Posted by Bob Dold on Saturday, November 18, 2006 10:21 PM
The first test runs I did on the trike resulted in a bent lower right a-arm, I straightened the a-arm out as best as I could after each incident but it was apparent the a-arm was not strong enough.
The original a-arms were made from .405 diameter by .068 wall 6061-T6 aluminum with a yield stress of 15,000 psi. While the stress analysis of the arms did not show any problems, the testing must have produced a load case not accounted for in the analysis. I also noticed the right rocker arm was not moving freely which may have restricted the shock movement, putting undue stress on the a-arm. Whatever the cause of the bending, it was apparent the arm needed to be stronger. A search of McMaster found a .500 diameter by .120 wall 2024-T3 tubing with a yield stress of 45,000 psi – this will be more than 3X stronger than the original tubing. The only downside is this tubing is much more expensive than the original tubing, almost $10/ft vs. $1/ft. Because the OD is close to the old tubing size I will be able to use the existing pushrod mount by boring it out to .500 diameter and bonding the new arms in.
The above pictures show the new a-arm on the right compared to the old one on the left, you can also see how much thicker the wall is on the new part.