Physics of rollover crashes

Physics of Rollover Crashes

mpaineATtpgi.com.au

Based on a research report "Passenger Car Roof Crush Strength Requirements" (updated link) by Michael Henderson and Michael Paine, prepared for Federal Office of Road Safety, December 1995

Update: June 1998: A common cause of single vehicle rollover crashes - the driver "overcorrects" after veering off the edge of the tar. See K.W. Ogden, The effects of paved shoulders on accidents on rural highways, Accident Analysis & Prevention 29 (3) (1997) pp. 353-362.

From the TV program "Crash: What Happened?" Channel 4 Television, 1998.

[animatation:off the edge]

Updates (see also Road Safety News)

23 Oct 01 Auto.com: U.S. considering tougher crush standards for car and truck roofs.
13 Feb 02 LAT: A Lonely Quest for Safer Cars (roof strength).
17 Aug 04 AI: Feds link injuries to weak roofs NHTSA rollover page now has a similar animation (no sign of the new roof crush report on the NHTSA website).
1 Apr 05 AI: New study ties deaths to crushed auto roofs - the roofs collapsed before injuries occurred to crash-test dummies
6 Jun 08 IIHS: IIHS testifies on the relationship of roof strength and injury risk in rollover crashes (PDF)
12 Sep 08 SAE: Rollover Symposium - Sydney 2 Oct + Center for Auto Safety: Press Conference to Demonstrate Inadequacy of Federal Roof Strength Rule + Public Citizen: Government, Auto Industry Drag Feet on Critical Roof Strength Standard, Squander Opportunity to Save Lives + IIHS: Rollover and roof crush + General Motors: Rollover Crashes Backgrounder + + NHTSA: Supplemental notice of proposed rulemaking (SNPRM)f for Roof Crush Resistance.
Roof Strength and Occupant Protection in Rollover Crashes - 2009 Australasian Road Safety Research, Policing and Education Conference

ANALYSIS OF VIDEOS: RALLY HITS Vol 2 & CRASH KINGS

There were a total of 129 incidents where vehicle at least rolled onto roof.

40% were "pure" side rolls, at 90 degrees to the longitudinal axis of the vehicle.
42% were corkscrew actions where the vehicle had substantial forwards motion as well as the rolling action. These tended to involve complex crash dynamics.
18% were end over end where the main motion was a pitching action (mostly forwards but some rearwards after the vehicle spun around 180 degrees).

36% were less than one full roll (most of these were a half roll onto the roof)
40% were at least one full roll but less than two full rolls
13% were at least two full rolls but less than three full rolls
11% were at least three full rolls
8 out of the 10 end-over-end crashes involved a half roll onto the roof. These crashes tended to be much slower than the other types but the final impact with the ground tended to be very severe.

The average time per roll for corkscrew and side-on crashes was very similar. A typical single full roll took 2.3 seconds. A typical double roll took 1.5 seconds per roll and a typical multiple roll ( 3 or more) took 1.1 seconds per roll. The average for crashes involving one or more rolls was 1.7 seconds.

Slow motion analysis of the videos revealed that substantial changes in the angular velocity occur as parts of the vehicle contact the ground. This results in high tangential forces on the occupants. In some video frames the head and arms of occupants can be clearly seen extended well outside open/broken side windows. This appears to be a whipping action which, fortunately, tends pull to occupant back inside the vehicle just before the adjacent roof edge makes contact with the ground.

Case 1: Occupant's head well outside window during the second complete roll. His head whips back inside the vehicle just before it hits the ground. Note the occupant is wearing a harness seat belt and helmet. He looked unharmed after the incident! [case 1 frame 1] [case 1 frame 2] [case 1 frame 3] [case 1 frame 4] [case 1 frame 5] [case 1 frame 6]

.
GIF OF MOTION

Case 2: Occupant's helmeted head breaks the side window just before that side of the vehicle contacts the ground. Again the occupant was wearing a harness seat belt and was apparently unhurt - but probably had a sore head!

[case 2 frame 1] [case 2 frame 2] [case 2 frame 3] [case 2 frame 4]

SIMPLIFIED CRASH DYNAMICS IN A SIDE-ON ROLLOVER

Vertical Motion of Vehicle C of G

roughly approximated

[diagram ofrollover dynamics]

Key features of this analysis are:

As the vehicle rolls over a "corner" the C of G reaches its highest point and, if the speed of the roll is sufficient, the acceleration of the C of G might go positive (note that the effect of Earth's gravity has been included in the above values). This indicates that the vehicle loses contact with the ground. In some of the rally crashes this effect was so strong that the roof did not contact the ground at all during the first half roll.

