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| Re: Calculation (36695) | |||
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Re: Calculation |
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Posted by Jeff H. on Fri Jan 7 01:56:48 2005, in response to Re: Calculation, posted by Fred G on Thu Jan 6 21:54:11 2005. A spiral is a gradual introduction to a curve.Let's say you need to make a right-angle turn around a street corner and the available right of way limits the turn radius to 200 feet. A simple curve has an abrupt transition from tangent (straight) track to an arc with radius 200. Another way of saying this is a 28 degree curve, i.e. for every 100 feet along the curve, the angle changes 28 degrees. An example of a spiral would be the Searles spiral, in which that deflection of 28 degrees would be broken up into 10 segments. At each segement, the deflection would increase 2.8 degrees. So, at the beginning of the spiral, it would be a 2.8 degree curve, then a 4.6 degree curve, etc. One of the effects of a spiral is that the main curve radius needs to be tightened to get the track to wind up in the same place as it was with a simple curve. With regard to super-elevation, the amount of lift corresponds to where in the spiral you are. I.e. the lift gradually increases from 0 to the value appropriate for the main curve. Spirals do not increase the ultimate speed of the curve, but they do reduce the lurch. |
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