SMC Mirco u-rack

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SMC Mirco u-rack

Postby bhc » Jun 5, 2018 1:59 pm

Howdy -- just got an SMC Micro u-rack. First time using a breaker bar rack for me.

I was under the impression (from reading/researching online) that when my hands came off the device, the rope would gather the bars and prevent a fall, but with the three different ropes I tried (one is a pretty stiff static 11mm, one is ~12mm dynamic, one is ~11 or so, dynamic), no such thing happens; the devices just lets rope through freely. Was I confused? I'd be using an autoblock backup anyway, but I had the impression that these racks would seize up when released. It works fine otherwise (though I am surprised at how little friction it imparts... I need to wrap an additional time around a hyper bar to get enough friction for control, even after pushing the bars up, and I only weigh 150lb / 68kg).

The micro is a 9" 4-bar rack, so maybe the auto-arrest only happens with larger 6-bar racks?

Thanks for any input!
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Re: SMC Mirco u-rack

Postby CaverScott » Jun 6, 2018 10:47 am

I don't know of any auto-arrest for racks without an additional device. However, our Vertical techies will surely chime in soon. :cave softly:
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Re: SMC Mirco u-rack

Postby bhc » Jun 6, 2018 12:25 pm

OK, thanks. I must have just had bad info. If any techies come 'round, thanks in advance!
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Re: SMC Mirco u-rack

Postby NZcaver » Jun 6, 2018 10:03 pm

Racks don't really have an auto-brake feature unless you add a friction knot or similar device. They need a human to push the bars together, or to wrap the rope over a hyper bar and apply friction by gripping the end of the rope. However if you have heavy rope hanging below you (think very large descent), the rope weight alone will likely make it so you can't move without spreading the bars out. That may be what you're thinking of. Cavers generally use longer J-frame type racks for those kinds of descents, and "tune" the bars as they go down.

Also, racks are primary intended for descending (or lowering with) stiffer static/low-stretch rope, not so much with more supple dynamic ropes. There's a huge difference in friction between these different ropes. You're likely to get a faster (less friction) descent on 12mm dynamic than on 11mm static.
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Re: SMC Mirco u-rack

Postby bhc » Jun 6, 2018 11:39 pm

Thanks, NZcaver -- that makes sense. I've only been practicing with it at like 5-10 feet off the ground, so I have no rope weight. I'm also using a very stiff, slippery 11mm rope (PMI sport max-wear, static) so that contributes as well. I'll reserve further judgment until I'm using it for real, and on a different rope. :-)
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Re: SMC Mirco u-rack

Postby CaverScott » Jun 7, 2018 10:43 am

Please feel free to ask any questions. By the way, have you considered purchasing "on Rope" - It is sold by the NSS along with various others.
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Re: SMC Mirco u-rack

Postby bhc » Jun 7, 2018 12:44 pm

Thanks! And yes, I have and will consider doing so. Much obliged.
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Re: SMC Mirco u-rack

Postby GroundquestMSA » Jun 7, 2018 4:28 pm

NZ is partly correct concerning static/dynamic friction. But some devices can ride much faster with static rope. Bobbins and long racks often put only gentle bends in the rope, meaning that surface contact controls speed to a large extent. A hard stiff static rope, especially if new and unwashed, is probably going to be one of the fastest rides out there, depending on the device.

Whatever the case, your rack will not brake itself. U racks, especially short-frames. have very little room for manually spreading or mashing bars and are more or less two-speed devices; with and without hyperbar. The rest of the speed control comes from your brake hand, not from the "control" hand often mentioned with long-frame j-racks.
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Re: SMC Mirco u-rack

Postby bhc » Jun 7, 2018 9:54 pm

Thanks -- that adds up. I have a short length of softer dynamic rope and it grabs a bunch more. And my practices have borne out what you say about the control in the right hand.
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Re: SMC Mirco u-rack

Postby trogman » Jun 9, 2018 9:47 am

My experience with mini or micro rack is that they are great as long as your rope is fairly clean. If it is stiff and muddy, it is almost impossible to make any downward progress. As a result, the only way to get any momentum is to take out the bottom bar, leaving only 3 bars and the hyperbar, which will make for a very rather unsafe and possibly an out-of-control rappel. :yikes:

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Re: SMC Mirco u-rack

Postby bhc » Jun 9, 2018 12:26 pm

Interesting.Although currently my issue is not enough friction rather than too much, I'm interested to check it out in more realistic conditions. I read one thread where the question is raised "what do you do when you have your descent device in the '0' friction setting and it's still way too much friction?" E.g. with 300 feet of rope hanging under you, a stiff/muddy rope, etc. Seems like a wise thing to consider which isn't often brought up.
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Re: SMC Mirco u-rack

Postby GroundquestMSA » Jun 9, 2018 3:10 pm

trogman wrote:My experience with mini or micro rack is that they are great as long as your rope is fairly clean. If it is stiff and muddy, it is almost impossible to make any downward progress. As a result, the only way to get any momentum is to take out the bottom bar, leaving only 3 bars and the hyperbar, which will make for a very rather unsafe and possibly an out-of-control rappel. :yikes:

Trogman :helmet:


Be extremely careful with this idea. Micro-racks are not designed to use anything less than all four bars. In fact, with each of the designs I have seen, a three bar descent is impossible anyway. Disengaging the bottom bar will automatically cause the third bar to drop, putting you on two (effectively zero) bars. For this reason caution is important when feeding rope on a slow rappel. Pushing rope up from below can allow accidental disengagement of the bottom bar and disaster.
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Re: SMC Mirco u-rack

Postby trogman » Jun 12, 2018 4:08 pm

GroundquestMSA wrote:
trogman wrote:My experience with mini or micro rack is that they are great as long as your rope is fairly clean. If it is stiff and muddy, it is almost impossible to make any downward progress. As a result, the only way to get any momentum is to take out the bottom bar, leaving only 3 bars and the hyperbar, which will make for a very rather unsafe and possibly an out-of-control rappel. :yikes:

Trogman :helmet:


Be extremely careful with this idea. Micro-racks are not designed to use anything less than all four bars. In fact, with each of the designs I have seen, a three bar descent is impossible anyway. Disengaging the bottom bar will automatically cause the third bar to drop, putting you on two (effectively zero) bars. For this reason caution is important when feeding rope on a slow rappel. Pushing rope up from below can allow accidental disengagement of the bottom bar and disaster.



Agreed! That was sort of what I was getting at, although my use of the phrase "rather unsafe" was probably a bit of an understatement.
I personally try to keep my ropes relatively clean, although sometimes I go with others who don't hold to the same standard. I hate having to feed a rack, but have found it necessary when using a mini rack on a very dirty rope.

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