SMC 4-bar micro rack for rappelling

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SMC 4-bar micro rack for rappelling

Postby bobby49 » Nov 8, 2019 7:37 pm

For the last several decades until now, I've used one form or another of Figure 8 device for rappelling. However, some are sensitive to the rope diameter. Lately I wanted to try something more like a micro rack that is supposedly more adaptable to varying ropes. I got a micro rack and tried it out with a smaller diameter rope, and it worked fine. Today I tried it with a stiff 11mm Max-Wear pit rope, and I discovered a problem.

There are four brake bars on this. Without any rope, the four brake bars can be moved near the top half of the U-bar (the springy area), so the bottom brake bar can be "snapped" on or off to lock it into position. With a small diameter rope, the four brake bars are spread out more, but the bottom brake bar can still snap into lock or unlock. With the 11mm pit rope, the four brake bars are spread out even more, and it forced the bottom brake bar so close to the bottom of the U-bar that there is no springy action left, and the bottom brake bar cannot be forced to lock or unlock. What am I doing wrong?

I found the interfering edge of the bottom brake bar (stainless) and filed it down a bit. Now I can get it to lock or unlock with the 11mm pit rope. I find it hard to believe that every user has to do this.
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Re: SMC 4-bar micro rack for rappelling

Postby NZcaver » Nov 11, 2019 5:56 am

Is yours the BMS micro rack? Does it have the longer frame, or the shorter frame? I suspect it's the shorter one. I acquired one of those years ago, but like you I had issues with stiffer 11mm rope. I don't remember problems with clipping the bottom bar, but it was certainly slow and awkward to rappel with it because I couldn't spread the bars enough. I promptly got rid of that one and got the slightly longer framed version, which solved the problem.

Here's a previous discussion on the micro rack, including differences between the short and long frames: viewtopic.php?f=5&t=14674
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Re: SMC 4-bar micro rack for rappelling

Postby trogman » Nov 11, 2019 9:45 am

NZcaver wrote:Is yours the BMS micro rack? Does it have the longer frame, or the shorter frame? I suspect it's the shorter one. I acquired one of those years ago, but like you I had issues with stiffer 11mm rope. I don't remember problems with clipping the bottom bar, but it was certainly slow and awkward to rappel with it because I couldn't spread the bars enough. I promptly got rid of that one and got the slightly longer framed version, which solved the problem.

Here's a previous discussion on the micro rack, including differences between the short and long frames: http://forums.caves.org/viewtopic.php?f=5&t=14674


Same here- I ended up buying the longer frame, and transferred the bars to it. It works much better than the short frame.

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Re: SMC 4-bar micro rack for rappelling

Postby bobby49 » Nov 11, 2019 11:10 am

NZcaver wrote:Is yours the BMS micro rack? Does it have the longer frame, or the shorter frame? I suspect it's the shorter one. I acquired one of those years ago, but like you I had issues with stiffer 11mm rope. I don't remember problems with clipping the bottom bar, but it was certainly slow and awkward to rappel with it because I couldn't spread the bars enough. I promptly got rid of that one and got the slightly longer framed version, which solved the problem.


SMC 8.5" frame
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Re: SMC 4-bar micro rack for rappelling

Postby NZcaver » Nov 13, 2019 3:20 am

bobby49 wrote:SMC 8.5" frame

Yeah, that's too short for most practical caving use unless you only use 10mm or thinner rope. And I don't think SMC make a longer version.

BMS makes an 8.5" (which I briefly owned) and the 10.5" I've used for years. Mine has a single hyperbar, but they make a dual version too:
https://www.innermountainoutfitters.com ... -hyperbar/

Suggest you either upgrade racks and sell off your old one, or change to slightly thinner and more supple rope.
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Re: SMC 4-bar micro rack for rappelling

Postby trogman » Nov 13, 2019 6:26 am

NZcaver wrote:
bobby49 wrote:SMC 8.5" frame

Yeah, that's too short for most practical caving use unless you only use 10mm or thinner rope. And I don't think SMC make a longer version.

BMS makes an 8.5" (which I briefly owned) and the 10.5" I've used for years. Mine has a single hyperbar, but they make a dual version too:
https://www.innermountainoutfitters.com ... -hyperbar/

Suggest you either upgrade racks and sell off your old one, or change to slightly thinner and more supple rope.


He might be able to use the same bars from the SMC rack and switch them over to a BMS long frame. As long as the diameter of the frame material matches that of the holes on the bars, it ought to work fine.

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Re: SMC 4-bar micro rack for rappelling

Postby bobby49 » Nov 13, 2019 11:00 pm

I guess my Rescue 8 device works pretty good after all.
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Re: SMC 4-bar micro rack for rappelling

Postby NZcaver » Nov 14, 2019 7:16 am

bobby49 wrote:I guess my Rescue 8 device works pretty good after all.

Except for twisting the rope, and limited ability to vary friction. But for simplicity on shorter drops, an 8 will usually work just fine.
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Re: SMC 4-bar micro rack for rappelling

Postby Mountain Gnome » Jun 3, 2020 9:16 pm

An old friend of mine, no longer a caver so I hear (surprisingly), went on a multi-cave trip one day in Alabama back in the 1990s. He was using 4 bars on an old (dirty, high-friction) rope all day, then at another cave, they rigged with a brand-new shiny-clean rope. As a vertical caver for over 10 years, he hopped on with 4 bars, and lost control, rocketed down, hit the wall, flipped upside down, and smashed a rock-pile on the floor with his head. His buddy tried to rebelay him, but it didn't slow him down. Yea, he was unconscious and all, and had to be rescued and was hospitalized.

I am really surprised that they sell a 4-bar rack. I guess because it is such a short rack it's ok? I only use 4 bars on very long drops when the weight of the rope pulls it so tight through the rack that you don't move with 5. But I add the 5th bar on the way down, when the rope weight gets lighter.

Just something for newbies to think about......
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Re: SMC 4-bar micro rack for rappelling

Postby NZcaver » Jun 6, 2020 7:16 am

Mountain Gnome wrote:An old friend of mine, no longer a caver so I hear (surprisingly), went on a multi-cave trip one day in Alabama back in the 1990s. He was using 4 bars on an old (dirty, high-friction) rope all day, then at another cave, they rigged with a brand-new shiny-clean rope. As a vertical caver for over 10 years, he hopped on with 4 bars, and lost control, rocketed down, hit the wall, flipped upside down, and smashed a rock-pile on the floor with his head. His buddy tried to rebelay him, but it didn't slow him down. Yea, he was unconscious and all, and had to be rescued and was hospitalized.

I am really surprised that they sell a 4-bar rack. I guess because it is such a short rack it's ok? I only use 4 bars on very long drops when the weight of the rope pulls it so tight through the rack that you don't move with 5. But I add the 5th bar on the way down, when the rope weight gets lighter.

Just something for newbies to think about......

Yep, something to be aware of. But when you say "short rack" be aware 4-bar U-racks come in different lengths, and the longer versions are more likely to result in an uncontrolled rappel than the shorter ones. Especially if you don't know how to "drive" the rack by pushing the bottom bar upwards to increase friction. Or, use a 4 bar hyperbar version like the BMS micro rack. Problem solved! (Assuming it's used correctly.)

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