Bicycles/Maintenance and Repair/Wheels and Tires/Fixing a flat
This page explains how to repair punctures in bicycle inner tubes, and gives some advice as to the most likely causes. In addition, there are notes peculiar to the use of tube sealants.
The basic parts of interest are these:
- Tires are the rubber outer parts that contact the road's surface. The rubber part is called cladding, and it covers the internal structure of the tire. The cladding has a tread, smooth or patterned, depending on its use. The edges that fit into the wheel rims have wires within the cladding called beading or beads, and various fibres run between the beads to give strength within the cladding. Although tubeless bicycle tires exist, they are still quite rare on pushbikes, so this page deals only with tires having inner-tubes.
- Inner-tubes are the inflatable parts between the tire and the metal parts of the wheel. They are invisible apart from their valves that protrude through the wheel-rims. There are three valve types in use; Schrader valves, Presta valves, and the lesser used Dunlop valves. Refer to the adjacent image for examples.
- Rim-linings protect the bottom of the inner-tube. The ends of the spokes enter the rims, and this lining in the rim-well covers them so that the tube is not damaged.
- Anti-puncture linings protect the top of the inner-tube. Some high-end tires have protection built-in, but most tires have none. As accessories, they can be easily added between the tire and the inner-tube. They prevent most punctures from thorns, glass, flints, and other sharp objects that contact the running surface of the tire, though they can rub on the tubes and be the cause of punctures in their own right.
- Self-sealing tubes. These are tubes with a clotting fluid inside, (Slime). They automatically seal punctures as and when they develop. They can be bought with the fluid in them or for tubes with Shrader valves the fluid can be added separately.
The Causes of Flats
A flat tire or puncture is most often caused by glass, thorns, flints or nails when they cut through the outer rubber tread of the tire and damage the inner tube. These deflations are usually quick, at least when the object is removed. Although this is the most obvious way to get a flat tire there are other ways too. Consider these:
- Internal abrasion on metal parts. If the spoke ends are not covered by the rim-tape properly, then this might happen. Other possibilities include a rubbing valve stem, or a bare edge wire in the rubber parts. Inspect the inside of the tire for these and for old inclusions that are sometimes missed. Also, make sure that the valve stem is straight in the rim opening.
- Rough edges from tube liners or worn internal surfaces in tires. Tube liners have been reported to cause punctures, and this author has experienced a slow puncture at the free end of a liner in a two month old tube. When a tire's cladding is frayed or holed, it can rub on the tube and cause damage. Tires can sometimes be made good with an internal patch or boot; this is a specially made patch or one improvised by adding a loosely fitted section of old inner tube over the inflated one at the site of the repeated abrasion.
- Recurrent movement of the tube. This happens mostly when tires are soft. The tires flatten or bottom-out on encountering bumps and form double faults or snake-bites in the tire. Tires inflated to near their maximum have fewest of these.
- Incorrect seating of the tube. If the tube is not seated well before inflating the tire, it is exposed to unusual stresses that lead to single faults called pinches. When inflating tires, first add a little air to the tube then press both sides of the tire around the whole perimeter to check that no tube rubber will be trapped.
- Rough handling of the tube. Some outer tires leak fluid when they are punctured, as do tubes with built-in puncture repair fluids. These cause the tube to become glued to the inside of the tire. If the user does not know this and pulls the tube too hard when removing it, a crescent shaped weakness in the rubber results. These areas usually become punctures, often after only a few days. Expect binding of slimed tubes when removing them and prise any sticky sections apart carefully.
- Manufacturing faults. These happen sometimes though it is difficult to tell these from other punctures. If such a fault is obvious then return it for a replacement at the store where it was bought. Commonly available tubes have a thickness of about 0.9mm though racing tubes are thinner; some downhill tubes are are made up to 2mm thick. The thicker tubes are slower to produce pinches and snakebites though they have a greater rotational mass, (heavy pedalling), and in any case are rarely stocked by bike shops.
- A series of recurrent punctures, where there seems to one every other day suggests that either the user is at fault in not properly clearing the cause of punctures, or that a long series of pinches and snakebites are just starting to break through. Replace the tube, check the state of the inside tire, or perhaps install a tube sealant.
- Valve leakage: Some valve cores can develop slow leaks when the fibres of tube sealants settle within them, and not all valve cores can be removed for cleaning. If a valve is not positioned squarely where it protrudes from the rim of the wheel, then the rubber will be stressed at that point and a puncture is likely to develop.
Replace or Repair? 
It is undoubtedly easier to change a tube for a new one than it is to find the puncture in the original. For this reason many riders carry both puncture repair materials and a new tube when they travel for any great distance.
