diff --git a/src/analysis.rs b/src/analysis.rs
index 38e350a..074c5e4 100644
--- a/src/analysis.rs
+++ b/src/analysis.rs
@@ -25,6 +25,74 @@ use crate::recording::UseCase;
 use crate::telemetry_frame::{gear_display, TelemetryFrame};
 use crate::track_model::{Corner, Sector, TrackModel, TrackModelSource};
 
+// ---------------------------------------------------------------------------
+// Frame trimming utilities
+// ---------------------------------------------------------------------------
+
+/// Split a run's frames into segments by lap_count transitions.
+fn split_by_lap(frames: &[TelemetryFrame]) -> Vec<&[TelemetryFrame]> {
+    if frames.is_empty() {
+        return Vec::new();
+    }
+    let mut segments = Vec::new();
+    let mut seg_start = 0;
+    let mut prev_lap = frames[0].lap_count;
+    for (i, f) in frames.iter().enumerate() {
+        if f.lap_count != prev_lap {
+            segments.push(&frames[seg_start..i]);
+            seg_start = i;
+            prev_lap = f.lap_count;
+        }
+    }
+    segments.push(&frames[seg_start..]);
+    segments
+}
+
+/// Return frames belonging to complete laps only.
+///
+/// A complete lap covers most of the track (spline spans ≥ 0.9 of [0, 1]).
+/// The first segment may start mid-track (driver joined before start/finish)
+/// but still complete a full revolution — spline wraps from the join point
+/// through 1.0 back to near 0. The last segment is often a partial out-lap
+/// (driver stopped before completing the lap).
+fn complete_lap_frames(frames: &[TelemetryFrame]) -> Vec<TelemetryFrame> {
+    let segments = split_by_lap(frames);
+    if segments.is_empty() {
+        return frames.to_vec();
+    }
+
+    let mut result = Vec::new();
+    for seg in &segments {
+        let splines: Vec<f32> = seg.iter().filter(|f| f.spline_position > 0.0).map(|f| f.spline_position).collect();
+        if splines.is_empty() {
+            continue;
+        }
+        let min_s = splines.iter().cloned().fold(f32::INFINITY, f32::min);
+        let max_s = splines.iter().cloned().fold(f32::NEG_INFINITY, f32::max);
+        let span = max_s - min_s;
+        // Complete lap: spline covers ≥ 0.9 of the track, or wraps around
+        // (min near 0, max near 1).
+        let wraps = min_s < 0.1 && max_s > 0.9;
+        if span >= 0.9 || wraps {
+            result.extend_from_slice(seg);
+        }
+    }
+
+    // If no segment qualifies (e.g., no spline data), fall back to longest.
+    if result.is_empty() {
+        let longest = segments.iter().max_by_key(|s| s.len());
+        if let Some(seg) = longest {
+            return seg.to_vec();
+        }
+    }
+
+    if result.is_empty() {
+        frames.to_vec()
+    } else {
+        result
+    }
+}
+
 // ---------------------------------------------------------------------------
 // Shift Point Analysis
 // ---------------------------------------------------------------------------
@@ -80,9 +148,28 @@ pub fn analyze_shift_points(
     for (run_idx, run) in runs.iter().enumerate() {
         let label = run_labels.get(run_idx).cloned().unwrap_or_default();
 
-        let valid: Vec<&TelemetryFrame> = run
+        // Trim to the WOT window: find the first and last frames where
+        // throttle is near 100%. This removes pre-launch and post-lift data.
+        let wot_start = run.iter().position(|f| f.throttle >= 0.9);
+        let wot_end = run.iter().rposition(|f| f.throttle >= 0.9);
+        let trimmed: &[TelemetryFrame] = match (wot_start, wot_end) {
+            (Some(s), Some(e)) if e > s => &run[s..=e],
+            _ => run.as_slice(),
+        };
+
+        // Filter for valid WOT acceleration frames.
+        // Exclude rev-limiter plateau: when speed stops increasing (delta < 0.5 km/h
+        // over the window), the engine is bouncing off the limiter and acceleration
+        // data is meaningless.
+        let valid: Vec<&TelemetryFrame> = trimmed
             .iter()
-            .filter(|f| f.throttle >= 0.9 && f.gear >= 2 && f.speed_kmh > 1.0 && f.engine_rpm > 500.0)
+            .filter(|f| {
+                f.throttle >= 0.9
+                    && f.gear >= 2
+                    && f.speed_kmh > 1.0
+                    && f.engine_rpm > 500.0
+                    && f.acc_g_frontal > 0.01
+            })
             .collect();
 
         if valid.len() < 10 {
@@ -292,7 +379,7 @@ fn lerp_search<T>(
 // Track Map Analysis
 // ---------------------------------------------------------------------------
 
