When is a Rim Worn Out?
 

How do you determine when a rim is worn to the point that it needs to be replaced?

Feeling the braking surface, it definitely seems to be concave, but thats not a precise measurement...
So whats the right way to go about it?

I'd rather not find out by having it crack while braking hard downhill...

Thanks

If your rim is noticeably concave on the braking surface, it is time for a replacement.

+1

And if the rim has a wear indicator, use that.

Replacing wheels is generally cheaper than the co-pay for a visit to the ER, plus follow-up ortho visits.

Only if you don't have a public health care system

Well, it's less painful, anyway. :)

Yeah, with a public health care system, you don't have to worry about the co-pay because you'll die from the road rash.

The rims can bet quite concave before they actually split. I found on my commuter bikes that the brakes became noticably more grabby over a few months until they were just almost impossible to only apply lightly or with any sort of lineararity at all. They just wanted to grab and jam on fully all the time. It was so annoying that I bit the bullet and replaced them. As they were well carved out and obviously not useable anymore I cut them up. There was still enough wall to hold things together but there's no doubt that they were on their last legs.

Mind you I'm more easily annoyed when something doesn't feel right. I knew of another regular at the LBS that I like to hang out at who had no compunctions at all about running his wheels until they split. I don't think he did it on purpose, rather I just don't think he thought about them or inspected the wear. He's one of those types that rides a lot but never looks at a thing on the bike until it breaks. I wouldn't work on his bike on a bet due to it being so grungy and gritty all the time.

Yep- the nonlinearity of the brakes comes from bulges where the rim is about to fail. On mine, this didn't show up until about 10 miles before failure. Fortunately, it let go at managable speed on a straight, smooth, bike path. It did leave me dripping wet in a bagel shop for 3 hours while my wife rode home to get the van to get me home:
http://home.comcast.net/~jeff_wills/...s/rites042.htm

I caught one of my rims (13 year old Mavic 217) before failure. I had a 4 inch spreading crack in the machined braking surface. The brakes still felt fine, but I started to hear a "tick-tick" sound at very low speeds, both pedaling and coasting. The sound was so muted that I couldn't hear it above a certain speed due to wind noise. On the bike stand you could see how much fatter the rim got (splayed out) on the side with the 4 inch crack. After I replaced the rim I sawed the bad section out and sawed it in half at the center of the crack. You literally needed a jeweler's loop to see the crack and the thickness of the concave section was only fractions of a milimeter less than the un-braked on rim section. In other words, in the failure region the rim was indeed cracked all the way through the braking surface but the cross-section width was not appreciably thin by any means.

I recently bought one of these at a local bike store to objectively and consistency measure rim wear on my fleet of bikes and those of friends that I do work on:

http://bikeforums.net/attachment.php...hmentid=129014

I've not seen anything similar sold elsewhere, apparently it is meant to be used by dentists, here is the product page of the Swiss dealer, in German. Translation by Google Translator, and tidied up by me:

"As used by dental technician to precisely measure the thickness of crowns and bridges, this is ideal for measuring brake discs and wheel rims. Thickness can be measured to an accuracy of 0.1 mm. Made of stainless steel.

"When are wheel rims worn out?
"To avoid surprises from ruptured wheel rims, it is advisable to periodically measure the braking surface. When new, the wall thickness is about 1.5 mm, when it is below the limit of 1 mm, a replacement should be considered.

"When are brake discs worn out?
"Avid does not disclose the minimum material thickness of their brake discs. Magura recommends a replacement when falling below 1.7 mm, Hayes at 1.52 mm, and Shimano at 1.5 mm. For safety reasons, we recommend Avid and other manufacturer's brake discs to be replaced when they are less than 1.5 mm thickness. After assembly, degrease new brake discs thoroughly."

I've got a dental caliper cheaply on Ebay. In my memory, if the thickness at the narrowest point is about 0.5 mm, the rim absolutely needs to be replaced and possibly earlier.

