Dirty Talk: Forensic Flatology (part two)

Posted at 04:30PM on Mar 27, 2008 by web

Last week, I warned you against simply changing your inner tube if you get a flat tire. Today, we're going to run down the list of different kinds of flats, their causes, and what to do to prevent them from recurring.

The outer puncture flat

The most common reason for flat tires on a well-maintained bike. Slivers of glass, thorns, staples, and all manner of pointy crap can do you in without much trouble. This sort of flat will usually deflate fairly slowly. Giveaway: a single, irregular hole on the outer surface of your inner tube. What you must do: run your fingers lightly around inside your tire (once you've got it off of your wheel) looking for something sharp imbedded in the rubber. I know this sounds like a really bad idea, but it's necessary and as long as you're not rubbing your tire as if you expected a genie to appear you won't draw blood when you find the interloper. To narrow the search area you can hold your tube up alongside your wheel and match the location of your puncture to the tire. Fail to do this and that staple (or whatever it was) will very happily cut up your tube every time it happens to be against the ground when you hit a bump. Note that big puddles of car window glass aren't a threat — broken beer bottles definitely are.

The inner puncture flat

A common source of mystery flats. These are caused by something sharp on your rim (the metal hoop part of your wheel) such as a shard of metal from the bike shop or (more typically) a spoke that is too long. Giveaway: a single hole on the inside surface of your inner tube. What you must do: Carefully inspect the bed of your rim where the inner tube lies for sharp bits and protruding spokes. Install a tire liner if you don't already have one. Duct tape will work in a pinch. If the problem is a too-long spoke, you can either file that sucker down or replace it with a shorter one. This may be a job for the bike shop if you're not up for tackling spokes just yet.

The pinch flat

More common on bikes with skinny tires than bikes with big fatties, a pinch flat occurs when your wheel hits something (usually a curb or a pothole) with enough force to crush the rubber in your innertube between your tire and your rim. Hit with enough force and you may permanently dent your rim at the same time — bonus! Giveaway: two regular holes, rim's-width apart on the inner surface of your inner tube. What you must do: If you love your skinny tires (as I do), make sure they're pumped up to around 100 to 110 psi. This usually requires topping them up with a good bicycle pump (not a gas station hose – those don't go higher than 90 psi) once or twice a week, depending on the weather and the quality of your innertubes. If you end up with this kind of flat on your mountain bike — either your tires were nearly flat to start with or you really need to soften up on those curbs!

The inflation flat

Nearly always a result of improperly installing an innertube. Giveaway: a tear in the innertube, with the flat coming during inflation of the tire or shortly thereafter. What you must do: Make sure that no innertube is squeezing out from under your tire as you're pumping it up! Proper tube-changing techique will usually prevent this (we'll cover that in a later post).

The valve-stem flat

More common with Presta valve tubes (the long, skinny ones) than Schraeder valve tubes (the ones that look like car tire valves), this one is a heart breaker. You're almost done inflating a new tube, or you're quickly pumping up a tire before heading off, and then pssssssst, you're back to square one. The juncture of the valve stem with the main body of your innertube is delicate — getting too rough with the stem can cause an impossible-to-patch rupture. A small rip here can also be the cause of a tire that seems to deflate itself slowly over several days. Giveaway: a linear rip around the base of the valve stem. What you must do: Be gentle with your innertubes! Having to re-change a tire because you were moving too fast is humbling (good) and a waste of rubber (bad).

That does it for our tour of flat tires, and this introduction to forensic flatology. If you're thinking that all of this mucking about with tires is a little beyond what you're comfortable with, don't despair! We'll cover how to actually change a tire — and when to turn your wheel over to a mechanic instead — another time.