[case 3 frame 1]

The highest vertical acceleration occurs when the underside of the vehicle is in contact with the ground, at the start of the roll and at the end of a full roll. This is a manifestation of the location of the C of G of the vehicle, which is closer to the underside of the vehicle than the roof. In typical passenger cars the vertical distance from the C of G to the roof is similar to the transverse distance from the C of G to the side of the vehicle and therefore there is relatively little vertical motion of the C of G as the vehicle rolls from its side to the roof to the other side. Low aspect ratio vehicles such as sports cars have a smaller distance between the C of G and the roof and therefore the vertical loads occuring when the roof is in contact with the ground can expected to be higher. This might partly explain the finding by Moffatt (AAAM 1995) that high roof vehicles have generally less roof damage than low roof sports cars.

The change in vertical velocity during each quarter turn is estimated about 2m/s for this scenario.

Horizontal motion of Vehicle C of G

Note that during the initial tripping event it is likely that there will be substantial horizontal deceleration which will tend to throw the occupants in the direction of the roll. This appears to be the mechanism of ejection for the unrestrained dummy in the paper by Habberstad et al (1986) - the dummy was ejected before the vehicle reached the first quarter of the roll.

Rotational motion

After the tripping event which initiated the roll the vehicle will be moving sideways with a typical horizontal velocity of 11 m/s (assuming the trip speed was just sufficient to cause the roll - see Gillespie, 1992 p326) therefore the first two contacts of the roof with the ground will probably involve relative speeds of about 6 m/s (11 - 5) and the impact will tend to increase the speed of rotation of the vehicle. In effect, the occupants will experience angular accelerations in a direction opposite to the direction of the roll during these first two roof contacts (and opposite to the direction in which they were thrown at the start of the roll).

As the horizontal speed of the vehicle drops (due to the braking effect of the ground impacts) and the rotational speed increases, the tangential speed of the corner of the roof may eventually exceed the horizontal speed of the vehicle and the impact will tend to decrease the speed of rotation of the vehicle. In effect, the occupants will then experience an angular acceleration in the same direction as the roll. The observation of rally crashes where the occupants head and arms are extended outside the side window are probably due to this angular deceleration, the peak of which would usually occur as the vehicle tips over on its wheels near the end of the first roll or at the start of the second roll. At each of these points the C of G of the vehicle is passing through its highest point therefore the occupants tend to become "weightless". An unrestrained occupant has a high risk of being ejected at this point, if the side window is open or broken.

Rotational speed will have a similar step-like profile to horizontal velocity. To obtain a rough estimate of the change in rotational speed during each ground contact, assume that the maximum rotational speed is reached after half a revolution (at the third ground contact). For a maximum rotational speed of 6 rad/s (based on a average of 3.8 rad/s for a full roll) the change during each contact will therefore be about 2 rad/s. This is equivalent to a linear velocity change of about 2m/s in the region of an occupant's head, in a direction tangential to the C of G of the vehicle.

Combined effect

Summary

For simple side-on rollovers the vertical impacts with the ground are likely to be of sufficiently low speed to not be a direct problem with typical car roofs.
End-over-end rollovers and launching rollovers, where the car C of G gains significant height above the ground, involve much larger vertical impacts and roof strength is more critical.
Roof lozenging (side-sway) is a cause for concern. Possible effects are that the side window may shatter and that the occupant is exposed to greater risk of direct contact with the ground or partial ejection.
Restrained occupants who are partially ejected may be subjected to a whipping action which forces their upper body out of the side window. They are then violently pulled back inside the vehicle and at this time the inboard side of their head may contact the outside upper edge of the side window frame (crash investigators take note!).
Most of the energy absorption appears to take place when the wheels and underside of the vehicle contact the ground. Therefore a stiff roof structure is unlikely to prolong the roll.

Acknowledgements: Thanks to the Rallysport Promotions (Rally Hits Volume 2) and Duke Videos (Crash Kings) for the fascinating videos. The Federal Office of Road Safety sponsored the research project on which these notes are based. Dr Michael Henderson was the principal consultant on that project. Details of references are contained in the research report. The report was released in July 1999 as CR 176 - see FORS.

This web page resulted from my playing with a new toy - the Snappy Video Snapshot - which makes it easy to capture stills off video. The resolution of the pictures on this page was intentionally low so that JPG files sizes were manageable. For a clearer picture buy the videos!

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