What are typical values for Free Rolling Drag coefficient?

Topics related to Vehicle Specifications, Vehicle Acceleration Tests, Vehicle Braking Tests
brian
Posts: 499
Joined: Tue Jul 14, 2009 10:52 am

What are typical values for Free Rolling Drag coefficient?

Post by brian »

Q: Anyone aware of any testing for COF for a free rolling vehicle on pavement or grass, wet or dry??
A: See SAE 980368 The measured Rolling Resistance of vehicle for Accident Reconstruction by Cliff & Bowler
The numbers they cite are low but it is another source for consideration. Also, in collisions, one or more of the wheels may be locked or have additional rolling resistance due to collision related damage to the structure and/or drivetrain.
From the paper:
ABSTRACT
  • Knowledge about vehicle rolling resistance is required to calculate speed loss of accident vehicles during portions of their pre-impact and post-impact trajectory when they are not braking or sliding directly sideways. The accuracy of assumed rolling resistance values is most important in accidents with long post-impact roll out distances. Very little hard data are currently available and the accident reconstructionist must usually make estimates of drivetrain losses and normal and damaged tire rolling resistance to determine overall vehicle rolling resistance.
    In the first part of this study, the rolling resistances of various vehicles with different drive configurations are determined, based on accurate measurements made with a 5th wheel. In the second part, sensitivity analyses are done with PC-Crash2, a computer simulation program, to determine what effect the error in assumed rolling resistance has on speed calculations for various types of post-impact trajectories.
CONCLUSIONS
  • 1. Brake factors of 0.007 to 0.015 (of normal load) were obtained at 2 m/s for free-rolling wheels of the vehicles tested. These values increased to the 0.017 to 0.024 range at 12 m/s. This range does not include the Ford Ranger drag because the Ranger's drag was higher in neutral than in fourth gear (overdrive).
    2. Manual transmission vehicle drive wheel brake factors ranged from 0.035 to 0.064 in 5th gear to 0.097 to 0.364 in 1st gear.
    3. Automatic transmission vehicle drive wheel brake factors ranged from 0.034 to 0.082 in 3rd gear to 0.144 to 0.284 in 1st gear, at 12 m/s. However, below the speed at which the torque converter disengaged and the engine stopped rotating these brake factors dropped to values close to those of free rolling wheels.
    4. Flat tire brake factors of 0.12 to 0.21 were obtained.
    5. Accidents involving at least some post-impact spin, and for which measurements of post-impact tire marks have been taken, can often be reconstructed with a trajectory simulation program without detailed knowledge of brake factors.
    6. Accidents involving post-impact rollout only cannot be reconstructed with a trajectory simulation program without detailed knowledge of brake factors, unless vehicle damage is also taken into account.
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