The last two columns in the table are the internal width and available length. These are arguably the most important dimensions for determining how well a rack functions, as opposed to how bulky it is for transport. I defined and measured the entries in these columns as follows:

• Internal Width: This is the internal distance between the two sides of the frame. I suggest using this dimension as an aid to deciding whether the rack can handle single or double rope of the size you are interested in. Most racks will function fine if this dimension is slightly smaller than double the rope diameter (say, a millimeter or two), while a large excess in this dimension doesn't add much except making the bars heavier. This does help with heat dissipation, to a point, but a 50 mm. wide rack intended for short drops on a single rope wouldn't be justified. As an example, I usually rappel on 9 mm. or 11 mm. line, and occassionally on doubled rope. For me, a 22- to 25 mm. frame internal width is probably close to ideal for my needs.
• Available length for bar motion: A longer rack allows one to spread the bars out more, giving a greater control range. This is more important for lighter rappellers who, all other things being equal (and they never are), should use longer racks. In order to have a basis for comparison, I assumed that the rappeller was using five bars and that the combined space in the top four bar pairs (1-2, 2-3, 3-4 and 4-5) was 45 mm., a reasonable number for 11 mm. rope (When a rack only had three bars, I used 24 mm. instead). This is different than the assumption I made for O-frame & U-frame Racks, where I assumed four bars would be in use (odd numbers not being practical for them). Where possible, I moved any bars beyond the first five as low as possible on the rack frame (If I could have filed the bars to move them in back where they belong but didn't, I assumed that I had). I then measured the total bar motion and subtracted the 45 mm. to get the numbers given here. I entered 0 mm. for racks with fixed bars or combined bar spacings less than the 45 mm. standard, effectively saying that these are constant-friction devices. The dimensions are for the racks, bars and spaces shown in the pictures on this web site. Naturally, if you change to different-size bars and/or spacers, the available length decreaseses by the increase in bar diameters/spacer length.