Many bicycle models in the low and mid-range come with an aluminum frame and sometimes even fork if they are fully rigid and designed for the road.
My MTB commuter is also made of A6+ alloy which is why I have always been interested in reading about aluminum as a frame material.
A common question that often causes heated discussions is whether aluminum frames wear out or their susceptibility to fatigue is overblown.
Aluminum frames do wear out as a result of accumulative fatigue which reduces the stiffness and tensile strength of the material over time. It’s believed that aluminum frames can pass through a limited number of stress cycles before developing cracks and eventually failing.
Understanding Metal Endurance Limit
The endurance limit designates the highest amount of stress that a material can sustain without deformation.
If the applied force is above the endurance limit it will cause gradual or sudden structural damage. But if it’s below the endurance limit, then the material would preserve its elastic behavior and integrity.
The other two terms for endurance limit are fatigue limit and fatigue strength.
Aluminum frames wear out because aluminum does not hot have a real endurance limit.
In consequence, even small stress cycles that other materials like steel wouldn’t register as harmful accumulate over time and cause the material to fail.
This is why the fatigue strength of aluminum is presented as a number of cycles that the material can endure before deforming.
Steel vs. Aluminum
Unlike aluminum, steel has a more defined endurance limit. If the stress applied to a steel alloy is below the endurance limit, then, in theory, the material can endure an endless number of stress cycles with no structural damage.
There’s a strong correlation between steel’s tensile strength (the limit of stress that a material can sustain before breaking) and its endurance limit.
The higher the tensile strength of the alloy, the greater the endurance limit.
Usually, the endurance limit of steel is about 50% of its tensile strength.
For instance, the high tensile strength of cold finished 4130 Cr-Mo Steel is about 690 MPa (100076 PSI) and its fatigue strength is 370 MPa (53663 PSI).
Endurance Limit Steel
The graph above illustrates the endurance limit of 4130 steel. When the applied stress reaches over 370 MPa, the material gradually begins to deform.
However, before that point, there’s no damage to the material and it can theoretically survive an unlimited number of stress cycles at that intensity.
Endurance Limit Aluminum
Graph 2 illustrates the fate of aluminum under stress. If the stress doesn’t stop, then the material will continue to accumulate fatigue even when the stress levels decrease. Therefore, on a long enough scale, aluminum has no choice but to always fail.
Those Are Perfect Conditions
The stress testing above works only in perfect conditions.
In a “sterile environment”, steel may indeed have the ability to sustain an unlimited number of stress cycles under the endurance limit.
However, there have been cases when steel fails when the stress cycles reach exponential levels.
How many cycles of stress can aluminum sustain?
It depends on the actual design of the element, the temperature, and the grade of the aluminum.
The literature that I could find says that basic aluminum is expected to fail after 500 million cycles.
How Is The Industry Dealing With Aluminum’s Fatigue?
To increase the tensile strength of aluminum as well as the number of stress cycles that it can go through, engineers design the frames in a way that mitigates some of the stress, thicken the tubing significantly and reinforce the most vulnerable parts of the frame like the head tube.
How Do Fatigued Aluminum Frames Fail?
An aluminum frame that’s about to fail usually loses its stiffness and develops creaking noises and small cracks at various places. The welds, the bottom bracket area, the chainstays, and the downtube are among the most susceptible places, but failure can occur at every point.
Aluminum is also known for the possibility of unexpected and sudden failure. Or in other words, when it breaks, it does so totally. E.g., The head tube disintegrates and falls off from the rest of the frame.
Meanwhile, steel often bends before breaking and thus gives the rider more signs of what’s about to happen.
Nonetheless, steel frames can also break suddenly and completely. A common example would be the hi-ten steel frames found on many lower-level BMX bikes. When ridden aggressively, those frames fail unexpectedly and hard too.
What Is The Lifespan of An Aluminum Frame?
The lifespan of an aluminum frame depends on the following factors:
Quality and Construction
The grade of the aluminum, the construction, and the craftsmanship of the welds greatly influence the longevity of an aluminum frame.