-#[derive(Debug, Clone, Serialize, PartialEq)]
+#[derive(Debug, Clone, Serialize, serde::Deserialize, PartialEq)]
 pub enum PositionMode {
     Spline,
     Coordinate,
@@ -362,9 +449,15 @@ pub fn analyze_track_map(
     let mut boundaries: Vec<TrackBoundary> = Vec::new();
 
     for (run, label) in runs.iter().zip(labels.iter()) {
+        // Trim to complete laps only — partial laps at the start/end of
+        // the recording produce artifacts when spline-sorted (the car
+        // was not at the boundary edge during those segments).
+        let trimmed = complete_lap_frames(run);
+
         let mut points: Vec<TrackPoint> = match &mode {
             PositionMode::Spline => {
-                run.iter()
+                trimmed
+                    .iter()
                     .filter(|f| f.spline_position > 0.0)
                     .map(|f| TrackPoint {
                         x: f.world_x,
@@ -375,7 +468,7 @@ pub fn analyze_track_map(
                     .collect()
             }
             PositionMode::Coordinate => {
-                let mut pts: Vec<TrackPoint> = run
+                let mut pts: Vec<TrackPoint> = trimmed
                     .iter()
                     .filter(|f| f.world_x != 0.0 || f.world_z != 0.0)
                     .map(|f| TrackPoint {
@@ -909,6 +1002,195 @@ fn downsample<T: Clone>(items: Vec<T>, target: usize) -> Vec<T> {
     result
 }
 
