The rim of my front wheel split today (I didn't hit a pothole or any other wheel-damaging obstacle: just normal street riding). Fortunately it held the tire's bead until I got home, 15 miles later. My last front rim split and stopped holding the bead in 1 fell swoop 6 years (37K miles) ago. I noticed a bulge (from the sound made while braking) 10 months ago. I squeezed it flat with vise-grips. When I started today's ride it braked normally. It's a Sun CR-18 rim.

Is there a rim-wall-thickness I should use to determine when to replace a rim? Or some other measure?

I ride to minimize braking: slowing down when approaching stops, looking ahead for problems. Is 37K too-few miles?

Did you measure it ten months ago?

the internet says this rim has a wear indicator, but I'm not sure what it is. Maybe a hole in the sidewall?

37k miles is more than I would expect to get out of a rim with rim brakes. I always manage to do something else to ruin them before they wear out, but a couple have been close after more like 10-15k miles.

I did measure it 10 months ago. It's about .01 mm thinner; I wouldn't trust a measurement this precise because I don't measure in the exact same spot. I hypothesize that it hasn't so much thinned over the last 10 months as that it has weakened from bending. 2 years ago I had a bulge in a different spot on the rim, which I squeezed back to flat, which hasn't bulged out again.

This rim has a wear indicator? A few months ago Sun's website's entry about it doesn't mention that.

Now that the OP has suffered this twice perhaps some measuring is due this time around. I'd first place a straight edge on the old rim's brake track running across it (or roughly parallel to the spokes). You will likely see that the rim has a concave surface, the straight edge will contact the top and the bottom of the track but the center will be worn in some. How much wear there is is the key. One could actually measure this with a depth gage or feeler gage or thin wire but more to just visualize how much wear there is.

As the rim accumulates miles periodic checking of this wear is easily done. When the wear gets close to the same then...

Another aspect is that as the rim wears the outer edge of the brake track will bulge out with tire pressure. With a higher pressure of a narrow roar tire this will be more apparent then with a low pressure fatter tire. So one could measure the rim's outside width with no pressure when new then repeat over the miles and when the rim begins to get wider with pressure...

Different rim brands and models will have different wall thicknesses so this measuring is only a guide. Andy.

That's considerably fewer miles than I've been getting - but brake track wear is so dependent on riding conditions (how much braking, number of rides in wet conditions, amount of dirt and dust, etc.) that there will be large variations in rim mileage. Was the previous bulge a result of hitting a pothole or other obstacle or just spontaneous? I've been replacing rims whenever there's an unexplained bulge or when I notice a clearly visible concave shape of the brake track.

Jan Heine reckons that 0.7mm is about as much as you need to keep the rim from self-destructing: https://janheine.wordpress.com/2012/05/09/rim-weight/

Consider investing in one of these dental gauges: https://www.amazon.com/Stainless-Iwan.../dp/B0087HKWCO They're cheap and work well for sidewalls.

You can't predict brake track wear by miles.

My tubular tire road bike is ridden on open roads and can see miles upon miles without me touching my brakes. If I hink back, I can remember 100 miles where I used the brakes fewer then 5 times total (if that). Those rims stat out only 1mm thick, and don't have much to give up, yet brake track wear is a total non issue.

OTOH- my commuter sees tons of brake use, including more hard stops in an hour than the road bike might see in a week of touring, so I see plenty of brake track wear. The last rims go to where failure would have been soon, but a T-bone crash put both out of their misery just in the nick of time (I was OK).

As a rough rule, I'd allow rims to wear about 1mm or roughly 1/2 the original thickness before worrying. But a realistic guideline, even if you measure precisely, is impossible, because a major factor is the "hoop stress" caused by tire pressure, That's proportional to the Pressure X the tire width, and so can vary tremendously rider to rider.

When I expected that my commuter's rims were near the end, I used a "safety method", pumping tires to 15-10% above riding pressure, then bled them off before riding. My theory was that if they held at 15% higher, that would be a safety margin when riding, and if all worked as intended, the failure (if it happened) would be when pumping rather than riding. Had it not been for an inattentive driver, I might have been able to report on whether my idea worked.

Ooh, I love destructive testing and seeing how much life I can get out of parts. Have you considered grabbing a cheap used wheel from a co-op that is already worn concave but still has a little life left in it for this experiment?

No, I've never been curious enough to test this for the sake of testing. Plus because of the large hoop stress variance, any information gleaned would be pretty useless.

As for riding older stuff, I'm pretty selective and will take my own stuff much close the wall, than I would something whose history i didn't know. Riding the same stuff gets you pretty attuned to it, and sensitive to any small changes that might be meaningful. That doesn't happen when you're riding unfamiliar stuff.

I can't count how many possible or imminent blowouts I've avoided over the years simply because the tire didn't sound or feel right, and on checking found a torn sidewall ready to fail completely.

The rim is obviously concave, to the eye and touch. Concavity is probably the best measure of about-to-failness.

