The Traffic Accident Reconstruction Origin -ARnews-

Re: Drag Sleds

Ed Phillips (edphill@aol.com)
Tue, 17 Dec 1996 14:32:21 -0500 (EST)

I think we need to define some issues here. A coefficient of friction is the measure of the resistance between two objects in contact. The value may be in the static region or dynamic (kinetic) region where one or both objects are in motion. The peak of the static region is higher than the dynamic values so there is a drop off once enough force is applied to put the object(s) in motion. A drag factor is NOT the same as a coefficient of friction unless the vehicle is behaving like a sled and slipping across the pavement with its entire weight supported by tires which are not rolling. Braking efficiency 100%. It is not proper to assume a drag sled defines other than an event of 100% braking efficiency. There are certainly fluctuations in any friction measurement when contrasted to the real world event. E.g., the g-analyst samples at about 100 times per second then averages ten samples and gives a reading every 0.1 seconds. The issue is can you get a reasonable estimate of the f
riction coefficient of a fully braked vehicle from a drag sled that is properly used? Empirical testing tells us you can IF the device is used properly. The device should not be used on surfaces where the weight of the vehicle contributes to the amount of interaction (on a thick sand or dirt surface) where the weight of the vehicle will will sink the tires to obtain a better interface between the surfaces. Barring a contaminated surface and the like, my Physics, Statics and Dynamics books all show drag sled devices to have a considerable history of measuring F/W to obtain a coefficient of friction.
Ed Phillips
edphill@aol.com

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