Thats a rough question to answer as the strength of rope depends on the material its made from, the thickness of the rope, the way in which it is made, the braiding and the wear and tear it has already undergone.
Some values for nylon rope, for example, can be found here: http://www.engineeringtoolbox.com/nylon-rope-strength-d_1513.html
I know its not much help, but i guess you could assume that the PCs would buy rope that would be strong enough to hold the whole party at once. Or at least multiple members of the party.
Also keep in mind the kangaroo is not going to keep still. That bouncing will put excess strain on the rope. The excess force will translate as the kangaroo seeming to way much more than he actually does.
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I can't find anything in the rulebook, except for break DC, which is 23, but I don't know how weight would correlate to a break DC.
With a feat of strength one can can burst a rope. The same amount of strength should burst the rope whether the situation is good or bad for the character.
Look what STR it takes to break a rope taking 20. DC 23 is a STR of 16. So to me it is reasonable that the load a STR 15 could bear is safe for the rope, while the load a STR 16 could bear might break the rope. Looking at what that STR can lift and stagger around with tells me the danger zone is 400 to 460LB {Double the normal max load for a STR 16]
So a load 400-460 LB has a 5% chance of breaking the DC 23 rope.
Some may say this is too low, so to achieve a happy medium, have the load on the rope take 10 at most times.
However on the round the rope is exposed to a major jarring, roll the STR check. All other times the "taking 10" result is used. Thus a hemp rope has a safe use of 400 LB and if no funny business is going on it can handle 6400 LB.
460 Lbs. has as an effective breaking STR of 16 +3
520 Lbs. has as an effective breaking STR of 17 +3
600 Lbs. has as an effective breaking STR of 18 +4
700 Lbs. has as an effective breaking STR of 19 +4
800 Lbs. has as an effective breaking STR of 20 +5
920 Lbs. has as an effective breaking STR of 21 +5
1040 Lbs. has as an effective breaking STR of 22 +6
1200 Lbs. has as an effective breaking STR of 23 +6
1400 Lbs. has as an effective breaking STR of 24 +7
1600 Lbs. has as an effective breaking STR of 25 +7
1840 Lbs. has as an effective breaking STR of 26 +8
2080 Lbs. has as an effective breaking STR of 27 +8
2400 Lbs. has as an effective breaking STR of 28 +9
2800 Lbs. has as an effective breaking STR of 29 +9
3200 Lbs. has as an effective breaking STR of 30 +10
3680 Lbs. has as an effective breaking STR of 31 +10
4160 Lbs. has as an effective breaking STR of 32 +11
4800 Lbs. has as an effective breaking STR of 33 +11
5600 Lbs. has as an effective breaking STR of 34 +12
6400 Lbs. has as an effective breaking STR of 35 +12
7360 Lbs. has as an effective breaking STR of 36 +13
8320 Lbs. has as an effective breaking STR of 37 +13
9600 Lbs. has as an effective breaking STR of 38 +14
11200 Lbs. has as an effective breaking STR of 39 +14
12800 Lbs. has as an effective breaking STR of 40 +15
14720 Lbs. has as an effective breaking STR of 41 +15
16640 Lbs. has as an effective breaking STR of 42 +16
19200 Lbs. has as an effective breaking STR of 43 +16
22400 Lbs. has as an effective breaking STR of 44 +17
25600 Lbs. has as an effective breaking STR of 45 +17
29440 Lbs. has as an effective breaking STR of 46 +18
33280 Lbs. has as an effective breaking STR of 47 +18
38400 Lbs. has as an effective breaking STR of 48 +19
44800 Lbs. has as an effective breaking STR of 49 +19
In the odd occurrence that a 30000lb load is suddenly forced onto the rope and it holds, remember that is just for a single round, next round when the load starts "taking 10", the ropes is a goner. Assume fate smiled on that moment allowing for the party to have a few seconds to use magic to save the load or to get out of the way.
Well, 1 inch hemp rope can have something like 8-10,000 lbs of tensile strength.
A break DC of 23 needs a Strength score of 36 to break on a take 10 (Str +13 modifier). Dragging load of Strength 36 is 18,400 lbs.
I'd go with that.. if it weighs more than 18,400 lbs, it'll break when attempting to drag something.
Silk rope would be over 20,000 lbs.
A kangaroo would likely only be a Medium sized creature, and the largest type of kangaroo weighs around 200lbs. If you have some special one for a mount, it might be "large-sized", in which case it would have 8x the weight, at around 1600 lbs (a horse is 1000-1500 lbs, so I'd go with that).
So a person, with equipment and armor, and a large-sized mount, would likely still be under the tensile strength of the rope involved (going by the break DC info). Even if you don't go with the drag values (such as going by lift instead, etc), it'd still be well within limits.
A quick check on google found an article that had a similar equation, but forgot to factor the 5x drag values. So I'd say it's probably "enough to not matter in most situations".
Edit
@Frankthedm: Lift values are for lifting over your head, which doesn't equate to the action of lifting for purposes of pulling someone along with the rope.
The concern is the ultimate strength (or tensile strength) of the rope before it breaks. The ultimate load of 36 strength (taking 10 on a DC 23), the drag value at 5x the max load.
What the people can actually pull will be dependent on lift weights, because they are lifting the creature into the air.
Now, if they don't have a pulley, then I'd make it so that the rubbing along the edge might reduce this amount a bit, but we are still looking at a ridiculously large number. DM hand-waving time.
You can also see some modern day values for hemp rope at this site.
I had written a response very close to Frank's but my computer futzed and it didn't survive. I think Frank has it right.
The tensile strength of spidersilk rope would be ridiculously strong, yeah. I'd say there's no reason to worry about the details on this one.
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The tensile strength of spidersilk rope would be ridiculously strong, yeah. I'd say there's no reason to worry about the details on this one.
And since it only lists spider silk ropes as :"Spider's silk rope has 6 hit points and can be burst with a DC 25 Strength check."
which is only 2 points higher then a hemp rope, that is either a weak spider silk rope, or really strong hemp.
Another fun fact, is the DC to break rope is 23 (as we know), the DC to bend iron bars is 24.
Or very thin rope. Spidersilk weighs 4 lbs for the same length, and still has effectively ~14,000 lbs more upper strength (32,000 lbs vs 18,400 lbs).
It has about 75% more strength, at only 40% of the weight.
Well, I was referring to the "weak" spidersilk rope. It's possible it's only 1/4 inch thick or less, considering how light it is, and how it's "not-quite-double" strength of the inch thick hemp rope.