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After reading this question: Cheap MTB with future upgrades v/s costly fully ready MTB, I myself had a question.

What are the most fundamental parts of a bicycle? What are the things which we have to look for in a bicycle prior to buying which needs to/can be changed over a period of time but cannot be upgraded? I know we can change derailers, drivetrains, and stuff but is there a part in a Bicycle that cannot be changed/upgraded over a period of time?

I am not the best to phrase a question so to further explain my question, I'll try to provide few examples.

Example #1: There are two cycles - Cycle A's frame is made of Steel and Cycle B's frame is made of Aluminium. We know that frames made of Aluminum are generally stiffer than steel which saves few fractions of a second count. Let's say I bought a Steel Framed Bicycle and want to make it more track-oriented and save a few microseconds off. I cannot have a stiffer frame because I cannot upgrade the material of the frame. If only someone would have told me that I should have invested more on a bicycle which had an Aluminium frame because I cannot change it.

Example #2: I want to install a larger size of rims from 26" to 27.5" because Larger wheels hold more speed than smaller wheels due to rotational inertia and more grip because the larger wheel has more rubber touching the ground. Let's say my suspension is not allowing me to fit the larger wheels which would not allow me to harness the advantage of more speed and grip. If only someone told me to get a cycle with bigger wheels.

Example #3: I want to upgrade the derailer from 24 to 27. I know it's possible, thanks to Bicycle Stack Exchange, I got good guidance for the upgrade and got lucky with the fact that my cycle allowed me to upgrade the derailer. If my cycle didn't have space for a 27-speed crank then it would not have been possible for me to upgrade. Let's say I bought a 21-speed bicycle because it's cheaper then I mostly wouldn't have had any space for a 27-speed crank or even a 24. I would have been stuck with my 21-speed until I buy a new cycle.

I am so sorry if you find these examples very silly. I made up these examples and personally I don't have both the issues. My bicycle is made of Aluminium, has space for a 27.5" upgrade and has a newly upgraded 27-speed derailer, crank, and shifters but I am a noob when it comes to cycling and who genuinely wants to ask this question. Are there any other factors such as these which I should consider before buying a new cycle so that I won't regret it?

I am trying to curate a list of these factors from your expertise which needs to be considered before buying a bicycle to prevent anyone from making a mistake that cannot be rectified in the future. I know I did a bad job explaining the question so edits are happily welcomed.

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    @ojs I built a bike of Theseus once: Took my bike and a new frame and moved everything I could over to the new frame. Over the years, everything else was also replaced bit by bit. In the end, I had only one or two individual bolts and nuts remaining from the original bike. So, I would whole-heartedly answer this question with a clear "None!" – cmaster - reinstate monica Sep 28 at 7:22
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    My local Decathlon offers "replace frame" service, so it's not like you really can't. It may just be more expensive than buying the right one in the firs place. – Mołot Sep 28 at 7:55
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    I just have a bike where the only original parts left are frame and headset. I've been thinking about replacing the frame, probably with one that isn't compatible with the old headset, but the original one works quite okay. – ojs Sep 28 at 8:14
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    A light steel frame could easily be quicker than an entry-level aluminium frame; wheel size doesn't relate that closely to speed, and more gears aren't necessarily an upgrade - a lot of questions recently have been about converting something like 24 (3x8) gears to 1x11 as an upgrade. That said there's an interesting answer to be written (though probably not by me) about which parts are unreasonably expensive to upgrade – Chris H Sep 28 at 8:38
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Certain parts of a bicycle are more easily upgradeable than others, while other types of upgrades require special tools, much more money investments or are limited to whatever standards are used in its design. For some parts, the opportunity of an upgrade coincides with the older part being worn out; in other cases, the replacement is not warranted by this fact.

Let's go from the easiest replacement to the hardest ones. I'll organize them in tiers.

  1. Non-essential stuff that does not affect key function of a bicycle: lights, bells, fenders, phone holders, cycling computers etc. These can be replaced very cheaply and easily.

  2. Contact points: pedals, handlebar grips/tape, saddle. These parts are often a part of one's personal preferences and get swapped immediately after a purchase of a new bike. Many bikes are sold without pedals for this reason, and some people argue the same should apply to saddles and grips.

  3. Regular wear and tear items: chain, tires, tubes, brake pads, cables, bottom brackets. One can upgrade to a lighter chain when replacing a worn one. Tires are often subject to personal preferences the same way as pedals are. One can prefer lighter or sturdier, narrower or wider, smoother or knobbier, all black or tan wall etc. tires depending on what and how they ride. On tubes, one can buy lighter or sturdier ones, or even go tubeless as an upgrade. The same applies to pads and cables: there are aftermarket products which are better in performance/longevity.

