Exploding wheel!
 

I rode in the Tour de Cure this morning. In the staging area a guy showed me his rear wheel that had, in his words, "simply exploded." No joke, either! He was wheeling his bike from the parking lot when he heard what sounded like a shotgun blast; he looked down and saw this:

http://distilleryimage11.s3.amazonaw...0a1f9e5b_7.jpg

I've never seen a rim disintegrate like this before, literally splitting down the center. As I checked over the rest of the rim, there were other stress fractures (I guess they were stress fractures!) all around the rim, in a line with the segment that burst. Anyone else ever seen something like this?

Thank God he wasn't riding it, that's crazy!

More common is that the brake track gets worn so much that one side of the rim pulls away. Usually that gives some warning in advance from the brake getting grabby as the rim edge bows out in spots.

But in this case it appears that cracks at the spoke holes propagated between holes until the rim split down the middle. Surprising that the wheel wouldn't have gone way out of true from uneven spoke tension long before the cracks got that extensive.

Besides cracks in the rim, I've got to think it must've took a beating over RR tracks or something to make it give way suddenly like that. Never saw anything quite like that myself. I guess it pays to put a tension gauge on those spokes after all.

That is interesting. It must be a single-wall rim run with extremely high pressure. What type of bike was it?

Never on a bike, but I've seen alloy wheels on cars blow out.

Hmm, offset spokes, and the crack didn't involved the opposite-side spoke hole right in the middle of the tear. Well, it must have been fatigued. A neighbor noticed longitudinal cracks in a rim he'd been tightening, and probably over-tightening, for years. If it had let go it might have looked just like that, but that's just a guess. FWIW, Jobst Brandt's book says that the rim is the weakest part of a well-built wheel and this it will eventually fatigue. This wheel looks to have given all it had to give, lived a long life.

I didn't pay much attention to the bike - some kind of hybrid, if I recall correctly. I also didn't check to see what rim it was either but it strikes me that it might've been CR-18. Jeez, can you really put that much pressure in a tube to blow apart the metal?

Doesn't look like a "box" rim. The two spokes at the end look like they are the drive side, higher tension. There must have been a stress riser somewhere along the break. If were able to examine the surfaces of the failure, you might be able to see where it started and if there was a any propagation prior to the failure.

We don't know what pressure it was running, but 120psi means exactly that, 120 pounds for every square inch! That it pushed the rim apart once the crack formed isn't surprising. We could question how it went that far before blowing the tube, but the explanation is simple. The rim strip did its job. Probably a wide strip of thick rubber or strong cloth. The loud bang would have been the tube finally finding an opening.

Shouldn't the title of this thread be: "Asploding Wheel!" ?

Nice Instagram image by the way...

Fortunately, I have never experienced this first hand, but I have heard similar stories from others. This is one reason I avoid carbon rims, which are even more sensitive than aluminum rims to stress risers from surface scratches and material fatigue.

Looks like a double wall rim to me. My guess is that if one were to look at the extrusion die there was part of that die that would show why that area could have been a weak spot. Any extrusion die that has a hollow cavity at some point has webbing to place that cavity, the metal flows around the web and under heat and pressure is "welded" back together. My view is a "cold" fusing of the metal happened, and over the years those micro voids became enlarged under pressure from the tire tube and oxidation. Lack of review by the owner and or mechanics let this percolate to the point of failure. Who knows how much pressure the fellow had in the tire too.

Someone gave me some wheels where the rear rim has longitudinal cracks, though they're not in the center. I plan to replace the rim eventually. I have not ridden it, and I don't plan to ride it as it is. I guess this is a good argument in favor of using spokes that are gauged right for the weight of the rim. Excessive tension probably played a big part in this.

Only if the metal is already about to fall apart... which it was in this case.

A rim can get fatigue cracks at the spoke holes, which typically propagate along the center of the rim... just like the big crack in this rim. It looks like cracks at two adjacent DS spokes eventually got so big that they "joined up" when extra stress was added.

Unless you are in the habit of ignoring your equipment, I wouldn't worry about this happening to you. There should have been ample warning.

Tom, I'm not a wheel builder myself so forgive my ignorance. Given that the bike was a major brand hybrid, it's a fairly safe bet these are (were!) the original wheels. In your experience have you seen other examples of incorrect gauge spokes paired up on factory built wheels? The thought of experiencing this sort of wheel demise while riding simply scares the ****ake mushrooms right out of me!

This is an outlier in my book. That said, test, don't guess. Big thick spokes, even typical ga. spokes can allow a wheel to be over tensioned, the typical spoke tension of yesteryear is far less than the new modern stuff, which most often has fewer spokes to take the load. Also, a fresh rim, NOS or new production is always the safest bet. That said, I have some wheels built with cheap cad plated spokes and used rims that I built decades ago that are still in service.

This reminds me of the Calvin & Hobbs strip where Calvin asks his dad how they know the maximum weight of a bridge. Dad says they drive heavier and heavier trucks over it until it collapses. Then they weigh that truck and build another bridge exactly like the first one.

What is the stress limit of a suspect wheel? Ride it until it breaks, then build another exactly like that one and just don't ride it quite that much or that hard the next time.

I agree with others who suggest this wheel likely had cracks at the spoke holes which propagated into this failure. I was given an old wheel with a (super lightweight) Mavic OR-7 rim. Every DS spoke hole had a .2-.5 cm crack on each side of the spoke. If I had ridden the wheel for an extended amount of time, I'd bet it would have met a similar fate. In the OPs example, I believe regular inspection of the wheel would have revealed cracks in advance of this failure.

Something similar happened to a friend of mine while he was riding. Afterwards the wheel looked like this:
http://www.bjmchugh.net/tv/wheel.JPG

Full story here.

This was not a matter of brake wear.

Rims more commonly fail on account of brake wear. That has happened to me twice, where my rear rim wore to a point where it started to crack and bulge. Both times I felt a whumpwhumpwhump when I hit the brakes and on inspection found a section starting to give way. Nothing catastrophic.

Hard anodized rims with high spoke tension contributes to this type of failure.

Just to hazard a guess.. the wheel was probably quite worn on the brake tracks and chances are the spokes were overly tensioned for the amount of abuse the wheel had withstood.. and if the wheel had been left sitting there deflated and he suddenly aired it up to a high PSI, that could easily happen. But this is all just speculation on my part.

Rats Azorch - Now I gotta go check all my rims...

Thanks Man Good Post...

AZORCH, no I don't typically see oversized spokes on factory-spec'd wheels. So thank you for asking, because it makes me realize it could be something else. Perhaps the spec was for spokes that typically aren't tightened quite that much. Maybe there is a bad assumption about how much tension will be put into the wheel. When I build a wheel, I make it absolutely as tight as I can.

Could riding hard create excess tension, if only momentarily? I suppose, but I don't know enough to say so.

Cheap wheels usually have straight 14g spokes, and they are not highly tensioned because that is more difficult. Torque will increase the tension on pulling spokes, but a wheel with that many spokes and all of them crossed would see only a miniscule increase. Vertical loads only decrease detension.

Most likely either the rim was faulty, or there was serious corrosion.