Most modern rims will have indicators for telling you when the sidewalls are getting to thin. There are basically two types: Either it is a thin groove machined into the sidewall, visible from the beginning. With that type it's time to change your rim when the groove is no longer visible. The second type is a little hole not visible on a new rim, and only once enough material has been taken off the rim it will appear. Since the first type can cause some problems with brake pad wear, the second type is more common. The issue with that type, however, is that you basically have no way of telling how close you are to the point of having to change the rim. Additionally, sidewalls often wear unevenly, caused for example by slightly untrue wheels. That means that even if the indicator hasn't appeared yet it's possible that some parts of the sidewall are already dangerously thin.
|My somewhat sad attempt of a DIY measuring tool|
Consequently, it would useful to have a different way of measuring the thickness of your sidewalls. Unfortunately, you can't just use your calipers because of the lip that holds in place clincher-type tires. Some people use calipers on the outside of the rims, taking one measurement with a deflated tire and then one with a fully inflated or overinflated tire. As a general indicator this is certainly useful, but not very precise. A more precise measurement can be had by using a nifty DIY tool made from an old spoke that will allow you to measure the wear of your rim using calipers. You can find extensive instructions here, but the basic idea is that you have two pieces of a known diameter -- a regular straight gauge spoke measuring 2.0mm --, put them on the rim, measure with the calipers and the subtract the diameter of the spokes times two. I have tried building that tool according to the instructions but even my second attempt doesn't look nearly as nice as the one shown in the article, and that made the measurements tricky and not very precise.
|Iwanson gauge on a rim with about 1.25mm sidewall left|
If you're similarly mechanically challenged there is hope, though. Fortunately, dentists have similar measurement needs, and they have come up with a dedicated instrument, the Iwanson gauge or Iwanson calipers. Cheap ones can be had for less than 20 dollars shipped on Amazon or on Ebay. With the Iwanson gauge you can measure quickly and directly. Jobst Brandt thinks that 0.5mm is the absolute minimum, and other sources suggest that anything under 1mm is problematic. This, of course, refers to the thinnest spot on the rim. So make sure to take several measurements all around the rim, no matter which measuring method you use. In addition, it might be a good idea to measure your sidewalls when they're still new, in order to have a point of comparison for later measurements.