What is the durability of a bicycle wheel?

First, we need to define what the durability of a wheelbuild and the failure of a wheel mean. For example, bicycle tires easily puncture in few thousand kilometers. This does not however mean the wheel failed. Similarly, a pressure washer can rust the hub bearings in a wheel that hasn't been used in riding at all. This, too, is not considered a wheel failure.

It is interesting to investigate only failure modes that only relate to the structural properties of the wheel, are not a user error or a case of natural wear and require wheelbuilding skills to fix. A large fraction of cyclists maintain the bicycle themselves and are able to do maintenance operations such as hub cun and cone bearing repacking, but many of those cyclists do not have wheelbuilding skills.

The failure modes of a bicycle wheel include:

• The rim starts to crack, usually around the spoke holes
• Hub flage fails, usually due to radial spoke lacing
• The wheel becomes untrue
• The wheel suffers a total loss of tension in the spokes
• A spoke breaks
• A nipple breaks

These are not considered wheel failure:

• Wearing out of rim brake track (a replacement rim obviously requires wheelbuilding skills to install but this is a case of natural wear and not a failure)
• Rim damage due to encountering an obstacle at too great a speed (user error)
• Puncture or any other failure in the tire system
• Hub axle failure
• Hub bearing failure
• Hub freewheel failure
• Brake disc failure
• Brake disc wearing out
• Cassette failure
• Chain jamming between the cassette and the spokes, damaging the spokes (user error)

How long does a wheel last then? The answer is that it depends. Anyone who weighs over 100 kg and has used a poorly built wheel knows perfectly well that even a 36-spoke wheel that does not have adequate spoke tension reliably suffers a total loss of tension in the spokes in as little kilometers as 300 km. This total loss of tension in the spokes is more likely on disc brake front wheels (due to disc brakes alternately increasing and decreasing the spoke tensions a lot during braking) and asymmetric wheels such as rear wheels on most derailleur bicycles that do not have an asymmetric rim and are not used on an asymmetric frame (such as Cannondale AI frame).

On the other hand, we know that a well built 36 spoke wheel with crossed spoke lacing, double butted 1.8mm/1.6mm/1.8mm stainless steel DT Swiss spokes, brass nipples, aluminum hubs (Campagnolo Record) and aluminum rims (Mavic MA-2) lasts over 200 000 miles (300 000 kilometers) requiring one to only replace the rim when its brake tracks have been worn away and occasional bearing service. This has been observed by the author of The Bicycle Wheel, Jobst Brandt (R.I.P), who rode his wheels over 300 000 km.

So based on this wheel longevity, the difference between well-built wheel and poorly built wheel, varies by at least three orders of magnitude (from 300 km to 300 000 km).

However, these observation leave a few questions:

• The brakes of Jobst Brandt wore out several brake tracks in rims. In theory, swapping the rim could have improved wheel longevity. For example, a rim used 100 000 km could develop cracks around spoke holes whereas swapping a rim every 50 000 km due to brake track wear could lead to the wheels lasting infinitely long
• The fact that wheel lasts "at least 300 000 km" does not tell what the actual upper limit for wheel longevity is.

To tackle these problems, several observations can be made.

Firstly, it has been observed that rims lacking double eyelets can usually be ridden at least two thousand km until the rim cracks around the spoke holes. A rim experiences a load cycle in approximately 2 meters (rolling circumference). So a wheel lacking double eyelets can withstand at least million load cycles.

According to aluminum fatigue curve, the tested aluminum alloy fatigues in million cycles with 25 ksi load.

A double eyelet rim distributes the spoke load to both walls of a double wall rim, so this halves the stresses in the rim. So the load in a double eyelet rim is half of the load in a rim lacking double eyelets, or 12.5 ksi. The fatigue curve in Wikipedia does not extend so far, but we can at least see from the curve that a double eyelet rim with 12.5 ksi load ought to last at least billion cycles, or two million kilometers.

Secondly, it has been observed that hubs using a low count of spokes (example: 24) can withstand spoke tension for at least 20 000 km, or ten million cycles. The fatigue curve shows a load of 20 ksi at ten million cycles. A 36-spoke wheel has two thirds of the spoke tension of a 24-spoke wheel and the dynamic load is reduced accordingly too, so a 36-spoke wheel has at most around 13.33 ksi load on the hub flange. This gives a fatigue life of more than a billion cycles too, or at least two million kilometers.

