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Q: Acceleration to deploy air bag, meaning of 20-30 g's? ( Answered,   0 Comments )
Subject: Acceleration to deploy air bag, meaning of 20-30 g's?
Category: Science > Physics
Asked by: mit_joel-ga
List Price: $40.00
Posted: 19 May 2005 18:46 PDT
Expires: 18 Jun 2005 18:46 PDT
Question ID: 523539
I need well documented (ie- government regulations, technical papers,
etc.) references for the following question: How much acceleration is
needed to deploy a car airbag? Also, what physical phenomenons can be
equilibrated to an acceleration of 20 g's and 30 g's to get an
understanding of this magnitude?

Request for Question Clarification by welte-ga on 19 May 2005 20:14 PDT

Air bag deployment algorithms are actually a little more complex, and
take into consideration a vehicle's deceleration, but also information
about deformation, weight of passenger, vehicle velocity, etc.  This
is one reason tests are performed using crash test dummies (usually
Hybrid III's) of various ages and sizes - to test the algorithm and
equipment under different conditions.

Would you be interested in technical documents describing how testing
is done?  I have found several documents that discuss this in detail
and include multiple accounts of the time between impact and
deployment of air bags, which (along with the initial velocity) gives
a measure of the deceleration incurred during the impact.


Clarification of Question by mit_joel-ga on 20 May 2005 05:52 PDT
Ideally there would be a published range telling when the MEMs were
activated and what minimum acceleration is required to deploy, given
that all other requirements (deformation, etc) are met. I am really
searching for a good physical phenomenon to help people understand how
severe 20 - 30 g's is. I thought that air bags would go off around
this range. i would love to say "20-30 g's is about the acceleration
you would find if you were exposed to ______________ (insert scary
event here)". Mentioned papers on testing could be part of the answer.
Subject: Re: Acceleration to deploy air bag, meaning of 20-30 g's?
Answered By: welte-ga on 20 May 2005 08:18 PDT

Thanks for the clarification.  

The following page has an excellent summary of acceleration required
to deploy airbags, based on the Federal Motor Vehicle Safety Standards
from the National Highway Traffic Safety Administration (NHTSA):

From the above site, and the NHTSA records, the acceleration that was
the initial maximum allowable before deployment of the air bag was 36g
in 1974.  Subsequently, this requirement was modified to 60g in 1998,
meaning that airbags became more difficult to deploy based on
acceleration alone.

The NHTSA information on which the above is based can be found here:

National Highway Traffic Safety Administration
49 CFR Parts 552, 571, 585, and 595
[Docket No. NHTSA 99-6407; Notice 1] 
RIN 2127-AG70
Federal Motor Vehicle Safety Standards;
Occupant Crash Protection

The Australian Road Safety Research, Policing and Education Conference
(2001) also uses the 60g number as a recommendation for Australian
regulations.  The report may be found here:

As a confirmation of the above information, the following document,
from, gives details of crash tests in various light
passenger vehicles.  As an example, on Page 104 of this document,
average chest accelerations for female dummies were in the range of
16.7 to 23.4g.  On Page 112, Figure D-10 gives a range of passenger
chest accelerations for various 1998 and 1999 autos between about 35
to 63g, for a 30 mph unbelted barrier test.  Figure D-13 on Page 114
gives a range of 45.4 to 48.4g for driver chest acceleration in the 30
mph unbelted rigid barrier test.



From an article by James Kowalick analyzing the potential dangers of
airbags, the minimum velocity for deployment of airbags can be as low
as 7 mph.  Here is an excerpt:

"Ralph Nader's Center for Auto Safety (he is the Founder) seeks bags
that will not inflate for crashes lower than 10 miles per hour. The
Center points out that all fatalities and serious injuries caused by
air bags were in low-speed crashes, and that in high-speed crashes,
either the air bag protected people, or they would have died anyway.
Most current-design manufacturers have chosen between 12 to 15 miles
per hour as the "trigger point" for air bag deployment, but actual
crash records indicate that some air bags deploy even at collision
speeds as slow as 7 miles per hour."

by James Kowalick, Ph.D., P.E. Renaissance Leadership Institute
Tel: (916) 692-1944; Fax: -1946; E-mail:


Also of potential interest, a manual for the acceleration sensors made
by Star Micronics Co., Ltd. and used in many of the MEM circuits can
be found here:


So, to put the accelerations into perspective, the most obvious is
comparison to the forces exerted by the earth.  20g is 20 times the
acceleration due to gravity, however, this is (I hope) not something
commonly experienced.  g-force can also be related to force (F=ma or
F=mg in this case); the force felt when experiencing 20g is the same
as 20 times one's weight.  This may be the easiest to grasp way of
expressing these relatively large accelerations to a general


The following page from gives some examples of low g
aeronautical limits.  Also, note that one of the definitions of gee is
incorrect - it describes an acceleration, not a force.

From Wikipedia:

"One often hears the term being applied to the limits that the human
body can withstand without blacking out, sometimes referred to as
g-loc (loc stands for loss of consciousness). A typical person can
handle about 5 g (50 m/sē) before this occurs, but through the
combination of special g-suits and efforts to strain muscles ?both of
which act to force blood back into the brain? modern pilots can
typically handle 9 g (90 m/sē). Resistance to "negative" or upward
gees, which drive blood to the head, is much less; typically in the
2-3 g (20 to 30 m/sē) range the vision goes red, probably due to
capillaries in the eyes bursting under the increased blood pressure."

This means that sustaining 60g is about 6 times what a fighter pilot
can sustain, 50g is 5 times, etc.

More information on the accelerations sustained by stunt pilots can be
found here (the links to the figures appear to be broken, however:


The Physics Hypertextbook gives a small table of examples of g-forces at this site:

Other examples in the Hypertextbook can be found here (for rockets):
Rocket launch: 20g
Space Shuttle: 29g

and here:
Car acceleration:
0-60mph in 7.0 seconds: 0.4g
Drag racer (0 to ~302mph over 1/4 mile track): 2.9g

Included are the following: 
Roller coasters:  4-6g for a few seconds at a time
Princess Diana of Wales in 1997 car accident, pulling the pulmonary
artery away from the heart: estimated 70-100g

More info on roller coaster g-forces can be found here, from the Saferparks group:


Information on Personal Water Craft collisions, with tables showing
speeds and equivalent g-forces, can be found here:
Particularly dramatic is Table II, which shows g-forces for side-swipe
collisions along with total force of impact in pounds (assuming a 150
pound rider).  For 15g, the impact feels like 2250 pounds.  Amazing! 
For 30g, the force would be 4500 pounds, and so forth.


You may also be interested in the NHTSA Dept. of Transportation
Biomechanics of Trauma set of software and technical reports, which
can be found at this page:

I hope this information was helpful.  Feel free to ask for any clarification.

There are no comments at this time.

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