+// ---------------------------------------------------------------------------
+// Racing Line Analysis
+// ---------------------------------------------------------------------------
+
+#[derive(Debug, Clone, Serialize, serde::Deserialize)]
+pub struct RacingLinePoint {
+    pub x: f32,
+    pub y: f32,
+    pub z: f32,
+    pub speed_kmh: f32,
+    pub spline: f32,
+}
+
+#[derive(Debug, Clone, Serialize, serde::Deserialize)]
+pub struct RacingLap {
+    pub label: String,
+    pub lap_index: usize,
+    pub point_count: usize,
+    pub points: Vec<RacingLinePoint>,
+    pub lap_time_ms: i32,
+    pub length_m: f32,
+    pub avg_speed_kmh: f32,
+    pub max_speed_kmh: f32,
+}
+
+#[derive(Debug, Clone, Serialize, serde::Deserialize)]
+pub struct RacingLineAnalysis {
+    pub laps: Vec<RacingLap>,
+    pub position_mode: PositionMode,
+    pub total_laps: usize,
+    pub total_frames_analyzed: usize,
+}
+
+/// Split a run's frames into individual laps based on lap_count transitions.
+fn split_into_laps(frames: &[TelemetryFrame]) -> Vec<Vec<TelemetryFrame>> {
+    if frames.is_empty() {
+        return Vec::new();
+    }
+
+    let mut laps: Vec<Vec<TelemetryFrame>> = Vec::new();
+    let mut current: Vec<TelemetryFrame> = Vec::new();
+    let mut prev_lap = frames[0].lap_count;
+
+    for f in frames {
+        if f.lap_count != prev_lap && !current.is_empty() {
+            laps.push(std::mem::take(&mut current));
+        }
+        current.push(f.clone());
+        prev_lap = f.lap_count;
+    }
+    if !current.is_empty() {
+        laps.push(current);
+    }
+    laps
+}
+
+pub fn analyze_racing_line(
+    runs: &[Vec<TelemetryFrame>],
+    labels: &[String],
+) -> RacingLineAnalysis {
+    let all_frames: Vec<TelemetryFrame> = runs.iter().flat_map(|r| r.iter()).cloned().collect();
+    let mode = detect_position_mode(&all_frames);
+    let mut laps_out: Vec<RacingLap> = Vec::new();
+    let mut total_frames = 0usize;
+
+    for (run, label) in runs.iter().zip(labels.iter()) {
+        let laps = split_into_laps(run);
+
+        for (lap_idx, lap_frames) in laps.iter().enumerate() {
+            if lap_frames.is_empty() {
+                continue;
+            }
+
+            // Skip partial laps: check spline coverage. A complete lap's
+            // spline values span ≥ 0.9 of [0, 1] (the car went almost all
+            // the way around). Partial laps at the start (joined mid-track)
+            // and end (session ended early) are discarded.
+            if laps.len() > 1 {
+                let splines: Vec<f32> = lap_frames.iter()
+                    .filter(|f| f.spline_position > 0.0)
+                    .map(|f| f.spline_position)
+                    .collect();
+                if !splines.is_empty() {
+                    let min_s = splines.iter().cloned().fold(f32::INFINITY, f32::min);
+                    let max_s = splines.iter().cloned().fold(f32::NEG_INFINITY, f32::max);
+                    let span = max_s - min_s;
+                    let wraps = min_s < 0.1 && max_s > 0.9;
+                    if span < 0.9 && !wraps {
+                        continue;
+                    }
+                }
+            }
+
+            let mut points: Vec<RacingLinePoint> = lap_frames
+                .iter()
+                .filter(|f| {
+                    if matches!(mode, PositionMode::Spline) {
+                        f.spline_position > 0.0
+                    } else {
+                        f.world_x != 0.0 || f.world_z != 0.0
+                    }
+                })
+                .map(|f| RacingLinePoint {
+                    x: f.world_x,
+                    y: f.world_y,
+                    z: f.world_z,
+                    speed_kmh: f.speed_kmh,
+                    spline: f.spline_position,
+                })
+                .collect();
+
+            if points.is_empty() {
+                continue;
+            }
+
+            total_frames += lap_frames.len();
+
+            // Compute track length from consecutive point distances
+            let mut length_m = 0.0f32;
+            for i in 1..points.len() {
+                let dx = points[i].x - points[i - 1].x;
+                let dz = points[i].z - points[i - 1].z;
+                length_m += (dx * dx + dz * dz).sqrt();
+            }
+
+            // Sort by spline if in spline mode for a clean path
+            if matches!(mode, PositionMode::Spline) {
+                points.sort_by(|a, b| {
+                    a.spline
+                        .partial_cmp(&b.spline)
+                        .unwrap_or(std::cmp::Ordering::Equal)
+                });
+                points.dedup_by(|a, b| (a.spline - b.spline).abs() < 0.0001);
+            }
+
+            // Downsample for efficient rendering (target ~400 points/lap)
+            let point_count_before = points.len();
+            points = downsample(points, 400);
+
+            let max_speed = lap_frames
+                .iter()
+                .map(|f| f.speed_kmh)
+                .fold(0.0f32, f32::max);
+
+            let avg_speed = if length_m > 0.0 {
+                let best_lap_s = lap_frames
+                    .last()
+                    .map(|f| f.lap_time_ms)
+                    .unwrap_or(0)
+                    .max(0) as f32
+                    / 1000.0;
+                if best_lap_s > 0.0 {
+                    (length_m / best_lap_s) * 3.6
+                } else {
+                    let frame_count = lap_frames.len() as f32;
+                    lap_frames.iter().map(|f| f.speed_kmh).sum::<f32>() / frame_count
+                }
+            } else {
+                0.0
+            };
+
+            let lap_time = lap_frames
+                .last()
+                .map(|f| f.lap_time_ms)
+                .unwrap_or(0);
+
+            laps_out.push(RacingLap {
+                label: label.clone(),
+                lap_index: lap_idx,
+                point_count: point_count_before,
+                points,
+                lap_time_ms: lap_time,
+                length_m,
+                avg_speed_kmh: avg_speed,
+                max_speed_kmh: max_speed,
+            });
+        }
+    }
+
+    let total_laps = laps_out.len();
+
+    RacingLineAnalysis {
+        laps: laps_out,
+        position_mode: mode,
+        total_laps,
+        total_frames_analyzed: total_frames,
+    }
+}
+
 // ---------------------------------------------------------------------------
 // Dispatch
 // ---------------------------------------------------------------------------
@@ -939,6 +1221,14 @@ pub fn analyze(
             serde_json::to_value(analyze_track_map(frames_by_run, run_labels))
                 .unwrap_or_else(|_| serde_json::json!({"error": "serialization failed"}))
         }
+        UseCase::RacingLine => {
+            if frames_by_run.is_empty() || frames_by_run.iter().all(|r| r.is_empty()) {
+                serde_json::json!({"error": "no frames provided"})
+            } else {
+                serde_json::to_value(analyze_racing_line(frames_by_run, run_labels))
+                    .unwrap_or_else(|_| serde_json::json!({"error": "serialization failed"}))
+            }
+        }
         _ => serde_json::json!({
             "status": "not_implemented",
             "use_case": format!("{:?}", use_case),
diff --git a/src/api.rs b/src/api.rs
index 634290f..d4343f4 100644
--- a/src/api.rs
+++ b/src/api.rs
@@ -636,6 +636,10 @@ async fn build_track(
     let game = req.game.unwrap_or_else(|| manifest.game.to_track_dir_name().to_string());
     let track_name = req.track_name.unwrap_or(manifest.track);
 