Also some people use radial spoking on the front wheel, a practice that does not have a good reason apart from fashion. Reputable hub manufacturers such as Shimano typically forbid this, but it has been observed that aluminum hub flanges do not immediately fail from radial spoking but rather the failure happens over time. A radially spoked wheel has all spokes pulling with great force away from the flange, whereas in a cross-3 wheel the spoke tension at the hub flanges mostly cancels away. A cross-3 36-spoke wheel has 63.8 degree flange exit angle so the flange only experiences cos(63.8*pi/180) of the force or 44% of the force. So the fact that radially spoked wheels do not immediately fail also gives credibility to the theory that hub flanges in reasonably spoked (not radially spoked) wheels can last a long time.

Spokes are not an issue in durability unless they fail very soon after wheelbuilding process. Spokes are built from steel, a material having non-zero fatigue limit at infinitely many load cycles. If a spoke has lasted million load cycles, it's going to last infinitely many load cycles.

So, theoretical analysis shows that Jobst could have ridden his wheels nearly ten times as much as he did. The only problem is, there are probably no people on this planet who have ridden two million kilometers on a bicycle during their lifetime.

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So, a summary: a wheel properly built will outlast every cyclist on this planet by lasting at least two million kilometers. Such a properly built wheel will never ever require any adjustment of spoke tension or truing, apart from the initial setting of spoke tension and truing during the wheelbuilding process.

Brake tracks can and do wear out, but this effect is eliminated by disc brakes. Also a wheel may suffer rim damage requiring a rim change, or hub bearings may wear out requiring a rebuild of the wheel due to a replacement hub not being available from which compatible bearing parts could be scavenged.

Good quality spokes that have been handled by a good wheelbuilder should never fail because they're operating inside their fatigue limit. In practice it could happen eventually anyway because no material object has perfect purity or forming, but on the scale of decades or more.

Rims vary in their fatigue resistance. There are some good reasons for this and some bad ones.

The Bicycle Wheel by Jobst Brandt is an excellent book about high-end vintage road rims. This category of rim had very high fatigue resistance because of their double eyelets and relatively (by modern standards) high-ductility materials. Since they were used with rim brakes, a premium quality rim of this type in the hands of a good wheelbuilder making good choices about spoke type and rim weight for the application would easily last the span of its brake track life without fatigue failure. Brandt details a method for arriving at an optimal spoke tension that is based on testing a rim to find the maximum tension it can sustain without structural failure once additional tension loads from braking are added (he uses finite element analysis to calculate those additional loads). This method works well for the rims he is writing about because, as he states, they will not prematurely fail in fatigue if ridden with this tension. On such a rim, his method is a near-perfect metric for optimum tension.

Modern rims use fancier alloys with fancier heat treatment. In contrast to the past, they have more strength and less ductility. This lets them be lighter and higher-performing, which are the metrics that drive cycling product development and marketing, and are factors most buyers weigh against longevity. Rims of this sort should be chosen based on the user's performance versus longevity requirements. There are not as many longevity-focused rims on the market today as there probably should be.

Rims of this type must never have Brandtian methods for determining optimal spoke tension applied to them. They will take much higher tension structurally than they can sustain in terms of fatigue resistance. It is for this reason that The Bicycle Wheel must be understood as a period piece, and that contemporary rims should be built using a manufacturer's recommended spoke tension.

Furthermore, many rims now are for disc brakes, so they will never age out from brake track wear. A disc rim dies by violence or fatigue, so cyclists need to understand that there will come a point in mileage where even an excellently built disc wheel will fatigue out, which was less or not true in the past. However, as noted above, truly longevity- oriented disc rims are lacking in the market.

In many cycling disciplines, it's expected that a rim will be destroyed from crashing, getting cased, etc long before it would fatigue out. In these applications, choosing a higher strength, lower ductility material usually makes sense in wheel longevity terms, because the strength will delay that failure as long as possible, and the fatigue resistance doesn't matter past a reasonable point. Most MTB rims are and should be made with this ethic.