When on the road it is sometimes difficult to find the point on an inner tube that is punctured. If it is a slow puncture it might be possible to pump some air into the tube, perhaps more than once, enough to reach home or a more convenient place to do the work. If is it a quick deflation, then a repair or replacement becomes necessary. When an object penetrates a tire it is best not to remove it immediately if it is the only thing that keeps air in the system. Instead, use the remaining air to get the bike home before removing it.
If the tires are fitted with self-sealing fluids it might be that the puncture will have been fixed already and that the tire just needs to be inflated. A puncturing object should be removed from such a tire only when the part with the inclusion has been rotated to the six-o'clock position; this allows the internal sealant to fully reach the puncture. A different method is needed for slow-to-seal sidewall pinches; allow the fluid to pond at the bottom of the tire then tilt the wheel, slowly rotating it, so that the fluid can reach the internal sidewalls.
If the tire is not self-sealing, and must be repaired rather than replaced, then the remainder of this page will explain how.
The Basic Tools 
This section lists the basic requirements for tube repair. The primary items are these:
- A pump that fits your valves. Adaptors too for some pumps.
- Puncture patches. The kind that need glue or the ones that are pre-glued.
- Rubber vulcanising solution. This is the glue that sticks rubber together. It is not needed for some kinds of patches. You also need a pin to puncture the top of the tube.
- A small piece of fine oil paper or sandpaper.
- A pen that can make marks on rubber, or a wax crayon. Users please comment on what is best. I have found that a pen with 'metallic silver' ink works very well for marking inner tubes.
- Tire levers You need at least two of these; plastic or metal. Some repair kits include these.
- A talcum block This is included in some puncture kits with a scraper.
Also useful are these:
- Wrenches . If the bike has nutted axles instead of quick-release skewers, you'll need some tools to remove the nuts so you can take the wheel off, e.g. "ring spanner" or some other type of wrench. Some bicycles require two wrenches. Try out your tools before riding to be sure that they are the right ones to carry.
- Hex wrenches You might need to release the brakes to get the wheel off.
- Valve core removal tools. These are needed only for tubes with removable valve cores, but are useful when self-sealing fluids have clogged the valves, or the cores have become loose. Removable Presta valve cores such as those of Schwalbe and Bontrager have flats on them that can be turned with pliers.
- A spare new inner-tube; in case all else fails.
The Repair Procedure 
Examine the flat tire carefully to find any sharp item that may be responsible. If a nail, thorn, shard of glass, or a flint is found in the outer tire then mark the rubber of the outer tire with a wax crayon or suitable pen so that the location is easily found. Continue to check the tire in case there are more.
Some punctures of inner-tubes also cause damage to the fabric of the outer tire, so look for any potential problems such as a bulge, that suggests a torn carcass, or for exposed carcass cord. If such damage is excessive then it may cause repeated punctures to inner tubes. At this time the method to correct such damage involves the surfacing of the inside of the tire with a so-called boot; a pre-glued patch that keeps the fibres clear of the tube. A piece of an old inner tube can also be used as a temporary fix, if loosely wrapped around the tube in the location of the damage. This latter fix suggests that it might be a good idea to carry an old piece of inner tube in your repair kit. At this time there is no rubber-based filler for cuts in outer tire cladding, so water ingress is likely.
If there is no clear cause of the puncture and the location remains obscure, or if it is a slow puncture, then the wheel will need to be removed to get proper access to the inner tube. Methods are given below that include both wheel removal and repair with the wheel in place.
Remove the wheel? 
Wheel in place 
Most punctures need the wheel removed, but if you are sure that you know where the hole is, you can do the repair with the wheel still on the bike. This method is popular on bikes that need wrenches to remove the wheels, and for rear wheels, even when they are of the quick-release type. However, if the front wheel is of the quick-release type, you will usually find it more comfortable to remove it anyway. The sequence for an on-the-bike repair is just:
- Prop up the bike in a stable position against a wall, fence, or use the bike stand.
- Move the punctured part of the wheel to a convenient angle where it is easiest to work.
- Deflate the tire.
- Remove one bead of the tire from the rim using tire levers as necessary, then gently pull out the section of tube with the puncture.
- Correct the problem, then re-inflate the tire. The methods for doing so are described in the sections below, though readers should ignore the parts that do not apply to their situations.
Wheel Removed 
- Set the bike in an upright position against a wall or fence, and because the bike will become unstable when the wheels are removed, the frame might need to be secured. Some risk scuffing the paintwork and turn the bike upside down for wheel removal.
- Remove the wheel from the frame. On some bikes, you need to spread or detach the brakes to do this, though deflation of the tire first will usually make it possible.
- If your bike has Quick release wheels, remove the wheel by loosening and unscrewing the quick-release bolts (skewers).
- If your wheel is attached to the frame by ordinary nuts, undo them with a ring spanner (wrench).
