Cavechat.org

I am looking to get my first rack for longer rappels (my petzl stop bottoms out at about 100M). I am planning on using onrope1's "build your own rack" option
http://onrope1.com/store/index.php?p=pr ... &parent=14
I'm thinking about getting either an 18 or 24 inch with 6 bars. Now, I'm curious to know which style of brake bars is better for longer rappels, the tubular style or the U shaped ones. To me, it seems the tubular shaped ones would ABSORB more heat, while the U bars wouldn't as much because they have more surface area to dissipate off. Please respond with any insight and help me understand! Thanks in advance.

You are correct- the round bars do absorb more heat. It is a combination of greater surface area, as well as greater overall mass. It is simple physics. The tradeoffs are that they weigh more, they cost more, and they are easier to rig backwards. Many people split the difference by using round bars on the top 2 or 3 bars, and U-shaped bars for the remainder. A hyperbar at the top is always a nice addition.

Trogman

Everything Trogman says is correct. But, in my experience, it doesn't really matter that much. The biggest contributor to heat is speed. If your bars are getting too hot, simply slow down. The longest drops in the world have been done safely with many styles of bars. Still, if you want the best, top of the line, ultimate set up, two or three round bars on top with U bars under and a hyperbar is hard to beat.

dont bother with an 18" rack. Too long, too bulky. A 14" will get you on any drop you will realistically do. LOTS of people do Bridge Day's 850' rappel on a 14" 6 bar rack, including myself, with stamped U bars. 18" = 24" racks are completely unnecessary unless you are going to golandrinas every weekend ;)

Wish I had read this post before buying my new rack with all open bars. I've always like the tubular bars but was advised to use the open bars for quick recogination of correct loading. Mixing the bars is a great idea.

acountrycaver wrote:Wish I had read this post before buying my new rack with all open bars. I've always like the tubular bars but was advised to use the open bars for quick recogination of correct loading. Mixing the bars is a great idea.

If you rappel enough you will eventually wear through the u shaped bars. Buy more bars and keep them for replacements.

It seems to me that a tubular bar would have no more thermal capacity than a u-bar, as you can think of a u-bar as nothing more than a flayed-open tubular bar, assuming identical wall thickness. My guess is the two have a similar thermal capacity but the u-bar has slightly greater convective and radiative cooling due to its inner surface being exposed. Unless perhaps you're referring to a solid rod bar when you say "tubular", in which case the thermal capacity of the solid bar would be much higher and therefore would not get hot as quickly. However, the surface area of a solid bar is not as large as a tubular bar or u-bar and would not cool down as quickly if it did get hot. Hopefully that makes sense.

This is assuming that there is as much steel in a u bar as a round bar there isn't. They are lighter. Much less steel.

Lava wrote:It seems to me that a tubular bar would have no more thermal capacity than a u-bar, as you can think of a u-bar as nothing more than a flayed-open tubular bar, assuming identical wall thickness. My guess is the two have a similar thermal capacity but the u-bar has slightly greater convective and radiative cooling due to its inner surface being exposed. Unless perhaps you're referring to a solid rod bar when you say "tubular", in which case the thermal capacity of the solid bar would be much higher and therefore would not get hot as quickly. However, the surface area of a solid bar is not as large as a tubular bar or u-bar and would not cool down as quickly if it did get hot. Hopefully that makes sense.

Assuming the same wall thickness, a round bar would be roughly twice the surface area and the mass as the half round bar. Of course the weight would be more as well, which is one of the trade-offs. If the bar were solid, it would obviously have more mass, but the surface area would be about the same as the half-round bar.

Trogman

trogman wrote:Assuming the same wall thickness, a round bar would be roughly twice the surface area and the mass as the half round bar. Of course the weight would be more as well, which is one of the trade-offs. If the bar were solid, it would obviously have more mass, but the surface area would be about the same as the half-round bar.

Trogman

But u-bars aren't half-round bars, they have much more material than that. If you cut a tubular bar in half there would not be enough material to drill holes for the rack frame and still maintain the same face radius for the rope to contact. U-bars much more closely resemble a tubular bar cut and flayed open, which would mean a roughly equivalent exposed surface area.

Lava wrote:

trogman wrote:Assuming the same wall thickness, a round bar would be roughly twice the surface area and the mass as the half round bar. Of course the weight would be more as well, which is one of the trade-offs. If the bar were solid, it would obviously have more mass, but the surface area would be about the same as the half-round bar.

Trogman

But u-bars aren't half-round bars, they have much more material than that. If you cut a tubular bar in half there would not be enough material to drill holes for the rack frame and still maintain the same face radius for the rope to contact. U-bars much more closely resemble a tubular bar cut and flayed open, which would mean a roughly equivalent exposed surface area.

I agree, they are more than a half bar; although I suspect that they are probably about equal to 3/4 of the surface area of a tubular bar. I do think the primary benefit from tubular bars is a result of their overall greater mass, due to thicker walls.

Trogman

there is no doubt in my mind that a u shaped bar would dissipate heat more quickly than a tubular........next obvious question is which would dissipate more per unit of time and over time......next is which would melt the rope first if disaster struck............whichever cools down first?......that don't count....

so, it appears that the u shaped bars are the best choice.......and another thing......why has no one commented that the tubular bars have a side of the bar that is completely unused when the bar is retired?...where are the entrepanuers here?.....why aren't confused people asking where are left handed raacks?....while others wonder why no one has suggested it before......

if convenient use the longest rack you can......24 or more.......the rappel is soooooo much more controllable when using a longer rack........it is also much harder to lose control of the speed of the rappel with a longer rack.......