# Optimize Substep values

**URL:** <https://forum.itascainternational.com/t/optimize-substep-values/1995>\
**Category:** FLAC3D\
**Tags:** tips, suggestions, flac3d\
**Created:** [March 18, 2025, 5:00am UTC](https://forum.itascainternational.com/t/optimize-substep-values/1995 "2025-03-18T05:00:21Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![sowmyaramanis](https://avatars.discourse-cdn.com/v4/letter/s/839c29/32.png) [@sowmyaramanis](https://forum.itascainternational.com/u/sowmyaramanis)\
**Post date:** [March 18, 2025, 5:00am UTC](https://forum.itascainternational.com/t/optimize-substep-values/1995/1 "2025-03-18T05:00:21Z")

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In consolidation analysis, how to choose the substep values for mech and fluid?

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**Author:** ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)\
**Post date:** [April 15, 2025, 7:54pm UTC](https://forum.itascainternational.com/t/optimize-substep-values/1995/2 "2025-04-15T19:54:05Z")

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Are you talking about a fully-coupled, fluid-mechanical analysis? Usually the fluid timestep is significantly bigger than the mechanical timestep, so you want to set fluid to be the leader and mechanical to be the follower. This will then execute one fluid step and multiple mechanical steps. In theory, you should cycle each mechanical part to equilibrium, (set the number of mechanical steps to a large number and it will stop cycling when it reaches equilibrium). However, this can be slow, so you probably want to limit the number of mechanical steps. The number of mechanical steps depends on how accurate you want to be and how much time you have to run your model. I wouldn’t go less than 10.

Note that in the subscription version, all of this coupling logic is much simpler (see [here](https://docs.itascacg.com/itasca930/flac3d/docproject/source/theory/fluid-mechanical/solving/solving.html?node2574#coupled-flow-and-mechanical-calculations))