Tuesday, April 10, 2012

Rim, Shot

As promised, here are a couple close-up shots of the rim that was felled by ninjas. First, the obvious point of failure:
The rim itself is a fairly low-budget OEM Weinmann with no eyelets but three chambers -- basically, a quasi-Deep V exterior shape with a couple tunnels thrown in for added girth and (one would assume) strength. It popped right at the seam, which was (as near as I can figure) behind and above the tire's contact patch, perpendicular to the impact, and right in the worst of the blast zone.

I assume from what little I know of rim construction that this is what's called a "pinned" rim, since there's certainly no sign of a weld here. That wiry looking bit spanning the gap (and barely holding the rim together) is a metal pin with spiral grooves running in opposite directions away from the centerline. My guess is that the pin plus some sort of industrial sticky stuff plus a lot of pressure holds the rim together, at least until some idiot hits it from the side.

(Aside: What you can't see is the big rim decal on the reverse, still valiantly holding across the chasm of failed rim. The moral of the story, kids, is that big, ugly brand labels are actually supposed to be a safety feature.) 

There was also an interesting failure mode away from all the excitement:
I didn't even notice this part until I took off the tire and tried to take off the (miraculously unscathed) tube. Seems that as the rim failed at the bottom, it lost a bit of diameter, squeezing down the valve hole and spoke holes at the top. See the tell-tale wrinkled decal at each hole and funky reflection at the un-decaled hole? So when I tried to extract the tube, I ended up yanking off the valve stem, thus making it unscathed no more.

I dearly hope no one takes this as a shot at Weinmann or their products. Remember, this rim took a pretty nasty lateral smack, the sort of thing that would put just about any hoop in the recycling bin. Even if the joint had been welded, I suspect the force would have potato-chipped it past the point of re-true-ability.


Chuck Davis said...

Sure looks shot to me

Unknown said...

I recall a Mavic guy at my shop in 1992 commenting that examination of an unspoked rim is akin to looking over a squid out of the water. Neither communicate their reality.

Spoke tension and inflation pressure make the pins and their glue hold things in circles. Pretty much out of balance as a result; the valve hole is opposite in even the cheapest rims to overcome that.

Mavic answered the implications of structure with the S.U.P. rim process:


Other than those, any rim banged hard enough to exceed the component strength of any piece will do some impressive self-disassembly. Wheels taking a hit out of the plane of the rim will uncover some unwitting points of failure. The amount of force is surprisingly low and a wheel using spoke tension deviation to "true" the wheel with an experienced rim is really a time bomb waiting to blow.

Too bad you were Darwin's collection agent on that idiot.