Vehicle Performance Metrics

Vehicle metrics are ways to characterize a physical system. They are not a single universal score, and they are not meaningful without the maneuver, operating region, signal definitions, and extraction method that produced them.

BobSim exports a practical set of metrics today. Those exported metrics are listed on this page, but they are not the end-all reference for vehicle characterization. Other metrics can be equally valid when they answer a clear engineering question and remain tied to the original vehicle response.

The right question is not "which metric is best?" The right question is:

• what vehicle behavior is being characterized
• what maneuver or data region supports the metric
• what assumptions are hidden by the reduction
• whether the metric reflects the physical system in the region being used
• how much uncertainty remains

Reading A Metric

A useful vehicle metric should define:

Requirement	Meaning
Signal source	Simulation, test data, competition telemetry, or reduced-order model
Maneuver	Steady-state sweep, transient steer, four-post sweep, envelope map, track section
Operating region	Speed, load, acceleration, steering, tire state, aero state, surface
Extraction method	Peak value, gradient, fit, trim condition, frequency response, time-domain event
Units and sign convention	The physical direction and scaling of the reported value
Coverage	Where the data supports the claim and where it does not

The same number can mean different things if the maneuver or extraction method changes. A steering gradient from a ramp-steer path is not automatically the same as a trim steering gradient from a YMD map.

Current BobSim Metric Families

BobSim currently exports metrics from StandardSim, FourPostEval, EnvelopeSim, and OptSim sensitivity workflows. These names are implementation artifacts, but they also show the major characterization categories BobDyn currently supports.

Steady-State Handling

SteadyStateEval characterizes quasi-steady lateral response from ramp-steer velocity isolines.

Current exported groups:

Group	Current metrics
Run health and source	n_cases, n_successful_cases, n_failed_cases, standard_sweep_max_ay_mps2, metric_target_velocity_mps, metric_source_velocity_mps
Lateral range	ay_min, ay_max, limit_ay_mps2
Linear response gradients	sideslip_gradient_rad_per_mps2, sideslip_gradient_deg_per_g, understeer_gradient_rad_per_mps2, understeer_gradient_deg_per_g, handwheel_angle_gradient_rad_per_mps2, handwheel_angle_gradient_deg_per_g, roll_gradient_deg_per_g
Limit response gradients	limit_sideslip_gradient_rad_per_mps2, limit_sideslip_gradient_deg_per_g, limit_understeer_gradient_rad_per_mps2, limit_understeer_gradient_deg_per_g, limit_handwheel_gradient_rad_per_mps2, limit_handwheel_gradient_deg_per_g, limit_roll_gradient_rad_per_mps2, limit_roll_gradient_deg_per_g
Steering effort	peak_handwheel_torque_Nm, peak_handwheel_torque_ay_mps2
Fit quality	roadwheel_fit_nrmse, handwheel_fit_nrmse, steer_excess_fit_nrmse, roll_fit_nrmse, sideslip_fit_nrmse
Velocity sensitivity	sideslip_gradient_velocity_slope_deg_per_g_per_mps, limit_sideslip_gradient_velocity_slope_deg_per_g_per_mps, understeer_gradient_velocity_slope_deg_per_g_per_mps, limit_understeer_gradient_velocity_slope_deg_per_g_per_mps, handwheel_angle_gradient_velocity_slope_deg_per_g_per_mps, limit_handwheel_gradient_velocity_slope_deg_per_g_per_mps, roll_gradient_velocity_slope_deg_per_g_per_mps, limit_roll_gradient_velocity_slope_deg_per_g_per_mps

Useful related metrics people may compute:

• roadwheel angle gradient versus measured lateral acceleration
• handwheel torque gradient and steering effort linearity
• curvature or radius tracking error
• yaw-rate gain versus lateral acceleration
• front/rear tire utilization at each lateral acceleration
• tire normal-load range and load-transfer split
• steering hysteresis or compliance steer if test data supports it
• local linearity loss or saturation onset by signal derivative
• confidence intervals on gradients from repeated sweeps

Common formulas:

where is lateral acceleration, is speed, is turn radius, and is a steering measure such as roadwheel or handwheel angle.

Transient Handling

TransientEval characterizes response to step steer and continuous sine steer inputs.

