Friday 24 June 2011

How To...Mend a Puncture

Not too long ago, I met a lad aged 18 who was taking his bike to the bike shop for a repair. "What's wrong with it?" I asked.

"It's got a puncture," he told me.

I was, naturally, quite incredulous that anyone could reach the age of 18 without having learned how to mend a puncture somewhere along the way. I saw him again later in the afternoon and he told me the repair had cost £5. Now, £5 isn't a lot of money, but since a puncture repair kit costs less than £2 and a puncture takes about five minutes to fix, it seems a bit costly.

The next day, I happened to see the man who runs the bike shop and I asked him if many people bring bikes in to have punctures repaired. He informed me that it happens seven or eight times in an average week.

People, come on! Read this and find out how to mend punctures for yourself!

You're going to need:
A puncture repair kit (including patchs, a piece of sandpaper or emery cloth, a little crayon, some chalk and some vulcanising cement)
A pump
You can use tyre levers, but they're not essential
Tools to remove the wheel (most modern bikes have quick release wheels and no tools will be required)

Puncture repair kit. Some will include tyre levers and perhaps a really rubbish spanner that will snap in half should you ever try to use it.
What you do:

1. It's easier if you turn the bike upside down. Now, if you have rim brakes, release them on the wheel with the puncture. Most brakes have an easy way for you to do this, such as by freeing the nipple on the end of the straddle wire in the case of most centre-pull cantilever brakes. If it's a side-pull cantilever, (also known as a V-brake), release the noodle (the metal tube with the 90 degree bend) from the left cantilever. Some side-pull calipers will require you to loosen the cable clamp - usually accomplished with a 10mm spanner - while good quality models and dual-pivot side-pull calipers may have a quick release clamp. Magura hydraulic rim brakes will either need to be unbolted on one side or will be fitted with a quick release lever. With disc brakes, you can leave the brakes alone.

You don't need to repair punctures in beautiful natural surroundings, but chances are you  will sooner or later.
2. This done, you can remove the wheel. Most bikes are fitted with quick release wheels, in which case it's a simple matter of flipping down the lever, twisting the nut a few times and lifting the wheel out of the drop outs (unless it was fitted by one of the surprisingly large number of people too stupid to use them properly). Other bikes may use nuts on a solid, threaded axle, in which case you'll need a spanner - the majority of bikes made in the last 30 years will need a 15mm spanner, but you may also come across other sizes or Imperial nuts for which you will require either the correct size spanner or an adjustable spanner. Using it, loosen the nuts either side but don't remove them from the axle until you are able to remove the wheel from the drop outs. Some mountain bikes have large, hollow axles which are clamped in place by the drop outs. Some of these clamps have quick release mechanisms, others will require you to use an allen key (in most cases, a 5mm one) or a spanner.

3. Once the wheel is off, it's time to remove the tyre. This can be achieved by working round the wheel rim, flexing the rubber with your thumbs to loosen it. Thus done, it should be a simple matter to free it from the rim's clincher, the ridge around the inner edges of the braking surfaces into which the tyre beads lock when inflated. However, the internal diameter among tyres of the same size varies quite widely and so whereas one tyre may be very easy to take off, another can be very tight. They'll all come with enough flexing, but if you have tyre levers use them. Some mechanics will tell you never to use tyre levers because they can pinch the inner tube and create another puncture, but by taking care to avoid the tube I've managed to avoid this happening even once during my three decades in cycling so I'm going to say use 'em if you want. Use one lever to, er, lever the tyre off the rim, then hook it under a spoke - most types have a special hook allowing you to this. Use the other lever to lever off section a few centimetres from the first and keep going until the tyre is free. You only need to free the tyre from one side of the wheel for the time being.

4. You can now remove the dust cap from the valve stem and push it through the hole in the rim, which will make it easier to take hold of the tube and pull it out of the tyre. Once this is done, use the pump to partially inflate the tube - in the majority of cases, you'll know where the puncture is when you hear the air escaping. If you can't, hold the tube a few centimetres from your lips and pass it across them - your lips will feel escaping air that your fingers cannot. If even that doesn't work, get a bucket or large bowl of water, immerse the tube and look for air bubbles. Before deflating the tube, mark the position of the puncture with the crayon or with a ballpoint pen.

Mark the puncture's location with a cross extending a good way outwards. That way, you'll know precisely where to put the patch even if the sandpaper and vulcanising cement remove the middle of the mark.
5. Select which size patch you're going to use. The smaller ones, 4 square centimetres or so, are large enough for the majority of punctures caused by thorns and suchlike. The larger ones are used for bigger holes which resemble slits rather than pinpricks.

