Rims wear out from brakes rubbing on the rim. Failures may surprise riders who ride mostly in the dry, and sometimes surprise even riders in wet areas.
Here is a worn rim. When new, the sides of the rim were approximately parallel. Here, you can see the rim is badly worn away.
The photo show the brake track is thin. The maker's diagram suggests it was originally thickest between hook and web, thinner between web and spoke bed. Now, it is thinnest near the web. Also, the diagram suggests the brake track was originally thicker than the sloping parts but is now worn to be thinner than the sloping parts.
This rim was replaced before it failed. If it had been used more, one of two failures would be likely: either it would crack at the thinnest part of the box, and the wheel would suddenly loose true and develop a radial "lump". Or, the part with the hook would crack off. Sometimes, such cracks lead to a lump. Other times, the tire pulls hard enough that the hook tears off a long section of the sidewall, often as much as ¼ of a turn, and the tire goes flat with a bang.
Rim failures can strand you, and sometimes hurt you. So it is a good idea to replace worn rims. However, wear is unpredictable, making it hard to just replace rims according to the calendar. For example, one rider often gets 50,000 km from a rim, and replaces it due to flat spots and other damage from hitting obstacles, not because the rim fell apart. Another rider goes through several rims every year, with as little as 500 km from a rim during wetter parts of the year.
Some things that affect wear-out rates include:
For example, the rider above who gets 50,000 km is riding mostly on-road; during rainy weather, he tends to replace the freewheel with a single sprocket and ride a fixed gear, and thus avoid brake use. The rider getting 500 km is riding off-road in wet/sloppy conditions that coat the rims with abrasive mud, the rider is fit and fast from lots of on-road cycling, rides lots, but is a novice off-road cyclist, so often brakes more than a rider with extensive off-road experience.
As noted above, rim failures vary with tire and rim sizes and tire inflation pressure:
Wear rate is highly variable, so "replace by the calendar" is impractical. Thus, it is important to look for signs of wear before you are stranded or injured. A typical sign of wear is the inward curve of the brake tracks, shown in the picture above. (There is some parallax distortion from photography, but the gray bars on either side are parallel.) The "curved hollow" happens for two reasons:
Ideally, manufacturers would publish rim dimensions — similar to the above, but including dimensions when worn.
For example, the diagram above shows "21.6", but could include "worn-out if the width anywhere is under 20.0 or if the maximum dimension exceeds 22.0".
They don't, but they could. We are paying their salary, after all.
Many newer rims have wear indicators. One common type of wear indicator is a groove or dimple in the sidewall; when the groove/dimple disappears, the rim is worn out:
Many rims lack wear indicators, but can be checked easily for wear. Many rims start new with sides that are nearly parallel, and you can check for wear by putting your thumb on one side, a finger on the other, then moving your hand radially between hook and spoke bed. If the width changes substantially, it is likely time to have the rim checked by a mechanic.
Unfortunately, you need to know the rim shape and dimensions new in order to figure out when it is worn. Here are three rims from Araya, each showing the shape when new. The left one is bulged-out (convex) and is widest at the bead when new; it is worn out before it is flat and before the sides are parallel. The center one (RM-17) is curved-in (concave) when new. So it is curved-in long before it is worn out. The third (395 Team) is also curved-in, but is narrowest at the bead and is worn out long before the sides are flat.
The practical upshot is you need to know the rim shape and brake track thickness when new before you can know whether a specific rim is in good shape or is worn out.
Here are some rims with worn brake tracks:
One obvious way to build longer-wearing rims is a thicker brake track. Unfortunately, many press articles encourage buyers to focus on low weight. In turn, makers give us lighter weight -- but pretty much everything else gets worse; in this case, a faster-wearing rim.
A rim is worn out when it is much more likely to fail suddenly. Suppose you ride for 1 year, then your rim fails. You cut up the failed rim, measure it, and the thinnest point is 0.5 mm. Rim life varies with rim shape, material, tire size, tire pressure, and several other things. But let's say for your use, "worn out" was 0.5 mm.
You go to the shop and get the same rim to replace it, and the new rim has a brake track 1.4 mm thick and weighs 500 grams. This gives you 1.4 - 0.5 = 0.9 mm of brake track to wear away before it fails again in about one year.
