UKC

I have a 8.7 mm rope, 50m, bought as a single rope for use on easy ground such as the Cuillin ridge, and rated in fact as a single rope.  If I get another rope as its pair for double-rope climbing, does it have to be another 8.7mm rope or would an 8.5m one do?  My main worry is to do with abseiling. If ropes are subtly different widths, can they be knotted safely, and are they in danger of feeding through the belay device at different speeds thus creating the 'feed-through failure' hazard.

Use the thin rope as the "pull" rope. That way if it does go through the plate faster, the knot butt's up against the anchor preventing any problems. 

some people absail with a standard half/single rope and 6mm chord, so an 8.7 and an 8.5mm should be fine! 8.7mm is very narrow for a single, my halves are about that, so it shouldnt feel wierd climbing double roping with it.

It depends on the ropes... there will be some difference in speeds through the belay device, but more often than not it's not a major concern.

As has been mentioned, always rap so that the thinner rope is the "pull cord" meaning that the knot will block the thinner rope to slide throught the anchor. So you can rappel "normally" and not worry about it much. Wouldn't do any speed rappels though.

There is no standard measure for rope diameter so quoted figures are simply approximate guidelines. For all practical purposes 8.5mm and 8.7mm are the same diameter.

More generally, differences in diameter only seem to become noticeable when pairing different classes of rope, such as a sturdy single with a skinny half - and even then I've never known it to cause problems beyond half a metre or so of knot creep.

The 8.5 could be thicker than the 8.7 and still be within spec so it’s not something i’d worry about.

As others have said the difference in diameter you mention is too small to matter much, and for abseiling even very different diameters are OK if set up correctly.

However I'd be interested to know what effect a larger difference eg 9.8 single and 7.8 mght have holding a leader fall when using half rope technique. This has been discussed in forums but I don't remember much consensus. I often used 11 and 9mm together and they worked, but I suppose that doesn't mean there wasn't greater slippage than belaying with equal diameter ropes, which could have been serious if there was decking potential. I think Jim Titt found that if using same thickness half ropes where only one took the weight then arresting was harder than if only one rope was in the hand, and it is easy to imagine that gripping a thin rope would be even more difficult if there was a thick stationary rope beside it. The Sticht plate for half ropes with different size holes for 9 and 11 possibly partially addressed this.

In your 9.8mm 7.8mm example, they could be 10mm and 7.6mm. Is there a device that works for both diameters?

I don't know. A quick look on the DMM website for the Pivot says: "Suitable for ropes from 7.3mm to 11mm. Optimised for 8mm to 10.5mm "

However I think that many accept that manufacturers' quotes of minimum diameter are not worth a lot and not based on standard testing. Trying a combination of skinny rope (which may vary from others of same diameter in slickness etc) and belay device oneself under safe conditions is invaluable.

I suppose the Sticht plate with 2 different holes for two specific diameters could be regarded as two joined plates. I wonder if using two different modern devices, possibly lashed together would be feasible. Not recommending this by the way....I have almost no knowledge of engineering etc.

DMM website again, on Mantis (similar belay plate features to Pivot): "Despite its low weight, the Mantis is a high performance belay device, giving confident control over ropes from 11mm down to a super skinny 7.3mm." But in Tech Spec; "half/twin: 7.3mm-9.2mm | single: 8.5mm-11mm"  Implying thick singles should not be used with skinny halves with this device. Presumably other manufacturers make similar claims.

Entirely academic, but interesting - that's got me thinking, and I wonder if it actually is 'creep' as such.  It occurs to me that perhaps your grip on the 'dead' rope forces them to feed through the belay device at exactly the same rate, then when it's above you the same 'unweighted' length of the more stretchy rope stretched half a metre more than the other under load.  (Or maybe that's what you mean by 'knot creep' in the first place - but I would tend to think of that as the ropes feeding through the belay at slightly different rates.)

8.7 and 8.5 are virtually the same diameter, so I wouldn't worry about that. 

