interesting, i think i break everything else but the lower arm!
i bend the aluminum screw long before the arm cracks. are you using a steel or ti screw?
Nope...I use the aluminum screw. It's a known issue with the IRS arms...many people have had problems with them cracking. If you've bent the aluminum screw it is likely you cracked the arm and didn't realize it because the crack is under the caster block.