Current exported groups:

Group	Current metrics
Run health and source	n_cases, n_successful_cases, n_failed_cases, representative_testVel_mps, metric_target_velocity_mps, n_velocity_groups
Lateral acceleration step response	ay_peak, ay_ss, ay_rise_time_s, ay_peak_response_time_s, ay_gain_dc, ay_overshoot_pct
Sideslip step response	sideslip_ss, sideslip_rise_time_s, sideslip_gain_dc
Yaw step response	yaw_peak, yaw_ss, yaw_rise_time_s, yaw_peak_response_time_s, yaw_gain_dc, yaw_overshoot_rad_per_s, yaw_overshoot_pct
Roll step response	roll_peak, roll_ss, roll_gain_dc, roll_overshoot_rad, roll_overshoot_pct
Settling	settling_time_s
Frequency response gain	ay_gain_dc, yaw_gain_dc, ay_gain_peak, ay_gain_peak_freq, yaw_gain_peak, yaw_gain_peak_freq, bandwidth_hz, gain_variation_pct
Frequency response phase and lag	ay_phase_1hz, ay_phase_0p5hz, yaw_phase_1hz, yaw_phase_0p5hz, ay_lag_0p5hz, yaw_lag_0p5hz, yaw_to_ay_lag_0p5hz, ay_lag_1hz, yaw_lag_1hz, yaw_to_ay_lag_1hz, lag_steer_to_ay, lag_steer_to_yaw, yaw_to_ay_lag, ay_phase_45_freq, yaw_phase_45_freq
Frequency trend and fit quality	ay_gain_slope, yaw_gain_slope, ay_phase_slope, yaw_phase_slope, ay_fit_error, yaw_fit_error
Velocity sensitivity	BobSim also exports _velocity_slope versions of the main transient response and frequency response metrics

Useful related metrics people may compute:

• peak yaw acceleration and time to peak yaw acceleration
• steering input rise time and actuator delay
• 10 to 90 percent rise time instead of 50 to 90 percent rise time
• damping ratio and natural frequency from a fitted reduced model
• control effort required to hold a path after the initial input
• steering correction count, reversal rate, or correction energy
• response repeatability across repeated runs
• tire relaxation contribution to lag
• transient tire load variation
• driver-to-vehicle closed-loop response if driver data is included

Common formulas:

where is yaw rate, is steering input, is yaw-rate response, and is steering input in the frequency domain.

Frequency-Domain Characterization

Frequency-domain metrics describe how the vehicle responds across input frequency rather than at a single time or trim point.

Current BobSim frequency metrics are exported through TransientEval, but the valid characterization space is broader:

• gain and phase from steering to yaw rate
• gain and phase from steering to lateral acceleration
• bandwidth
• resonance peak and peak frequency
• phase crossing frequencies such as -45 degrees
• equivalent time lag
• gain and phase slopes versus frequency
• coherence or fit quality when using measured data
• uncertainty bounds from repeated sine sweeps

These metrics are especially useful when comparing driver feel, steering response, and whether a setup change shifts response timing in a desirable direction.

Suspension, Kinematics, And Compliance

FourPostEval characterizes heave and roll response through K&C-style output records.

Current exported groups:

Group	Current metrics
Heave alignment gains	camber_gain_heave_rad_per_m, toe_gain_heave_rad_per_m, caster_gain_heave_rad_per_m, kpi_gain_heave_rad_per_m, trail_gain_heave_m_per_m, scrub_gain_heave_m_per_m
Roll alignment gains	camber_gain_roll_rad_per_rad, toe_gain_roll_rad_per_rad, caster_gain_roll_rad_per_rad, kpi_gain_roll_rad_per_rad, trail_gain_roll_m_per_rad, scrub_gain_roll_m_per_rad
Motion ratios	avg_motion_ratio_front, avg_motion_ratio_rear, avg_stabar_motion_ratio_front, avg_stabar_motion_ratio_rear
Anti and jacking behavior	avg_anti_dive_pct, avg_anti_squat_pct, avg_anti_roll_front_pct, avg_anti_roll_rear_pct, avg_lateral_jacking_coeff_front, avg_lateral_jacking_coeff_rear, avg_longitudinal_jacking_coeff_front, avg_longitudinal_jacking_coeff_rear
Roll stiffness and LLTD	spring_roll_stiffness_front_Nm_per_rad, spring_roll_stiffness_rear_Nm_per_rad, arb_roll_stiffness_front_Nm_per_rad, arb_roll_stiffness_rear_Nm_per_rad, elastic_roll_stiffness_front_Nm_per_rad, elastic_roll_stiffness_rear_Nm_per_rad, avg_lltd_front_frac, avg_lltd_front_pct

