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| Re: Determining braking distance (581318) | |||
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Re: Determining braking distance |
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Posted by Jeff H. on Wed Mar 5 20:28:57 2008, in response to Re: Determining braking distance, posted by RonInBayside on Wed Mar 5 20:13:25 2008. It takes less than that. 1 second is a design assumption, butas I said from a signals standpoint the braking distances are based on a graph or lookup table, not calculated. The much tossed-about number of 3.0 or 3.2 is just a way of reducing a non-linear graph to a single number which managers can quote. Think about it: during an emergency application, all braking effort is due to the brake cylinders (since dynamics are disabled). Cylinder pressure is held constant. If the relationship between cylinder pressure and braking rate were constant for all speeds, then one could use a single emergency brake rate number and plug it into the equation and accurately predict distance for any speed. But, in fact, the relationship is not constant. Brake rates increase as speed decreases. This effect was more pronounced with cast iron shoes than with the modern composition shoes. Also, there are secondary effects such as heating of the shoe which changes the friction coefficient. Therefore it is not correct to plug into the equation for constant acceleration. |
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