Seriously cracked rim: How does this happen?

[MattyCiii]

The side wall on this rim has had a bulge for several weeks... I knew its days were limited. Then last night I put some air in the tire with a floor pump, and the rim side wall bulged out a lot. I let a bunch of air back out of the tire.

This morning I saw the extent of the problem (pix below). How does this happen? Some info about the wheel:
1) I built the wheel myself, using stainless 13/14 ga butted spokes of about 5" length.
2) I've been riding the rim maybe a year, all season, all weather. But last winter after the first snow, when I arrived home with slush all over the place, I bought a garden sprayer and I now wash the bike down whenever dirty/slushy. That said, I see a lot of aluminum corrosion in the rim.
3) I normally run the tires at 60-80 psi.

Perhaps I over tightened the spokes?
I think the initial bulge started when I nearly dragged the e-power drive chain tensioner through the running gear. Bike stopped in an instant (lucky it happened at near zero mph). One of the spokes got a nice dent & ~20 degree elbow bend. Maybe this was the beginning of the end.

I really want to get more than a year out of my rims. Any general advice, before I go ahead and re build this wheel?

This is the wheel

Here are the cracks:

This one is more like a canyon than a crack:

Any thoughts?

 

[Drunkskunk]

Looks like it split from the inside. That doesn't look like damage from the spokes, as the spokes have no tension on that part of the rim. Realy hard braking on a cheap rim in very cold conditions might have caused that. Running higher tire pressure might have as well. Or a combination of the two.

 

[amigafan2003]

Simple manufacturing defect by the looks of it - or just bad design.

 

[dak664]

Something was doing a lot of in and out flexing of that metal. Road shocks, many many reinflations, huge temperature or barometric pressure changes every day. Maybe whoever drilled the holes left a lot of microfractures to start the process.

That's why I don't trust airplanes

 

[MadRhino]

This rim was just not strong enough for your hydro rim brakes, very few are.

 

[john7700]

Congratulations !

That is the worst damage I have ever seen done to a rim without blunt impact. I have
never seen even the trials guys bugger one like that. You may want to look in to what
those trials guys are running these days as I think they are the only ones left using Hydro
rim breaks any more.

 

[amberwolf]

The side bulge here:
https://lh4.googleusercontent.com/-_l04PfA6Qco/UEJCsy5G4cI/AAAAAAAABgQ/8tCecbfbFrQ/s720/IMG_1640.JPG
looks like rims I have seen where braking has worn away the surface of the rim until it is too thin, and begins to flex with tire inflation/bumps/etc, cracks, and breaks. From what people have said, when braking isn't really all that hard or often, or rims aren't really all that old mile-wise, it seems to be caused by grit getting trapped in the pads, which then seriously grind away the rim surface, sometimes very quickly depending on what the grit is.

The rest of it...Iv'e never seen anything like that even in rims I've crashed up. I don't see cracking in the spoke holes thesmelves at the inner rim circumference, only at the outer, where it's actualy broken apart. It does not look like a single crack that spread, either, but rather cracks from every (or nearly) hole drilled in the rim. I suspect manufacturing defect or process issue.

Hmm...further looking at the pics shows that the cracks are often pushed upwards like crustal plates in collision at mountain ranges. This supports MadRhino's statement implying the hydros crushed the rim by squeezing too hard on it--but that seems odd, as it seems to me that to squeeze that hard you'd have to be locking up the wheel at that point (which means you would be skidding the rear wheel all the time). So I don't really know what cuased it, but it does indeed look like it was hugged to death and then named George.

 

[MattyCiii]

Thanks for the input guys.

Indeed, the spoke holes themselves are all intact - so yeah, very likely not the spoke tension.

I think I'll stick with the hydraulic rim brakes for now, I don't think I put too much of a squeeze on them ever. They are rear brakes after all and only provide 1/3 or less of breaking power.

I have a rim on hand I'll use for the rebuild. I don't want to be off the road any longer than I have to, of course. But I'll shop for a stronger rim, and consider swapping back to a cable brake.

 

[dogman dan]

Make a bike into a motorcle, and you beat stuff. It's pretty much that simple. Suspension helps with that a lot though, but still that alloy was flexing awhile. I was suprised by the location though, I thought it was going to be a rim edge seperating not a crack up the center.

Combination of many things, rim designed for light as possible is a major part of it. After that, just the pounding of riding above 20 mph on a few potholes. The powerfull rim brakes could easily have been a factor as well. Might look for a thicker cargo bike rim? Or something mondo for DH.

 

[MadRhino]

...I don't think I put too much of a squeeze on them ever. They are rear brakes after all and only provide 1/3 or less of breaking power.

I have a rim on hand I'll use for the rebuild. I don't want to be off the road any longer than I have to, of course. But I'll shop for a stronger rim, and consider swapping back to a cable brake.

It is when you are stopped uphill that you put too much brake pressure on your rear wheel. With the hydro rim brakes you don't even notice that your rim is squeezed to death. Every time you start a micro crack, after a while normal braking is enough to complete the destruction.

You can put a lot of pressure on a rolling rim at speed, but be careful once you are sopped.

 

[MattyCiii]

Thanks again everyone.
This is my mainstay bike, so I don't want any down time. I've laced, trued and installed my "spare" rim. All things being equal, I'll have about a year before this one fails. But I won't wait that long...

I'm going to strongly consider returning to cable pull rim brakes. I like the Avid Single Digit brake I put on the front. I can standardize on that brand/model. Then there's the e-brake switch. I have not (yet) gotten the hydro e-brake switch to work, so maybe this is an opportunity to try cable brake lever with e-brake switch (either off the shelf, or DIY per the several techniques shown here).

Finally, I'm shopping for some beefier rear rims. The NuVinci N360 is heavy, and I'm sure that has part to do with the rim fatigue (all that unsprung mass). I've never been a weight weenie - especially with this bike - so a heavier rim is not a problem. Better design, better materials or just plain greater wall thickness are all just fine in my book. A few more grams or dollars is no bother.

That said, it took only about 2.5 hours to re build the wheel all told. I'm getting better at this through experience. But still, if the changes above can move me from a wheel that lasts a year to one that lasts till whenever, that's where I want to be.

 

[full-throttle]

I've seen sidewalls wear and fail from rim brakes many times

The split in the middle of the rim is from over-inflation. What is the tyre width you're running, 2.0? 80PSI is prob too much for cheap rims.

 

[MattyCiii]

Thanks Full Throttle.
I'll bet it was over inflation, I'll be keeping its replacement at 60psi and lower. I'll bet the side wall failure was just a side effect of that.