- If you have to take off the back wheel, it is usually a bit more complicated, because you need to get the wheel past the chain. If your bike has multiple gears Derailleur gears, change into the very top (smallest) rear and front gears to slacken the chain as much as possible. This makes it easier to get the wheel off.
Remove the Inner Tube 
- If the tire is not fully deflated, open the valve and let the rest of the air out, as any residual air will make it difficult to get the tire off and the tube out. If you have Schrader valves then remove the plastic cap and press the center-pin in the middle of the valve. If you have Presta valves, remove the plastic cap, undo the metal locking screw all the way to its end, then press the end of the valve to release the air. Later, during patching, the Presta valve can be prevented from refilling again, if necessary, by re-tightening the metal locking screw; no such procedure is needed for the Schrader since it has an internal spring.
- With the tire deflated, insert the smooth end of a tire lever under the bead of the tire at a position far from the valve. While doing this be certain that the inner tube is not trapped by the tire lever. The tire lever is used with the bent end facing inward so that it can be hooked onto the spoke to keep it positioned. With it so positioned, insert another tire lever about four inches or so from the first, in the same way. Continue moving and inserting the tire levers until one bead of the tire is free of the rim. In the case of wide tires this task is fairly easy but for slim tires another tire lever may be useful.
- Remove the inner tube very gently, whether it is just a local section or the whole tube. Always anticipate that the tube is sticking to the inside of the tire, and that wrenching it will cause a stretch-weakness. If the tube seems stuck, then use extra care in separating it from the tire, perhaps softening any glued-up tube sealant with some water before easing the two apart.
Be careful too, with the valve when it is removed from its rim, and remember to first remove the locking screw on the rim if the valve is of the Presta type.
Find the Puncture 
- If you intend repairing the tube without removing the wheel, the location of the puncture will be already known, but otherwise the puncture still needs found.
- Remove any penetrating objects, and carefully inspect the inside of the tire, visually and by hand contact to make sure that there are no remaining causes of punctures.
- A fast leak in the tube can be found by inflating the tube and listening for the escaping air. Another method is to dampen a hand, then detect the cooling effect of the leak by feeling around the tube. Sometimes a small hole is visible, especially near a stretch mark on the tube, or in a place that was sticking to the inside of the tire. Another likely place is at the free end of a tire liner. If the leak is still not found then submerge the inflated tube in a basin or a sink to look for air bubbles. During this process, it helps if the tube is folded and compressed with the hands to aggravate the leak. Use a ball point pen, felt marker, or the chalk supplied with your puncture repair kit to mark an X on the location of the puncture. Make the X somewhat wider than the patch diameter. This will enable you to centre the patch over the hole.
Repair the Tube 
- In the area of the puncture, lightly roughen the surface of the tube with the sandpaper to expose clean rubber for the bonding process. Rubber-to-rubber patches make continuous bonds that extend though the glue layer. Although the stresses on tube patches are fairly low, there are nonetheless optimal conditions for success:
- Surfaces must be clean, dry, flat, and oil-free. This assures a good connection.
- The glue layer must be thin. Thin layers are strong and airtight and bonding is fast.
- The bonding must be undisturbed. Escaping air or movement will weaken it.
- The best patches are pressed while they set. Press the parts together.
- Make sure that the tube is completely deflated before applying a patch as escaping air might spoil the effort; Presta valves can be locked if need be, to prevent their re-filling. There are different kinds of patches in use, and each kind is accompanied by instructions of its own. A few of these are mentioned below.
- Pre-glued rubber patches with foil backings usually require that a thin film of glue be applied to the tube, slightly larger than the intended patch, and left until tacky. Then the backing is removed and the patch is stuck firmly onto the puncture. To avoid handling the glued surface, expose one corner of the patch first and apply it to the tube, then withdraw the remainder of the backing while completing the connection. Some have a handling film that can be left in place. If too much glue has been used, be sure to press the parts together for an extended period before use; hours rather than minutes.
- Recently, thin clear-film patches have been made available. These do not need a priming layer of glue, and are pre-glued. After surface preparation, these are transferred to the tube directly, using the progressive removal of the backing as described above. Like other methods, they depend on a clean, dry, oil-free, flat surface for the greatest success. They are perhaps the easiest to use, and do not need a separate glue supply.
- Conventional rubber patches need a thin priming layer for the tube and also for the patch, then when tacky, the patch is made.
- After making the patch scrape some of the talcum block onto any visible glue to prevent sticking.
- The bond of the patch strengthens with time. Although a perfect patch can be used almost immediately, if there is no real hurry, leave the patch to set as long as possible before inflation.
- The most difficult patch to make is one located near a ridge in the rubber; the ridge spoils the airtight connection, and ridges are hard to remove or make flat enough, even with sandpaper. Sealant-filled tubes can repair such punctures without difficulty.