Useful related metrics people may compute:

• bump steer and roll steer by wheel and axle
• camber recovery at expected ride and roll states
• compliance steer, compliance camber, and compliance toe under load
• ride rate, roll rate, and pitch rate
• damper motion ratio and damper velocity distribution
• tire normal-load variation through heave and roll
• wheel-center migration and track or wheelbase change
• spring, anti-roll-bar, damper, and jacking contributions to load transfer
• K&C metric uncertainty from hardpoint or compliance uncertainty

Envelope And Limit Metrics

Envelope metrics describe capability limits from reduced-order calculations. They are useful when they remain tied to the physical system and are not treated as a replacement for high-fidelity response validation.

GGV Metrics

Current exported GGV groups:

Group	Current metrics
Speed coverage	reference_speed_mps, n_speed_slices, speed_min_mps, speed_max_mps
Envelope size and reference slice	ggv_volume_g2_mps, ggv_area_ref_g2, ggv_cornering_ref_g, ggv_accel_ref_g, ggv_braking_ref_g
Mean and minimum capability	ggv_mean_cornering_g, ggv_mean_accel_g, ggv_mean_braking_g, ggv_min_cornering_g, ggv_min_accel_g, ggv_min_braking_g
Shape and balance	ggv_area_fill_factor_ref, ggv_longitudinal_balance_ref, ggv_min_area_g2, ggv_min_area_speed_mps, ggv_max_area_g2, ggv_max_area_speed_mps
Peak capability	ggv_max_cornering_g, ggv_max_cornering_speed_mps, ggv_min_cornering_speed_mps, ggv_max_accel_g, ggv_max_accel_speed_mps, ggv_min_accel_speed_mps, ggv_max_braking_g, ggv_max_braking_speed_mps, ggv_min_braking_speed_mps
Track-profile summaries	track_corner_speed_mean_mps, track_corner_speed_min_mps, track_velocity_mean_mps, track_velocity_peak_mps, track_path_distance_m, track_straight_distance_m, track_straight_speed_mean_mps, track_accel_capacity_mean_mps2, track_brake_capacity_mean_mps2, track_longitudinal_capacity_mean_mps2
Normalized track indices	track_lateral_performance_index, track_longitudinal_performance_index, track_accel_performance_index, track_brake_performance_index, track_combined_performance_index

Useful related metrics people may compute:

• tire utilization at each GGV boundary point
• power-limited versus grip-limited regions
• braking bias sensitivity
• aero balance sensitivity with speed
• combined acceleration occupancy for a real track or test section
• uncertainty bounds from tire, mass, aero, or surface assumptions

YMD Metrics

Current exported YMD groups:

Group	Current metrics
Map health	speed_mps, converged_fraction
Peak lateral acceleration	peak_lateral_accel_mps2, peak_lateral_accel_g, peak_lateral_accel_abs_g, yaw_moment_at_peak_lateral_accel_nm, beta_at_peak_lateral_accel_deg, hwa_at_peak_lateral_accel_deg
Yaw moment authority	peak_abs_yaw_moment_nm, yaw_moment_peak_signed_nm, lateral_accel_at_peak_yaw_moment_g, max_positive_yaw_moment_nm, max_negative_yaw_moment_nm, yaw_moment_range_nm, yaw_moment_balance
Local yaw moment gradients	yaw_moment_hwa_gradient_nm_per_deg, yaw_moment_beta_gradient_nm_per_deg
Zero-yaw trim	trim_points_count, trim_peak_lateral_accel_mps2, trim_peak_lateral_accel_g, trim_peak_lateral_accel_abs_g, trim_beta_at_peak_lateral_accel_deg, trim_hwa_at_peak_lateral_accel_deg, trim_max_positive_lateral_accel_g, trim_max_negative_lateral_accel_g, trim_lateral_accel_range_g, trim_lateral_accel_balance, trim_steer_gradient_deg_per_g, trim_beta_gradient_deg_per_g
Zero-input sanity	zero_input_lateral_accel_g, zero_input_yaw_moment_nm
Speed sweep	speed_sweep_min_speed_mps, speed_sweep_max_speed_mps, speed_sweep_peak_lateral_accel_abs_g, speed_sweep_peak_lateral_accel_speed_mps, speed_sweep_peak_trim_lateral_accel_abs_g, speed_sweep_peak_trim_lateral_accel_speed_mps, speed_sweep_peak_abs_yaw_moment_nm, speed_sweep_peak_abs_yaw_moment_speed_mps