My front wheel I've been putting about 100 miles/week on has an early 80s Ambrosio 19 Extra rim, 36h; it came to me on a used Tommasini racing bike and already had some visible signs of braking/use. No concavity on the sidewalls, and they are fairly smooth aside from a few small vertical linear scratches (perpendicular to the length of the sidewall). They are not nearly as smooth as my new rear wheel sidewalls
(Open Pro CD) and there are no wear indicators I can see. The inner surface around the spoke nipples show no cracks.

I am guessing there are at least few thousand miles on the wheel, probably more....given I am doing a lot more riding and want to have the hub serviced (Campy Record), would it be prudent to just have the rim replaced? and what 36h rim would you go with for a heavier rider (210#) on bad roads? I was thinking the same Mavic OP CD as i am running in back.

The wearout point is also determined by the tire section and pressure. On wired-on tires the rim makes up part of the cross section of the tire and must keep the tire from expanding. As you inflate the tire it'll pull the rim a bit wider at the flanges, and as the rim thins it'll reach a point of not being strong enough and fail.

This was a rarity in the days of narrow HP road tires, but mtn tires with the larger x-section and high pressure ratings create much higher rim loads leading to failure. Add to that the higher braking forces applied to the rim, accelerating metal fatigue and you can see the problem.

As another poster mentioned, there are wear indicator grooves cut into some rims, or you'll get some warning of a coming failure by uneven brake modulation as the rim begins to widen unevenly under the pressure load.

Folks riding large section tires at higher pressure need to be most attuned to the problem, which in any case can be helped somewhat by mounting the brake shoes so they press hardest on the box area of the rim rather than the flanges.

If you replace rims when the track feels concave, you'll go through a lot of rims and spend a lot of time or money, though you'll be very safe. I let mine go until I feel throbbing in the brake lever, then I stop using that lever for the rest of the ride and replace the rim before riding that wheel again. Once I let a little air out of the tire because it seemed close.

It's also good to make a habit of pumping your tires at home before every ride, then spinning the wheel while watching the track. I've caught a couple of thin rims that way, too.

I wouldn't start a major brevet with bad tracks, though.

Going back to my old post.... The grabiness I felt as the rim gets concave was not a throbbing sort of feel that you would get if the rim was deforming in bumps. It was an even thing brought on by the concavity grabbing the pads more aggresively. It happened evenly over the whole rim with no sign of a once per rev "hernia" of the rim. Like I said before. I tend to be very critical of stuff like this and replace it just because life is too short to ride poorly performing parts.

If I were less fussy I don't doubt that I would have gotten a couple to three more months of commuting from the rim before it would have been in danger of erupting.

What I started noticing with my old rim (especially on descents) was the sound changed - hard to describe in words, but it just sounded like a hissier sound, louder and more like white noise, as if the rim was thinner. Not very confidence inspiring.

I reckon that could also be the brakes themselves (I have a different brake/pads on the front) but since I know exactly how old all my other rims are and have no clue about the life this one led for a decade+ before it came to me around 1996, I figured, why take the risk? The old rim was probably still fine and had plenty of safe miles left in it but better safe than sorry.

I had a rim fail (the outer edge blew off while I was pumping up the tire) so I purchased the Iwanson Dental Gauge Caliper (which is really cheap, BTW; $5.70 on Amazon. A beautiful, well-designed tool) and measured the blown rim. The rim thickness at the narrowest point ran around 0.7 mm. So I would say you'd be safe replacing the rim when the thickness gets down to 1 mm.

You're always safe replacing a rim. The question is how long you can put it off.

Generally -- but not always -- rims will not wear to uniform wall thickness, and you'll begin to feel flex as uneven or pulsed braking before the rim reaches the breaking point. However, pulsed braking can indicate other things besides rims nearing the end of their lives, and as I said rims don't always pulse before breaking, so this is only a coarse indicator.

