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Consideration for buying a used entry-level road bike.

What is better: Aluminum vs Carbon vs Steel ?

I don't care regarding the weight +/- 2-3 kg.

The durability is important for me.] I know that it's very difficult to repair an aluminum. Also to repair carbon seems to be easier, but it's not cheap. Seems to more than a half price of the used entry level road bicycle.

The steel (chromoly), seems to be more durable and easier to repair, but it's relatively old model without modern derailleurs. And probably to find replacement might be a bit tricky.

What have I missed? Any brand recommendations?

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    Frames usually don’t break. If you have to worry about the repair-ability of the frame you are too heavy or traveling with too much luggage. You really don’t want a frame to break.
    – Michael
    Aug 22 at 12:28
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    A decent aluminum frame should last a lifetime, barring a serious crash or overloading. Aug 23 at 7:15
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    @Michael Having broken two frames and three forks, all on flat terrain, I have to disagree. Frames can and do break. And it's a big reason to prefer diamond shape over Y-shape: Failures in diamond frames are typically not catastrophic as there is always a second tube that holds the thing together. A Y-shape frame failure is always catastrophic. Aug 23 at 7:55
  • I've broken a frame before, it was old and well-used but I was not overweight or into touring. Fortunately that one was steel and the local bike shop got it welded in a day or two. Aug 23 at 7:56
  • @Michael having broken three (very different) frames in 20 years, in the city, I also disagree. The frames were from a steel street bike (64 cm), a cheap steel bike and an aluminum all terrain bike, both smaller frames. The street and the all-terrain bike were decent quality in the $800-$1000 league. The "department store" bike was smaller-framed, cheap, heavy and I got it for free from a friend. I didn't even know such frames can break. When I tried to take load off the frame when I went down the curbs by leaning on the handle bar, the bar broke. Aug 24 at 1:26

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If the question is limited to "entry-level road bike" (I assume you mean "drop bar sport bike" to be used on road), there's not so much choice: alumiminum. It's the only material you'll find on entry level bikes from reputable brands.

Carbon is considered to be a performance material, so you will only find it on mid-range bikes and up. In term of durability, carbon is also probably the material where you have the most custom parts, so on the long run, it might be difficult to find some parts (seat posts, stems, forks,...)

Steel on road bikes is a niche, quality steel bikes are at the same price as carbon bikes (typically on long distance touring and some gravel bikes). You can easily find steel on department store bikes, that should be avoided at all cost. You can also find quality steel city bikes.

About your comment on repairability: you didn't indicate the use of the bike, but I would say that repairability of the frame is dominantly a concern for long distance touring bikes (that are used with heavy loads on bad roads).

About the fatigue: the fatigue on aluminium is a "characteristic" that is known in the industry. Frames are designed against some standards, that specify the number of cycles the frame has to endure before failing. The lifespan of the frame then linked to "your use vs the standard" (and maintenance, care, conditions you are riding, your weight, the load,...).

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    Long distance touring bikes also might have the bad luck to break near Zavkhanmandal, where the local smith might handle steel but not aluminium or carbon.
    – gerrit
    Aug 23 at 7:07
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If you care about durability, not weight, you should prefer good quality (e.g. chromium molybdenum) steel. Avoid cheap gas pipe steel.

If you don't care about weight at all, it shouldn't matter whether the tubes are butted or not, but I don't believe there are a large number of non-butted chromium molybdenum steel frames. Chromium molybdenum is a sign of quality, being butted is a sign of quality, and those two occur together very often.

Aluminum has three problems:

  • After doing weld-repairs to it, you need to heat treat it, which requires stripping the paint, stripping all components and re-painting. This is so expensive it would be just cheaper to replace the entire frame with a new one, or to replace the entire bike. However, this shouldn't be your major concern since for weld-repairs to steel frames aren't easy or cheap either. Finding an experienced welder could be hard.
  • Aluminum doesn't have a fatigue limit. This is the major concern. If you construct an aluminum load-bearing structure, and subject it to loads, it will eventually fail. How quickly it fails depends on the magnitude of the loads and the number of load cycles, but it will fail. For steel, there is a limit and if all loads are below the limit, it will never fail.
  • All dimensions and threads in bicycle frames and forks have been originally designed for steel. So for example 1" or 1+1/8" head tubes -- steel was the design material. Bottom bracket or cantilever brake threads -- steel was the design material. A 1+1/8" head tube could be too weak for aluminum, and threads could be not strong enough. Even bottle cage threads on aluminum are very easy to damage. Today this has changed for head tubes -- tapered 1.5" to 1+1/8" head tube is good for aluminum and compatible with normal stems. The easiness of damaging aluminum threads still continues to be a problem.

Carbon fiber doesn't have a fatigue problem at all (i.e. not just having fatigue limit, but fatigue being entirely a non-problem). However, carbon fiber has another very large problem: it's designed only for certain types of loads. Specifically, it's not designed for crashing. Crashing on a carbon frame can put loads in undesigned directions that cause permanent damage. In some cases, the permanent damage can be entirely invisible to human eye, so there's always a danger that if there have been such undesigned loads in the history, it could suddenly fail "just riding along" hurting you severely without there being no way to beforehand see that it would fail in such a manner. For a used bike, that's a very great danger since you can never know the history. For a new bike, perhaps if you replace all carbon parts including frame and fork after every crash, perhaps that could be ok, but I don't think replacing frame and fork after every crash is good use of money.

It's true however that steel is going out of fashion, so finding a reasonably recent high quality steel bike may be getting hard. There are still brands like Surly that make steel bikes.

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  • Finding new steel road bikes isn't hard, but if you're looking 2nd hand it's another matter (except tourers - steel is really common there)
    – Chris H
    Aug 23 at 5:52
  • If you replace all parts including the frame you have built a new bike.
    – gerrit
    Aug 23 at 7:11
  • A lot of other machines are still made of steel so finding an experienced welder of steel isn't that hard in my experience. The average welder might not be qualified to rebuild a mangled frame but a cracked joint can be easy and cheap to fix. Aug 23 at 8:14
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    By "doesn't have a fatigue limit", did you mean "does have a fatigue limit"? Or did I misunderstand what "fatigue limit" means?
    – mkrieger1
    Aug 23 at 11:20
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    does have a fatigue limit = there is a value of tension that never breaks the material. doesn't have a fatigue limit = every tension applied will eventually break the material -- it might take one ride or more than the age of the universe, but theoretically it will eventually break.
    – juhist
    Aug 23 at 11:51
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The frame’s durability may be overrated as a consideration, provided the frame isn't an ultralight one. In crashes, bikes will frequently strike their handlebars or derailleurs, or the rider, before the frame. It's theoretically true that aluminum has a fatigue limit and steel doesn't, but in practical terms, an aluminum bike probably won't fall apart even if the frame gets dented - again, unless it's a very thin tubed frame.

It's also true that steel can be repaired, but depending on the damage, repair might actually be pricey relative to the cost of the frame. Often, repairs will involve taking out an entire tube and re-brazing or welding it. That said, some dents in steel can be rolled out by a framebuilder, whereas I don't think the same is true for aluminum.

Buying a carbon bike used can be tricky. It is true that it's possible to get invisible damage. Even a frame that's dropped in the wrong way can get a small crack internally, which then propagates. That said, just because you drop a carbon frame doesn't mean that it will definitely break while riding, it's just that this is possible. I would generally prefer a bigger discount if buying a used carbon bike. Also, newbies often drop their bikes in the process of learning how to handle them properly, so I would be less inclined to get a carbon bike as your first ever bike. That said, if you have a competent repairer, carbon is quite a repairable material.

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