Refit the Tube 
- Before the replacement of the inner tube, make sure that the causes of the puncture have been removed. In addition, run fingers around the inside of the tire to find any rough spots that could cause damage to the tube; if found, they can be fitted with a boot, a durable seal that goes over the rough bits of the inner tire, or a loose piece of old inner tube around that section of the tube itself. Check that the rim tape, (liner), covers the spoke ends and clean any leaked sealant from the inner tire with a damp cloth.
- Some tires have arrows on the sidewalls to indicate which way round the tire tread is to be aligned; make sure that this is right. When all is well, add a little air to the tube to give it some shape, and fit it into the tire. Take care in feeding the valve stem through the rim cutout.
- Starting at the section with the valve, work around the wheel, pushing one bead of the tire onto the rim, using your hands only. If you use a tyre lever it is difficult to avoid trapping the tube between the rim and the tyre. Make sure also that the valve is straight at the end of this stage, and if it is not then gently straighten it by shifting the tube's position on the rim; not by pulling on the valve.
- Repeat the process to set the second bead of the tire onto the rim following the same procedure as for the first. Re-check the straightness of the valve stem and make any corrections before proceeding beyond this stage.
- Work around the tire, pressing both sides toward the centre of the rim. If this is easily done then it is clear that the tube is not trapped at any point.
- Inflate the tire and install it on the bike, paying attention to the inflation limits marked on the tire's sidewall.
Valve Problems 
Any inner tube can be checked for leaks by first inflating it, and submerging it in water. Telltale air bubbles will seen emerging from the site of the leak. This process can be made more effective by forcing sections of the tire between the hands to increase the leakage.
A slow leak can sometimes be caused by the valve itself. Sometimes a leak can be seen in a valve by wetting its various parts with a soapy solution and looking for bubbles.
To try to fix a leaking Schrader valve, deflate the tire, unscrew the valve body with a keyed valve cap or valve tool, and examine the seat or rubber sealing ring for cuts or nicks, dust, lint, or fibers that prevent the valve from closing fully. Likewise check the valve seat and the bore of the valve stem. Clean if necessary. The valve body may be replaced. If no spares exist, an emergency fix can be had by inflating the tire with the valve sealed with silicon rubber, caulk, or cured epoxy resin. Obviously the tube must be discarded after such a process.
If the Presta valves cannot be disassembled, then tubes with leaking valves must be replaced. Some Presta valves, for example, those made by Schwalbe and those of Bontrager have removable cores, but most do not. Such valves can be recognised since they have flat sections on them to allow slackening and tightening of the cores with a valve removal tool.
Internal tube sealants can block both valve stems and cores. However, stems are easily cleared with a thin object after the cores are removed, and sealant can be removed from the cores by washing them in water.
Self Sealing Tires 
Sealants will seal most small punctures in tires up to about one eighth of an inch in size. The product (Slime), is already in some tubes bought from bike shops, but can be poured into existing tubes, provided that they have removable valve cores. There are a few points peculiar to the use of tires with internal sealants.
- New tires with sealant: When a tube with sealant is first installed, rotate the wheel quickly several times, occasionally operating the brake, to fully spread the product.
- Parking position for the wheels: Because valves might become clogged with sealant when the bike is stationary for any length of time, the wheels should be routinely set to a position that allows the solution to drain from the valve. The position that does this best is either the five o'clock or seven o'clock positions. See the adjacent diagram.
- Inflating sealed tires: When the tires are being inflated there is a significant airflow. To avoid any unnecessary clotting of the sealant, position the valves as for parking, in the five o'clock or seven o'clock positions.
- Clogged valves: Despite best efforts, and sometimes at the point of purchase, the valve core or the valve stem will be blocked. For this reason it is always best to use tubes with removable cores; these can be rinsed in water to clean them, or the stems can be unblocked with a thin object. A valve removal tool is useful.
- Removing puncturing objects: Before removing a penetrating object from a sealed tire, first rotate the wheel to the six-o'clock position to give maximum exposure to the sealant pool. This will minimise the air loss before the puncture is sealed.
- Slow puncture repair: Sidewall punctures, snakebites and pinches are sometimes slow to heal. This is because the airflow is low and the conventional rotation speed might be excessive. To make sure that slow punctures are fixed, first allow the sealant to pool at the bottom of the tire, then tilt the wheel and slowly rotate it so that the sidewall has full access to the sealant. Repeat for the other sidewall.
- Patching a sealed tube: Sealant leaked into a tire can be removed with a wet cloth. Punctures too big for the sealant to handle can be patched in the usual way, but the patch will fail unless all traces of sealant are first removed from the surface. During the sealing of punctures, a small amount of sealant can cause sticking of the tube to the inside of the tire. When such tubes are being removed, try to anticipate this so that the tube is not weakened by the inevitable stretching. Rather than tugging the tube, consider using water to soften the gluing effect.