Bicycle Mechanics - theory on rim failure in cold weather

I usually post in the commute forum but thought this observation was appropriate for the gearheads in the mechanics forum. Both my front and rear rim failed in the same way in the past month, sidewall of rim cracked on one side, both on super cold days, say below 10F. My theory is that the metal in the rim was subject to extreme temperature change from heat due to friction of brakes to cold, sitting at stop light in freezing temps. These rims are probably 2-3 yrs old and suffer daily use from a 16 mi commute so they have been pounded pretty bad. It just seems like too much of a coincidence. Comments? Rear rim was a Sun CR-18 36-spoke and front some generic 32 spoke rim. The CR-18 just failed yesterday.

Both my front and rear rim failed in the same way in the past month, sidewall of rim cracked on one side, both on super cold days, say below 10F. My theory is that the metal in the rim was subject to extreme temperature change from heat due to friction of brakes to cold, sitting at stop light in freezing temps.

More likely the rims were subject to extreme crud and metal shavings imbedded in the brake pads.

More likely the rims were subject to extreme crud and metal shavings imbedded in the brake pads.

That'd be my guess, too. They use a lot of sand & salt on the roads you commute on? Road salt & aluminum aren't friendly with each other.

Were the sidewalls of the rims thinned down due to break wear?

If you use your front and rear break simultaneously all the time, then seems like they would each have similar wear and might fail around the same time. I don't think you go through such an extreme temperature fluctuation that would cause cracking. It seems to me this would be an epidemic across the northern half of the country and into Canada.

Yeah, the brake pads were pretty bad. I haven't check the rim walls.