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they also tend to shear off at the bar mount, leaving a jagged hunk of metal flailing around your leg.
Yup, I've seen this happen way too many times to feel safe with the Renthals 7/8" bars.
Typical oversized bars can still do that, but it's less likely. The Twinwalls are specifically designed not to do that. Since the outer wall is what comes into contact with the clamping area it will be the wall to develop cracks and other defects. The inner wall provides the strength but is shielded from that sort of thing, so they're virtually unbreakable at the clamping surface (which is the point of highest stress concentration).
Most bar manufacturers recommend that you change your bars after a bad crash, just like helmets. If the bar flexes too much, it may eventually fatigue and break. Aluminum bars are most specifically affected by this since aluminum is prone to fatigue failure. Also, if you flex a bar and develop a crack (no matter how small), you've put a defect into it that'll continue to grow even if you don't crash. Renthal now shot peens the surface of the 7/8' bars (and Fatbars) so that helps with stopping defects from propogating. But its still possible to snap them in the right situation.