Wednesday, November 17, 2010

A Broken Rim Mystery

It was a dark and stormy night...

Really, it was. It was a Monday evening in November. The wind was whipping the orange and yellow leaves off the trees and the wind was howling up the valley. The lights in the shop had flickered ominously several times, but it so far the power was still holding and the phones were still working. It was the kind of night where I'm glad that my current commute is only four blocks long.

One service we don't offer at our bike shop is on the road pick up, but when the call came in the fellow on the other end of the line sounded so soaked, stranded and stuck I told him I'd see what we could do. As luck would have it Chris, one of our off-duty mechanics, lives not far from where this guy was broken down and Chris happened to be home, happened to answer the phone when I called him and happened to have a truck big enough to bring man and bike back into Issaquah on this particular dark and stormy night.


What stranded man and bike wasn't a routine puncture, it was a rim blowout. I've written previously about rim blowouts, but they are usually caused by the rim being thinned by braking. In this case, the rim failed at the rim wear indicator but this bike uses disk rather than rim brakes, so the rim should not be subject to any wear at the rim. The bike's owner tells me that this is the second rim failure he's had on this bike in the year or so that he's owned it. Rather than having his recumbent shop replace the rim again with the same thing, he's opting to have us rebuild the wheel with what we all hope will be a stronger rim. The new rim has no wear indicator, and stronger construction. Since the new rim has a different E.R.D., we're replacing all the spokes as well.


The mystery of why this bike is blowing out rims is still a mystery. I have theory, but I'm posting this story and a couple of photos here in the hopes that folks might say "oh, I've seen this before" or "I bet this is what's going on."

The bike is a Rans Cruz semi-recumbent and it has a Bionix electric rear wheel. The design of the bike puts a lot of weight on the back of the bike. The wheel weighs about 16 lbs and the battery (which the customer took with him and I didn't get a chance to weigh) rides on a rack right above the rear wheel. Most of the rider's weight is carried by the rear wheel and the electric motor puts out a lot of torque.

My best working theory (which is really just a wild-ass guess) is that the combined weight and torque is placing a greater stresses on the rim than what an unassisted rider would generate on a conventional bike. Unsprung weight over the rear wheel means that every bump transfers more pressure via the tire bead to the rim. The wear indicator on the existing rim not only serves no positive function on a bike with disk brakes, the groove of the wear indicator actually creates a stress riser in the rim. Over time, the combined stresses lead to a blowout.

So like a Stephen King mystery that I bothered me a lot when I read it but I still sticks in my head, I have a problem and a theory but no certain solution. We've got a rim and spokes on order and we'll lace 'em up, advise running a bit lower pressure in the tire and hope for the best. I guess that's how life goes. We never get all the answers but we get interesting questions.

Keep 'em rolling,

Kent "Mountain Turtle" Peterson
Issaquah WA USA

16 comments:

Eric said...

Hey Kent,
Don't know if you've already rebuilt this, but the BionX hubs are drilled for 13ga spokes. You need to use 13/14 butted spokes, otherwise you'll get lots of spoke breakage at the elbow. Sapim makes them.

Kent Peterson said...

Thanks for the heads up Eric. We haven't rebuilt the wheel yet. One more thing to factor in!

Michael R pdx said...

I had the same problem on my small wheeled, folding bike. Two rims split on it.

The problem seemed to be related to two factors:

more weight on the rear wheel - the folder weight distribution was biased rearward. (demonstrated by my ability to wheelie and an occasional rear endo when climbing a steep grade)

Wide tires placing greater horizontal, 90° to wheel plane, force on the rims.

The recumbent seems to share the weight distribution problem. Perhaps it shares the tire issue also.

John said...

I've seen some discussion among the 29er crowd having this issue when using tires that are too large for the rim. The rumor is that oversize tires on not oversized rims are okay if the pressure is kept low...

Pedals Cycling said...

I haven't ridden a recumbent all my life but my wild guess would be because his weight was put mainly on the rear wheel? Hope you find the right answer to your mysterious question. Did you fixed it already?

Bob said...

No experience with this but your theory seems sound. It makes sense that a crease in the metal would create a flex point that would eventually weaken and break.

Anonymous said...

as John alluded to, perhaps an oversized tire inflated to a high pressure to compensate for the rear weight bias of the bike plus the added weight of the hub and battery.

Th said...

If it were my bike, I'd look for a way to move the battery into the front triangle. I pity that back wheel

kfg said...

My theory, which you are free to incorporate into your own as you see fit, is that gouging a grove into the side of a rim is such a dumbass thing to do that a lawyer must have thought it up.

Jason T. Nunemaker said...

Hey, Kent, one more crackpot theory for you...

I'm seeing a "wheel within a wheel" effect in my mind here, with the wear indicator groove as the dividing line between the two. There's a lot of weight on the tire, so it doesn't want to budge. Then the hub applies a ton of torque via the spokes to the inner portion of the rim. I imagine the effect on that rim would be similar (although less extreme) to when an underinflated tire gets spun around the rim, damaging the valve stem.

That's not going to blow the rim by itself, but combine it with the weakening factors others have mentioned (tire pressure flexing that wear groove) and kerblammo.

Glad the rider's OK -- these sorts of blowouts can be deadly if they happen at the wrong time.

Jason "Armchair Mechanic" Nunemaker, in Des Moines, IA

charlie said...

He also needs a wider rim with a larger volume tire and lower pressure. Not sure if the Rans will take a wider tire though.

Johann Rissik said...

@kfg:..or an engineer? And the word verification says "jokedie". Should I exercise caution?
Kent, my 5 cents is on all factors combining to overload the poor wheel.

Surly Dave said...

That is one seriously ugly bike. Might be cheaper and kinder to tell him it died on the operating table.

Dave Abercrombie said...

I suffered a similar problem on a $500 mountain bike that was only seven months old.

http://www.rahul.net/abe/bike/rim-split

Sure, I ride on bumpy trails, but I am not abusive. I chalked it up to just cheap, substandard construction, typical on bikes of this class. I've never had a stock, machine-built wheel last more than about half a year.

Anonymous said...

Same thing happened to me on Giant MTB. Brakes were not worn so that is not problem. I think its because of the groove in the rim.

David Smith The Bicycling Store said...

The Consumer Product Safety Comission, CPSC has filed my report on potentially dangerous bicycle rims with a groove cut on the inside of the rim: http://saferproducts.gov
where you can search the report number: 20121207-34770-1289152
For pictures of a blown-out rim and a new rim with the grove, at the report end click the related files for this report: Attach_-_H12C0062_att_1[1]_Redacted.pdf