Slime Smart Tubes are inner-tubes containing a thick liquid which instantly repairs punctures. They work well, and are a very handy option for winter riding, but there is a weight penalty.
This kind of inner-tube is well known in the mountain-bike world, but Slime Smart Tubes are also available for road bikes in two sizes: 700c x 19-25mm and 700c x 28-35mm, both with presta valves.
Here’s the theory: you get a thorn or flint in your tyre and it makes a hole in the inner-tube. As the air starts to escape, a bit of the thick liquid (aka slime) comes out too and then solidifies, instantly sealing the hole.
In practice, these Smart Tubes seem to work. I’ve been testing a pair in my winter training bike for a couple of months and haven’t had a puncture yet.
Fitting is exactly as usual, and you can pump up the tyre to your usual pressure: in my case that's 100psi inside 700x23 tyres.
The Slime Smart Tubes weighed in at 170g each on the road.cc scales, so there is some extra weight to consider (50 to 100g more than usual, depending what you usually use), but this is no problem on a winter training bike.
I didn’t notice a major difference in ride ‘feel’ – just a tad less lively – but once again no big problem on a winter training bike. If it makes you work harder now, you’ll go even faster in the summer, right?
For the first few hundred miles I was running the tubes inside some reasonably heavy tyres (Bontrager All Weathers) which obviously provided some additional outer protection. Later I tried the tubes inside a lighter pair of tyres (Schwalbe Duranos); still puncture free.
Of course, maybe I would’ve been lucky anyway, so after the road-trials it was time for some further testing.
First test: I deliberately rode over a drawing pin and kept going along the road. I could hear the clicking of the metal on the tarmac as the tyre went round, and there was a slight drop in pressure, but I could keep going without trouble. Back at the road.cc lab, the gauge revealed a fall from 100psi to stabilise at 80psi.
Second test: Take the pin out, and immediately ride down the road again. There was a sound of escaping air for a few seconds, and there was another drop in pressure, but once again I could keep riding. Back at the lab the gauge showed the pressure had fallen again from 80psi to stabilise at 60psi.
Third test: Drawing pin in, then out straight away, then ride down the road. This time the pressure fell from 100psi to stabilise at 70psi.
(Incidentally, the self-healing does not seem to work if the bike is stationery. I assume the spinning wheel needs to spread the slime around, and centrifugal force may play a part too.)
So, on the road, these tubes seem to work (although maybe I was lucky), and this performance seems to be backed up by my drawing-pin lab tests. It seems that the slime works better if the object causing the puncture stays in the tyre (which is often the case in the real world, as anyone who’s struggled to pull out a thorn from the inner casing at the side of the road will tell you).
If you’re a tourist encountering flints and thorny hedge cuttings, or a commuter dealing with glass and those tiny bits of metal that come off worn car tyres, then these tubes are worth considering. If you’re riding on good roads in clean conditions, and like to keep the weight down, or if you’re happy to ride heavier tyres in the first place, then they’re something you may not need.
On cost, the tubes recommended retail is £9.99 each, though you can find them for a few quid less on-line. Obviously more than a normal tube, but for a better chance of puncture-free cycling this might be a price you’re happy to pay.
Inner-tubes to help reduce the chance of punctures. A handy option in winter, but they're not featherlight.
road.cc test report
Make and model: Slime Smart Tube Self-healing inner tube
Size tested: 700 x 19/25c
Tell us what the product is for, and who it's aimed at. What do the manufacturers say about it? How does that compare to your own feelings about it?
Slime smart tubes are designed to help prevent you suffering punctures. To be precise, you'll still get the punctures, but the slime is designed mend the holes as you're going along.
The manufacturer's website says: "All SLiME Smart Tubes are factory-filled with a precise volume of Slime Tube Sealant. Smart Tubes instantly seek out and seal punctures as they occur, preventing flats, repeatedly and continuously for up to two years."
This is a fair claim, although the website doesn't mention the fall in pressure. However, this is not a major issue. You can ride on on slightly podgier tyres, or stop and pump them back up to your usual pressure. Either way, it's better than fixing a flat at the side of the road.
The manufacturer's website also emphasises that Slime works only on punctures less than 3mm, and only in the tread area, not the side wall.
Tell us some more about the technical aspects of the product?
Under 'How does slime work?' the manufacturer's website goes onto say: "Slime coats the inside of the tire within the tread area, allowing the product to seal tread area punctures. When the tire is punctured, the escaping air carries the sealant to the puncture. The liquid portion of the sealant escapes and the fibers and binders build-up and intertwine to form a flexible plug. This will allow the product to instantly seal any puncture that may occur.
Construction seems very good. No blemishes in the tube which might develop into weak patches in future.
Performance is very good. No punctures in a few hundred miles of winter riding on dirty Cotswold lanes. Deliberate punctures in test conditions show the slime quickly repairs holes, with or without sharp object remaining in the tyre, although there is a drop in pressure.
Rate the product for weight, if applicable:
These tubes weigh more than a normal tube, but this might be a penalty you're happy to have for fewer punctures and peace of mind.
No major difference in ride 'feel' – maybe just a tad less lively – but this was no problem on a winter training bike.
On cost, the tubes recommended retail price is £9.99 each, though you can find them for a few quid less on-line. This is on a par with similar products from other manufacturers. Obviously more than a normal tube, but for a much better chance of puncture-free cycling this might be a price you're happy to pay.
Tell us how the product performed overall when used for its designed purpose
These tubes worked well. I've been running a pair of these tubes in my winter training bike for a couple of months and haven't had a puncture yet - although I might have been lucky. Lab tests with deliberate puncures show the slime repairs holes, although there is a drop in tyre pressure. These tubes won't prevent every puncture, but they'll definitely reduce the chances of having to stop and repair a flat at the side of the road.
Tell us what you particularly liked about the product
Easy to fit. Puncture-free riding at best. Reduced chance of puncture at worse. Peace of mind either way.
Tell us what you particularly disliked about the product
Slightly heavier than normal tubes. Slightly less lively ride.
Did you enjoy using the product? yes
Would you consider buying the product? yes
Would you recommend the product to a friend? yes, especially for winter riding.
Anything further to say about the product in conclusion?
Overall, these tubes do the the job as claimed by the manufacturer, so on that level they score 10. They're not cheap, and not as light as normal tubes, so that knocks a point off - giving an overall score of 9.
About the tester
Age: 51 Height: 5ft 10 / 178cm Weight: 11 stone / 70kg
I usually ride: an old Marin Alp, or an old steel classic My best bike is: an old Giant Cadex (can you see a theme here?)
I've been riding for: Over 20 years I ride: A few times a week I would class myself as: Experienced
I regularly do the following types of riding: touring, club rides, sportives, general fitness riding,