Most curling games treat sweeping as a single dial: harder sweep, more distance. Curling Ice adds a second axis. Sweep the side of the stone the curl is bending toward and you hold the line straighter. Sweep the far side and you exaggerate the curl instead. It's a real layer of shot-shaping most curling simulators don't model at all.
This page explains the gesture, the formula behind it, and when a curler would actually reach for each side.
In the game
Sweep straight up and down across the stone's path and you get conventional sweep: less friction, more distance, a small amount of straightening. Sweep at a sustained diagonal, consistently biased to one side of the stone for several strokes, and Curling Ice reads that as a committed directional sweep: inside or outside. Alternate strokes back and forth and the two cancel out, so the stone stays on conventional sweep. This mirrors how real sweepers commit a broom to one edge of the stone rather than sweeping dead center.
Under the hood
Directional sweep modulates the stone's curl magnitude only, layered on top of the base sweep model described on the Ice Physics page (where sweeping already reduces friction and trims curl by up to 65% at full effort, regardless of side). The directional layer decides whether that curl gets pushed further down, or pushed back up, based on which side of the stone you're sweeping relative to the direction it's already curling.
Because sweepSide is curl-relative rather than left/right, "inside" always means the same thing whether you threw in-turn or out-turn: the side the stone is bending toward. Sweep that side (a factor below 1) and curl is suppressed. Sweep the opposite side, the outside (a factor above 1), and curl is enhanced. The model is inspired by published research on sweep-induced pivoting, where uneven friction from one-sided sweeping causes the stone to swing around its higher-friction, unswept edge.
Inside vs. outside
Neither side is "better." They answer opposite questions: is the stone about to curl too much, or not enough?
Sweep the side the stone is curling toward and you fight the curl, holding the stone straighter for longer. Reach for this when the read is already tight and any extra bend takes you somewhere you don't want to be.
Sweep the far side and you exaggerate the curl instead of fighting it. Reach for this when the stone needs to bend more than its natural path to get where it's going.
Practical tips
Go diagonal, not vertical. A straight up-and-down sweep always reads as conventional. You need a sustained diagonal motion across the stone's path before the game reads it as a side.
Commit to one side. The system accumulates a vote across several strokes before committing. Back-and-forth strokes that alternate sides cancel out and you stay conventional, on purpose, so an accidental wobble doesn't flip your sweep mid-shot.
Harder sweeping means a stronger effect. sweepStrength scales with how hard and fast you're actually sweeping. A light directional sweep barely nudges the curl; a full-effort one gets much closer to the directionalGain ceiling.
Conventional sweep is still the default, and that's fine. Most shots don't need a committed side. Directional sweep is a precision tool for the moments where the read is close and a small shift in curl changes the outcome.
Raise curlMaxEffect to make side choice matter more. On the Ice Physics settings panel, a higher curl suppression ceiling gives directional sweep more room to work with on every shot.