Useful related metrics people may compute:

• yaw moment reserve at fixed lateral acceleration
• trim steering requirement over speed
• stable versus unstable trim regions
• yaw moment sensitivity to steering and sideslip over the full map, not only near zero
• asymmetry between left and right capability
• robustness of trim against tire, aero, or mass uncertainty

Sensitivity And Design Exploration Metrics

OptSim does not create a new physical metric family by itself. It compares vehicle metrics across design variables.

Current BobSim sensitivity outputs include:

• StandardSim sensitivity results from selected SteadyStateEval metrics
• EnvelopeSim sensitivity results from reduced envelope metrics
• refined response-surface results
• tornado-style effect sizes
• Pearson correlation tables when enabled
• percent or absolute metric deltas from a baseline

Useful related metrics people may compute:

• local derivative of any metric with respect to a design variable
• normalized sensitivity by realistic manufacturing or setup range
• interaction terms between two or more design variables
• Pareto fronts across competing metrics
• confidence intervals on response-surface fits
• robustness metrics under uncertain tire, mass, aero, or driver assumptions

Sensitivity metrics are not automatically design recommendations. They show how a chosen metric moves under a chosen set of assumptions.

Driver And Telemetry Metrics

Driver and competition telemetry can characterize behavior that pure vehicle models do not capture alone.

Useful metrics include:

• steering correction count
• steering reversal rate
• steering, throttle, and brake smoothness
• control input entropy
• time spent near saturation
• yaw-rate correction after entry
• brake-release timing
• throttle pickup timing
• segment repeatability
• response-space coverage against test sections
• driver-to-driver variance

These metrics become especially useful when paired with the FSAE connection workflow: a test section supports a competition claim only where its response fingerprint overlaps the competition fingerprint.

Reliability, Energy, And Operational Metrics

Vehicle performance is not only handling response.

Useful metrics include:

• energy per distance
• power-limited time or distance
• brake energy and temperature margin
• tire temperature and pressure window
• tire degradation or grip fade
• damper temperature margin
• motor, inverter, and battery thermal margin
• setup repeatability
• failure or derate occurrence
• event-operation robustness

These metrics matter whenever the vehicle must deliver repeated performance, not just peak performance.

Model Quality And Uncertainty Metrics

A metric extracted from a model or reduced tool should carry information about how much trust it deserves.

Useful metrics include:

• fit normalized root-mean-square error
• residual bias
• residual distribution by operating region
• confidence intervals from repeated tests
• sensor noise and calibration uncertainty
• coverage of the response space
• simulation-to-test error by signal
• simulation-to-test error by metric
• extrapolation distance from measured data
• failed-run count and convergence fraction

BobSim already exports some of these, such as fit NRMSE, sine-fit errors, failed-run counts, and YMD convergence fraction. More can and should be added when the workflow needs stronger uncertainty accounting.

Choosing Metrics

Choose metrics by the claim being made:

Claim	Useful metric families
The car has more lateral capability	GGV/YMD limits, measured steady-state lateral acceleration, tire utilization
The car is easier to place at corner entry	yaw and lateral acceleration rise times, phase lag, steering correction behavior
The setup is more predictable near the limit	limit gradients, yaw moment reserve, sideslip behavior, repeatability
The suspension supports the intended platform	motion ratios, roll stiffness, LLTD, camber/toe gains, jacking coefficients
The result should transfer to competition	response-space fingerprint coverage, telemetry overlap, uncertainty bounds
The model is trustworthy	controlled maneuver correlation, residuals, fit quality, coverage, failed-run rate

The current BobSim implementation is one useful implementation of these ideas. It is intentionally open to extension. A new metric is valid when it is tied to the physical vehicle response, extracted consistently, and honest about its coverage and uncertainty.