+    if game.is_empty() || track_name.is_empty() {
+        return err(StatusCode::BAD_REQUEST, "Game and track name are required. Specify them in the request or set them when starting the recording.");
+    }
+
     let boundaries = {
         let mgr = lock_sessions(&state);
         let session = match mgr.get_session(&req.session_id) {
diff --git a/src/session.rs b/src/session.rs
index 1948832..465cd3a 100644
--- a/src/session.rs
+++ b/src/session.rs
@@ -161,8 +161,13 @@ impl SessionManager {
     }
 
     /// Atomically write bytes to a path via temp file + rename (same directory).
+    /// Uses a unique temp file name to avoid collisions between concurrent writes.
     fn atomic_write(path: &Path, bytes: &[u8]) {
-        let tmp = path.with_extension("json.tmp");
+        let tmp = path.with_file_name(format!(
+            ".{}.{}.tmp",
+            path.file_name().map(|n| n.to_string_lossy().into_owned()).unwrap_or_default(),
+            uuid::Uuid::new_v4()
+        ));
         if let Err(e) = std::fs::write(&tmp, bytes) {
             eprintln!("[WARN] Failed to write temp file {}: {}", tmp.display(), e);
             return;
@@ -173,8 +178,8 @@ impl SessionManager {
         }
     }
 
-    /// Persist session metadata (compact JSON, excludes frames) synchronously,
-    /// then write frame data off-thread to a separate file.
+    /// Persist session metadata (compact JSON, excludes frames) and frame data
+    /// synchronously. Both files use atomic write (temp file + rename).
     fn persist_session(&self, id: &str) {
         let Some(session) = self.sessions.get(id) else {
             return;
@@ -188,17 +193,17 @@ impl SessionManager {
             Err(e) => eprintln!("[WARN] Failed to serialize session metadata {}: {}", id, e),
         }
 
-        // Frames are large — clone under lock (memcpy) then serialize + write off-thread.
+        // Frames — clone under lock, then serialize + write synchronously.
+        // Must be synchronous: off-thread writes race on the same file,
+        // and an earlier call (with empty frames) can overwrite a later one.
         let runs_frames: Vec<Vec<TelemetryFrame>> =
             session.runs.iter().map(|r| r.frames.clone()).collect();
         let frames_path = self.frames_file_path(&id);
-        std::thread::spawn(move || {
-            let frames_file = SessionFramesFile { runs: runs_frames };
-            match serde_json::to_vec(&frames_file) {
-                Ok(bytes) => Self::atomic_write(&frames_path, &bytes),
-                Err(e) => eprintln!("[WARN] Failed to serialize frames for session {}: {}", id, e),
-            }
-        });
+        let frames_file = SessionFramesFile { runs: runs_frames };
+        match serde_json::to_vec(&frames_file) {
+            Ok(bytes) => Self::atomic_write(&frames_path, &bytes),
+            Err(e) => eprintln!("[WARN] Failed to serialize frames for session {}: {}", id, e),
+        }
     }
 
     fn delete_session_file(&self, id: &str) {