Tree Supports in Bambu Studio: Why Default Branch Distance Fails on Steep Overhangs

Default tree-support branch distance in Bambu Studio is 5mm. That works fine for 60° overhangs. On the 75-85° overhangs you find under cat-mask jaws or behind build-a-hero shoulders, 5mm is too sparse — the unsupported gaps between branches let the overhang surface sag. Here's the parameter sweep that fixed it on 8 catalog models.

What Branch Distance Actually Controls

Tree supports work by generating a 'trunk' of support filament near the model and branching out at the top to multiple contact points that touch the supported overhang surface. Branch distance is the spacing between those contact points where they meet the overhang.

At 5mm spacing, an overhang gets one support point per 25mm² of overhang surface (5mm × 5mm grid). For mild overhangs (60-65°), this is enough — the print can bridge 5mm with minimal sag. At 75°+, that 5mm gap is too wide; the molten plastic droops noticeably between contact points.

Why Default 5mm Was Chosen

Bambu's default optimizes for support material cost vs surface quality on typical use cases — usually 55-65° overhangs. At 5mm spacing, support material use is modest (10-15% of model volume on average) and surface finish is acceptable for most models.

The problem: cat masks, helmets, and humanoid figures (anything modeled after biological shapes) tend to have steep overhangs concentrated in specific areas. The default optimization fails these specific models even though it's fine for the catalog average.

Parameter Sweep on 8 Catalog Models

We tested branch distance values 1.5mm, 2.0mm, 2.5mm, 3.0mm, 3.5mm, 5.0mm (default), 7.0mm on 8 models with known-problematic overhangs. Bambu A1, PLA Basic, 0.20mm layer, Tree (Organic) support style.

• Captain America Cat Mask (75° jaw overhang) — 5mm: visible sag. 2.5mm: clean. 1.5mm: clean but 2× material.
• Stormtrooper Cat Mask (78° cheek overhang) — 5mm: failed (sagged into hole). 3.0mm: minor sag. 2.0mm: clean.
• Mandalorian Cat Mask (72° helmet brim) — 5mm: acceptable. 3.0mm: clean. Stayed with 3mm for production.
• Predator Cat Mask (80° dreadlock overhang) — 5mm: catastrophic failure. 3.0mm: still failed. 1.5mm: clean. This model is the limit of tree supports.
• Mario Hat (65° brim) — 5mm: clean. No change needed.
• Hollow Knight Hornet (88° spike overhang) — 5mm: failed. 2.0mm: failed. Switched to Normal supports — tree supports cannot handle 88° at all.
• Urban Skull Helmet (70° eye socket) — 5mm: minor texture. 3.5mm: clean.
• Bugs Bunny Ear Overhang (60°) — 5mm: clean. No change.

Pattern: branch distance correlates strongly with overhang angle. Rough rule:

• 60-65° overhangs — 5mm default works fine
• 66-70° — drop to 3.5-4.0mm
• 71-75° — drop to 2.5-3.0mm
• 76-80° — drop to 2.0-2.5mm
• 81-85° — drop to 1.5-2.0mm AND check 'Maximum branch angle' setting
• 86°+ — tree supports may fail entirely; switch to Normal supports

Print Time and Material Impact

Reducing branch distance increases support filament use. Real numbers from the 8-model sweep (averages):

• 5mm (default) — support uses 12% of total filament. Reference.
• 3.5mm — support uses 16% (+33%). Print time +8%.
• 2.5mm — support uses 21% (+75%). Print time +15%.
• 2.0mm — support uses 28% (+133%). Print time +22%.
• 1.5mm — support uses 38% (+217%). Print time +30%.

The trade-off: tighter spacing wastes filament but recovers it on PRINT QUALITY. Failed prints from default 5mm spacing waste 100% of filament (whole print is scrap) and 100% of time. A 30% time/filament increase on supports to AVOID failed prints is net positive.

Other Critical Tree-Support Parameters

Branch distance is the most impactful parameter, but two others matter for steep overhangs:

Maximum branch angle (default 45°)

How much branches can lean from vertical. At 45°, branches reach widely (covering more overhang area per trunk) but tip over on tall thin supports. For overhangs > 75°, reduce to 30°. Branches become more vertical, less likely to fail mid-print.

Branch diameter (default 2.0mm)

Thickness of each branch. Default 2.0mm. For VERY steep overhangs combined with small branch distance, branches can be too thin to support the load. Increase to 2.5mm — adds 10-15% support material but prevents branch failure.

Top z distance (default 0.20mm)

Vertical gap between top of support and the overhang surface. Default 0.20mm = 1 layer at 0.20mm layer height. Too low: support fuses to the model, hard to remove. Too high: poor surface finish because support is too far from overhang. Keep at 1 layer height. For 0.16mm prints, set to 0.16mm; for 0.28mm prints, set to 0.28mm.

Using Modifier Blocks for Localized Tight Support

If your model has one critical 80° overhang but the rest is at 60°, halving branch distance globally is wasteful. Bambu Studio supports modifier blocks that change settings for a specific region:

1. Right-click model → Add Modifier → Block
2. Position the block to enclose the steep-overhang region (use Move and Scale tools)
3. Click the modifier block (it appears as wireframe)
4. Inspector → Process → Support → set Branch distance = 2.0mm (or whatever your steep area needs)
5. Outside the block, the global 5mm setting still applies

Saves 20-30% of support material compared to global reduction. Especially useful for build-a-hero figures where 1-2 joints have steep undersides and the rest of the body is mild.

When to Use Normal Supports Instead

Tree supports are usually better than normal supports (cleaner surface, less material, easier removal). But three cases where Normal still wins:

• Overhangs > 85° — tree branches physically can't span the gap without sagging. Normal supports' line pattern handles this.
• Overhangs inside deep pockets — tree algorithm can't navigate into pockets, leaves them unsupported. Normal supports fill any enclosed space.
• Very large flat overhangs (> 50mm × 50mm) — tree supports tip over from their own height; normal supports' uniform grid stays stable.

Per-Material Adjustments

Material affects how forgiving an overhang is, which affects required branch distance:

• PLA Basic — base values (most forgiving — cools fast, holds shape between contact points)
• PLA Silk / Matte — same as basic (similar cooling behavior)
• PETG — reduce branch distance by 0.5mm vs PLA (more sag between supports due to higher viscosity at print temp)
• TPU 95A — reduce by 1.0mm (much more sag — TPU drapes between supports)
• ABS / ASA — same as PLA (in heated chamber); add 0.5mm in unheated chamber
• PA-CF — increase branch distance by 0.5mm (carbon fibers add stiffness, can bridge longer gaps)

Summary

• Default Bambu tree-support branch distance is 5mm
• 5mm works for 60-65° overhangs; fails on 75°+ overhangs
• Rule of thumb: drop branch distance by 0.5mm for every 5° steeper than 70°
• Reduced branch distance costs 30-70% more support filament — worth it to avoid failed prints
• Use modifier blocks for localized tight support without global cost
• For overhangs > 85° or inside pockets: switch to Normal supports instead
• PETG and TPU need tighter branch distance than PLA at same overhang angle