6UL wheels still cracking...
09-16-2019, 09:59 AM
#62
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Tire Rack article on checking your wheels for track use.
https://m.tirerack.com/tires/tiretec...jsp?techid=110
https://m.tirerack.com/tires/tiretec...jsp?techid=110
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09-16-2019, 11:02 AM
#64
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I don't think anyone is arguing in good faith that aluminum wheels should never fail, at least not after you show them an Sn diagram for aluminum. The issue is that the endurance limit of the 6UL in particular seems to be much less than expected, and much less than competitor options which are similarly light and inexpensive.
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09-16-2019, 11:30 AM
#65
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I don't think anyone is arguing in good faith that aluminum wheels should never fail, at least not after you show them an Sn diagram for aluminum. The issue is that the endurance limit of the 6UL in particular seems to be much less than expected, and much less than competitor options which are similarly light and inexpensive.
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09-16-2019, 12:48 PM
#66
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I have no dog in this fight, but there is a lot of trash engineering speculation going on in this thread.
We keep talking about “aluminum”, but there are lots of different aluminum alloys, and we have no idea which alloy these wheels are. I would speculate that they are 6061, but there are even different variations of 6061. The mechanical properties of the different alloys vary dramatically.
Aluminum doesn’t have less “elasticity” than steel. It has a lower elastic modulus, which means that a similar size/shape object will be less stiff if it is made from aluminum.
The failures being discussed are fatigue failures, not static failures. This makes things like elastic modulus almost irrelevant.
There are two steps to fatigue failure, crack formation and crack propagation. Aluminum is more notch sensitive than steel, meaning that a small scratch or defect can generate a fatigue fracture more easily. Unless there’s a sharp step or corner designed into the wheel where the cracks are starting, there is probably not a design defect per se. The recommendation to inspect wheels frequently is good advice.
The most important characteristic with regard to fatigue is how cracks propagate through the crystal lattice of the metal. As Sav pointed out, all aluminum alloys are inferior to steel in this regard, because cracks propagate more easily. Steel has a way of partially “healing” these cracks, where aluminum does not. Therefore steel has a finite fatigue limit, where aluminum does not.
https://en.m.wikipedia.org/wiki/Fatigue_limit
None of this is unique to 6UL’s, every manufacturer has to wrestle with these inherent properties and design accordingly. The “fix all” for these problems is to make the heaviest smoothest wheels possible, which none of us would probably buy.
Small differences in wheel design can make big differences in stresses. For instance, a 15x9 wheel is going to apply a lot more stress to the spokes than a 15x8 wheel because the inside edge of the barrel is a cantilever, and the stress on the spokes goes up exponentially with the length of the cantilever.
Manufacturing methods play a critical role in fatigue properties. Good wheels are forged or cold formed because it work hardens them, which basically means that the crystal lattice gets all tangled up and the material gets stiffer. If the work hardening is not evenly distributed then the stresses won’t be properly distributed either, which could lead to point stresses and crack initiation. I am in no way implying that there is an obvious issue with 6UL’s, but I would guess that there are substantial differences in how different wheels are made, and that differences here could be important to fatigue life. OEM auto manufacturers have teams of PhD’s that do things like forging and mold flow analysis.
We keep talking about “aluminum”, but there are lots of different aluminum alloys, and we have no idea which alloy these wheels are. I would speculate that they are 6061, but there are even different variations of 6061. The mechanical properties of the different alloys vary dramatically.
Aluminum doesn’t have less “elasticity” than steel. It has a lower elastic modulus, which means that a similar size/shape object will be less stiff if it is made from aluminum.
The failures being discussed are fatigue failures, not static failures. This makes things like elastic modulus almost irrelevant.
There are two steps to fatigue failure, crack formation and crack propagation. Aluminum is more notch sensitive than steel, meaning that a small scratch or defect can generate a fatigue fracture more easily. Unless there’s a sharp step or corner designed into the wheel where the cracks are starting, there is probably not a design defect per se. The recommendation to inspect wheels frequently is good advice.
The most important characteristic with regard to fatigue is how cracks propagate through the crystal lattice of the metal. As Sav pointed out, all aluminum alloys are inferior to steel in this regard, because cracks propagate more easily. Steel has a way of partially “healing” these cracks, where aluminum does not. Therefore steel has a finite fatigue limit, where aluminum does not.
https://en.m.wikipedia.org/wiki/Fatigue_limit
None of this is unique to 6UL’s, every manufacturer has to wrestle with these inherent properties and design accordingly. The “fix all” for these problems is to make the heaviest smoothest wheels possible, which none of us would probably buy.
Small differences in wheel design can make big differences in stresses. For instance, a 15x9 wheel is going to apply a lot more stress to the spokes than a 15x8 wheel because the inside edge of the barrel is a cantilever, and the stress on the spokes goes up exponentially with the length of the cantilever.
Manufacturing methods play a critical role in fatigue properties. Good wheels are forged or cold formed because it work hardens them, which basically means that the crystal lattice gets all tangled up and the material gets stiffer. If the work hardening is not evenly distributed then the stresses won’t be properly distributed either, which could lead to point stresses and crack initiation. I am in no way implying that there is an obvious issue with 6UL’s, but I would guess that there are substantial differences in how different wheels are made, and that differences here could be important to fatigue life. OEM auto manufacturers have teams of PhD’s that do things like forging and mold flow analysis.
Last edited by Dietcoke; 09-16-2019 at 12:58 PM.
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09-16-2019, 01:05 PM
#67
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How Strong Is Your Wheel
The point of my post was not to defend or indict 949. There seems to be some practical experience that indicates there's an issue here. I'm only pointing out that half-assed "engineering" explanations don't really add to the discussion. If someone wants to put forth theories on WHY the wheels are breaking, they need to show their work.
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09-16-2019, 01:09 PM
#68
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I have no idea, hence the term "speculate". They don't explicitly say the alloy on their website. After giving it a quick read, they don't explicitly say whether they are cast or forged either.
How Strong Is Your Wheel
How Strong Is Your Wheel
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