As to replacing it at the first sign of a bulge: that's sensible. You won't catch me being sensible. I got 2 more years out of this rim past the first bulge.

I'm trying an Alex Adventurer rim this time; it has 3.5mm thick walls. The Sun CR18 has 2.5 mm thick walls. Will they both fail at the same thickness? If they both last to 1mm the Alex should last 40% longer. It has a wear indicator. Alex's docs read:

.3 mm ?! They expect me to discard a rim with 3.5mm walls when they're 3.2mm thick? Do I misunderstand or do they make money by selling rims?

Rim wear indicators and early wear/replacement recommendations have nothing to do with selling more rims. It's about lawsuit protection.

The vast bulk of any rim maker's business is from OEM sales, with the total replacement market being tiny. If that tiny segment, the vast majority is because of theft, crash damage, and other wheel failures. If rim makers depended on the few people who rode enough and were lucky enough to have a wheel last to where brake wear is an issue, there wouldn't be a rim business in the first place.

Also keep in mind that brake track wear on road bikes isn't that common at all. The entire issue didn't come to the fore until mtn biking sped up the process to where significant numbers of riders could wear out a rim.

As to whether the thicker rim is all extra wear material, or some more is needed than the thinner one, that depends on the alloy used, and the shape of the extrusion. It also depends on the shape and location of the wear zone, plus whether it's smooth or striated. A thin smooth wear area can survive better than a thicker one with striations or a single gouge, even if it's minimum thickness is greater.

Here is the biggest reason for disc brakes. They dont wear out expensive rims.

Get your hands on a new CR-18 rim and compare the thickness to your broken one. That will give you a pretty good idea of how far they can go before failure, though not every rim is going to fail at the same thickness.

The thicker Alex rim should last longer but it's probably heavier too. Your choice. Personally I think 37K out of one rim is pretty good. If your spokes are in good shape I'd just get another CR-18 and swap it over using the method of taping the rims together.

Lol. If rim wear is the "biggest reason" for discs, how many rims did you wear out before you got tired of it and switched to discs?

As FBinNY notes, rim wear is seldom an issue with road bikes. I'm still riding wheels I built more than 30 years ago, and they're holding up fine. Off-road is another story, and that's why disc brakes are predominately found on off-road bikes.

So your rim began to fail ten months ago and you measured it. That's not a good enough answer? Then there is no answer that will satisfy you. Troll indeed!

See what I mean about people hating first hand experience -- it gets too close to the truth.

I don't see the appeal in trying to push a rim to its absolute limit in the name of frugality. Take the case above, it is a $30ish rim that lasted well over 30k miles, the cost per mile is incredible there already. Should that rim fail at speed it could cause a nasty crash that could easily cost you thousands in medical bills or at the very least a few days down because of being hurt. All to save $30 ($70 if you have to pay someone to build it). I get trying to squeeze the most out of expensive components like cassettes, chain rings, hubs, etc but tires and rims are too critical for me to get it. You did claim we would not see you being sensible though.

I thought Mr FBinNY exaggerated but this new Alex Adventurer rim's label reads: I had never seen anything on a rim other than name and size before.

Dental calipers have been mentioned before. Then, you Measure Thickness..

I agree. Mine lasted about 8,000 miles, including a couple of Pacific Northwet winters. Then things go boom:
rites042
Like the OP, this was a Sun CR-18 rim.

Discs are a great engineering advance. Were you against click shift when it came out in the middle 80s too. A lot of "real" cyclist were.

Random Troll, My old post on the same subject: https://www.bikeforums.net/bicycle-me...zzy-logic.html

Honestly, if I had a rim last as long as yours did before failure I'd be happy.

Brad

Nice job at avoiding the question and trying to drag it even more off track. To answer yours, I appreciate both friction and indexed shifting.

I'll try one more time. If rim wear is the "biggest reason" for discs, how many rims did you wear out before you got tired of it and switched to discs?

If and that is if you never ride when wet when rims are ground down really fast, yes rims will last a long time with rim brakes. However if used most every day wet or dry, they can wear out really fast. A broken rim put you on foot for maybe a long walk home.

My reason for supporting disc brakes are the fact that without rim brakes, rims can be made stronger and with a more aero profile. And the elephant in the room, disc brakes dont wear out expensive rims. May I remind you that probably many on this forum ride on a rim set that cost more than a large percentage of lower cost bikes.

Lastly with some buying carbon rims, rim brakes dont work very well. And any those against rim brakes appear to be on the losing side. More and more mfg are bringing out road bikes with disc brakes. Probably in 5 years they will be pretty much standard.

I'm as happy as I can be with a split rim; not only did I get to ride home but I got another 30 miles around town before I built the replacement. I didn't complain, I asked. I noticed the first bulge in the rim 2 years, more than 10K miles, ago. A different place eventually split. I'd rather get 2 more years out of a rim than replace at the first sign of a bulge. I won't recommend that to others.