The L663 Defender's are factory equipped with Aluminum thrust arms. These arms are subject to significant forces when driving over rugged terrain. If one breaks on the trail, you'll be left stranded with that corner of your suspension dislocated. The dislocated wheel will make recovery difficult, and could cause additional damage along the way. So for those who plan to tackle the toughest trails, Steel thrust arms are a worthwhile insurance policy. Don't let a weak link compromise your Defender's off-road prowess. Invest in our heavy-duty steel front lower thrust arms and experience the difference.