Weird behavior when using bevel and subsurf

Question

I'm trying to make a simple LEGO brick with smooth corners. I modeled the basic shaped and made it a little bit smoother by using the bevel modifier and to make the stud rounder I applied a subsurf modifier, everything went well except for the bottom part

The faces are getting all messed up at this point, I realized it was because of the subsurf modifier and topology of the model, I tried several methods to fix it.

Adding loop cuts to the problematic areas

This solves the issue but adds a lot more of useless geometry, specially when you consider the subsurface modifier.

Adding edge creses

Somehow it makes the problem even worse

I'm not a complete beginner to modelling but this got me stumped. I'm trying my best to make a non-destructive workflow so I can layout the basic shape and leave detailing to modifiers.

EDIT: The bottom hole has to be squared

Here's the model without any modifiers:

If the problem is bad topology what would a good topology be for this workflow?

Thanks in advance if anyone can help me!

Answer 1

I would do this slightly differently. Rather than manually beveling the edges, I would use a bevel modifier. Here's a step by step with a 1x1 brick, not to scale.

• add a default cube
• go into edit mode
• select all faces
• subdivide
• select all four top faces
• inset
• deselect the center vertex
• use loop tools to make the eight vertices into a circle
• extrude the four inner faces outward to give the top peg
• select all four bottom faces
• inset the same amount
• deselect the center vertex
• extrude the four inner faces inward to give the bottom hole
• add a bevel modifier
• set it to two segments
• set the method to angle
• set the angle to 60 degrees.
• add the subsurf modifier below the bevel modifier in the stack.
• add two loop cuts to the top peg

At the end, the block will look like this

and the modifier stack will look like this

EDIT:

The above approach works if you want two circular connectors; but the question asked for a square bottom. To do that, I would

• Use two bevel modifiers, in each case, using a vertex group to identify what was modeled.
• inset the bottom and extrude it inward
• but not make it into a circle.

This method requires a couple of control loops to force the bottom to behave well. Here's what the square bottom looks like without the modifiers:

Here's the vertex group for the outside bevel modifier:

Here's the vertex group for the inside bevel modifier

Here's the modifier stack

Answer 2

Note:

This answer was being made before Marty edited his answer, I'll accept his answer because it contains very good information and solves the issue. But I will also keep mine here for sake of completion and extra info in case it might help someone. Again, great help from Marty, thanks!

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I managed to "solve" this issue by using a bevel on the problematic areas. To keep the workflow as simple as possible, here's what I did.

• Made the topology a little bit better by using a technique similar to what Marty Fouts suggested (image 1)
• Selected all the EDGES connected to the VERTEX where the problem happened like in image 1
• Added the EDGES to a VERTEX GROUP
• Applied a second bevel modifier with 2 segments, limited by the VERTEX GROUP with 2 segments. This is between the first bevel and the subsurf, the modifier stack can be seen on image 2
• Result is much more pleasant to look at, keeps hard edges, no visual inconsistencies and you can control how sharp those edges are (images 3 and 4)