Originally Posted by CargoDane

Aluminium can be non-jarring as well. It's about geometry (and wheel base) mostly. That said, my mini-velo (Dahon Smooth Hound) can be jarring too under the right circumstances, but that is not due to the material.

I've had loads of alu bikes, and apart from the mini-velo none of them was jarring. Least of all is the Bullitt (a long john). It is by far the least "jarring" bike I own (it even feels "soft" at times). The Omnium cargo (Ti) feels much stiffer than the Bullitt, but it is not jarring despite the oversized tubes compared to the steel version (steel omnium). It just feels stiff as it should be when you carry a lot of weight.

I think you're confusing lack of flex with "jarring". I don't think flex is a good thing, regardless of the material. In fact, you have to have a minimum of stifness to the rear frame to use a Rohloff hub, if not, it will jump gears or not shift properly. I have seen some titanium bikes with the same diameter as a steel version, you don't want that as that is making the whole bike more flexy than the steel version.
Carbon can be made as stiff as you want it, or as flexible as you want it. It all depends. It's not the material choice that makes the difference, it is how you implement it.

But it doesn't matter. My preferred metal is titanium. When it does flex due to loads, you don't fatigue it like you would an alu frame, and you don't have the weight or corrosion of steel. Carbon is great for some things too.

My fork on the Omnium is steel. Luckily, it isn't a super expensive fork, so if it begins to rust, is otherwise damaged, or I suspect it is rusting inside, I can buy a new one for little money. That said, I'd prefer a fork made from (oversized) titanium for weight, ease of mind, strength, and corrosion resistance. I'd consider a BMX vert fork from CF if I could figure out how to attach a tab for the steerer rod (both my cargo bikes employ steerer rods).A custom titanium fork would not have that problem.