Learning how to build a bike: Is it better to buy a used bike and disassemble it?

I would recommend building up a bike from scratch. Disassembling a super cheap bike isn't going to teach you much as the standards and interfaces used on those are going to be quite different from a multi thousand dollar bike's. A good place to start is all the "dream build" videos on YouTube. You can see much of the process there, albeit with highly experienced professional mechanics doing the work.

You'll also need to consider the cost of tools. There are some jobs better left for the local bike shop (installing pressed components, facing and chasing), but the smaller things you can realistically do yourself are going to require some specific tools.

A used bike won’t teach you how to build a bike from the ground up but they are often in need of serious maintenance.

On a worn down and beaten bicycle you’ll likely have to replace:

• Chain (sometimes cassette/freewheel as well)
• Brake pads
• Tires
• Seat
• Cables
• Pedals

You’ll probably also have to re-grease some bearings, tighten all screws, true the wheels, set up the seating position to fit you and so on.

Once you’ve done all that you’ll have gained a lot of experience and a perfectly fine bicycle for little money (assuming you pay an appropriate amount for a bike which needs all those components replaced, i.e. very little).

If you want to bring a used bicycle back to life I’d go for a worn (i.e. relatively high mileage) but relatively high quality bike. Can be a bit older as well. Don’t go for bikes which were cheap to begin with. Look for rust, severe rust can weaken the frame and makes getting the seatpost or quill stem out a pain.

Other answers makes a lot of sense to me, and I post this one just to expose an other possibility you may want to consider.

In my opinion (and experience), if you want to learn something, you may want not to try a single approach, but try to have the widest background as possible. @Argenti Apparatus' answer says:

The disadvantage of working on an old, cheap bike is that the components will be old designs and standards which does not give you experience with more modern stuff.

which is true. Yet, if you want to have as much experience as possible, you may want to work on older bikes as well as on newer ones.

The second point of learning whatever is that it is much more efficient (i.e. you learn more stuff for the same amount of time) if you are not alone. Again, Argenti Apparatus suggested watching videos, and I can't emphasize enough that this helps (in particular Park Tool's videos are great). However, what helps even more is to actually learn from other (more experienced) bike guys, to which you can ask questions, who can actually show you stuff on the actual bike your working on.

Finally, I'd emphasise that, sure, bikes improved over times and older components are different from the newer ones, but essentially, bikes are still the same than a century ago (imo, the last fundamental improvement is the derailleur): you'll have to learn how to adjust a bearing, how to true a wheel, how to adjust the saddle, etc. My point is that there is a lot of things that are in common to all bikes.

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So, to summarize, in my opinion, what you may search first is: (i) people to help you; (ii) work on as much different bikes as possible^[1]; (iii) start working on what is universal on a bike, and specialize (toward newer/older/whatever kind you prefer) when you have this common background.

You may say: "sure, but where can I do that?". The good news is that there are a lot of community workshop all around the world. In those workshop, you'll meet a lot of benevolent people that are waiting to share their passion of building bikes, and most of the time, they are very good teachers^[2], which will not only teach you the mechanics, but also the history of biking, the culture that goes with it, etc... Usually, they have a lot of bikes they can afford to lose in case you accidentally destroy some parts when working on the bike.

Also, having a dedicated (shared) workshop, with other people to share discussions, meals and drinks improves your motivation a lot!

You may want to peruse BikeCollectives.org if you are in the US, Heureux Cyclage if you are in (western) Europe, or otherwise discuss with a local bike shop, they may indicate you where the closest workshop is.

