Stress resistant steel. All shafts are heat-treated alloy steel and are designed to resist stress and deflection. It has a degree of flex so instead of bending, they flex...sort of like leaf springs on a car/truck. They will bend but the degree of it is minimal and it happens near the threads so the change in the steering is minimal. Chances are you will not even know you bent one until you spin it out and roll it on a flat surface. How much stress can these take...launch a 600lb quad plus rider over 100 foot jump...landing front wheels first, full collapse of suspension and drive away with no damage...pretty much more than any average rider/driver is going to push them.
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My testing grounds... 75ft double.
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Using different set ups...flat track, MX, hare scrambles, trails, dunes...the rods held.
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I produced over 1000 pairs, only had 2 failures and those were flaws in the metal...both under extreme stress under racing conditions. G out on one and hitting hole on the track at 45mph...the G-out only bent the rod, racer continued on to finish...the hole caused the rod to snap at the thread, complete failure (yes it hut...it was me) truth be known, I don't think anything else would have held together either...LOL
One issue though for some is the weight of these...they are much heavier than a stock rod and really overkill for the average rider/driver... I do know a guy who makes them from stainless steel alloy...very cool looking, not over-kill on strength but still much stronger than a stock or sleeved tie-rod and less weight than the HD rods...It really all depends on what your needs/use and desire for bling bling are...