csport

I had a strange rim failure. There are cracks on both sides of the rim (photo). On both sides the outer (larger diameter) part is pushed to the right. No loose spokes, no visible cracks near spoke holes. There is another crack like this on one side in a different region.

Velocity Chukker 36H rim with about 10,000 miles on it. Originally it was a prebuilt wheel with Shimano T610 rear hub, then it was rebuilt with M6000 hub for disc brakes. Now it has thicker spokes on the DS, 2.3/2.0 double butted vs 2.2/1.8/2.0 triple butted on the non-drive side.

Heavy rider.

Any idea of what can cause this?


Both sides of the rim failed. The outer part is pushed to the right: to the outside on the drive side and inside on the non-drive side.

Desert Ryder

rim brakes?

Russ Roth

Rim brakes will wear through rims, and velocity has seemed to use some softer aluminums that wear faster than others, old velocity aeroheats seemed to wear faster than open pros which seemed like a harder aluminum but more prone to cracking at spoke holes. Commuters that experience a lot of wet use where road grit can mix in can quickly wear through rims, a lot of muddy MTBing pre-disc could do the same thing fairly quickly. 10k miles is a little low for this kind of failure but not unheard of especially if used a lot in the wet and through the winter. Cleaning the pads of any grit and the rim after a wet ride helps.

79pmooney

Have you laid a straight edge on the rim? Say a credit card oriented radially. Those rim sides (the braking surface) started off flat. I'll bet the middle of the braking surface now makes a channel perhaps much of 2mm deep. That's aluminum that once kept your rim strong and is now distributed along those 10,000 miles.

Now, both broken rim portions, drive and non-drive are pushed to the drive side? Weird. Weakened rims from brake wear typically explode outward and usually violently. You do not want to be riding and have flesh in its path. (Calves of legs.)

If this failure is from brake wear, not how deep the channel is and in the future, never get within (oh, say) 2/3s of that dept Riding conditions have a lot to do with how fast rims wear from brake use. Road grit speeds it up a lot. Wet roads means more grit sticks to the rims. The sand and dust of different regions can be quite different in abrasive qualities. Those of us who ride in the Pacific northwest in winter see brake wear at very high rates due to the lava dust that is everywhere. In my commuting days, I wouldn't go two full winters on a pair of rims.

A thought - it looks like both sides are worn but only one side has cracked through. So cracked through side has bulged out. Intact side is worn in. (All tough to judge from the photos without having seen the bike.)

FBinNY 

While brake wear is the most common cause or rim failures like this, it's not the only cause.

I don't see obvious brake wear, plus you mention disc brakes, so let's start with 2 questions.

1-how many miles with rim brakes before moving to discs?
2 - how many miles or months since the switch to disc brakes?

The non brake possibility is metal fatigue, possibly combined with the hoop stress of a wide section tire at high pressure.

As you ride, the rim flexes. Because it's a complex shape, that flexing causes uneven deflections and tends to cause stress where the b shape changes, ie. where the b flange joins the box. Your higher weight speeds the process.

csport

Need to clarify that I used the original wheels with rim brakes for about 1,000 miles and then rebuilt with disc hubs and used them with disc brakes after that. Rim brake pads were some Promax ones, not Kool Stop, but the photos do not show significant damage (left photo). I have seem some worn brake tracks on rental bikes, does not seem to be the case here.

Both sides are cracked through and pushed to the drive side. Looks like there is some lateral force pushing the inner part of the rim to the right.

No significant groove in the rim edge.

csport

1,000 miles with rim brakes, brake track does not look worn (photo in post #6), 9,000 miles with disc brakes.

Rim inner width = 19mm, I ran 38-42mm tires at 70-80 psi and also toured with rear load. The crack is at the bottom of the flange, everything checks out. Thank you!

panzerwagon 

Rim looks more worn on one side. Perhaps those first 1000 miles were with brake caliper not centered, rim wore deeper on one side, as well as being put under lateral stress during braking? Stress cracks built along bead edge inside, might have just been biding time whilst on discs? Just a shot in the dark.

Jeff Wills

Relatively fat tire on a narrow rim means the tire exerts a lot of leverage on the rim flange. After 10,000 miles- ccrrrrraaaccckkkk…

GamblerGORD53

I think that model rim was the defective one sold about 10 years ago.
Something seriously gouged that brake track tho. 42 mm tires are too fat for it, IMO. 80 lbs air is a lot for that width tire.
I have had nothing but Dyad rims, the 700c all have 35 mm tires. The 584 rims have 38c, only the front wheel has crummy calipers.
The others look as good as new, with 20 and 32,000 miles.

oldbobcat

Dirt on the pads and rims will speed rim wear, too.

SurferRosa

That's nothin'. This happened to me earlier this month:



It's a vintage Mavic MA40 rim from the early '90s and the second MA40 rim I've lost this year to brake track failure. I found a Mavic replacement for one wheelset and relaced the second set of hubs to new rims (Sun m13ii).

Monkitoucher

Same thing happened to me with a 90s MA40 too.

sweeks

I'll see your Mavic rim failure and raise you this Kinetix rim fail...


Rear brake application caused "thumping". (Kinetix 406 rim)

LV2TNDM

Cool Pic! I've had the same happen to the MA-2 and Mavic mountain bike rims. This most likely is not a failure due to the brake track. It's most likely fatigue failure, similar to what breaks spokes. Dan Burkhardt has expounded on the problem. Basically, when you have the spoke to go zero or close to zero tension during its rotation, fatigue life suffers for spokes AND rims! This is a common problem with NDS spokes and wheels with high dish. Straight-gauge spokes also worsen the problem. And it sure looks like those are straight-gauge spokes in your wheel!


Oh and to the OP:


10,000 miles on a mountain bike wheel? Well that kinda explains your failure! Don't know why you'd expect an aluminum rim to last much longer than that. Especially if it was ridden as a "pre-built" wheel before being rebuilt. And I don't really understand why the wheel was rebuilt with different gauge spokes on DS vs. NDS. This is usually a no-no in wheel building. Plus, there's no reason in my book to NOT have used the 2.2/1.8/2.0 spokes for the entire wheel. Those were definitely the best choice and it's too bad they weren't used throughout. Your other spokes aren't really double-butted - they're straight gauge with beefy elbows.


Very common misconception is that thicker spokes build a "stronger" wheel. They don't. Butted spokes build stronger and longer-lasting wheels.

grumpus

That's a lot of pressure for a tyre that wide. Plug your numbers into the Silca pressure calculator to see what that suggests.