(Original post: 16.01.2018)

I’ve been wondering for some time about the state of my rims. They are as old as my bike, that is, the wheels were built in 1990. They are the old Mavic Modul 3 CD which have a ceramic coating. The coating is well gone on the sides and there are no wear indicators. I’ve been thinking about getting new wheels, but getting a modern pair of wheels opens up a certain amount of followup which I don’t want to pay for just now: possibly a hub dynamo (and therefore a dynamo headlight, if not a rear light as well); a cassette hub for the back, which would mean spreading the forks from 126 to 130 mm; and if the frame’s going to the framebuilder, there are a couple of extra braze-ons and a paint job I’d want.

Or should I just get new rims built on my existing hubs? I ran into H., my mechanic of choice and Super Randonneur, at a Christmas party last month and he hinted to me that if I wanted wheels built I should order them before the spring rush. But was it even necessary?

To check the thickness of rim sidewalls, you need some kind of measuring gadget that can get past the thicker lip and measure the thinnest spot. A bit of googling turned up the name of the instrument that can do this: it’s an Iwanson caliper, which seems to be borrowed from dentistry. I found out about it on this page of the excellent Ride or Pie blog.

The rather elegant little gadget arrived today (it cost all of € 12 including postage) and my front rim turns out to have about 2.4 mm of metal left in the sidewalls – no need to worry about them for a while yet! I’ll leave the back wheel till I get a pair of metal tyre levers. My new tyres are ferociously tight. Later: back rim is also fine though a bit more worn, around 2 mm.

Update Autumn of 2018: I was moving stuff out of my cellar and had a look at some old rims I had down there – not very worn ones – and they were about 1,4 mm. And looking around the Internet, it seems that 1.5 mm is a not uncommon thickness even on brand new rims. So these ones really ought to be good for a while.

Update #2: Jan Heine mentions 1.6 mm as a quite usual sidewall thickness for a new rim and says 0.7 mm is the lower limit: here.