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| Re: Question about the Queens Blvd CBTC. (1532165) | |||
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Re: Question about the Queens Blvd CBTC. |
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Posted by Stephen Bauman on Mon Dec 9 19:46:33 2019, in response to Re: Question about the Queens Blvd CBTC., posted by AlM on Mon Dec 9 18:24:10 2019. Your analysis assumes that a can't be improved as V is improved (not necessarily proportionately, of course). That is patently absurd.a is the emergency braking rate. NYCT brakes only the wheels. This means the acceleration is limited rolling friction. It's sliding friction, if the wheels lock. Sliding friction braking isn't controlled. Therefore, the emergency acceleration is limited to less than the minimum sliding friction achieved by the steel wheel on steel rail. The coefficient (c) is between 0.35 and 0.5 for a dry contact. That's the ratio of the weight on the rail to the braking force. From 8.01 (F = ma) this means on level track that F = ma = cmg, where m is the mass, c the friction coefficient and g the gravitational acceleration (32 ft/sec). The mass cancels out on both sides of the equation. a = cg = 0.35 * 32 = 11.2 ft/sec-sec = 7.6 mph/sec So, increasing a as a function of V isn't a realistic hypothesis. |
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