  4. Hard wear and tear items: cassettes and chainrings, braking discs. It takes more time, but ultimately, these parts wear out and are bound to be replaced/upgraded. Here compatibility issues are becoming more prominent (even though they already play role in the earlier tiers), and not everything is clearly compatible with everything else. Some parts are likely to be of limited availability. As an example, replacing an 8-speed cassette with 11-speed might bring you to the next tier of costs/troubles, as it will drag everything drivetrain-related with it.

  5. Non-wear components that are at higher risk of breaking: derailleurs, brake calipers and levers, shifters, wheels. Normally these parts do not get worn per se, but they run higher risks of being damaged in minor crashes. Replacement options are often limited by the bicycle design. E.g.: there is no easy way to switch from QR-skewers to through axle hubs unless the frame supports it. Similarly, no safe way to put disc brakes on rim-braked bike.

  6. Non-wear components that are at low risk of breaking: handlebars, cranks, forks, frames, headsets, seatposts. Unless you have a specially nasty hard crash, these parts do not wear out. Replacing them is always justified by the owner's desire to "upgrade" something in any meaning of the word (even if that means changing the color). At this point, the compatibility is the main issue: there are dozens of seatpost diameters and lengths, a hundred of headset dimension combinations, many frame sizes and innumerable variations in geometry, several handlebar diameters and lengths etc. etc. Unless something is explicitly compatible in every aspect of its parameters, it is not.

The presented classification is not universal of course. Certain items can be moved up or down a bit. For example, wheels with rim brake surfaces are rather at level 3 (i.e., eventually worn out). For some people, bottom brackets live for years (thus being on tier 3 or 4), while others replace them every season (tier 2), and so on. I am likely to have forgotten a lot of essential parts as well.

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    5. These are not always justified by a desire to upgrade: After breaking two handlebars and three forks, I've taken to replacing these safety critical parts routinely after 10000km. These items don't wear, but they may develop fatigue cracks with sufficient probability over time that I classify them as untrustworthy. – cmaster - reinstate monica Sep 28 at 13:53
  • It should be noted that high quality fenders can be in $100 range. The width of the chosen fenders can be fine for the current tires, but may limit future tire size. – Saaru Lindestøkke Sep 29 at 16:08
  • Low risk of breaking? The fact that you don't break these components at a high rate doesn't mean that riders who put lots of hard miles have the same experience. pardo.net/bike/pic/fail-001/000.html – juhist Sep 29 at 18:03
  • @juhist So what? Anything can fail. In particular after a crash. But Grigory is right, those components do not just wear out. – Vladimir F Sep 29 at 18:05
  • Rear derailleurs (well, the jockey wheels at least) definitely do wear out. And it took 50 000 km, but I did have an aluminum handlebar fail on me from accumulated fatigue. – DavidW Sep 29 at 18:12
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Any and every part of a bicycle can be replaced, including the frame. (Many people would probably regard a different frame as a different bicycle though.)

You ask whether there are components that 'cannot be upgraded'. I think you are misusing that word. Anything can be upgraded, i.e. replaced with a equivalent but better quality version.

I think what you mean is that you cannot change the basic design of the bicycle. As you note in one of your examples frames are designed for a specific wheel rim diameter. You cannot put in a larger (or smaller) wheel because it will not fit or you will mess up the steering geometry.

By the way, your assertions about aluminium frames being stiffer than steel, larger wheels holding more speed and larger diameter tires having a larger ground contact patch are all untrue.

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  • I am sorry if my assertions are false. I personally am not an expert when it comes to bikes and stuff. Just a curious guy trying to learn new things. With that said, I have found a few pages which back my claim. Can you please help me to understand where I made mistakes on those examples? I think those examples were fundamental and having any conceptual error on them is a bad thing from my side. – TikolaNesla Sep 28 at 14:29
  • Sources to back my claim: Aluminum Frames stiffer than Steel: missionbicycle.com/blog/oversimplified-steel-vs-aluminum Larger Wheels gives relatively more Speed and Larger Ground Contact: rei.com/blog/cycle/advantages-riding-27-5-wheels – TikolaNesla Sep 28 at 14:31
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    "Many people would probably regard a different frame as a different bicycle though." Heh, reminds me of the classic bit in Only Fools and Horses where Trigger (a road sweeper) gets an award for saving Council money by using the same broom for 20 years. What he didn't think to mention to the Council was that the broom had had 17 new handles and 14 new heads. ;-) youtube.com/watch?v=56yN2zHtofM – T.J. Crowder Sep 28 at 15:16
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    @KarthikSrivijay: notice the word "oversimplified" in the very link you posted. Everything is tradeoffs. Aluminum can be faster in some situations, steel in others. Stiffer does not necessarily mean faster. And if larger wheels were always faster, without any tradeoffs, racers would be riding penny farthings. – whatsisname Sep 28 at 18:19
  • @whatsisname To be fair, research is being conducted into 32” and 36” wheel sizes! I don’t know how’d they’d work for smaller riders though. – MaplePanda Sep 29 at 17:39
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The bicycle's serial number is usually found under the bottom bracket. if you regard the serial number as equivalent to the VIN number on a car, then changing the frame makes it a new bike.

