miles per hour. These tires are not designed for autos like a family sedan, but for a vehicle with very high speed capabilities, such as an exotic, high-performance car.



Considerations

The load index and speed rating are for tires that are working adequately. The load capacity designation is for a tire that is inflated at its maximum air pressure. According to information on the Tire Rack website, the speed rating only applies to tires that "have not been damaged, altered, under-inflated or overloaded."
:3tens: to everything.
 

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While interesting analysis, it prob just comes down to lightweight (low mass) cast aluminum wheel get damaged easily by potholes and road debris, and this becomes more likely the lower the profile of tire.

You see this all the time with lightweight track wheels like an rpf01, cast or forged.
 

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Discussion Starter #9
Looking at the OP's wheel/tire config makes me question a couple things.
1) Isn't the stock rear wheel width 9"? The OP appears to have changed them to 9.5" but kept the OEM tire size.
2) The rear tires now on the car are Michelin Pilot Sport A/S 3+ having a size/load rating of 275/35ZR19 96Y. The stock Bridgestone rear tires carry a size/load rating of 275/35R19 100Y XL. Even the stock fronts carry a higher rating with a 98Y XL.

I'm wondering if it's possible having a lower load rated tire on a wider wheel could cause this type of impact wheel failure to happen more easily? The OP didn't say what tire pressure he was running with, but if it was below 35 psi, I would imagine it wouldn't take much of a sized object to bottom out a 35 sized side wall tire at any speed.

Here's Tire Rack's rating explanation:
Load Index

The "96" represents the tire's load index. The load index basically shows how much weight a tire can support. A load index of 96 gives the information that the tire can handle a load of 1,565 lbs. Passenger cars and light trucks normally have tires with a load index between 70 and 110.


Speed Rating

The "Y" at the end of the tire information tells the speed rating, or the tire's maximum speed capability. These ratings are initially calculated in kilometers per hour, then converted to miles per hour. A "Y" goes on a tire with top speed capabilities up to 300 kilometers per hour, or 186 miles per hour. These tires are not designed for autos like a family sedan, but for a vehicle with very high speed capabilities, such as an exotic, high-performance car.



Considerations

The load index and speed rating are for tires that are working adequately. The load capacity designation is for a tire that is inflated at its maximum air pressure. According to information on the Tire Rack website, the speed rating only applies to tires that "have not been damaged, altered, under-inflated or overloaded."
stvmoe75, thank you very much for the great insight and info!
A couple of comments:
1. Tire load rating: Great catch! Rear tire rating spec is indeed 100Y. I "inherited" the tires with the car and did not bother to check them against specs. Will try to take it up with the Chevy dealer who sold me the car, although it's been more than a year (good luck, huh?).
On the other hand, tire load rating is taking into consideration max load (GAWR), and I am driving 99% of the time 1 or 2 up, empty trunk. During the incident I drove alone, empty trunk. Also, the load and resistance to impact is carried by the air pressure and not the tire itself; the tire needs to be able to support the pressure, heat and pressure spikes without bursting; that's where the load rating comes in. In my case, the tire did not burst, so ultimately I see no clear correlation of this incident to the tire rating.
In any event, I will likely replace both rears with Michelin's of the correct load rating. Thanks again for catching this!


2. Rim width: I can see possible correlation of the incident to the rim width, although the increase vs. OEM is just over 5%. In some charts I found, 9.0" rim width is below the minimum recommended for 275 tire width, 9.5" being the minimum. Note in our cars the nominal 12.7 mm difference in front to rear rim width vs. 30 mm in tire width. I realize Holden had some reason for this, but having been in this industry for 4 decades, I know it's not always about performance, there are various cost, weight, packaging, supply and other compromises. I could see no technical reason against it, so I decided on 9.5" for the rears.


3. Tire pressure: I pump mine to 39 psi cold all around (have compressor and precision gage at home), and keep a watchful eye on the pressures (have the dash tire pressure display always on). High tire pressures are a must for low profile tires and alloy rims. Tire pressures were good.


4. I tend to think the main culprit is the wheel design and manufacturing, as pointed out by thewrx. I'm still wondering whether there is a metallurgical flaw in this wheel which contributed to the rather dramatic failure, but it would not be easy to prosecute...


Thanks again for the insights, guys.
 

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Wow! That is an extreme failure. Makes me think again about a getting set of Forgestars. I do like your setup but wheel destruction like that with no obvious tire damage is not reassuring about the wheel's strength despite what FS says.
 

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A couple of comments:
1. Tire load rating: Great catch! Rear tire rating spec is indeed 100Y.
The only reason I happen to catch this is when I went on TireRack to look for the specs of your tire, it didn't list it as an offering. Then on the side menu I noticed it stating the SS requires an XL rating for the rears.

2. Rim width: I can see possible correlation of the incident to the rim width, although the increase vs. OEM is just over 5%. In some charts I found, 9.0" rim width is below the minimum recommended for 275 tire width, 9.5" being the minimum. Note in our cars the nominal 12.7 mm difference in front to rear rim width vs. 30 mm in tire width. I realize Holden had some reason for this, but having been in this industry for 4 decades, I know it's not always about performance, there are various cost, weight, packaging, supply and other compromises. I could see no technical reason against it, so I decided on 9.5" for the rears.
I will agree with your reasoning here and would probably do the same. If I had 9.5" wheels, I'd probably look at going to the 285's for width, but would make sure to find tires having the XL rating. The only thought I had concerning the wider wheel was thinking that a 96Y tire might not have the facial strength that a 100Y XL does and would likely flex more easily in the middle given a 1/2" more distance between sidewalls. (I know very little about tires and can only apply my limited theory to a possible cause.)

