What Triggers Side Curtain Airbags

Side curtain airbags deploy when the vehicle’s crash-sensing system detects a severe side impact or an actual/imminent rollover, based on inputs such as lateral acceleration, door pressure changes, and roll rate/angle; once thresholds are met, the control unit commands deployment within milliseconds, typically on the struck side for side impacts and on both sides during rollovers. In brief, they are triggered by the airbag control unit interpreting sensor data that indicates a high likelihood of head injury or occupant ejection.

How the System Knows to Fire

Automakers use multiple, redundant sensors feeding an airbag control unit (ACU). The ACU continuously analyzes how hard and where the vehicle is being hit, plus whether it’s rolling. The following sensors are commonly involved and work together to confirm a qualifying event.

• Side-impact accelerometers: Measure sudden lateral acceleration (g-forces) in the body structure (e.g., B-pillars, rockers).
• Door pressure sensors: Detect a rapid rise in internal door pressure as the outer panel/deformation intrudes during a side hit.
• Yaw- and roll-rate gyroscopes: Sense vehicle rotation; roll rate and roll angle help identify actual or imminent rollover.
• Longitudinal/lateral accelerometers in the ACU: Provide central, corroborating data on crash severity and direction.
• Wheel-speed and stability control data: Help the ACU understand vehicle motion and corroborate a rollover trajectory.

Taken together, these sensors allow the ACU to distinguish a true crash or rollover from routine bumps, potholes, or door slams, reducing false deployments while reacting fast enough to protect occupants.

Deployment Logic and Thresholds

While exact trigger thresholds vary by make and model, the underlying logic is similar across modern vehicles. The ACU “votes” across multiple sensors and looks for a distinctive crash pulse—how quickly forces rise and how long they persist—to decide if deployment will prevent head injury or ejection.

• Side-impact events: If a rapid lateral acceleration and/or door pressure spike exceeds calibrated limits, the curtain on the impacted side typically fires.
• Rollover events: If roll rate and predicted roll angle indicate a rollover is happening or imminent, curtains on both sides usually deploy and remain inflated longer for containment/ejection mitigation.
• Speed and severity gates: Many systems include minimum vehicle speed or severity conditions so minor contacts don’t trigger deployment; the exact numbers are proprietary and differ by vehicle.
• Timing: Detection can occur within about 10–20 milliseconds; the bag inflates in roughly 30–40 milliseconds. In rollover-capable systems, inflation may be sustained (often up to several seconds) to keep the curtain in place.
• Side selection: In a side impact, only the near-side curtain typically deploys; in rollovers, both sides generally deploy. Some systems may also deploy the far side if data indicate risk.

This multi-criteria approach helps ensure curtains deploy in truly hazardous side crashes or rollovers, not in routine jolts that pose minimal head-injury risk.

When They Do Not Deploy

Side curtain airbags are specifically tuned for side-impact and rollover protection. The ACU filters out events that do not match those signatures to avoid unnecessary deployments.

• Low-speed parking bumps, curb strikes, or potholes without a severe lateral crash pulse.
• Front or rear collisions that lack significant lateral acceleration or rollover indicators.
• Door slams or body vibrations that do not produce the distinctive pressure rise or acceleration profile of a side crash.
• Glancing scrapes with minimal intrusion or force.

By ignoring these non-qualifying events, the system preserves components, reduces repair costs, and keeps protection focused on scenarios with real head-injury or ejection risk.

Special Case: Rollover Detection

Rollover-capable systems use gyros to monitor roll rate and estimate roll angle and time-to-roll. If the algorithm predicts the vehicle is about to tip or is already rolling, the ACU triggers both curtains and may keep them inflated longer to help prevent occupant ejection and reduce contact with glass or hard structures.

What Happens During Deployment

Once triggered, a pyrotechnic or hybrid inflator rapidly fills the curtain, which drops from the roof rail to cover the side windows. It’s designed to create a protective barrier for the head and upper torso, often spanning multiple rows. In rollovers, some systems maintain pressure longer—often several seconds—to provide continued containment. Airbags are single-use; after deployment, components must be replaced and the system inspected.

Conditions That Can Inhibit Deployment

Although designed to be highly reliable, some conditions can prevent or delay curtain deployment if they compromise system integrity or the ACU’s ability to confirm a qualifying event.

• System faults: An illuminated airbag warning light indicates a fault—such as a sensor issue, wiring problem, or module error—that can inhibit deployment.
• Low or unstable electrical power: Severe voltage issues after an initial impact may affect additional deployments.
• Aftermarket modifications: Non-OEM window tinting near sensors, altered headliners, or removed pillars/trim can interfere with sensor operation or airbag deployment paths.
• Flood/water or collision damage: Prior damage to sensors or the ACU can impair detection.

If the airbag light remains on or a fault is suspected, prompt professional diagnosis is essential to restore full protection.

Vehicle-to-Vehicle Differences

Trigger strategies and sensor suites vary by automaker, model year, and trim. Some vehicles use both door pressure and pillar accelerometers; others rely primarily on accelerometers and gyros. Rollover capability and inflation duration can also differ. For precise details, consult your owner’s manual or manufacturer service information for your exact model.