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When you speak about "upgraded over time" I think you mostly mean "ages well". Every time I buy a bicycle I want to make sure that it stays up to date for as much time as possible. And I try to sell it before the technology behind it gets out of date. The scope narrows down to the frame, pretty much, but components are also important, as they can make upgrade financially unreasonable.

Practical tips:

  • Avoid proprietary stuff, old or unique standards, and other unique essential components that may leave you with a frame that has no spare parts, or nobody wants it due to bad reputation of some parts. Examples: older Lefty forks, custom seatposts, frame materials, custom headsets, Trek's BB90 that they just abandoned, older axle widths, brake types, and even "not cool" geometry and "old" looks. There are many cheap steel frames on eBay, but if you buy them you'll find that there are no compatible components available anymore.
  • Don't get into the components trap. Some component setups may lead to a very expensive upgrades if/when needed. There could be no smooth way to upgrade to the latest trends, but rather a cliff. For example, older Shimano DI2 have no compatibility with newer versions, so potentially you'll need to replace every single thing. Sram AXS release made all the older Sram components totally unsexy, I see people having problems selling bikes with Sram Red, and there is no way to upgrade without changing EVERYTHING, up to a freehub body.

Real life practical story:

  • I decided to sell my expensive rim brake carbon superbike with 2x11 Ultegra 6800 before it's too late and price goes down. Rim brakes are losing demand drastically on the 2nd hand market, plus Shimano is about to release 12 speed for road, so while Ultegra 6800 doesn't look old against newer R8000 Ultegra today, that bike would look truly ancient when 2x12 comes in, even being 3 years old flagship bike!
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    So upgraded/replacement rim brakes are going to become really cheap? That's good news! – Jeremy Boden Oct 14 at 13:17
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The problem I see with this line of questioning is that it assumes the existence of some ideal perfect bike that has every conceivable upgrade on it. Then you are measuring a bike you're considering buying against that ideal with the idea that you could upgrade part-by-part until you have the ideal. That just doesn't exist because bikes are built for different purposes (road, cross country, downhill, etc) and have different design and cost considerations. Also, the ideal changes over time as new innovations come on the market. Some of those innovations are backwards compatible and others are not. As others have said, ANY part CAN be upgraded to a better quality one. However, there are compatibility issues that will always come up (wheel size, bottom bracket and headset standards, etc). You can never have a bike that allows for ALL possible upgrades.

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I'm just going to point out some minor things the other answers have missed.

The original question correctly pointed that you may be unable to change the wheel size, and you certainly can't go from small to big wheels because there won't be room. You will also change the handling in ways you didn't expect. Many gravel bikes are designed to accept 700c and 650B wheels (these being equivalent to 29" and 27.5" in the MTB world). With 650B wheels, we would run larger tires. If your bike isn't designed to take 650B wheels with larger tires, you are likely but not guaranteed to run into clearance issues. You would certainly not be able to do this on a road bike. I have a Parlee Chebacco, which is designed for 700c alone, but I have heard of one person fitting 650B wheels with relatively narrow tires (which, in my view, defeats the point).

Related to the above, frames all have a maximum tire size they can fit. Many older road bikes from the 1990s and 2000s may have been designed for 23mm to 25mm tires. Road bikes are trending towards 28mm tires right now, because they are more comfortable and no slower than narrower tires. However, older bikes may be unable to upgrade to these. Similar issues might exist for some gravel bikes; mine is designed to accept 40mm tires, but the trend is towards 45mm or wider.

You can't change from rim brakes to disc brakes (although a handful of frames may have been designed to accept both rim and disc brakes). You can change the fork, which gets you one disc brake, but the tubing at the rear of the frame would not have been designed to accept the stresses of disc braking. Thus, even if you were able to drill disc brake mounts, your frame would probably be under more stress than it was designed for, and it may be too flexy, and it might even fail. (NB: drilling the frame also creates stress risers, which can lead to later failures - and under the stresses of disc braking, they very likely would do this.)

