I got bored and made a subframe.

[bastarddsm]

While I agree with boost98 to a point I dont think that you can just go around saying things can fail based on harmonics. As a Mech E I full understand the point you are trying to making but absoultely no one looks at every design established and challenges it based on harmonics.
If you have seen these designs fail due to those conditions then it makes sense to challenge it and not one person can say it wont fail untill its been tested. Ansys will give you an idea but its never perfect.
Plus the harmonics from road conditions are much less constant then a wind frequency across a bridge.

I think you need to stress test it and see where it fails.