FBinNY 

Feel free to disagree. Bulged is your word, not mine.

Duragrouch

I have cheap rims and have seen similar before. I put on a set of "new" brake pads that were takeoff from another bike, but (as I discovered later) they were aged enough that the rubber was hard, and after a few miles of downhill braking I heard bad scraping sounds and limped home. The whole of the sidewalls had similar bits on the surface, galling. I wrapped some emery cloth around a file and dressed all of it off, cleaned, replaced pads with actual new ones, was fine. I think that is a galling chip that welded itself to the sidewall, I'm surprised it was not pulled off already, mine looked more stable and spread out. I don't think there is a hole in the sidewall there. Carefully dress the surface flat. Also, pull off the brake pads, they may have tons of embedded aluminum bits in them. Flick those out with a knife point, but most likely the pads are old so replace with fresh rubber ones. Even with my fresh brake pads, I need to periodically remove to flick out tiny aluminum bits, which is why I went to cartridge pads so can slide them out and back in fast with no adjusting. The rims are worn well past any surface anodizing if they ever had (I doubt it) so the aluminum is soft. "Hard" anodizing like on good rims is I think always dark gray, aluminum oxide, like old Calphalon Commercial cookware, and alumina sharpening stones. However since that episode I have only found bits in the brake pads, no serious galling like before.

Kontact

Because you can't imagine that a rim with a flat spot might bulged in the process, and keep recommending against doing to minimum safety inspection of removing the tire.

Russ Roth

Going to agree with FBinNY, I've seen aluminum rims where pieces of metal and rock have gotten stuck in the brake pads and been transferred into the rim through extended braking, looks like what we're seeing here. I would use an awl and needle nose and try to pry it out, once out, take off the tire and make sure it wasn't going through the rim. If it didn't I would just keep using it. If the object caught in this particular spot due to there being a dent in the rim creating a bulge in the brake track I'd take a pair of channellocks and a hammer and massage it a bit flatter. If the object went through then I'd be worried its a stress point for a crack to start in the rim and I'd ditch it.

smd4

You got five more years, @Kontact! Consider yourself lucky.

I wonder what roused Bill's curiosity enough to take you off his ignore list, albeit briefly.

hhk25

Following up with more photos and a video. I'll warn you that this is a long post with a lot of attachments. Before I go on, I'll mention that I removed the tire and the inside of the rim looks intact.


From the video, you can see that the rim is dented in (I inaccurately described it as a hop) exactly at the location of the embedded metal.

https://media.giphy.com/media/v1.Y2l...ized-large.gif

You can also see it in the next two photos. You can see how much closer the caliper is to the edge of the rim in the first photo as opposed to the second.




Next photo shows that I found another piece of embedded metal on the same (right) side of the rim. This part of the wheel is true.



The brake pad is clean other than a light groove worn into it from the metal bits.




I then looked closely at the rear wheel and discovered another similar metal bit but this time on the left side of the rim (last photo). It's smoothed down quite a bit probably because the rider uses the rear brake more heavily. The brake pad is also clean with no metal bits and only a light groove from the metal bit.



Lastly, I'll mention that I tried to pick out the first piece of metal that I noticed with a dental pick and it does not want to come out. I don't want to dig at it more aggressively because I fear I will damage the brake track further.

I agreed to do a quick tune-up for my friend and look what this has turned into. Very interested in what the forum has to say about this. Thanks for all the input.

smd4

I'm guessing a manufacturing flaw.

Crankycrank

Ha ha, the dreaded "friend tune-up". Been there many times. You have my sympathy but good for you for trying to fix it for them.

hhk25

To be fair, she did offer to pay me but I asked her to donate to a charity. The last thing I want is a backyard bike tuning business.

Duragrouch

I don't know why bits like that appear welded to the rim. I'd never seen this phenomenon on my old race bike with (dark gray) hard anodized rims, but then I also never used it in my current venue of steep downhills. That, and my current cheap silver aluminum rims, and I get bits of aluminum in the pads, though not welded to the rim except for with those old pads. You could also flick out or cut off that chip with the tip of a knife, that's what I'd do with just a small bit like that. On my rims where I galled the total of it, emery cloth around a file was needed.

