Forgestar Wheel Failure?

[Pentalab]

http://www.forgestar.com/wheels/testdata/
So how come no test data for any of the 17" rims?
No test data for 18" x 10" rims either (nor 18" x 11")
All the independent test results contain NO impact tests.

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Test Procedure


Using computer SIMULATION it is possible to perfect the wheel design before the wheel goes into final production. However, the ultimate conclusions are generated during the test procedure. The test criteria checked by Forgestar Wheels SIMULATES the toughest road conditions.

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Overview of Forgestar test scenarios:
Dynamic cornering fatigue test:
The transverse forces which affect the wheel during driving through bends are simulated.
Dynamic radial fatigue test:
The stress to the wheel going straight or cornering is simulated.
Impact test:
The strength (center disc, rim flange, assembled part) against breaking and air-leak is SIMULATED.
We test every width in each wheel design family with the offset configuration that presents the greatest testing challenge and subject it to the EQUIVALENT of a full product LIFETIME of STRESS in our own in house testing laboratory.
Then we send one of each wheel design to a independent third-party laboratory for added tests. In this way, we insure that the recommended wheel fitment is safe at maximum rated load.


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## Ok, define .....'toughest road conditions'.

What does their.... 'Impact test' consist of ?

Define lifetime of stress, surely not hitting curbs.



All I was interested in, was what alloy + heat treatment are they using, that's it, simple question, nothing more.

 

[zquez]

hahaha You are the man Terry.

 

[Norm Peterson]

## Ok, define .....'toughest road conditions'.

What does their.... 'Impact test' consist of ?

Define lifetime of stress, surely not hitting curbs.

All proprietary information, best as I can remember the answer to a similar question on an automotive engineering forum . . . (though I'd also guess that contact with the kinds of curbs you typically see on city streets probably isn't part of it).


Norm

 

[Pentalab]

I talked to a metallurgist, and also another fellow who is a chemical engineer for a large Aluminum company, just in the last 3 days. That was an eye opener. If you want increased impact strength, it's doable..... with either a stronger alloy, and / or thicker material, esp any weak links, or failure prone stress points. End result is either you require deeper pockets, and / or end up with a heavier final product. Sound familiar?

 

[Pentalab]

All proprietary information, best as I can remember the answer to a similar question on an automotive engineering forum . . . (though I'd also guess that contact with the kinds of curbs you typically see on city streets probably isn't part of it).


Norm

A few years back, on the FRPP site, some of their wheels listed were also curb tested. Now they didn't elaborate how fast they hit the curbs, or how often.

 

[Boaisy]

^^ and in other news water is wet....

...

 

[Norm Peterson]

This picture showed up in a magazine that at least some of us get. There's a (too-brief) mention of fatigue in the accompanying article.

Felt it was appropriate to post in this thread . . . edited for picture resizing.

Norm

 

[Pentalab]

This picture showed up in a magazine that at least some of us get. There's a (too-brief) mention of fatigue in the accompanying article.

Felt it was appropriate to post in this thread . . . edited for picture resizing.

Norm

I assume they are using 39.885 K psi yield strength aluminum alloy? And that the load varies from 2.3 ksi to 10.8 ksi, per their pix above. My guess is they are using something like 6061-T6 alloy which is listed as 40 ksi. 6063-T832 is also listed as 40 ksi yield strength. 7075 alloy is listed as 70 ksi, but would be cost prohibitive.

However, a wheel spinning around, with varying loads on it, will produce a lot of load cycles on the wheel assy. Eventually, they will succumb to metal fatigue, but that would take a looong time. Hitting curbs is a whole other ballgame, with huge int dynamic shock loads. If their above pix is correct, perhaps increasing the spoke thickness by
.5 - 2mm in both width, and / or depth, would be just enough to alleviate issues, without adding hardly any overall weight or increased costs.

 

[Norm Peterson]

I suppose you could assume some load history made up of various specific loadings from mild to severe and how often they might occur. Then run it out until the sum of all the individual fatigue damages puts the wheel on the edge of failure. Not terribly different from an ASME NB3600 fatigue evaluation actually.


Norm

 

[CobraRed]

I assume they are using 39.885 K psi yield strength aluminum alloy?

That would be strange because I always used 45k for standard 6061-T6, and to my knowledge that doesn't factor changes in grain structure like rotary forging.

 

[Thenorm]

looking at all the dark blue regions of that FEA, maybe there is some weight that could be shed in the barrel and hub....

another point regarding aluminum, not sure if it was mentioned before, but there is no minimum stress where fatigue can be avoided.

at a tiny fractional load, it may be gazillions of cycles before it fatigue cracks, but it will crack eventually.
higher stresses will cause it to fatigue in fewer cycles.
this is different from steel which has a minimum stress below which it will never fatigue.

 

[Pentalab]

That would be strange because I always used 45k for standard 6061-T6, and to my knowledge that doesn't factor changes in grain structure like rotary forging.

No way in hell is 6061-T6 45K psi for yield strength. All the old books from various aluminum makers depicted 35K psi. Later on it has slowly migrated up to it's current 40 ksi. If you actually test it, it's typ 41-42 ksi. If you ever use any structural software that requires the yield strength to be entered, don't enter anything higher than 40 ksi. Usual deal is to enter 35 ksi, which allows for a bit of a safety factor.

You can now also get 6061-T8, but it's the same 40 ksi rating. That came out just a few years ago, and is typ used on tubing..with .120 wall thickness. Previously, Al tubing could only be had in .125 wall, and would not readily telescope into the next size up /down.

Like the other poster noted, aluminum is not like steel. Modulus of elasticity for Al is triple that of steel.... hence al will flex 3 times a much. (it will also twist 3 times as much).

If they are using 6061-T6, it will work, but it requires lotsa meat to it. It's a good alloy, but it does have it's limitations.

Their rotary forging process is something new to me. I still haven't quite figured it out. The blanks they start with are cast.

 

[Norm Peterson]

Don't you mean "one third that of steel"?


Norm

 

[Pentalab]

Don't you mean "one third that of steel"?


Norm

Senior moment, you are correct of course.

 

[Norm Peterson]

Don't feel too bad - I used to get paid pretty good money to catch exactly that sort of thing before I'd sign off on the report. Guess that makes me an ex-professional nitpicker these days.


Norm

 

[El_Tortuga]

This picture showed up in a magazine that at least some of us get. There's a (too-brief) mention of fatigue in the accompanying article.

Felt it was appropriate to post in this thread . . . edited for picture resizing.

Norm

Where is this from? What's the loading?

 

[Norm Peterson]

All I know is that the origin would have been inside Forgeline's engineering section. It's no better than secondhand info to me (via SCCA's monthly), and not surprisingly there wasn't any hard information concerning the specific loading.


Norm

 

[El_Tortuga]

All I know is that the origin would have been inside Forgeline's engineering section. It's no better than secondhand info to me (via SCCA's monthly), and not surprisingly there wasn't any hard information concerning the specific loading.


Norm

Thanks. Looks really odd to me with opposite spokes having peaks unless it's a Von Mises plot. Peaks on the sides of the spokes would seem to indicate torque application.

 

[MechE]

Thanks. Looks really odd to me with opposite spokes having peaks unless it's a Von Mises plot. Peaks on the sides of the spokes would seem to indicate torque application.

Maybe the gradients are due to gravity, compression, downforce? Seems like the more horizontal the spoke, the higher the stress; the more vertical, the lower the stress. The peak stresses seem to be on the edges which indicates bending rather than compression/ tension.