Summary

Side curtain airbags are triggered when the airbag control unit detects a severe side-impact crash pulse or a rollover through a combination of lateral acceleration, door pressure changes, and roll-rate/angle data. They typically deploy on the struck side during side impacts and on both sides in rollovers, inflating within milliseconds and, in rollover-capable systems, staying inflated longer to prevent ejection. Exact thresholds vary by vehicle, but the core principle is consistent: deploy only when data show a high risk of head injury or ejection, and remain inactive during routine bumps or minor contacts.

At what speed do side curtain airbags deploy?

Deployment thresholds can be as low as 8 mph for narrow object crashes (e.g., trees and poles) and 18 mph for the more widely distributed side impacts (vehicle-to-vehicle crashes). Side airbags also deploy in certain types of frontal crashes.

How do side curtain airbags work?

Side curtain airbags work by deploying from above the car’s windows during a side impact or rollover crash, creating a cushion between the occupant’s head and the side of the vehicle, window, or an external object. Sensors detect the collision and trigger an inflator that rapidly fills the folded airbag with gas, a process that happens in milliseconds. This protects the head and neck from severe injury and helps prevent occupants from being ejected in a rollover event.
 
This video demonstrates how side curtain airbags deploy in a crash: 46sTransport Accident Commission VictoriaYouTube · Aug 3, 2023
Here’s a step-by-step breakdown of how they work:

1. Detection: Specialized sensors, often mounted in the vehicle’s B-pillars (the structural area between the doors), detect a side impact or the tilting motion of a rollover. 
2. Signal to the Control Unit: Accelerometers measure the significant and rapid change in speed or angle, which is far greater than what occurs during normal driving or braking. 
3. Deployment Signal: If the crash is severe enough to meet specific criteria, the vehicle’s airbag control unit (ACU) sends an electrical signal to the airbag’s inflator. 
4. Inflation: The inflator contains a chemical propellant, such as sodium azide, that rapidly decomposes when heated, generating a large volume of harmless gas. 
5. Cushioning Effect: This gas fills the folded, fabric-like airbag at high speed, causing it to inflate rapidly and deploy from its storage location under the roof rail. 
6. Protection: The inflated curtain airbag hangs vertically between the occupant and the side window, providing a cushioning barrier. It absorbs energy from the impact and helps to prevent the occupant’s head and neck from striking the window or other hard objects. 
7. Deflation: The airbag is designed to remain inflated for a short period to continue providing protection, after which the gas escapes through small holes, causing the bag to deflate. 

This video explains the role of the airbag control unit in deploying airbags: 55sHistory of Simple ThingsYouTube · May 19, 2025

How hard do you have to get hit for side airbags to deploy?

Generally over 25 miles per hour (MPH). The impact must be frontal and not side. That’s 25 MPH as if you hit a wall, non-moving obstacle. If you hit a car from behind and it moves forward then the impact must be much greater to fire the bags.

What triggers side airbags to deploy?

Side airbags deploy when vehicle sensors detect impacts that meet a certain severity threshold, triggered by the force, angle, and direction of a collision, such as a side impact with another vehicle, a pole, or a tree, or even during some rollover events. These sensors monitor sudden changes in speed and impact forces, sending a signal to the airbag control unit to inflate the side airbag and cushion occupants from intruding vehicle parts or objects.
 
How Side Airbags Deploy

1. Sensors Detect Impact: Accelerometers, impact sensors, and pressure sensors are located throughout the vehicle to detect sudden changes in speed, deceleration, and impact forces. 
2. Signal to Control Unit: When these sensors register an impact exceeding a specific severity threshold, they send a signal to the airbag control unit (ACU), the “brain” of the system. 
3. Airbag Inflates: The ACU determines if the crash is severe enough for deployment and sends an electrical signal to a pyrotechnic initiator. 
4. Rapid Gas Generation: The initiator triggers a small explosive, which rapidly burns and generates a large volume of harmless nitrogen gas to inflate the airbag within milliseconds. 
5. Cushioning Occupants: The inflated airbag positions itself to cushion the occupants’ heads and chests, preventing them from striking the vehicle’s interior or intruding parts. 

Factors That Trigger Side Airbags

• Severity Threshold: Airbags are designed to deploy in moderate to severe crashes. 
• Angle and Direction: The angle and direction of the impact are critical factors, with airbags most likely to deploy in frontal and side impacts. 
• Type of Object: The type of object involved in a side impact can influence deployment. 
• Rollover Events: Side and curtain airbags can also deploy if a rollover is detected. 

Why They May Not Deploy

• Minor Crashes: Airbags are designed to deploy in moderate to severe crashes, not minor fender-benders. 
• Improper Seating: For optimal performance, occupants need to be properly seated and use seat belts, as airbags are supplementary protection. 
• Sensor Malfunctions: A malfunction in the sensors can prevent an airbag from deploying when needed.