(a) External foreign body picked up in tread : A thorn, flint, shard of metal/gless etc. becomes embedded in the tyre carcass and works through the layers to penetrate the air chamber.
(b) External foreign body that remains on trail : A nail, piece of barbed wire, or any other sharp object large or heavy enough not to remain lodged in tyre penetrates the air chamber as above.
(c) Internal foreign body introduced carelessly : Small bits of glass, thorns, stones etc can end up inside the gap between the tyre carcass and the inner tube. They remain trapped and can work their way into the inner tube eventually causing a puncture.
(d) Spokes/failure of rim tape : Occasionally a wheel builder can use an over long spoke. It shouldn't happen but it does. It can also happen if you use a replacement spoke without the proper care. Much less of a problem with double walled rims but something to consider if you keep getting punctures (remember to check the inner surface of the tube)
SCOTTISH MOUNTAIN BIKING ADVENTURES
PUNCTURES
Printer friendly pages to work with at your bike
THE AIR CHAMBER
Experience has shown that a substantial number of riders have little awareness of the importance of the air chamber and its roles in how tyres/wheels function.
At the most fundamental level, the system relies on a positive pressure difference between the air inside the tyre (a) and that outside it (b). When (a) is greater than (b) it pushes the tyre carcass (c) out and creates a cushion of air between the bike/riderand the ground. This positive air pressure also forces the beads (d) of the tyre into the rolled edges of the rim.
In the diagram opposite the blue line represents an inner tube. With tubeless systems this will not be present (duh!)and it's purely the air pressure against the carcass which provides the form and support.
The compromise of any of these elements, in either type of tyre, can lead to loss of pressure and tyre failure.
A lot of basic stuff here that experience shows is missing from many people's knowledge.
TYPES OF PUNCTURE
Penetration : A foreign body ruptures the air chamber and can be caused by the following.
Snakebite Puncture
Caused when impact compresses tyre onto rim at points marked (e) in the diagram opposite. Resulting pressure forces the inner tube against the upper surface of rim and it is cut through.
Tubed or tubeless, two ruptures occur opposite one another at the point of impact. The paired holes resemble a snake bite hence the name.
Often it is only one side of the rim that catches an offending object or protruberance. In the diagram it can be seen that the RH side is likely to survive but as the system rolls forward under compression then the LH side may be ruptured.
People will say you don't get them with tubeless tyres : you do, just not as commonly.
Sidewall rupture
Thes occur when the sidewall of the carcass fails. A tear can be caused by the sidewall brushing against a sharp flake (f) , pointed rock or anything else nasty enough to go through the tyre's thinnest part. It happens most easily when the tyre is heeled over in cornering as in the diagram. The material is in tension and this aides the process of cutting through it.
Occasionally failure can take place due to constant stressing of the area where the tyre meets the rim at (g). Invariably this area is reinforced but it takes a great deal of abuse, especially if you run low pressures. The outer rubber can be worn away exposing the ply which then abrades and eventually allows it to tear. It's nigh on impossible to fix so keep an eye on older and cheaper tyres!
Whatever the cause the inner tube will force its way out to burst.
Failure of inner tube/valve
A clumsily handled inner tube can sustain damage. If not protected when carried it can have a puncture before you even fit it. If not properly inserted it can be trapped between the bead and the rim causing it to burst. If it's been patched previously these repairs can fail.
With Presta valves, the threaded rod (h) can become bent. This can push the conical assembly with the rubber grommet (i) out line and and unable to make a seal in the core. Dirt can also get inside the valve core and prevent it sealing properly. Use a valve cover at all times to help keep the dirt out.
Over zealous pumping can rupture the seal where the valve enters the tube. This can be a case of the assembly coming loose in the moulding or abrasion/cutting at (j) caused by movement against the sharp edge of the valve hole. Although it's rare, the potential for damage around all valves can be minimised by screwing down the knurled ring that secures the barrel of the valve in the rim. Using it can also help keep a lot of dirt, and some water, from entering the void between the two rim walls.
Fixing punctures
We're working on pictures and graphics at the moment for a page shoing how to do this. It will be posted soon. Don't laugh, it's not as straightforward as many seem to think!
Many riders seem to head out without spare tubes and think they can get by with a couple of self-adhesive 'magic' patches. This will cause no great hassle at riding centres as there will usually be another party along who will come to your rescue.
You can't rely on samaritans when you're out and about in the wilds. There, you will discover that the magic that allows a magic patch to be applied to a wet or dirty tube hasn't been invented yet. The difficulty of actually pinpointing the location of the puncture without a basin of water to see the bubbles becomes apparent. Driving wind, rain, cold hands, sore backs and the accumulated frustration of the umpteenth mechanical of the day can all leave a soul wishing they were better prepared.
Unfortunatley, with current technology, punctures are a fact of life. For as long as we need to fill our tyres with air we'll get them. So, it makes sense to be prepared. On the pages to follow, not only will we try and help you to reduce the number of punctures you suffer but also how to sort them more efficiently.
With today's larger volume tyres, a pump that will bring you up to pressure quickly is needed. It will minimise the time spent in freezing rain on exposed hillsides, or in midge infested hollows. A pump that means your knuckles don't get mashed on the spokes is also an advantage. One where the chuck (the bit that sits over the valve) stays in place and doesn't leap off when you're putting your back into it is also a boon.
All the reasons above are why the Topeak Morph range of pumps (or anything similar) is a sensible thing to consider. They're bombproof and have a hose that connects to the valve and isolates any movement generated in the pumping process. This lessens the chance of the chuck jumping off. There's also a lever which, when activated, closes the chuck's sealing grommet around the valve. This makes it even more secure and efficient. A fold-out handle matched with a fold-out foot peg allows it to be used like a track/workshop pump and you can blast away to your heart's content.
These these features aid rapid inflation. However, they also allow the solo rider to inflate a tyre with one hand. This can be a real comfort if even a slight injury is present. So, (for the final reason alone) it's a bit of kit that riders venturing onto remote trails should consider. I've certainly been glad of it on a couple of occasions. If you're obsessed with weight you can get a mini version and if you' want to make sure that all your pressures are properly set you can get a model fitted with a pressure guage.
The Pump
Before dealing with any other aspects, it has to be said that nothing will hold you up like an inefficient pump. No matter what short barrelled mini-pumps promise they are awkward to use and the amount of effort that needs to be expended tempts many riders to leave tyres under-inflated. This affects the bike's handling. It also leaves you vulnerable to snakebite punctures.
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