Heat is a factor. Aluminum loses strength at much lower temp than melting point. Hard brake pads generate more heat, especially on long downhills. And, the more the rim sidewalls polish smooth, the more heat is generated for the same braking force. I'm having issues with that on my old worn rims. Also, I think my use of smaller road pads (because cartridge style so easier to clean) may also cause more local heat than larger mountain pads.

Kontact

They aren't welded to anything. They are chunks stuck in a hole.

Reynolds 

I'd just file it down and inflate to max pressure +10%. If no issues, deflate to desired pressure and ride it.

smd4

And your explanation as to how that happened?

Duragrouch

Perhaps welded is the wrong term. But I think somehow got heat bonded to the rim. Perhaps a chip of aluminum in the pad, under hard braking it heats up until suddenly grabs at a spot on the rim and stays there, perhaps just as the brakes are released. I don't know. I just know I get tiny bits of aluminum in my pads which I periodically clean out, as otherwise I'll start to see and hear it on the rim sidewalls.

It's an interesting issue, and common on cheap rims and old pads. I'll look online to see if this has been studied by anyone. EDIT: Yep, or at least occurance, if not studied:
https://motoredbikes.com/threads/rim...oy-rims.20907/

Notably, some say the aluminum material "gathers" on the pads in one spot (from tiny bits, not one big tearout), then bonds together and to the rim, my guess is from heat. That makes sense. Just like sintered metal.

FBinNY 

I'm always impressed with the certainty some have on an issue, while lesser mortals can only theorize. Doubly impressed by the absolute certainty with which two mutually exclusive explanations are offered by the same person.

In any case, keep in mind that the grades of aluminum used on basic rims are soft and very prone to galling This allows brake shoes to rub metal off in one place and redeposit in another. So, while I don't claim to KNOW the cause here, I've seen plenty of similar cases, and suspect it's a form of galling related to brake heat.

I'd flick it out with an ice pick, if only to prevent it getting picked up by the shoe and contining the process.

Camilo

Why I never offer to "tune up" or repair a friend's bike. I do help on the road with obvious adjustments - FD or rim brake rubbing, RD indexing are about it. Maybe help change a tire on the road. But never "tune up" - because it always turns into a goat rope.

grumpus

Looks like a flat spot rather than a hop, I used to regularly fix these with a rubber hammer - just remove the surrounding spokes and beat it back into shape - but I've never tried it on a deep section rim. Is it definitely a foreign body, rather than a scar from contact with road debris? As long as it's not caused any deformation on the inside of the rim and it doesn't want to move I'd just file it flat.

hhk25

This particular rim has two grooves machined into it and the metal bits are stuck in the grooves. I'm still wondering what the source is for these metal bits. I've never seen this in my many years of cycling. Is the wheel picking them up off the road? Seems odd.

Duragrouch

Good link, I bookmarked that, thanks.

I've seen galling before, but usually with sliding of two metals of similar low hardness under high surface pressure. Rubber brake pads against even soft aluminum, one would think not, but my only guess is that aluminum bits build up on the pads to the point that there is then aluminum-against-aluminum and galling starts and can be self-perpetuating and accelerating.

It's weird. My rims currently are very smooth (too smooth), no appearance of galling, and yet I get very tiny bits of aluminum in the pads, though very, very minimal with periodic cleaning, perhaps only 3 or 4 tiny bits per pad each cleaning. I clean with scrub sponge to remove the black schmutz, then the bits are very visible, and flick them out with a knife point. Cartridge pads make the process a whole lot quicker, unlike regular v-brake pads which require minutes to realign properly, I'm picky about correct position and alignment.

Kontact

Galling does not bind material permanently to rims. Galling is when the friction forces are great enough to bust some material loose or partially loose and possibly smear that partially attached material. Galling can't weld loose pieces of metal together at the temperatures that rubber brake pads exist at.

