Quote Originally Posted by Ama-fessional Molder View Post
A subframe for a motorcycle. I only weigh about 220, add another 10-15 lbs with gear, and then assume there will be bumps in the road.

I still intend to print the bulk of it, just reinforce it with aluminum. It will all be visible, and custom is cool. Plus I need to do up all sorts of brackets and stuff for wires, tail light, various electrical control bits, battery, and all that.

I see no problem just designing it with a channel all the way around to just drop in square aluminum tube from the bottom, then have the printed part just "sit" on top of it.
A couple of observations:
First, if you have a design payload of 250lbs, I think you need to design for 500-750 lbs minimum. That "bump" in the road could easily cause a 2G force making you weigh 500lbs for a moment.
FDM printing is inherently weak in certain directions as it is dependent on the tensile and shear strength of the interlayer bond, which is significantly less than that of most of the materials we use.
Tensile strength along the build's Z axis is almost non existent except for very large areas.
Compression directly along the build's Z axis will perform at or near the material specifications where directly supported, however, the beam strength (i.e. bending resistance between supports) is also reduced due the the weak interlayer bond.
IMO, FDM (at least with our hobby machines) is not suitable or safe for any kind of structural requirements in the ranges you need. It would be fine for enhancing the aesthetic appeal of an otherwise strong and well built aluminum structure that is properly designed to support all the loads.
Graham