Downhill Street Bicycle runs one bicycle at a time on a paved downhill course. The runs are timed. The course should be continuously downhill. The course should have about ten curves per mile with the curves relative to a bicycle speed of 30 MPH to 40 MPH. There should not be any jumps. Course width should be 25' or wider. The course should have shoulder areas on the outside of the curves and possibly need hay bales.
The tire sizes should be 700 x 35C or wider for lightweight bicycles. For mountain bikes the tires should be 26" diameter or larger with widths of 1.5" or wider. The maximum weight of the bicycle should be 32 pounds. The bicycle should have hand-lever brakes. The rider should stay in the seat unless pedaling. The rider should hold the handlebars at the full width of standard-width handlebars. Wheel suspensions are not allowed, fairings are not allowed, pedal-assist motors are not allowed, knobby tires are not allowed, and recumbent bicycle designs are not allowed. The riders must wear safety helmets.
The bicycle gearing could be limited for non-standard downhill courses so that bicycle control remains more important than pedaling and gearing. Otherwise, the largest chainring should be 55T or less. Also, the gearing should be a freewheel design.
More notes: Compliance suspension of seat and/or handlebar is allowed. Hydraulic disc-brakes are recommended.
More tire notes: A 700C tire for a lightweight bicycle is considered to be a 28" tire but also has a designation of 622mm. Then a 650B tire, as available for lightweight bicycles, is considered to be a 27.5" tire and has a 584mm designation. The actual tire widths should be 35mm or wider. A 26" tire for a mountain bike also has a designation of 559mm while a 29" tire for a mountain bike is the same designated diameter as a 622mm tire but expected in wider widths. Now 27.5" tires are also available for mountain bikes. The minimum tire widths for mountain bikes should be 1.5". Knobby tires are not allowed.
A single-element course is possible as a 200' to 500' downhill street into a ninety-degree intersecting street. The intersecting street could be downhill or uphill. The exit tangent should be about 200' unless uphill. The intersecting street should have sufficient width for margin-of-error. The street intersections must have smooth grade transitions.
Then possibly, the single-element course, as with 28' wide streets, sets the scale and nature of a full size course. Well, the central-angles would be ninety-degrees, the inside radius layout could be 11', the centerline radius layout could be 25', and the outside radius layout could be 39'. However, the radius of the bicycle path within the pavement width would then be a surprising 106.60'. For comparison, the radius of the bicycle path through the square corner layout would be 95.60'. Or with a 25' course width, central angles of ninety-degrees, an inner radius of 12.5', a centerline radius of 25', and an outside radius of 37.5', the radius of the bicycle path within the pavement width would be 97.86'.
First simple course design: A | | | |________ | | | | _________| | | | | |_________ | | | ________| | | | |________ | | | | _________| | | | | |_________ | | | | B A to B is downhill. Unfortunately, the lateral sections are only slightly downhill. A wide entry line, combined with an inside apex point, and combined with a wide exit line, makes a curve within the pavement width. Actually, square corners are not necessary since a radius combined with a central-angle swings a curve. Minimum pavement width is required. Well, the lateral sections are only slightly downhill unless cut into the side of the hill on a descending grade. Second simple course design: A * * * * * " * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * B A to B is downhill. Radius curves could be laid-out relative to the square points. The central-angles could be tightened to ninety-degrees. Minimum pavement width is required. The course design doesn't imply a transitional slalom but features tangent straights between curves. Third simple course design: A | | | | |_______ | | * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * B A to B is downhill. Radius curves could be laid-out relative to the square points and actually indicated at the last curve. Central-angles, near ninety-degrees, could be tightened to ninety-degrees. Minimum pavement width is required. A rider coming out of a curve onto a flat section might hold the bicycle tight so as to get the bicycle upright sooner for more effective pedaling. A rider coming out of a curve onto a continuing downhill might depend on the downhill to replace speed scrubbed-off and run a wide line. Or possibly there is a third riding technique of a turn-in to hit the curve apex but then a let-up in steering leverage at the apex to hit the exit flare.
The most difficult aspect to course design might be grade transitions. But one known element with a 6.25% grade just turns into an off-camber curve and is no problem. The street does twist back to horizontal as it leaves the grade intersection. Another known element with a 10.5% grade uses a vertical-curve to flatten-out for the alignment-curve section. Then a second vertical-curve could be used to go back into descent. Or the alignment-curve section could have a slight twist in side-to-side elevation to minimize off-camber on the turn-in and then have another slight twist in side-to-side elevation back to more natural elevation for the continuing descent. However, bicycle tires with tread areas down the sidewall of the tire, and with good tire width, can handle off-camber curves of some significant amount.
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