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Off Road Vehicle

Division

J-Turn Discussion
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Off Road Vehicle

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From safecar.gov Website

2

NHTSA Rollover Definitions

TRIPPED ROLLOVERS

NHTSA data show that 95% of single-vehicle rollovers are tripped. This happens when a vehicle leaves the roadway and slides 

sideways, digging its tires into soft soil or striking an object such as a curb or guardrail. The high tripping force applied to the tires 

in these situations can cause the vehicle to roll over.

UN-TRIPPED

Un-tripped rollovers are less common than tripped 

rollovers, occurring less than 5% of the time, and 

mostly to top-heavy vehicles. Instead of an object 

serving as a tripping mechanism, un-tripped rollovers 

usually occur during high-speed collision avoidance 

maneuvers.
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NPR Ay Requirement Can Not Prevent Tripping Rollovers

3

Off Road Vehicle Tripping Condition 

Environment

Sand

Soil Berm

Rock

Tree Stump

Incline

Resistance 

Center of Gravity 

Sliding Force

Moment 

Of Inertia Force = Mass times Acceleration (Spike Load Condition) 

Can easily exceed 2G

Tipping Force

Off Road driving conditions can result in a tripping condition

http://www.safercar.gov/Vehicle+Shoppers/Rollover/Types+of+Rollovers
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Video shown at half speed – actually traveling 25mph

4

Sand J-Turn Video
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Off-road behaviors can vary greatly from on-road – unintended consequences

5

Off-Road Testing

Notes:

1) On pavement, the locked differential is oversteer and the 

open differential is understeer

2) In sand and gravel, the understeered vehicle rolled much 

easier than the oversteered vehicle
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Off-road behaviors can vary greatly from on-road – unintended consequences

6

Off-Road Testing

Notes:

1) On pavement, the locked differential is oversteer and the open 

differential is understeer

2) As the surface roughness increased, less steering angle was 

required and the differential position had less effect
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Sliding Begins Well Below 0.7 G, Tripping Occurs Well Above 0.7G

7

Lateral Acceleration Comparison

Sand

(Peak Ay @ 1.1)

Gravel

(Peak Ay @ 0.9)

Pavement

(Peak Ay @ .72)

All runs shown ended in roll:

• Off-road runs begin sliding at less than 0.3G and can reach lateral accelerations well above 

1.0

• Spikes are noticeable of the tire tripping/skipping over the ground

• Once the vehicle begins sliding, well below 0.3G, it really doesn’t matter what its Ay on 

pavement is because it will trip and spike well above that value

0.7G

0.7G

0.7G

Tire Slip

Tire Slip

Tire Slip
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Sliding Begins Well Below 0.7 G, Tripping Occurs Well Above 0.7G

8

Lateral Acceleration Comparison

Plowed Dirt

(Peak Ay @ 0.96)

Rough Grass

(Peak Ay @ 0.87)

0.7G

0.7G

Pavement

(Peak Ay @ .72)

0.7G

Tire Slip

All runs shown ended in roll:

• Off-road runs begin sliding at less than 0.3G and can reach lateral accelerations well above 

1.0

• Spikes are noticeable of the tire tripping/skipping over the ground

• Once the vehicle begins sliding, well below 0.3G, it really doesn’t matter what its Ay on 

pavement is because it will trip and spike well above that value

Tire Slip

Tire Slip
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Steer input on Pavement Is A Better Pass/Fail Metric

9

Vast majority of Off-Road rollovers are tripped

On-Road J-Turn does not predict tripped rollover 

resistance

Off-road terrain causes tires to slip well below 0.7g, 

proposed threshold is not connected to the terrain failure 

limit

Once tires begin to slip, a tripped rollover is highly likely

Lateral acceleration at trip is well above .7g

Steer input at roll consistently higher off road vs on 

pavement

Focus vehicle designs to increase slip resistance & 

improve handling predictability

Summary