etherhuffer 

I see lots of posts of folks getting vintage or used bikes and wondering about rim wear. I have a set of calipers that are cheap and do a good job. I have yet to find a really worn rim in an old bike. Most bikes seem to die of weather, poor maintenance, or general disuse. That said, a pair of these are nice if you are not sure of a vintage rim.:

https://www.amazon.com/Stainless-Iwa.../dp/B0087HKWCO

Andrew R Stewart 

Machinist's calipers are not needed for detecting brake track wear. A simple straight edge that can be placed across the brake track to show concave wear works well enough. Does the concavity increase with inflating the tires? If so then the rim wall thickness is too little to support the air pressure and the rim brake track's upper/outer "half" flexes outwards. Time to replace the rims. Andy

rccardr 

Bonus points for using the word “Concavity”.

79pmooney

I like. Never thought about observing sidewall distortion after pumping.

And to those who and seen blown rims at the local bike shops (sometimes with the bike and resulting damage. I look at my winter bikes' rims and make a call in the fall as to whether I feel sure it is good till spring. If not, that rim gets replaced, A little early is far better than having a shard of aluminum ripping open my calf.

Here in the Pac NM we have lava dust that simply eats rims. (Most of Oregon is sheathed in lava. I'd love to see a study of what lava types have what abrasive properties and where those lavas are, but the pressing need for that info isn't reaching the right people. ) Fortunately for me, I get around this dilemma the easy way. I love building wheels! Those volcanoes put me in a good place.

Now there's an easy way to make wheels safer after extreme brake wear - replace the rims with tubular rims aka sewup rims. Now, it you wear clear through the sidewall, the rim gets quite weak and will dent in easily on small pothole edges but the sidewalls still will never blow out and your tire stays inflated and ridable. (I rode a pair of wheels 17,000 miles out of Seattle. Collapsed the rear rim on a pothole. Looked and saw so much brake wear you could have read a newspaper line through the brake track. But the tire didn't care so I just bumped the 10 miles home.)

etherhuffer 

Nice! I like that idea. That said, the calipers are 10 bucks or less which is a no brainer for my tool supply. And are sure a lot smaller than my other calipers and micrometers. Out of interest I perused a bunch of rims at home and at the recycled bike shop. Nothing was less than 1.8-1.9 mm wall thickness.

sweeks

Another thing to look for is a "thumping" sensation when the brake is applied. This means a portion of the rim is thin enough to expand under tire pressure; failure is imminent. Reduce the pressure, get home and replace the rim.


This rim is getting ready to fail... it won't be pretty! Brake is thumping.

Andrew R Stewart 

"Nothing was less than 1.8-1.9 mm wall thickness." etherhuffer

Could you describe exactly what/where you are measuring? As in do the rims have a hook/lip to the rim's inner sidewall top? Is the measurement across this lip? Is it under this lip but above the bead seat?
When we start stating actual numbers it's nice to know where they are coming from.
The next step if using measurements is to spec a new rim and periodically remeasure to gain some understanding of the amount of the change (from wear) as well as one's rate of change. Of course riding conditions will influence these measurements greatly.

Back in the 1990s when I had my shop in Cleveland and bike messengers were considered gods I serviced quite a few routinely. We had a number of rim sidewalls wear out and split off the rest of the rim. The Mavis MA2 rim became well known for doing this sooner then other brands (no surprise as Mavic tended to use fairly thin extrusions to gain their low weight). We likely saw a couple a winter season do this. These days I see a lot less but still feel for any sidewall concavity on transportation bikes' rims. Andy

etherhuffer 

Good point to clarify. I basically look at the rim and find the area of flat below hook or sidewall top. The bead seat area is more problematic as they are mostly curved. Every extrusion is different. I look for an inner to outer segment of rim that is flat. The taller the rim, the more reliable a spot you can see. On less tall rims it is not always clear. The nice part of the tiny dental calipers is you can just go from inner to outer and take multiple readings. Its obvious that a thick reading is good, but a thin reading is bad. So with 3-4 readings, you basically find the low and high spots. And of course you should have an idea of the manufacturers original thickness. Funny you should mention the MA2. I got a few sets to build up for a vintage ride. They were thinner than my touring rims, which makes sense. I can't remember the original spec on those but found it online somewhere.

byscott

My wife’s Trek hybrid came with rims that have a small circular divot on the side of the rim near the valve stem. The idea, as I understand it, is that the rim is ready for replacement when the divot is no longer visible. I took the wheels out of service when the rims had worn down to that point … I didn’t do any measurements, but they felt pretty thin and did have some concave wear. It wasn’t a great set of wheels anyway, so she was pretty happy with the higher-quality replacements.

JohnDThompson 

A dental caliper is an inexpensive tool to measure sidewall thickness:

sweeks

Also known as a "crown thickness gauge", as that's what they're used for by dentists. Ask me how I know!
FWIW, these are accurate to 0.1mm if they are reasonably well made.