Likewise measuring isn't ultra reliable insurance, because the worn wall may not be uniform, and the minimum safe limit will depend on tire pressure and width (which determine hoop stress), and where the shoes track.

When I had a rim reach the point where I was concerned, I adopted a "safety method" pumping tires to about 20% above my riding pressure, then bleeding it back. While nothing is absolutely foolproof, I feel that this offers a safety margin of that 20% or so, and pretty much ensures that a rim will fail while pumping rather than riding.

Not to worry is you have disc brakes.

Yes, disc brakes address this problem -- that is one of the main reasons they exist.
I am more comfortable using an inexpensive, reliable measuring device than waiting for behavior like pulsing that may or may not happen. If you measure the rim width, you can judge how long it will be before it's time to replace the rim. And if you notice one side is wearing out faster than the other, that suggests you might want to adjust your brakes. And, finally, measuring at multiple points, taking the minimum, and then adding a 50% margin is a reliable, standard way of ensuring safety.

Not mentioned is another rim wear/failure mode. Cracks. Some rims can and do crack, usually starting at the designed in stress risers called spoke nipple holes. These holes have a spoke/nipple that is pulling/stretching the rim's spoke bed toward the hub all the time. The stress grows and falls to a degree with the wheel's revolutions and is also dependent on spoke tension, rider/bike weight, rider's smoothness, road hazards and other factors. Rims which have been hardened, by heat treating or anodizing, will be more brittle then lower cost rims which don't usually see the more costly manufacturing treatments. Lighter weight rims usually have a greater chance of cracking. Wheels with fewer spokes then common counts and then run the spoke tensions to a higher level tend to crack more easily. Rims with reinforcing eyelets tend to reduce the chance of cracking.

We rarely see low cost rims crack because they are made with heavier/thicker amounts of material, often have less spoke tensions, usually have more spokes sharing the stresses. We usually see cracking in the more expensive wheels where fewer spokes, lighter/thinner cross sections (for lower rim weight) and higher spoke tensions combine.

We sell a brand which has a known issue of their wheels (of years ago) cracking at the rim/nipple holes. We check dozens of these wheels each year as the bikes come into the shop for service and find a number each year with cracks. To the credit of this brand is that they did warranty these wheels for at least 5 years after initial sale. But this offer is long past now.

As an interesting tangent but somewhat related- I had a well known and reputed (and costly) hub fail from the drive side shell bearing coming loose within the shell's seat. I could not adjust the bearing so no rim play remained (the hub uses an angular and adjustable cartridge bearing). I serviced the hub, replacing the bearing, using their special tool set. The rim play remained. I called the hub manufacturer and they offered to look at the wheel. I sent the rear wheel to them. They replied with the assessment that I had too many spokes and too high a spoke tension level for the hub to handle (36 2.0/1.8 spokes at an average of 110 units of tension, drive side, as measured with my Park tension meter) and the shell had expanded out allowing the bearing to rock around the seat a bit when under stress. They did "fix" the problem with a bearing which has a slight oversized OD. They charged me for this bearing and their labor (and shipping). Now a couple years later the bike is quiet and slop free and I'm happy enough to not do anything about this until the rim/wheel needs work. BUT I won't be using this brand's hubs (I still use their headsets) any longer and don't feel comfy suggesting them to my customers. I feel that they have built a hub which under performs for a user segment which was the standard not too long ago. I learned, once again, that choosing low weight over durability is a fools path. The brand's reputation suggested this wasn't going to be the case... Andy.

Yeah, I saw recently where Jan Heine of Bicycle Quarterly suggested using the over-inflate method to determine if a rim is about to fail. The higher pressure distorts / warps the weak rim. Better to have this happen in the shop than on the road.

Jan and I haven't spoken in over 5 years. So, in your mind, am I validating what he said?, or is he validating me?

Dunno. I accepted what he had written, because I had accidentally done it before I read about it. For all I know, he could have learned it from you.