6. Once deflated, dry the tube if you used water to find the puncture. Now locate it again using the mark you made. Take the sandpaper/emery cloth and use it to rub the tube over and around the puncture, sanding an area slightly larger than the patch. If the puncture is situated near raised parts of the tube such as the brand name, take a little extra time to flatten the surface as this will help the patch to stay in place. Remember that you're roughening the rubber and removing dirt - there's no need to sand so firmly that you reduce the thickness and damage the tube.

7. Now take the vulcanising cement, which most people call glue. Apply a thin layer to the sanded area (you can also get self-adhesive patches, but in my experience they don't work very well). Now wait for it to dry. One of the commonest mistakes made by people who don't know how to mend punctures is sticking the patch onto the tube before the cement dries. Vulcanising cement is not glue - it doesn't work by sticking the patch in place, but uses sulphur and other compounds to create crosslinks between individual polymers in the tube and the patch, "melding" them together. Press the patch firmly into position using your thumb. If you tried to save time by not deflating the tube at the end of Step 4, you will now find air bubbles have formed under the patch and have to return to the start of the same Step.

An expertly-applied patch.
8. On the back of your puncture repair kit you should find a raised, rough section rather like a file (if you have a puncture repair kit you've put together yourself, you probably know what you're doing and have an alternative. I use fag ash, personally). Use this to finely powder some of the chalk, then rub this over the area where you applied the cement to prevent it forming crosslinks with the inner surface of the tyre as this will rip the inner tube next time you need to remove it.

9. Before replacing the tube, remove the tyre completely from the wheel. Look around the inner surface to see if there's anything stuck in the rubber which will immediately puncture the tube again. The nose of a pair of pliers can be used to push foreign bodies back through the tyre so that they can be grasped and removed using the same tool. If you can't see anything, run your fingers around the inside instead. This will transfer the thorn/razor-sharp shard of glass/tetanus-ridden flint particle from the tyre into your skin if you're not careful, so be careful. Use the pliers to remove it from the rubber.

10. Place the tyre back onto the wheel, pushing one bead over the rim into the gap between the braking surfaces. Locate the valve stem aperture. Feed the valve stem through it, then work around the circumference feeding the remainder of the tube into the tyre. Once in place, work the bead of the tyre onto the rim, making sure both sides engage with the clincher all round. Reinflate, reattach the wheel making sure the chain is properly located on the sprockets if its the back wheel and job's a good 'un

11. Oh, and by the way - don't forget you unhooked your brakes in Step 1. A lot of people forget that, much to the delight of dentists.


A. Repairing punctures in tubular tyres, which are glued to the rim, is much more difficult and requires the tyre to be cut and later rejoined. That's why few cyclists use them except for in competitions when spare wheels with inflated tyres ready fitted are available.

B. Tubeless tyres are becoming common on mountain bikes. If you have them, chances are you know what to do when you get a flat. If you don't, you'll need a special puncture kit for tubeless tyres with patches to be affixed to the inner surface of the tyre. Tubeless tyres require a seal between the beads and clinchers to be achieved very rapidly otherwise air will escape - for this reason, it may be necessary to use a large capacity track pump or a compressed carbon dioxodge cartride able to pump a lot of gas into the tyre in a very short period, thus forming the seal.

C. If you suffer a catastrophic puncture, ie one that cannot be repaired and you don't have a spare tube with you, remove the damaged tube and pack the tyre with grass and leaves. Keep your speed low and though it won't work very well, it will get you home.

D. If your tyre continually goes flat but you can't find a puncture, chances are the valve is at fault. Woods and Shrader valves can be removed (by hand for the former, with a special tool for the latter) and replaced, but most people will just replace the entire inner tube. Most Presta valves cann't be removed, so faults will require replacement of the entire tube.

E. If you get a lot of punctures, check your tyres aren't worn - if they are, replace them. If they're not, it's likely that the wheel has no rim tape, a cloth or rubber strip running around between the braking surfaces which protects the tube from the spoke nipples or holes allowing access to the spoke nipples. Your bike shop will be able to supply you with a new one. If the rim tape is in place and in good condition, you probably just ride in an area with a lot of thorns so consider fitting tougher, puncture-resistant tyres such as those featuring Kevlar inserts.

(Images by Mike Warren at WikiHow)

No comments:

Post a Comment