Year of Production
The great demand for aluminum frames stimulates more sophisticated manufacturing. Newer aluminum frames tend to be better than the old ones.
Style of riding
An aluminum bicycle used for “calm” commuting meets less stress than a mountain bike machine spending a lot of time on trails.
Riding Frequency
A frame that’s ridden occasionally would need a lot longer to accumulate fatigue than one used simultaneously for commuting and racing throughout the entire year.
Weight of the rider
The heavier the rider, the higher the stress on the frame. Nonetheless, light cyclists can also impose great demand on a frame during stunts.
Weather and Temperature Shifts
Big temperature changes harm aluminum because the material isn’t very flexible. As a result, aluminum frames used throughout all seasons age faster.
Humidity matters too. Aluminum doesn’t rust, but it corrodes, especially in the presence of salt.
Due to the high number of involved factors, it is difficult to predict the lifespan of an aluminum frame.
An aluminum mountain bike used primarily for commuting may last a long time because the beefy frame is not all that stressed from regular street riding.
However, an aluminum road bike that’s ridden frequently and intensely may break in five years.
Are Aluminum Frames Weak?
No. Aluminum frames can be exceptionally strong. Hence why they are used for extreme mountain biking and dirt jumping too.
Many respectable manufacturers of aluminum frames even give a lifetime warranty. If the frame breaks, they replace it. Companies wouldn’t do this if they didn’t believe in the strength of their products.
Of course, that doesn’t mean that aluminum frames don’t disintegrate. They just do so rarely enough not to hurt the lifetime warranty business model.
Can An Aluminum Frame Be Repaired?
If the frame is fully broken (e.g., a tube has completely snapped), repairing it won’t make much sense as the repair will be expensive and won’t bring the frame to adequate strength.
If the break is smaller (e.g., a tiny crack) then the frame could be repaired through welding. However, sometimes it’s difficult to find welding shops willing to do that for three reasons:
- Aluminum frames are heat-treated during the production process. Further welding of heat-treated aluminum weakens the frame.
- Aluminum requires a special welding process and has to be heat-treated afterward.
- Some welders refuse to work on a bike frame due to possible liability issues in case the frame breaks again.
Another problem with repairing aluminum is that one crack is rarely alone. If the frame has cracked at one place, chances are that other segments are about to give up too. This reduces the incentive to repair an aluminum frame even further.
Ultimately, repairing an aluminum frame is a tricky, ungrateful, and uncertain process. Hence why many aluminum frames are abandoned upon cracking.
Frequently Asked Questions
Do aluminum frames get tired and dead?
Some people claim that after a decent amount of use (e.g., 5 years) aluminum frames get “softer” and lose their initial responsiveness. This is the so-called “dead” state.
This statement is unproven, but the available anecdotal evidence suggests that it’s not a fairy tale either.
How can the life of an aluminum frame be extended?
1. Use the bicycle as intended.
It takes a long time for a quality aluminum frame to degrade as long as the bicycle is used as originally intended.
For example, if you have an aluminum road bike, stick to road riding and don’t use it for touring or gravel.
If you have a Cross Country mountain bike, don’t ride on downhill tracks.
2. Clean the frame
Salt and water could speed up the corrosion of aluminum.
Therefore, if you want to baby an aluminum frame, you can park it in the garage during the winter and use a beater bike instead.
But if you have only one bike and want to rely on it all year, you should clean the frame regularly, especially during the winter when the roads are covered in anti-freezing substances.
3. Use frame protection
You can reduce the risk of scratching the frame by putting protection on it.
One of the most vulnerable areas is the head tube because the brake and shifter cables often peel off the paint and expose the aluminum underneath.
It’s also highly advisable to use a chainstay protector if you have a mountain bike. The job of the protector is to defend the chainstay from the chain which bounces when the terrain is uneven.
Tip: You can make your own chainstay protector by using an old inner tube.
