How to Patch a Bicycle Tube
Every cyclist with a well-stocked saddle bag is, of course, carrying a patch kit. But, since you also have an extra tube, what’s the point of the patch kit? Well, if you can get one flat, you can certainly get two. And even if you simply carried a theoretically infinite number of tubes with you on every ride, at some point the pile of tubes in your garage with small holes in them would start to pile up. Of course the reality is that you can’t carry an infinite number of tubes. So learning to properly patch a tube is an essential item in the cyclists’ toolbox of skills.
Diagnosing the Leak
The first step is to assess whether you should, or even can, patch the tube. If the tire went flat suddenly, it’s likely that the tube is too damaged to patch. Conversely, if the tire has been slowly leaking air for some time, you probably don’t need to patch it on the road. So long as it is holding air long enough to get home, just pump it up and keep going. In the middle ground between major damage and ultra-slow leaks, you’ll find plenty of tubes to patch.
Preparing the Tube
So, you’ve already removed the tube from the tire, following the steps in the How to Replace a Bicycle Tube guide. Pump enough air into the tube to inflate it to about twice its normal size. Quickly look for the spot on the tube where the air is leaking out. This can be harder or easier (see the suggestions below for extreme cases), but there are a few easy tips:
Finding a Puncture in a Tube
1. Look – carefully examine the tube.
2. Listen – rotate the tube through your hands close to your ear, the characteristic sound of air escaping should be obvious.
3. Feel – if all else fails, carefully move the tube close to your lips and try to feel any air leaking out.
If none of these work, you've got a leak that is slow enough to make it home!
Ideally you would be able to mark the spot with a little chalk or something, but most patch kits don’t contain anything to mark the tube. So just put your finger over the spot that is leaking and move onto the next step.
At this point, if the tube won’t even hold air long enough for this inspection, it’s likely that the hole is too large to patch. While the patch includes large size patches, the reality is that they don’t work very well and patching larger holes isn’t worth the effort since the failure rate is high. If the hole is more than a few millimeters across or it is a slice more than a few millimeters long, the tube is garbage.
In contrast to the large hole, slow leaks are not really worth patching on the road. It’s almost impossible to find the hole since the air is coming out so slowly – you can’t hear or even really feel it. And if you can’t easily find the hole, then the tube is going to hold air plenty long enough to get you home.
Finding a Slow Leak
The only way to find this kind of leak is by submerging the tube into some water and looking for bubbles. Obviously impractical on the road, wait until you get home to find the leak and proceed from there. Though harder to find, it is worth patching a slow leak at home since it is the easiest, with so little damage to the tube to repair.
With the hole marked on the tube, take out the little bit of sandpaper included in your patch kit and rough the area around the hole. Make sure to rough up an area larger than the size of the patch. Now open the tube of glue, tap a little bit right on the hole and spread a thin layer in the area all around the hole. Try to leave the glue mark intact as it will be the only way to tell where the hole is once the glue is on the tube. Then peel the foil backing off of the patch (leave clear plastic cover in place) and spread a thin layer of glue on the patch. Make sure the glue goes all the way up to and over the edge of the patch. Set the patch and tube aside for a few minutes to let the solvent evaporate. You need to let the glue get mostly dry to the touch so that when you put the patch on the tube the glue can immediately cure. If there is too much solvent left in the glue when you put the patch on, the patch might not adhere properly.
After waiting, the tube and patch are ready for final adhesion. Locate the glue spot you left on the tube marking the hole. Center the patch on the hole and press down firmly onto the tube, making sure that the patch goes on evenly and that you are pressing all around the edge. Now take the rounded lip of a plastic tire lever and carefully press all over the patch to ensure that it is fully adhered, with special focus on the edge.
All Patched Up and Ready to Roll
Check over the patch, making sure no section is loose or gapped. The plastic cover will still be on the patch – leave it! It will protect the patch somewhat and removing the plastic risks pulling the patch off with it. Ideally let the patch cure for a while and then slap it into your tire and it will be ready to go.