FVD++ : Cubic roll instead of Quartic ?

Hi there ! I was told that i should use cubic function for my roll instead of quartic, so i tried to see,
but cubic makes the roll weirder and all kinds of weird things...
What function type do you guys use most of the time for your roll ?
Thanks

Depends on what you want to do, really. I recently made a woodie from geometric sections and quartic curves suited that application well. Most steel rides will be better off with cubic though.

A mistake I see a lot is putting two quartic curves next to each other for a direction change when you should have three cubics instead. This creates a sharp roll change when really you want it to be smooth in almost any application.

Here's an example of what I mean; left of the highlighted section is two quartics, each are 10m long peaking at 10*/s and -10*/s respectively, on the right is three cubics with the first being 5m long peaking at 10*/s, the middle is 10m long peaking at -20*/s (to get it down to -10) and the third is 5m long peaking at 10*/s. The left one will bank left, pause, then bank right back to 0*. The right one will bank left then right in a fluid motion. Ultimately they both end up at the same roll and in the same distance, but one of them is more 'comfortable.'

I'll use quartics to get a quick 'sketch' of my roll sometimes, but will almost always use the cubic method in its place once I have it sorted where I want the track to go.
All of the curves, even linear, have their applications. Sine is good for transitioning from geometric to force sections as you're usually going from some constant radius to a non-constant radius, plateaus can be handy for vertical loops, quartic is good for long turns with quick entrance and exit banking, cubics are good for fast and smooth transitions from left to right, and so on.

Continuous roll can be tricky, but it is leagues ahead once you understand how it works.