FAQ: Can I use stickers instead of frame protection?
Stickers may look cool, but they have two downsides – they are thin and could hide a potential crack. Nonetheless, if you don’t have anything else available, stickers may be a good temporary solution. You may also use duct tape.
That’s what I did back in the day when the shifter cables were digging into the head tube of my bicycle. Another option would be to wrap the affected area in inner tubing. This technique offers substantial protection but can also hide a crack.
If you use one of the “camouflage methods”, you could occasionally “undress the area” to inspect it for cracks.
If aluminum frames accumulate fatigue and are harder to repair why are they so popular?
Aluminum is a popular frame material for the following reasons:
- Lightweight
- Affordability
- Doesn’t rust
- Suitable for mass production
Aluminum frames may accumulate fatigue, but when they’re designed and produced with attention to detail, they will need a lot of time (10+ years) to fail when used with medium intensity.
At what places do aluminum frames crack most frequently?
A frame can crack anywhere, but the most common places would be:
- The intersection where the head tube and the down tube meet.
- The bottom bracket
- The chainstays’ welds connecting to the bottom bracket.
- The places where the seat stays connect to the seat tube.
- The seat tube.
In general, the most fragile places are the welds and the inserts. It’s rare for the tubing of an aluminum frame to break in the middle, but similar scenarios are not unheard of.
Nonetheless, with extreme riding, a frame can break in very imaginative ways regardless of its material.
Can a dented aluminum frame be repaired?
Aluminum doesn’t tolerate deformation. Once dented, the frame is often too compromised to continue riding it.
Pulling out the dent will further reduce the integrity of the frame because aluminum does not like to be bent.
In most cases, the safest and probably the best option would be to just retire the frame and replace it.
Of course, if the dent is minuscule, the frame may still be usable.
If your frame is dented, and you’re unsure what to do, get an expert opinion. Don’t ride your bike without argumentative confirmation that it’s safe to do so.
Is it true that aluminum frames lose their tensile strength when welded?
Welding significantly weakens the tensile strength of aluminum. The weakened area includes the welding points and “the heat-affected area” which refers to the tubing in proximity to the welds.
To battle this problem, manufacturers use thick aluminum tubing to increase the surface area of the weld and subsequently its strength.
Also, welded aluminum frames undergo heat-treatment meant to recover the lost tensile strength and hardness during welding.
Is it wise to buy a second aluminum frame?
It depends on the history of the frame. If the frame is ancient, and you have no way of knowing how it’s been used, then it may be wiser to stay away from similar offers due to the uncertainty that they hold.
But if the frame is fairly recent (e.g., a few years old) and has no signs of previous or current failure, then it may be worth considering. If you are uncertain whether a particular offer is worth it, you can post a link to it on a forum to get another opinion.
In general, steel holds it’s value on the second-hand market better than aluminum for two main reasons:
- it doesn’t suffer from excessive fatigue;
- steel frames are more “repairable”;
Those steel properties lower the risk associated with purchasing a used frame.
What are the most common aluminum alloys used for bike frames?
The two most common alloys used in the cycling industry are 6061 and 7005.
7005 is slightly stronger than 6061, but 6061 is easier to work with and therefore commonly seen on mass-produced aluminum frames.
Why are most touring bikes made of steel rather than aluminum?
Steel is the most common material for touring bicycles because it’s strong, flexible, compliant, fatigue-resistant, and more “repairable” than aluminum. In case of failure, it’s easier to find a welder willing to work with steel than with aluminum.
Having said that, welding a steel frame is not as easy as many would like you to believe because the tubing of modern steel frames is thin and more difficult to work with.
Another reason for the touring industry to rely on steel is respect for tradition which serves as an incentive for producers to continue working the way they know instead of trying to push new frame materials that may upset the crowd.
References:
Askeland, Donald R.; Pradeep P. Phule (2003). The Science and Engineering of Materials (4th ed.). Brooks/Cole. p. 287
