Friction Demand Research Articles

Relationship between Transitional Acceleration of the Whole Body Center-Of-Mass and Friction Demand Characteristic during Gait

A laboratory study was conducted to examine the gait changes associated with aging and the effect of these changes on initiation of slips, initial friction demand, and slip distance utilizing newly defined biomechanical parameters of slips and falls. Twenty-eight subjects from two age groups (young and old) walked around a circular track at a comfortable pace wearing a safety harness. Slippery floor surface was automatically placed on the walking track over force platforms at “unexpected” random time intervals utilizing remote controlled floor changer. Synchronized kinetic and kinematic measures were obtained on both slippery and non-slippery floor surfaces. The results indicated that older subjects step length was significantly shorter, and transitional acceleration of the whole body center-of-mass (COM) was significantly slower than their younger counterparts. Older subjects initial friction demand as measured by required coefficient friction (RCOF) was not significantly different than their younger counterparts. Additionally, older subjects slipped longer and faster than younger subjects. Bivariate correlation analyses suggested that transitional acceleration of the whole body COM was significantly related to friction demand characteristic (e.g., RCOF). These findings suggest that gait changes associated with aging (especially slower transition of the whole body COM) may affect initiation of slip-induced falls among older adults.

Effects of age related sensory degradation on perception of floor slipperiness and associated slip parameters

A laboratory study was conducted to determine how sensory changes in elderly people affect subjective assessments of floor slipperiness, and associated friction demand characteristics and slip distance. To relate these parameters to actual slip and fall incidents, 30 subjects from two age groups (young and elderly) walked around a circular track on the slippery and non-slippery floor surfaces, while wearing a safety harness to prevent injury in case of a slip or fall. Prior to the walking experiment, the Sensory Organization Test was performed. During the experiment, subjective assessments of surface slipperiness of the floor were obtained prior to walking and after walking on the floor. Slip distance, required coefficient of friction (RCOF) and adjusted friction utilization (AFU) were assessed utilizing motion analysis and force platform systems. The results indicated that sensory changes in the elderly increased the likelihood of slips and falls more than their younger counterparts. This was due to incorrect perceptions of floor slipperiness, and uncompensated slip parameters such as slip distance and adjusted friction utilization.

Side Friction and Speed as Controls for Horizontal Curve Design

This paper describes an examination of the relationship between vehicular speed and side friction demand on horizontal curves. Models of speed and friction demand are described. The curve speed model explains the effect of approach speed, radius, and superelevation rate on curve speed. The side friction model explains the effect of approach speed and curve speed reduction on side friction demand. The terms in the side friction model reflect a general desire by motorists for a lower side friction demand at higher speeds. Model terms also reflect a willingness by motorists to tolerate slightly higher side friction demand in an effort to minimize the amount of speed reduction required by relatively sharp curvature. This model is recommended as a rational basis for defining the maximum side friction factors for use in curve design.

The Tyre as a Sensor to Estimate Friction

SUMMARY Continuously updated knowledge of the friction potential and the friction demand can help to improve manoeuvrability and thereby safety of vehicles under slippery road conditions. An on line friction estimation method is presented using a simple brush type tyre model. First the method is verified by outdoor experiments using a tyre test trailer. Then the setup of a low-cost measurement system in a vehicle is discussed and results from both simulations and outdoor experiments are presented. Neural networks have been used both for modelling and identification of friction potential and friction demand.

Friction Recognition Using the Tyre as Sensor

Continuously updated knowledge of the friction potential and the friction demand can help to improve manoeuvrability and thereby safety of vehicles under slippery road conditions. An on line estimation method, based on recognition of the pneumatic trail, is presented using a simple brush type tyre model. The method is verified by outdoor experiments using a tyre test trailer. Disturbances such as camber, residual forces and moments, combined slip of the tyre and dynamic behaviour of the tyre are considered. Neural networks have been used both for modelling and identification. Finally, a low-cost implementation of the system in the front wheel suspension of a passenger car is discussed.

The influence of surface slope on human gait characteristics: a study of urban pedestrians walking on an inclined surface.

Human locomotion on inclined surfaces has been little studied. This work investigated uphill and downhill walking of urban pedestrians on slopes of up to 9 degrees. During a 3-month period, the gait characteristics of 1200 female and 1200 male pedestrians were recorded as they traversed a ramp of naturally varying slope at Sydney's Circular Quay. Walking speed, cadence and step length were determined for each subject, and average population gait parameters, for each ramp angle, were also calculated. The most significant finding was that the pedestrians' step length was decreased during ramp descent. Since theoretical analysis concludes that a reduction in step length produces a reduction in the friction demand, a shortening of the stride length is probably a means of counteracting the higher friction demand that would otherwise be required at heel strike during downhill walking.

Driving Dynamic Aspects and Related Safety Issues for Modern Geometric Design of Non-Built-Up Roads

Safety issues affecting modern highway geometric design of roads in non-built-up areas are discussed. The status of knowledge of Safety Criteria I and II (achieving operating speed consistency and design consistency) for two-lane rural roads is briefly reviewed. Considerations for design consistency are also extended to multiple-lane rural and suburban road design to evaluate good, fair, and poor design levels for these road categories. Utilization ratios (n) for maximum permissible side friction factors are developed for different road categories, topography levels, and maximum and minimum superelevation rates. Relevant minimum radii for curve design are established and compared with present AASHTO values insofar as possible. Safety Criterion III (achieving driving dynamic consistency) is further developed, through which reliable and quantitative ranges between the side friction assumed and the actual friction demand at curved sites could be established. AASHTO's design policy is evaluated on the basis of Criteria II and III. The evaluation showed, for lower design speeds (30, 40, and 50 mph), an unbalanced degree of curve and superelevation rate ranges, which lead to poor design practices from a safety viewpoint. By heeding the three safety criteria and the recommended side-friction utilization ratios, sound alignments can be established for the various design cases in future highway geometric design and redesign and for resurfacing, restoration, and rehabilitation strategies.

Driving Dynamic Aspects and Related Safety Issues for Modern Geometric Design of Non-Built-Up Roads

Safety issues affecting modern highway geometric design of roads in non-built-up areas are discussed. The status of knowledge of Safety Criteria I and II (achieving operating speed consistency and design consistency) for two-lane rural roads is briefly reviewed. Considerations for design consistency are also extended to multiple-lane rural and suburban road design to evaluate good, fair, and poor design levels for these road categories. Utilization ratios (n) for maximum permissible side friction factors are developed for different road categories, topography levels, and maximum and minimum superelevation rates. Relevant minimum radii for curve design are established and compared with present AASHTO values insofar as possible. Safety Criterion III (achieving driving dynamic consistency) is further developed, through which reliable and quantitative ranges between the side friction assumed and the actual friction demand at curved sites could be established. AASHTO's design policy is evaluated on the basis o...

Driver behaviour at horizontal curves: risk compensation and the margin of safety

A study involving unobtrusive observation of drivers at horizontal curves before and after realignment is described. The speeds and path radii adopted by drivers in the curves before and after realignment are compared, as are the levels of side friction demanded by each driver while negotiating the curves before and after realignment. The results reveal substantial variations between drivers (with respect to speed, path radius, and side friction demand) and between the path and curve radii. While vehicle speeds increased markediy, the side friction demand was reduced for all curves except one. It is concluded that the margin of safety was increased for all curves, and this is supported by the accident data.