Today, you do not get to choose the thickness of the brake track. But suppose makers offered your model with a few options for thicker brake tracks. Here is a table showing (roughly) what happens for each 0.1 mm of added brake track thickness (0.2 mm total; 0.1 mm on each side), assuming a 700C rim, made of aluminum, weighing 500 grams, with a 10 mm-tall brake track:
|thickness||weight||+ weight||+ % weight||wear||+ wear||+ % wear||% life||life|
|1.4 mm||500 g||0 g||0.0 %||0.9 mm||0.0 mm||0 %||100 %||1.0 years|
|1.5 mm||511 g||11 g||2.1 %||1.0 mm||0.1 mm||11 %||111 %||1.1 years|
|1.6 mm||521 g||21 g||4.2 %||1.1 mm||0.2 mm||22 %||122 %||1.2 years|
|1.7 mm||532 g||32 g||6.3 %||1.2 mm||0.3 mm||33 %||133 %||1.3 years|
|1.8 mm||542 g||42 g||8.4 %||1.3 mm||0.4 mm||44 %||144 %||1.4 years|
|1.9 mm||553 g||53 g||10.6 %||1.4 mm||0.5 mm||56 %||156 %||1.6 years|
|2.0 mm||563 g||63 g||12.7 %||1.5 mm||0.6 mm||67 %||167 %||1.7 years|
|2.1 mm||574 g||74 g||14.8 %||1.6 mm||0.7 mm||78 %||178 %||1.8 years|
|2.2 mm||584 g||84 g||16.9 %||1.7 mm||0.8 mm||89 %||189 %||1.9 years|
|2.3 mm||595 g||95 g||19.0 %||1.8 mm||0.9 mm||100 %||200 %||2.0 years|
|2.4 mm||606 g||106 g||21.1 %||1.9 mm||1.0 mm||111 %||211 %||2.1 years|
Briefly, going from a 1.4 mm brake track to 2.4 mm adds 20% (well, 21.1%) to the weight of each rim, yet gives just over double (2.1x) the rim life.
This does add about 210 g to the bike weight (both rims), at least when new — the more you wear it away, the lighter it gets! For racing, the lighter rim with shorter life is almost certainly better. But for most other riders, a balance of weight, cost, and durability is more important. And note that doubling the rim life means half as many rims and half as many wheel rebuilds; and that the labor cost to replace a rim is often close to the cost of the rim itself.
Further, a thicker brake track adds almost nothing to the rim cost: 20% more material costs 20% more; but 20% more material adds almost nothing to design, manufacturing labor, packaging, shipping, etc. Thus, added manufacturing cost is small — maybe 5%. So a rim might cost 5% more, add 2% to the weight of the bike, and give 210% longer service life.
Better yet, a thicker brake track makes the rim stronger until the brake track wears thin. For example, the 1.8 mm brake track is about 30% thicker than the stock 1.4 mm rim, but adds 8.5% to the rim weight — and is about 60% stronger against some pothole dents and makes some common rim shapes about 15% stronger against overload/potato chip. Put another way, if you buy a new light thin-wall rim every year (or however often they wear out), you spend much of your time riding with a weak rim. If you buy a thick-wall rim, you spend almost all your time riding witih a stronger and more reliable rim; it is only near the end that it is as weak as the thin/light rim.
Some rims use a ceramic coating on the brake track. The ceramic is much more abrasion-resistant than the base aluminum, so this seems like a good way to avoid rim wear.
Some advantages include:
Some dsadvantages include:
The price premium is hard to "sell", in the sense that it adds a significant cost and slight weight, but the main benefits are somewhat "hidden" -- better wet braking and possibly longer rim life.
Extending the rim life can be a good investment. Suppose a not-coated rim is US$60, a coated rim is US$90, and labor to replace the rim is US$40. A plain rim is thus US$(60 + 40) = US$100, while a coated rim is US$(90 + 40) = US$130. If the ceramic-coated rim lasts at least 130/100 = 1.3 times as long as a not-coated rim, the long-term cost is equal; and if it lasts more than 1.3 times as long, the ceramic rim is cheaper.
Relatively few rims are available with ceramic coating. If you need a specific rim shape or spoke count, you may not be able to buy it with a ceramic brake track.