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_{^[1] One may say that it would be quicker to start working on your (future) bike directly, instead of studying different bikes before actually starting working on your bike. To that I answer that (i) in my experience, I have seen no evidence of being actually quicker (nor to be slower); (ii) if your goal is to learn, your main goal should be to learn and not to build a bike. Also, since this is a summer project for you, you have plenty of time (once you have a bit of experience, building your actual bike is a matter of days if not hours).}

_{^[2] To illustrate the quality of what you learn, let take an example. I don't know for other countries, but in France where I live, there is an "official" professional training (CNPC) to become a professional bike builder/maintainer. The fact is that what you learn in this training is (i) more to sell bikes than to repair/build one; (ii) the maintenance is pretty basic (you don't necessarily even learn how to patch a flat, since what professional do most of the time is to change the tube); (iii) you learn the most when you actually do your internship in an actual workshop.}

Disassembling a lot of bikes is a great way to learn how to use bike tools. This is exactly how I learned to build bikes. The place I learned is a non-profit organization that takes in a lot of used bikes, and send them over to other countries.

Every week (before covid-19), this org hosts an event to process donated bikes. Some of them have damaged frame with good parts so someone has to take parts off from bike. Stripping parts taught me how to use the bike tools.

I now have all the tools, from crown remover, bearing press, frame alignment gauge, etc. Without the opportunities to take bikes apart, I wouldn't have learned it.

So, if possible, look for an organization that deals with a lot of bikes. People there can teach you how to use bike tools. You can learn about bikes while volunteering. I highly recommend it. If there aren't any, at least look for bike coop and they can give you some hands on experience as well.

What are your thoughts on the subject? Should I buy a used and disassemble it, or start from scratch?

Buy a reasonable condition bike, ride it, and maintain it yourself. After few years, you will know what you like on the bike and what you don't like. Thus, you will be able to select the best components for you for your eventual built-from-scratch bike.

Not only that, but you will also know how to maintain every component of your bike. To do that, you will start accumulating tools. The same tools can be used to build a bike from components.

I did that. I bought a cheap straight bar bike, rode it and maintained it myself for few years. At some stage, I decided I want a drop bar bike. I converted the straight bar bike to drop bars by buying the necessary components.

Eventually, I decided that a straight bar bike converted to drop bars is not fully ideal (it was the weirdest bike I have ever seen; a bike with drop bar and suspension fork at the same time), so I built a touring bike from the Surly Long Haul Trucker frame. Because I had ridden the bike for many years, I was aware of what kinds of components are ideal for my use.

I even built the wheels for the bike myself. The only part of the build I left for a bike shop to do was the installation of the headset because it is a one-time operation requiring specialized tools.

If you don't know anything about bike maintenance, now is not the time to build a bike from scratch.

There are lots of good answers here, but I wanted to add a few points to consider that go beyond the comment sections.

Keep in mind that bikes follow a law of diminishing returns: the difference between a €100 and a €400 bike is massive; the difference between a €400 bike and a €800 bike is noticeable but subtle; beyond that, most casual cyclists are not going to notice much difference at all.

So, the answer by @juhist is very good advice: get a relatively modern used bike for a reasonable price. Doing so avoids 2 major pitfalls:

1. by buying a reasonable condition bike up front, you can be fairly sure that all the parts are still widely used and are meant to work together on that particular bike.
2. you avoid overspending on a bike that may be more than you really need, so you can invest the money in tools later on if you choose to do so.

Speaking of tools, when you start looking into that you will probably be overwhelmed by the number of specialized tools and obscene price tags. There is no need to invest a ton of money to get everything necessary to completely tear down, service, and reassemble a modern bike. Many of the exotic tools can be substituted with generic hand tools which can be used for more than just bike maintenance: metric socket set, adjustable spanners, various sized locking pliers and channel locks, screw drivers, rubber hammer, chain saw (for when things get rough). Optional but highly recommended is a torque wrench (the beam type is cheaper but less accurate, whereas the "click" type costs more and is more reliable across its whole functional range). If you are clever and a bit crafty, you can rig up some things out of standard lumber and simple hardware (no need to buy a $200 repair stand or a $150 wheel truing stand). The only specialized tools that are really necessary are: bottom bracket and cassette lockring tool sockets, and a mini chain tool (chain tool is even optional, but they are cheap and will save you a level of frustration that would have you looking for the chain saw...).

And buy a good lock.