3. Tire pressure: I pump mine to 39 psi cold all around (have compressor and precision gage at home), and keep a watchful eye on the pressures (have the dash tire pressure display always on). High tire pressures are a must for low profile tires and alloy rims. Tire pressures were good.
I too, keep mine set at 40 psi. This bit of research has opened my eyes a bit when it comes time to get new tires.
I found this nugget on page 311 in the 2016 OM that sort of surprised me - not that I spent much time above 100 mph.

Vehicles with 245/40ZR19 98Y and 275/35ZR19 100Y tires are capable of high speed use.
Make sure front tires of size 245/40ZR19 98Y are inflated to 310 kPa (45 psi) before operating the vehicle at speeds of 160 km/h (100 mph) or higher.
Make sure rear tires of size 275/35ZR19 100Y are inflated to 340 kPa (50 psi) before operating the vehicle at speeds of 160 km/h (100 mph) or higher.
Return the tires to the recommended cold tire inflation pressure when high-speed driving has ended.


4. I tend to think the main culprit is the wheel design and manufacturing, as pointed out by thewrx. I'm still wondering whether there is a metallurgical flaw in this wheel which contributed to the rather dramatic failure, but it would not be easy to prosecute...
I don't think I'm quite in your camp yet with it being a poor wheel design or manufacturing defect. If there had been a 100Y XL tire on, then I'd be more on board.
 

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Discussion Starter #12
Thanks again for the good insights, stvmoe75.
I wanted to stay with the OEM size tires -- I was so happy with the handling balance I didn't want to screw it up, although increasing the rear rim width by 0.5 inch might have a small effect - none that I could detect on public roads.
I see your logic regarding the 100Y XL load rating potentially contributing to some more tire resistance to rim impact. I somehow doubt that the 100Y XL would have prevented the destruction.
I also found the tire pressure recommendations for high speeds in the FOM (an acronym I learned here) -- was actually astonished that the General's lawyers allowed it and did not force them to include multiple warnings and disclaimers about the need to obey all speed limits etc (or did I tune them out)... Unbelievable on a US market (GM!) car.
The way the rim broke with no tell tale signs on the tire (and the blow was not felt that hard) still looks to me suspicious... that's all I'm sayin'.
I did talk today to the service manager at the dealer where I bought the car about the under-spec tires. He said he will investigate and get back to me... will keep y'all posted as this is all so exciting...


BTW, what is OP? I assume it's me in this case but not sure whether it is a complimentary or derogatory term... OmniPotent? Old Pucker??



Cheers.
 
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Original Poster (thread starter) or Original Post (first in thread)
Seems to be used interchangeably here. I prefer the former.
 

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Discussion Starter #14
Thanks!! I'm assembling a glossary of Forum Acronyms for my own reference, unless there is one posted already in some Sticky?...
 
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Been mentioned by some mostly because of the paint name acronyms, but not aware that anyone carried through.
 

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For those that haven’t realized this, the optimal width wheel for the factory 275/35r19 is 9.5. The factory 9 is within specs but not optimal for tire size. Who knows what reason Holden had to use a 9, there are many possibilities. See screenshot from Tire Rack. I do understand his tire is not an XL with 100 rating but it still seems like the load rating of the 96 would be sufficient for our cars with proper inflation, it’s only a 91 lb difference.
 

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I should not have said “optimal”, my point was that tire manufacturers base their tread width, section width and diameter dimensions for the 275/35r19 on a 9.5” wheel. So 9.5” wheel width is not to blame for the failure. Unfortunately, this is not the first failure for a Forgestar wheel which concerns me as I have F14’s.
 

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For those that haven’t realized this, the optimal width wheel for the factory 275/35r19 is 9.5. The factory 9 is within specs but not optimal for tire size. Who knows what reason Holden had to use a 9, there are many possibilities. See screenshot from Tire Rack. I do understand his tire is not an XL with 100 rating but it still seems like the load rating of the 96 would be sufficient for our cars with proper inflation, it’s only a 91 lb difference.
I think you may want to check that difference again. The load rating of a 96Y tire is 1,565 lbs whereas the 100Y is 1,764 lbs - so when taken across all 4 corners of the car, you've lost 800 lbs of carrying capacity. I don't think that amount is trivial.
But the big difference (IMO) is the loss of having an XL load range rating which basically takes the tire from being a 4 ply down to a 2 ply type tire. A 4 ply tire is far stronger, and likely stiffer tire, and will have a much better ability to endure a road hazard like the one that took out the OP's wheel.

I'm sticking with my thought the weaker tire is what caused the wheel failure.
 

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I think you may want to check that difference again. The load rating of a 96Y tire is 1,565 lbs whereas the 100Y is 1,764 lbs - so when taken across all 4 corners of the car, you've lost 800 lbs of carrying capacity. I don't think that amount is trivial.

It is a significant loss of load capacity.


But the big difference (IMO) is the loss of having an XL load range rating which basically takes the tire from being a 4 ply down to a 2 ply type tire. A 4 ply tire is far stronger, and likely stiffer tire, and will have a much better ability to endure a road hazard like the one that took out the OP's wheel.

I'm sticking with my thought the weaker tire is what caused the wheel failure.
allowed

... the wheel to get whacked and fail.

Cast wheels are not optimal for performance cars.

GM learned this lesson years ago; but it has been "forgotten" by the young whipersnappers pushing for profits.

... ask the Corvette guys about their wheels ... sorry Doug :wink
 
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My mistake, it’s 199. I compared to the fronts 98 rating. Yes, XL is a stronger tire but a 3,130 rear axle capacity with the 96Y’s should be adequate with proper inflation and minimal load as he stated. I believe his front tires are still factory rated so he has not lost 800 lbs capacity, only 398. The 100 XL would be better choice but I still lean towards an issue with the wheel itself.
 
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