Stability analysis of bicycles by means of analytical models with increasing complexity

The basic Whipple-Carvallo bicycle model for the study of stability takes into account only geometric and mass properties. Analytical bicycle models of increasing complexity are now available, they consider frame compliance, tire properties, and rider posture. From the point of view of the designer, it is important to know if geometric and mass properties affect the stability of an actual bicycle as they affect the stability of a simple bicycle model. This paper addresses this problem in a numeric way by evaluating stability indices from the real parts of the eigenvalues of the bicycle's modes (i.e., weave, capsize, wobble) in a range of forward speeds typical of city bicycles. The sensitivity indices and correlation coefficients between the main geometric and mass properties of the bicycle and the stability indices are calculated by means of bicycle models of increasing complexity. Results show that the simpler models correctly predict the effect of most of geometric and mass properties on the stability of the single modes of the bicycle. Nevertheless, when the global stability indices of the bicycle are considered, often the simpler models fail their prediction. This phenomenon takes place because with the basic model some design parameters have opposite effects on the stability of weave and capsize, but, when tire sliding is included, the capsize mode is always stable and low speed stability is chiefly determined by weave stability

Feral Horses Disrupt Greater Sage-Grouse Lekking Activity in the Great Basin

Greater sage-grouse (Centrocercus urophasianus; hereafter, sage grouse) and feral horses (Equus ferus caballus) co-occur within Great Basin sagebrush ecosystems of western North America. In recent decades, sage-grouse populations have declined substantially while concomitantly feral horse populations have increased drastically. Although multiple studies have reported feral horses adversely impacting native ungulate species, direct interactions between feral horses and sage-grouse have not been documented previously. We compiled sage-grouse lek count data and associated ungulate observations from 2010 and 2013-2018. We used Bayesian multinomial logistic models to examine the response of breeding male sage-grouse to presence of native (i.e. mule deer, pronghorn) and non-native (i.e. cattle, feral horses) ungulates on active sage grouse leks (traditional breeding grounds). We found sage-grouse were 9.5 times more likely to be present on active leks concurrent with native ungulates compared to non-native ungulates. Of the four different ungulate species, sage-grouse were least likely to be at active leks when feral horses were present. Our results suggest that the presence of feral horses negatively influences sage-grouse lekking activity. Because sage-grouse population growth is sensitive to breeding success, disruption of leks by feral horses could reduce breeding opportunities and limit breeding areas within sage-grouse habitat

Dynamics of active membranes with internal noise

We study the time-dependent height fluctuations of an active membrane containing energy-dissipating pumps that drive the membrane out of equilibrium. Unlike previous investigations based on models that neglect either curvature couplings or random fluctuations in pump activities, our formulation explores two new models that take both of these effects into account. In the first model, the magnitude of the nonequilibrium forces generated by the pumps is allowed to fluctuate temporally. In the second model, the pumps are allowed to switch between "on" and "off" states. We compute the mean squared displacement of a membrane point for both models, and show that they exhibit distinct dynamical behaviors from previous models, and in particular, a superdiffusive regime specifically arising from the shot noise.Comment: 7 pages, 4 figure