How do you know when to replace a rim?

[fiamme red]

A friend of mine, who rides lots of miles in all sorts of weather, was doing a 400 km brevet this weekend. He went into a convenience store, and when he came out, he found that his Open Pro 36h front wheel had disintegrated (see photo).

How do you know when it's time to replace a rim, short of having it break?

[cribbit]

If you're asking whether that needs to be replaced, the answer is yes.

Alloy rims are easier to spot failures before they go catastrophic but it can still be hard to spot sometimes.

Usually cracks at the spoke holes are the first point of failure. But if you do gravel or have bad brake pads the braking surface can fail like this (one edge that disc brakes have imo).

A failure like this would be very hard to spot before it happened I would think.

[sitzmark]

Concavity (from wear) in the braking surface - looks a little concave but could just be camera artifact. Wear indicator is down but not completely worn. Might have been a hairline stress crack that was missed. Need to spend a little extra time inspecting high mileage frames/wheels/drive lines before setting out on major excursions - maybe he did, maybe he didn't. Sometimes "stuff" happens.

[fiamme red]

Originally Posted by cribbit

If you're asking whether that needs to be replaced, the answer is yes.

Alloy rims are easier to spot failures before they go catastrophic but it can still be hard to spot sometimes.

Usually cracks at the spoke holes are the first point of failure. But if you do gravel or have bad brake pads the braking surface can fail like this (one edge that disc brakes have imo).

So with disc brakes, as long as you don't dent a rim, it should theoretically last forever?

Originally Posted by cribbit

A failure like this would be very hard to spot before it happened I would think.

I guess that it would be possible to notice if the rim looks noticeably concave. But how concave does a rim have to be before it splits?

[quickfeet]

Originally Posted by fiamme red

So with disc brakes, as long as you don't dent a rim, it should theoretically last forever?

No, all spoke holes will fail at some point with enough riding in my opinion.

[m_sasso]

I don't think there is a definitive answer however straight edge on the braking side wall, if you see light between the two surfaces, it is concave and a substantial amount of side wall braking surface has been worn away, time to contemplate replacement. That rim looks quite worn, more towards the centre of the side wall at the darker grey band and not so worn at the position or height of the ware indicator. If your pad alignment is not such that you are making contact over the ware indicator the indicators are not going to tell you much.

How much air pressure was he riding on and was the bike parked out in the hot summer sun to substantially elevate the pressure? New rims are generally tested to withstand double or triple the normal maximum riding pressure.

[Ken Robb]

If a small hard stone gets stuck in a brake pad and goes unnoticed it can act like a chisel and cut a deep groove in the brake track pretty quick and that could cause a failure like this.

[AngryScientist]

The best scenario is a rim with built in wear indicator dimples.

I also think it would be reasonably easy to determine what the original wall thickness is of the rim braking surface and using a dial indicator at some frequency determine when the rim wall has lost enough surface at the lowest point. What the quantity of “enough surface” lost would have to be up to the rim mfg.

[Peter P.]

You really DON'T know when to replace a rim; you just take guesses. Yeah; some rims have wear indicators but we're inclined to think those dots are put there to sell more rims .

You can feel a rim as the sidewall goes concave, but how much concavity can you tolerate before it's too much?

One test I've devised is to measure the rim width at the tire bead with the tire infliated to X psi. Do this when the rim is new. Periodically, repeat the test and document the increasing rim width. As the sidewalls wear, tire pressure will force the rim wider. Sound time consuming-you bet. And what's the "replacement point"?

I've blown many rims through wear and they've usually popped while riding, fortunately without dire consequences.

Here was my friend's solution when it happened to him, so we could finish our ride. It worked! We stopped at a drug store and purchased a roll of medical tape. He didn't inflate the tire to full pressure, and he had to leave the brake caliper open so the rim would pass through cleanly.

[oldpotatoe]

Originally Posted by fiamme red

So with disc brakes, as long as you don't dent a rim, it should theoretically last forever?

I guess that it would be possible to notice if the rim looks noticeably concave. But how concave does a rim have to be before it splits?

1)...well, sorta..if the rim gets too distorted, spokes tension is too uneven, it may not stay true..altho lack of true in disc brake wheels isn't the issue that it is with rim brake rims.

2) no way to say, how much concavity is too much, a straight edge on the rim..see the gap, feel, wear marks are gone..

[ultraman6970]

Many manufacturers now put a tiny hole in the braking area, once that hole is done you have to swap the frame.

As for old rims, seen and i have stuff that is like like 30 years old, light use but either way is like 30 years old.

[unterhausen]

I think the wear indicators might not be proactive enough. Some manufacturers put a groove around the braking surface as a wear indicator, but also as a convenient place for the rim to crack.

My rims always seem to tell me when it's time. Or I just want a new wheel. I really don't like a concave braking surface, that also seems like a convenient place for rims to break