2_i 

I do not think this to be right. It is not the concavity that decides about the rim cracking but the minimal thickness of the rim. In my memory you start with the thickness of 2mm or so. You can easily shave off 0.5mm here or there and still have plenty of thickness left.

sweeks

At the first sign of thumping, the rim would be on borrowed time. A rim whose wear indicator had worn off (or shown up, as the case may be) might be good for a while yet... your test would probably be a good idea.

You know Murphy was an optimist, don't you?
Steve

southpier

are you using the new "blue slate" brake pads? all the cool kids have them!

sweeks

The concavity is from wear. The rim has to hold up under tire pressure from the inside and brake pressure from the outside. As the rim gets thinner, it will flex more (though still very slightly) when the brakes are applied, with the tire pressure acting as a "return spring". Aluminum doesn't flex very well, which is why the cracking eventually happens ("cyclic fatigue").

If you Google "bicycle rim wear", you'll find a number of references to "concavity" as an indicator of wear. Here's one: SAFETY - Check Your Rims for Wear - Bike Test Reviews

Another BikeForums thread (https://www.bikeforums.net/bicycle-me...-worn-out.html) where the consensus seems to support considering rim replacement when the thickness is about one millimeter.

I looked at several rims I have (either new or lightly worn Kinetix 20" rims for my folder) and measured their sidewalls with a crown thickness gauge. The thickness of the brake tracks is 1.5mm. At the wear indicator the thickness is 1.0mm. Now, it occurs to me that the wear indicators are probably designed to be *conservative*... so it's probably safe to say a rim at 1.0mm is probably good for a while.

Here's my suggested protocol: when the rim wear indicator (or a measurement) shows the rim is at 1.0mm or less, it's time to source another rim to keep handy. Then if the rim begins to thump during braking or a crack becomes visible, it's time to replace. Pumping the tire to pressure higher than rated can be done (wear safety glasses!!) to be extra cautious.
Steve

sweeks

I saw an indirect explanation on another site. Heavier radial loading, as expected from a heavier rider, means the brakes have to be applied harder, or for a longer time, to stop the bike. This translates to more wear, though the more heavily-used front brake would be expected to be proportionally affected.

It's probably a combination of this, the smaller wheel circumference and the abrasive kicked up by the front wheel. It's odd that no one else on the Tern, Dahon (AFAIK) or this forum has the same problem.
Steve