This is a specialty rack that I designed in 1977. My idea was to design a rack where the size of the bars was proportional to the amount of heat that they absorbed during a long rappel. I described the analysis in an article Design of Specialty Racks in Nylon Highway #9. The bar sizes that I eventually chose to use are as follows:
| Bar | Desired Relative Bar Size (#3=1.00) | Thickness | Height |
|---|---|---|---|
| 1 | 1.19 | 25 mm. (1 in) | 32 mm. (1-1/4 in) |
| 2 | 1.68 | 25 mm. (1 in) | 41 mm. (1-5/8 in) |
| 3 | 1.00 | 25 mm. (1 in) | 25 mm. (1 in) |
| 4 | 0.59 | 25 mm. (1 in) | 19 mm. (3/4 in) |
| 5 | 0.38 | 19 mm. (3/4 in) | 19 mm. (3/4 in) |
| 6 | 0.14 | 19 mm. (3/4 in) | 19 mm. (3/4 in) |
The last two bars needed to be made larger than the theoretical size based on thermal calculations. The rope groove on the top bars are not circular - instead, they are shaped to distribute the heat input into the bar better. Fortunately, the rope tends to wear the bars while maintaining approximately the right shape.
I've used this rack on 250 meter drops without cooling it with water. At the bottom, the bars were cool enough to lick without burning my tongue (but they tasted like aluminum).
I try not to laugh when I see people use an oversize bar on their rack, but put it in the top position. Not everyone understands that the second bar receives more heat than the top one, because the rope bends more around the second bar.
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©2007, Gary D. Storrick