I am working on a rack to hold a trunk to the back of a motorcycle. I guess this could be attached to an existing rack but they are not rated for more than 10 pounds. Also the trunk does not fit to the rack and a mounting plate must be used, mounting the plate to the rack isn't pretty.

My question relates to material strength. I would like to use 1/4" aluminum but am concerned with it breaking. The rack is a cantilever mount held to the bike with two bolts. I realize that the stress point will be the upper bolt as the weight will be felt downward.

The pic below shows the prototype bracket.

My question is this ....

Will 1/4" aluminum be sufficient? There is about 3/4" of material around the upper bolt at the most narrow point.

Let's assume the rack will be tasked to hold 80 pounds! Though I am certain it won't be THAT much!