What could happen is that you could have a divet that a piece of metal could be forced into and get it to wedge under enough pressure. Blacksmiths would fill forging voids by hammering a nugget of soft iron in without welding, so it is just mechanically retained.

If you inspected the inside of the rim and see no damage or thin spots, you could consider continuing to use the rim. If that larger blob pops out, it should produce any particular braking problem, though it may pulse.

Duragrouch

I don't think the rim would get that hot, but the aluminum bits at the pad might. The following analogy might apply, or might not ("I could be wrong"): Trying to grind a piece of aluminum on a bench grinder, aluminum will immediately embed into the grinding wheel, requiring hours of flicking out each piece, or dressing the wheel to remove both the aluminum and some abrasive. (Thus signs at machine shop grinders, "NO ALUMINUM".) Steel? Doesn't embed, even though, since harder, actually gets hotter at the contact point. The aluminum doesn't get hot enough to melt, but, either hot enough to soften and embed, or somehow the aluminum grinding bits ("swarf") bond together and then embed.

The aluminum bits in the rubber brake pads, I think might get hot enough to then attach to the rim. My limited experience has been, the harder the rubber (both in orginal durometer and due to age), the more likely this is to happen.

Stick arc welding steel, the splash(?) will often stick to surrounding metal that was cool. Not stuck very strong, those BBs will fall off when struck. I think the same is true with the bits stuck to the aluminum rim, not much bond penetration, they come off.

As I said, I could be wrong. And even if the above is true, galling might not be the correct description.

Kontact

Imagine you are grinding aluminum, but you're doing it with small bits held in your bare fingers. You can't heat a piece of metal up so much while holding it with a piece of rubber. The rubber will burn or melt.

The splash sticks to stuff for the same reason that brazing works - the temp is high enough to bind metal to metal without needing the passivation layer first removed.

A better explanation for metal embedded in metal (rather than embedded in a porous ceramic grinding wheel) is that material is jammed in open pores left from galling.

Duragrouch

Hmm, that's a good point. I think these mechanisms happen very fast, but I still take your point. I think the rubber may hold up at the temperature that the aluminum softens. And rubber transfers heat from the surface to the rest of the pad, terrible. Need to give more thought. Thanks for making that point. I'm interested in finding truth, not winning an argument.

FBinNY 

Sorry, but no.

Earlier, specifically to avoid endless argument, I linked an authoritive article explaining galling. I gather you didn't bother, and I get that because it's longish.

But here's the opening paragraph....

An important aspect of any part’s design and maintenance is the component’s resistance to wear. Adhesive wear is the product of two metal surfaces being rubbed together. Under a sufficient load, this will penetrate the oxide film on the surface of metals like aluminum and stainless steel. When the force exceeds the metal’s strength, adhesion or cold welding occurs.

It goes on and into greater depth. But the takeaway is that galling results from a metal's affinity for itself, not heat.

So, while we can all speculate, I'm a fan of Occam's Razor....

We have a material prone to galling, we have the right conditions for galling, we have an end result that looks like it might galling, so I'm going to guess that it's probably galling.

FWIW, it's a mistake to think only about rubber brake shoes. The shoes pick up all kinds of grit capable of abrasively machining rims, resulting in aluminum bits embedding in the shoe, giving the aluminum on aluminum friction that galling is about.

I've seen countless examples over the years, and it's most pronounced in certain alloys, so I see it on low end rims, and less so on better rims.

Duragrouch

Truer words were never said. I wish there was an easy way to hard anodize my rims without removing spokes and hub, and acid bath and electricity. If I ever cough up for a new bike, it'll have discs. Speaking of which, I saw a blurb about Dahon's "safety disc", clicked on Dahon's euro web article, it redirected me to the USA site in a second (with no info), I hate that! I wish I knew a way around that, tried a different more incognito browser, no help. But anyway, finally found info; It's discs mounted 15mm further inboard, so if the bike falls over next to a curb, less likely to damage the disc. That also may come in handy when transporting a folder like mine.