To sum up, a ceramic coating works well for riders who do not encounter hot braking; initial purchase price is higher; and chipping of the ceramic coating is common enough to make it hard to predict rim life with high certainty. A further practical problem is many common rim sizes/shapes are simply not available with a ceramic coating.
Disc/disk brakes are one alternative to rim brakes and can offer
A rim is closer to the ground/puddles/etc. than a disk brake, so rim braking is more variable than disk brakes. If predictable braking is the main issue, a rim brake may always be at a disadvantage.
Disc/disk brakes are heavier than rim brakes if you want the same heat ("hill") capacity. Some light but low-capacity rotors lead to complete brake failure in ordinary hilly riding; high-capacity rotors can avoid most problems but add over 100 g per wheel compared to the light rotors. Disc/disk brakes are also heavier than a rim brake for the same wheel strength; and even heavier if you want both as-good heat capacity and as-good wheel strength. Note the weight of a bicycle with disc/disk brake includes the caliper and rotor (the brake itself), heavier wheels, heavier cable and housing, and heavier fork and frame.[*]
[*] Hubs are heavier due to the brake mount. Brake loads are transmitted through the spokes in a way that slightly weakens the wheel in hard braking; to equal the wheel strength with a rim brake, heavier rims and/or spokes are needed. Front wheels are dished to make room for the brake, which further weakens the wheel; the front thus needs yet-heavier rims/spokes to get back to the rim brake wheel strength. The disc/disk brake cable and housing need to reach further, so weight is higher. Disc/disk brake mounts are similar in weight to rim brakes, but the disc/disk brake operates at higher leverage, so under equal braking power puts more force in the frame/fork, and in turn needs a heavier frame/fork to reach the same level of durability.
Disc/disk brakes are almost always more expensive to purchase. The long-term cost of disc/disk brakes may be less if you frequently wear out a rim's brake track — replacement rotors are often cheaper than replacement rims, and the labor cost to replace a rotor is often much less than the labor cost to replace a rim. Rim life with a disk brake is still finite: rims get bashed, and the spoke bed eventually cracks. However, riders with rim brakes in wet conditions often see 5,000 km service life, while riders with rim brakes in dry conditions often see 50,000 km service life. 50,000 kms is probably a fair estimate of rim life in disk brake service.
Disc/disk brakes are relatively easy to knock out of true and sometimes warp permanently (cannot be straightened). For many riders, bent rotors are a "parking" issue, e.g., they get bent when the bicycle is parked at a rack and knocked by another bicycle being parked. Disk brakes are immune to minor rim damage such as hitting a pot-hole or rock; whereas minor rim damage can make a rim brake un-usable. For many riders, such rim damage is less common than rotor knocks, but when it happens, is often much more expensive to repair.
Heat dissipation of a disk brake is limited by the rotor size, thickness, and by the pad materials. Heat dissipation of a rim brake is limited by the rim surface area. For many riders, widely-available disk brakes offer plenty of dissipation. For riders who need high dissipation (e.g., heavy riders in hills), a rim brake with deep-section "aero" rim offers more total heat dissipation than any available disk brakes. This limit of disk brakes is a market problem, not an engineering problem: makers could build disk brakes with more heat dissipation... but they don't.
For some riders and some riding, parts may be harder to find for a disc/disk brake than for rim brakes. Various rim brakes have been fairly standard for many decades, meaning "make do" cables, brakes, and brake parts can be scrounged from a parts heap or found at hardware stores. Since disk brakes are much newer, parts (both in heaps and stores) are much less common.
A crash that tears out a hydraulic hose is hard to service in the field, and damaged hoses are a not-rare problem. Cables somewhat more likely to survive a crash, and easier to fix in the field. Both disc/disk and rim brakes are available in both hydraulic and cable versions, but hydraulic rim brakes are rare, while hydraulic disc/disk brakes are common.
Disc/disk brakes can be a good way for many riders to avoid rim wear-out, and offer predictable wet braking. At the same time, they have some costs (money, wheel strength, heat capacity, weight). For some riders, a rim brake or rim brake and "heavy" rim are better than a disc/disk brake.
Fast, light, cheap: pick all three — just as soon as somebody offers you a rim with a thick brake track.