I think you're missing the point here, when using a pull cord, you abseil on your main rope set up as a single - with a grigri for instance. The cord is just to pull the knot down on your main line.

There is absolutely no problem with 8.7 and 8.5mm. Most people climb with partners who have different ropes to themselves so it's a very common occurance. 

i may well be, how does that work? i cant picture what you are on about? how do you untie a knot at a fixed anchor point form 50m away?

You don’t. You pull the knot down the ab rope using the tag line.

Same principle as this https://www.climbing.com/skills/two-rope-rappels-with-one-chopped-cord/

The diagram showing an abseil rig for a damaged rope is open to misinterpretation. 

This is not the set up for ropes with a widely different diameter, eg in a tag line setup. It is only safe for ropes of the same diameter when one is damaged. The difference is in the positioning of the overhand knot. The overhand knot in the diagram is taking all the weight and could be seriously compromised if the ropes are different diameters. The set up in the case of a tag line needs the carabiner link to be taking all the strain and the overhand knot should be below it in a non load bearing position.  

Don’t do what’s in that diagram with a piece of 6m cord.

In reality the biggest danger of trying to abseil on 6mm cord is not the overhand,,  which is a lot more tolerant of rope thickness than people realise (properly dressed with decent tails of course) - but simply trying to control it through a normal abseil drvice

I accept your point fully - I was just looking for a picture of a fig of 8 on a bight with a screwgate clipped into, and that then clipped into the main rope - closing the link and making it possible to ab on a single rope (feature 3 in the link I used). But that one is open to interpretation as it still joins two ropes.  A better drawing showing what I wanted to explain is here: https://www.flickr.com/photos/elcap2011/8246865699/

Of course if you are using a tag line, you attach the tag line to the knot, the krab or even the end of the rope coming out of the knot. Different people favour different solutions it seems - interestingly looking back at this old thread: https://www.ukclimbing.com/forums/rocktalk/abseiling_on_single_rope_with_pu... the general consensus is avoid tag lines if possible. I've abseiled on retrievable singles a good few times, but probably on a grigri hence needing a single, from bolted top anchors and just using the rope doubled, not a separate tag line. Pulling on a thin cord seems to be difficult.

But in what I was TRYING to explain no one is abseiling with the 6mm through the belay device. You abseil on the main rope, the tag line is just to pull it down afterwards.

What is the best knot for abseiling on two ropes of different diameters? Double fishermans? 

The best answer is - I have never abseiled on ropes that are different diameter tied together because it would be an added worry - so if in doubt I would use the tag line method which I have done - avoid added worry because big abseil retreats are freaky enough as it is.  But pulling down all the krab clutter with a tag line is a jamming risk (that I have experienced) on multiple abs so I wont do that again either.  So I always used matched ropes.  

There is a real risk with any written stuff about tying what to what then hanging on it over a big drop.

Double fisherman's is a safe knot for this purpose and most people are familiar with it. Obviously higher chances of jamming during retrieval. 
The overhand (EDK) is fine if the thinner rope can't roll over the thicker, ie thin one on side of knot furthest away from tails if that makes sense.

If someone was  really worried and unsure I suppose they could use fig 8 loops in end of each rope, either linked directly or with a screwgate. 

A second oerhand, on both tails together and snug up against the first, would seem to provide the best of all worlds if concerned about the security of the main overhand.

Agree with that, though the resulting bigger knot might be more liable to get jammed while pulling rope down than the single EDK.

With the joined fig 8 loops I was trying to think of a of knot for someone with little experience faced with an abseil retreat. Almost everyone is familiar with it and it wouldn't slip even if the ropes differ in diameter.

"Best" depends on circumstances. Is it too cold to take gloves off ? Too dark to see ? Abseil line with loads of rope jamming potential ? Rope diameters would have to be very substantial for me to consider anything other than a single overhangs.

I've been away for a  bit since posting this, and I have found the responses and the discussions very useful.  I can't tell by looking which is the thinner rope, so I am reasonably reassured.