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Leak down rate defined and discussed A tire and tube's leak down rate is a measure of their tendency to lose inflation pressure over time. All tires and tubes lose pressure when they sit around. Air passes through the membrane made up by the tire casing and inner tube. Their walls are permeable to air, just like a balloon. Standard weight tubes leak at a fairly low rate, and might only need to be inflated once a week. "Ultralight" tubes with very thin walls are very permeable, and have a noticeable leak down rate. A tire fitted with this type of tube must be inflated daily to be sure that it has the correct inflation pressure. Riding on a standard or tubeless tire at pressure that is lower than you think can cause problems. Experienced riders know that and inflate their wheels often enough to avoid problems. But a high leak down rate can be a problem for an inexperienced rider. They tend to forget to inflate their tires, and a set up that loses air too rapidly will make it likely that they will damage the rim, tire or tube when they ride it if it is underinflated. There are a few other potential disadvantages to riding with low pressure. It will cause the wheel to roll more slowly in some situations. And in an extreme situation it might let the tire come off the rim while it is in use. An acceptable leak down rate is not easy to determine. An experienced rider who is after maximum performance in a short event might be willing to live with a high rate if it means they can go a little faster. A long distance rider can't accept a high rate, unless he or she is willing to stop and re-inflate their tire often. An acceptable rate for a tubeless set up will not be much greater than it currently is for the parts that are in common use. That means that tubeless wheels are not going to be able to leak down much faster than the standard wheels people use now in a given application, at each component price level. In particular, low leak down rates will be required for less expensive wheels and tires used by less experienced recreational riders in order to minimize the problems that occur when riding a bike with excessively low air pressure in the tires. Curing leak down problems with a tubeless tire is more complicated than it is with a standard tube and tire. It's casing is permeable to air to some extent, and will leak air even if no defect or damage exists. This last source of air loss is similar to that of a traditional tire and tube. Lighter tires will leak more if they are made from the same material tires are currently made from, so there will be a compromise between tire weight and leak down rate. A tubeless tire can leak air through a puncture or imperfection in the casing. Experiments with Slime (a fluid sealant that you can put in the tire) show that this is manageable. Some tubeless rims can leak at the welded seam if there is porosity. They can also leak air under the tire bead at the point where it seals against the rim, if the bead is damaged or if there is any foreign object caught in this area. A scratched rim can also leak where it seals against the bead of the tire. All of these are manageable when the wheel is new, but can show up readily when the wheel is in use.
Fine print: The opinions expressed herein by Keith Bontrager are in the nature of general observations and should not be relied upon in your specific situation. Your question, problem, bicycle or component, may be different than the one he is discussing. This Page (Web Site) is not a repair manual and is not a substitute for having your bicycle examined and repaired by a qualified bicycle mechanic. It is not intended as a tutorial or course in how to safely ride or use your bicycle. See your local library or bicycle store for publications regarding how to ride and repair your bicycle. Think of Keith like your Aunt Edna. You wouldn't really do what she tells you to do, now would you? Bontrager Cycles is a division of Trek Bicycle Corporation. Bontrager and Trek are registered Trademarks of Trek Bicycle Corporation. Copyright, Trek Bicycle Corporation. All rights reserved.
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