Sunday, May 23, 2021

Occupational hand injury -- how to change an inner tube

 Every bicycle inner tube has it in the instructions printed on the box. Every experienced mechanic will tell you. Don't use tire levers to install a tire. If you're prying off a tire with a tube that's already punctured, levers are okay, particularly because the tire may be cruddy and the beads may be stuck to the rim. Installing, however, you risk putting a hole in the new tube if you use a tool to pry a reluctant bead over that last few centimeters of rim. Use your hands.

Your thumbs are important, but they're not the prime movers. Let me show you with this handy graphic what parts of your hand impart the most useful force.

This was from yesterday's wrestling match with a classic Wolber rim from the early 1980s. Some manufacturers seem to interpret the 622-millimeter bead seat diameter as 623.79, or maybe even 625. It isn't really, but you'd swear you were trying to put on a tire that was too small. Since these are almost always road rims for skinny tires, you don't have enough room to insert a tire lever anyway.

The bruised area indicates where the most power is applied to roll the tire over the edge. The blister and the split above it in the middle finger indicate how the pressure is applied. My thumbs are fine. Thumbs only provide stabilization and guidance, and a small amount of added force.

Interestingly, my left hand only showed minimal bruising and no blistering.

The key element is not strength alone, but directed strength. I do not have a particularly strong grip. And the technique does not usually lead to injuries like this. It does come in handy as a teaching aid here, though.

Be methodical. With the tube very slightly inflated, mount the first bead to the rim. Often, the first bead can be more difficult than the second. I've never figured out why, I've just observed it to be so. You can use a lever to pop the first bead on. For safety, you might want to pull the tube out of that section of tire before prying. Some mechanics mount the whole first bead and then insert the tube. Find what works for you.

Once the first bead is in place, push the valve stem into the the tire casing to make sure that you do not trap a section of tube under the beads at the valve. If it's a presta valve with a threaded stem, make sure that you have removed and discarded the stem-ripper nut thoughtfully provided by the manufacturer for no good reason. The same goes for threaded Schrader stems, but those are much less common. The presta stem rippers have the same thread as bolts in the brake system, so they can come in handy here and there as spacers and shims. Or you can make some nifty jewelry. Maybe hang them from various piercings. Braid them into your hair. They don't rust.

With the valve out of harm's way, push the first section of the second bead into the rim, and begin working the bead on in both directions, working toward the opposite side of the wheel. This is where you use your thumbs the most, and where the confusion sets in for the sore thumb crowd. You reach a point where it's too hard to continue by thumbs alone.

With a lot of bead on the rim, move your hands back toward the valve and place the other side of the wheel on the floor. Put the heels of your hands on the outside of the tire casing and push down hard as you work your hands down around the rim on both sides. This pushes the casing in and around, working slack toward where you are going to need it. Because wheels are round -- ideally -- you will reach a point where you can't apply downward pressure anymore. Here's the tricky bit, where you maintain as much pressure as you can while pulling the wheel up and wedging the top (valve) side into your lower abdomen, while moving your hands toward the goal line, pushing the casing and working the bead around. This ends with your hands in the grip position to roll the bead over the last bit to snap into place. You hope, anyway. You might have to work the slack around a few times before you finally get it.

Many tire and rim combinations that feel intimidating at first turn out to be quite manageable once you work the slack around. You can also let the tube get caught under that last bit of bead and use the slitheriness of the tube itself to help coax the tire over the edge. Then put the tiniest bit of air -- or just a tiny bit more -- into the tube, and pinch the casing repeatedly. The tube will want to expand into the casing. You can peek down alongside the bead to watch it retract into the tire where you want it.

Pinch and peek also describes how you want to inspect the entire circumference on both sides to make sure that the tube hasn't slipped into mischief anywhere else around the rim. Then apply pressure gradually, making sure that the bead seats securely and evenly before wailing it up to your desired max.

Rims that fit loosely to the tire are more hazardous to your hearing. If the tire flops on very easily, you need to be much more vigilant to make sure that the bead catches. Otherwise, the tire crawls off the rim and you get a blowout. Explosive failures like this not only destroy the tube, they can cause damage to the tire bead itself, so that it never will stay on. Follow your procedures. Check and double check. Inflate in short intervals.

Fatter tires may call for more thumb because they're too large and flexible to bring the power palm to bear. Fatter tires usually -- but not always -- fit more loosely, so you can work the slack around as necessary to get them over the top.

You can also judiciously sneak a thin tire lever in to help work the bead of a reluctant tire toward the finish line. The key here is never to use the levers for the final push, only to set up the final snap that you will do by hand. Be aware of where the tube is. You can push the tube further into the casing with the tire lever and then back the lever out to do the actual levering.

Tubeless tires eliminate the problems of pinched tubes, but can sometimes blow off during inflation, with hilarious results. In the case of a tubeless blowout, the entire area and any bystanders get slathered with sealant. You do need to shove a lot of air into the casing in a hurry to get the tire to snap to the bead seats, but then proceed with caution to make sure that everything stays where you want it. Some technicians recommend mounting the tire dry and then taking the valve core out to inject sealant, but it's so much easier to pour sealant into the casing with one section of bead partially dismounted. There's plenty of time later to gook up your valve stems with sealant. I've done it both ways, depending on other factors. You can still blow the tire off the rim if you get too frisky with the compressor. That I haven't done. I'm pretty cautious about blowing things up.

On the plus side, I shredded my hands last thing before going on my days off, so I have time to heal up on my own time before returning to the world of virtuous toil next week.


greatpumpkin said...

This is a helpful post. It would have been more helpful to read before I changed the tires on my trike a couple of weeks ago. However, reviewing what I did and these instructions, it looks like I got it mostly right. And no injuries (small wheels are so much easier).

greatpumpkin said...

I like your cartoon illustrating how to change a tire the clean hands way -- would make a nice illustration here.