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I recently had an inner tube on my mountain bike changed, but it was my wife that actually took the bike in to the shop for service. When it came back I noticed the stem has a blue cap rather than the typical black cap. I have seen slime inner tubes that have a green cap, so it got me to wondering if there was something different about the tire. Although I hadn't asked for slime and she didn't remember anything being said about it, it's possible they could have just thrown it in as a freebie. (This sort of thing has happened before where they quote one price over the phone and then the actual price winds up being less than what they quoted.) I have also noticed that at one point the tire went flat uncharacteristically fast (within a couple days of non-use compared to normal losses of a maximum of 1/3 pressure over a week), but after I pumped it back up it seemed to hold its pressure fairly well over a longer time period, which suggests to me that either there was a tiny leak which somehow sealed itself when the tube was re-pressurized, or slime was involved.

Aside from actually taking off the tire and taking out the tube and shaking it, how can I tell if a given tire has slime in it?

  • If you pour salt on the tire, it might curl up and warp, implying slugs have been crawling all over your bike since the last time you used it. – smeeb May 10 '16 at 1:21
  • I suspect you're being funny, so this would have been better as a comment rather than a real answer. – Criggie May 10 '16 at 1:26
  • No @Criggie once when I had a bike as a kid, I found slugs on my bike after it had rained. They had crawled all over my tires and the seat and it was slimy to the touch. I poured salt all over them and the bike was still slimy, but it was confirmed: they were slugs. – smeeb May 10 '16 at 1:31
  • Oh okay - in this context "slime" is a commercial brand name for a liquid puncture sealant added inside a tube (or inside the tyre of a tubeless setup) The idea is it will coat the inside of the tube/tyre and get pushed out through any puncture, and seals the hole somehow. – Criggie May 10 '16 at 1:37
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    Valves can often be flaky, and sometimes seal better than other times. So the fact that the leakage rate changed is not significant. But if this is a Schrader valve you should be able to remove the core and stick a toothpick inside, to detect slime. (It might be doable with a Presta valve as well, but it's iffier.) – Daniel R Hicks May 10 '16 at 2:34
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To find out if there is sealant inside the tube,

  • cut it open - kinda terminal for the tube

  • take it off the rim and deflate it then squish the tube with your hands. It will feel more slippery than an empty tube

  • Hang the deflated tube up for a night and the sealant will pool at the bottom. You should be able to feel it through the tube

  • If the valve core is removable, remove it and stick the end of a matchstick or toothpick through, see if it comes back damp.

Generally weight alone won't tell you, because there's only 20-50 grammes of sealant in a tube, when a tube weighs 5 times that.

  • If the core is not removable, can you still get slim in, or is that enough to show its not been slimmed? – mattnz May 10 '16 at 2:29
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    @mattnz Probably an indicator, but not guaranteed. Some people add sealant via a syringe through the tube, or tubeless can have the sealant put in through the bead before the tyre is inflated. – Criggie May 10 '16 at 2:35
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    If it has slime in it, the valve core is by definition removable. Thats how shop install it, and how a manufacturer adds it before packing. – zenbike May 10 '16 at 15:16
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    turns out before I had a chance to take the tube out to try any of these I found another way to determine if it contained slime. I was riding when all of the sudden I heard a hissing noise... long story short, was slime. – Michael May 15 '16 at 15:17
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    @Criggie I didn't realize there was actually slime in it at first and so I was trying to figure out why the area around the leak looked funny. I decided to try to put my finger on the leak while I was thinking of what to do about the junk around the hole, and then I decided to just get on the bike and start pedaling and see how far I could get before I ran out of air. I got about a block when I realized I didn't hear the leak any more, and the air still had about half its pressure left. – Michael May 16 '16 at 4:48
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If you pump up the tyres and then let a little air out you might be able to smell it, at least if it's quite fresh. I haven't used it for a few years but remember quite a distinctive smell compared to the standard rubber smell you get if you let air out.

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take the wheel off, put is verticaly and spin it. A tire with slime rotates "differently". You may feel the slime circulating in the tire.

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When I deflate the tube, often a bit of slime comes out as well at the end, and you can see the residue at the valve. Most brands seem to be very bright colour (green or orange) so it is very noticeable.

Also when I inspect the (inflated) tyre for cuts and damages (good idea to do that every week or so), I can sometimes see green goo where I had a small puncture that was sealed off before I even noticed it. So have a look and you may actually see it.

The other day I had a larger cut and the tyre spat out lots of green slime all over the place before it sealed the hole - good to have mudguards!

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