It’s vital to work out what caused the puncture before you fit a new tube. If the problem’s still there when you fit a new tube, you’ll puncture again straight away – which is even more irritating if you haven’t got a second spare tube.
Your first step is to inspect the tyre carefully. Look around the outside for thorns, shards of glass or sharp stones. If you can’t see anything from the outside, check the inside of the tyre too. The easiest way to locate the culprit is to feel around inside the tyre with your fingers, moving slowly and carefully to avoid cutting yourself. If you’re still unsure what caused the flat, pump air into the tube and locate the hole. You may be able to hear it rushing out of a big hole. Smaller holes can be harder to find – pass the tube slowly through your hands so that you can feel the air on your skin. You can put the tube in a bowl of water and watch for bubbles, but I don’t usually carry a bowl of water in my emergency toolkit. Sometimes you can use puddles as an alternative. Once you’ve found the hole in the tube, hold the tube up to the tyre to locate the area of the tyre where the puncture occurred, and inspect the tyre again carefully.
Remove anything that you find. It’s often best to push objects out of the tyre from the inside, rather than forcing them through the tyre from the outside and making the hole bigger. You won’t necessarily find something in the tyre because punctures happen in other ways too. Pinch punctures – also known as snakebite flats – happen when you don’t have enough tyre pressure. If you hit a rock hard, the tyre squashes, trapping the tube between the rock and your rim. Pinch flats are usually easy to identify; you have two neat holes in your tyre, a rim width apart. Check the tyre sidewalls as well because a hole here will turn into a fresh puncture immediately. If you have rim brakes, the most likely cause of the puncture is the brake block sitting too high and rubbing on your rim. Either your brake block is set too high or the wheel has been refitted crookedly in its dropouts so that it sits off to one side rather than neatly in the centre of your frame.
Booting your tyre
Big tears or gashes in the tyre will need to be repaired before you fit a new tube; otherwise the new tube will bulge out of the split when you inflate it. The tube is much softer than the tyre, so any bubbles will either scrape on the ground and tear or get pinched in the tyre split as the wheel rotates. Both will cause another puncture immediately, which is irritating, especially if you don’t have a second spare. Feel carefully around the inside of the tyre. Any holes big enough to push the end of your finger into will cause a problem. You can buy tyre boots – sticky-backed strips of plastic – but out on the trail, you’ll have to improvise. The air pressure trying to force the tube out through the tyre is high, so you’ll need something fairly stiff. Ideally, choose something sticky so that it stays in place as you refit the tube. Duct tape is ideal for smaller holes; it’s worth sticking a strip of it to the underside of your saddle for occasions like this. Ordinary tube patches will also help. For bigger holes, you’ll need something stiffer to bridge the tear in the tyre. This is a chance for you to use your imagination – try using food wrapping, cardboard, shoe insoles, whatever you have available.
Step 1 : Now for the new tube. Remove the nut on the valve, if there is one. Pump a little air into the tube – just enough to give it shape. This will prevent the tube getting trapped under the bead as you refit the tyre. Pull back the section of tyre over the valve hole and pop the valve through the hole. Work around the tyre, tucking the tube up inside it.
Step 2 : Returning to the opposite side of the valve, gently fold the tyre back over the rim. This gets tougher as you go. When there’s just a short section left, you’ll probably get stuck. Let a little air out of the tube again, and push the sections of tyre you’ve already fitted away from the sidewall of the rim and into the dip in the middle, like you did to get it off. You should then be able to ease the last section on with your thumbs, a bit at a time.
Step 3 : If you can’t hand-fit the last section, use tyre-levers. Work on short sections 5 cm (2 inches) at a time, and take care not to trap the tube between the rim and the tyre lever as it’s easy to pinch-puncture it. Once the tyre is reseated, push the valve up into the rim so that it almost disappears (to make sure the area of tube near the valve is not caught under the tyre bead).
Step 4 : Pump up the tyre. If you had a snakebite flat last time, put in a little more air. Once the tyre is up, retighten the thumb nut on Presta valves, screw the stem nut back onto the valve stem and refit the dustcap. Don’t fit the valve stem nut until the tube is inflated, as you risk trapping a bulge of the tube under the tyre bead.
Step 5 : Refit the rear wheel. With the bike upside down, stand behind it and hold wheel in your right hand with sprockets on left-hand side. Put a left-hand finger in front of the guide jockey wheel and your thumb behind the tension jockey wheel. Pull finger back and push thumb forward, then place wheel so sprockets are within the loop of the chain. Guide the axle into the dropouts, and secure by doing up the quick-release.
Step 6 : Refit the front wheel. This is easier. Drop the wheel into the dropout slots; make sure there’s an equal amount of space between the tyre and the fork legs, and tighten the quickrelease lever securely. Again, if you’re not sure about your skewers, read the quick-release skewer section.
Step 7 : If you have disc brakes, wiggle the rotor (A) into place between the brake pads before settling the wheel into the dropout slots. You need to check that the rotor is sitting centrally between the brake pads inside the calliper. If it’s hard to see, hold something light-coloured on the far side of the calliper as you look through. You may need to adjust the position of the wheel slightly so that the rotor is central.
Step 8 : For rim brakes, don’t forget to refit the brakes – it’s easy to overlook this vital stage in the excitement of fixing your puncture. Pull the brake units together and refit the cable. If you have V-brakes, take care to seat the end of the noodle (B) securely in the key-shaped nest.
Step 9 : Turn the bike back over and check that the brakes work properly: pull the front brake on and push the bike forward. The front wheel should lock and the back one should lift off the ground. Pull the back brake on and push the bike forward. The back wheel should lock, sliding across the ground. Lift up the wheels and spin them. Check they spin freely, and that rim brakes don’t rub on the tyre.
Checklist : what caused the puncture?
- Sharp things (thorns, glass, flint) cutting through the tyre
- Cuts or gashes in the tyre that allow the tube to bulge out – check both the sidewall and the tread
- Snakebite punctures – when the tyre, without enough air, gets trapped between the rim and a rock
- Rim tape failure – when sharp spoke ends puncture the tube or when the tube gets trapped in rim holes
- Valve failure – when underinflated tyres shift around on the rim
- Overheating from rim brakes – although rare, this can happen on long mountain descents
- Worn tyres – when tyres get old, the bead can stretch, allowing the tyre to creep out over the edge of the rim, where it will puncture
- Badly adjusted rim brakes – when blocks that are set too high rub on the tyre rather than the rim, cutting through the sidewall of the tyre in no time at all
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES