docs/impl/reports/Z27AG-DECODE-3.md

---
type: report
date: 2026-10-15
---
# Z27AG CAN: third pass, the drive back

Sessions `000011-20261015T212904` and `000013-20261015T214544` (the drive back, about 13 minutes,
no marks) together with pass 2's `000008` and `000009`: 20 km of driving with GNSS and the calibrated
IMU, no OBD-II. Nothing was marked, so this pass uses what any drive has: corners and distance.

## Confirmed

| Signal | Where | Evidence | Registered as |
|---|---|---|---|
| Wheel speeds | `235` four words, little-endian (pass 2): bytes 0–1 **front right**, 2–3 **front left**, 4–5 **rear right**, 6–7 **rear left** | from corners, below | `wheel_speed_fr_kph`, `_fl_`, `_rr_`, `_rl_` |
| Odometer | `570` bytes 4–6, little-endian, **1 km** | steps once per kilometre: 12 steps over 11.9 km (`000009`), 3 over 3.0 km (`000011`); 167966 on the 13th, 167988 at the end of the drive back; valid with the engine off too. Agrees with the cluster's (digital) odometer | `odometer_km` (the layout format gained 24-bit fields) |
| Distance counter | `14D` bytes 6–7, little-endian, **about 0.40 m** per count | tracks distance from the wheel speeds with correlation 1.0000 in both drives (103.2 and 103.6 m per step of byte 7); wraps every ~26 km | not registered. The cluster's two trip meters count 0.1 km and may run from this; it is also the finest distance on the bus (slow creeping, short manoeuvres) |

### Which word is which wheel

In a corner the outer wheels run faster than the inner ones, by about half the track width times the
curvature; and the fronts run on a wider arc than the rears, by about half the wheelbase squared times
the curvature squared. Fitting each word's share of the four (its speed ÷ their mean) against the
curvature from the steering angle (`steering_angle_deg` + 2.7° for the straight-ahead offset, ÷ 16.3,
over a 2.5 m wheelbase), 74 476 frames above 15 km/h:

| Word | Linear term (track) | Square term (wheelbase) | So |
|---|---|---|---|
| 0 | +0.72 m | +1.4 m² | right, front |
| 1 | −0.72 m | +1.4 m² | left, front |
| 2 | +0.75 m | −0.9 m² | right, rear |
| 3 | −0.75 m | −1.9 m² | left, rear |

Curvature is positive turning left, so a positive linear term is a wheel on the right (outside in a left
turn). The size, ±0.72–0.75 m, is half the Colt's track (1.45–1.46 m). The square terms split the words
into a pair above the mean and a pair below it, ±1.5 m² against an expected ±1.6 m² (2.5² ÷ 4). Medians
by curvature agree: in left turns tighter than 20 m (`κ > 0.05`) the words read 1.068, 0.956, 1.044,
0.932 of their mean.

Straight ahead the fronts read 0.6 % more counts than the rears, matching pass 2's per-word scales
(0.0578 against 0.0581 km/h per count). Driven wheels or tyre sizes, either is ordinary; each pair is
registered with its own scale, so all four read GNSS speed when cruising.

Jack stands (checklist 23–26) would still confirm this, but are no longer needed to find it.

## Candidates

- **`570` bytes 2 and 3: fuel, most likely.** Both fall slowly: byte 2 read 87 on the 13th and 76
  at the end of the drive back, 22 km later. Both read `7F` with the engine off. Outside temperature
  was the other reading, but the car shows it nowhere, and the bytes **slosh**. Take away their own
  60 s average, and what is left follows the car's motion in all three drives:

  | Byte | Lateral g | Longitudinal g | Lag |
  |---|---|---|---|
  | 3 | +0.70 to +0.81 | −0.42 to −0.48 | none |
  | 2 | +0.64 to +0.75 | −0.48 to −0.50 | about 3 s |

  Lateral g here is from the wheel speeds and the steering angle, longitudinal from the change in
  wheel speed. A temperature would not do this; liquid in a tank would. Byte 3 looks like the sender
  as it is, and byte 2 a damped copy (a gauge needle is damped the same way). The "rise while parked"
  (78 → 81) was the same thing: the drive before ended at 80, parked in the same spot (the same
  IMU tilt to three decimals). It read 76 a minute earlier, while still moving. The reading moves about
  ±4 with tilt and motion, so it needs averaging over minutes.
  Scale unknown: full is unlikely to be 100 or 255. A refuel gives two points (before and after,
  parked level, with the litres put in). Not registered until then.

## Notes on the recording

- `000012` is a 25-second parked session in between; `000013` ends with the PEAK unplugged.
- The recorder kept IMU samples as JSON lines (34 MB for `000009`). From this pass on it packs them
  (`imu.bin`, svc-core `imu/packed.rs`): the same samples are 8.5 MB, and all of `000009`'s samples fit
  the packed form exactly.