Monday, May 20, 2013

What Can Cause Your Wheel to Pop off at the Track

All 5 lug bolts busted off the hub under heavy braking

I had a fantastic student with a very unfortunate mechanical failure at speed at Watkins Glen last weekend. We were entering the braking zone for the toe of the boot ~ 90 mph when the front driver side wheel popped off and got stuck at a 45-degree angle inside the wheel well. We were just passengers at that point and went into the tirewall. Luckily, it happened at a turn that goes uphill, which slowed us down significantly before we hit the tire wall at < 30 mph. Failure analysis of the hub with broken studs revealed one stud failed in yield and was likely caused by a stress riser at the end of the hub thread. This is definitely one thing you may want to check in your car before your next track event.

4 comments:

  1. "likely caused by a stress riser at the end of the hub thread" what's this mean? Wasn't the stress riser mean to to hold the weight of the wheel? Was the wheel not compatible with the stress riser? Maybe I am referencing the hub lip.

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  2. "Stress Riser" is an engineering term for place where stress is concentrated within a part. Usually this is around sharp corners or edges.

    See the red in this picture:
    http://cr4.globalspec.com/PostImages/201006/plane_strain_notch_sxx_602C7DE2-DF92-8E44-3D50134923744790.jpg

    As in this case, stress risers are where a failure starts, then once the lug/beam/brace/whatever is compromised the whole thing comes apart quickly.

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  3. Still not sure how they could have prevented it. Were the studs not the right fit?

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  4. I have a feeling this is due to re-torquing hot lugs right after a session, which causes extreme over-torquing. Not a good idea. This person should probably replace all of their studs and nuts.

    -Brew

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