Internal cable routing is currently the dominant trend for drop bar bikes. Bikes designed for external routing can't generally be upgraded to internal routing. One possible exception is that if you're talking about electronic shifting, it's possible to drill steel and titanium bikes for internal electronic wiring. As noted earlier, drilling poses some risk due to stress risers, so this should be done by a professional, and it's at the rider's risk. (Of course, there's also wireless electronic shifting.)

In general, the standards for mounting components to frames sometimes change. On road bikes, we recently went from quick releases to thru axles. On mountain bikes, thru axles have been dominant for longer, but MTB frames are going to Boost spacing, which has wider spacing for both front and rear hubs. Head tube diameters are another example of a change like this (my custom bike is designed around a 1" steerer, most bikes in that era used 1-1/8" straight steerers, and even wider and sometimes tapered steerers are being used now). This should be distinguished from component standards changing (e.g. freehub spline design, like Shimano moving from Hyperglide to microspline, or Campagnolo changing its freehub design from 8s to 9s); with those, you just have to buy more stuff to make an upgrade.

The question is also inseparable from the issue of how economical it is to upgrade parts. Because bike manufacturers buy components in bulk, they get much better deals than consumers do. Past a certain point, it doesn't make economic sense to upgrade a bike, compared to selling it and buying a new one. Naturally, this may be irrelevant for frames with a lot of sentimental value, e.g. custom frames. Also, as pointed out, it is physically possible to change everything out barring incompatibilities above, including the frame. If you do the latter, you do raise an ontological question of whether you have an entirely new bike or not (I suspect most people would say yes, as alluded to in other answers).

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    A road bike with 650B may be good for small riders. I am 174 cm tall and I still do clip my foot on the wheel. One easily gets used to that, but there are riders who are 20 cm smaller than I. – Vladimir F Sep 29 at 18:12
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    @VladimirF agreed, but actually there's also the 650c standard to consider (popularized in triathlon at first, for reasons that turned out not to be substantively correct). Additionally, some bike manufacturers may, wisely or not, spec 26" wheels for some sizes. I recall Gunnar Bikes did, and Serotta (the newly relaunched Serotta, different corporation from the defunct one but the same founder) intends to spec them as well. – Weiwen Ng Sep 29 at 19:53
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I think you'll find that the second most difficult component of a bicycle to change is a wheel hub (unless changing the entire wheel obviously). This means if you are interested in for example dynamo hub lighting system, you should try to find it as stock hub in an existing bicycle. The most difficult component of a bicycle to change is the frame. Changing a frame might affect the possibility of using certain brake types (short reach caliper brake vs long reach caliper brake vs cantilever/V-brake vs different mounts of disc brakes). Also different frames have different axle mounts (normal axle and what over-locknut-distance? vs thru axle and if thru axle then what diameter and what length?). The front derailleur mount might also differ. Very likely you cannot use the same seatpost if you buy a new frame. Supported headsets also differ between frames. Bottom bracket mounts also vary.

Example #1: There are two cycles - Cycle A's frame is made of Steel and Cycle B's frame is made of Aluminium. We know that frames made of Aluminum are generally stiffer than steel which saves few fractions of a second count [citation needed]. Let's say I bought a Steel Framed Bicycle and want to make it more track-oriented and save a few microseconds off. I cannot have a stiffer frame because I cannot upgrade the material of the frame. If only someone would have told me that I should have invested more on a bicycle which had an Aluminium frame because I cannot change it.

You seem to assume lack of stiffness equals to power loss. Lack of stiffness doesn't equal to power loss.

Heavy touring cyclists hauling their heavy loads with heavy steel framed bicycles don't complain about lack of stiffness. In contrast, a typical racer plus racing bike weighs less than half than the touring rig plus heavy rider plus load.

Example #2: I want to install a larger size of rims from 26" to 27.5" because Larger wheels hold more speed than smaller wheels due to rotational inertia and more grip because the larger wheel has more rubber touching the ground.

I would say it's the 70 kg rider holding speed, not the 2 kilograms of tire, tube and rim at the wheel perimeters. Yes, rotating mass near the edge of the wheel is counted twice but it's still far less than the rider.

Also do note that rotating mass is counted twice regardless of the wheel circumference. You seem to assume larger wheels would count the rotating mass with a larger factor which isn't true. Larger wheels rotate at slower angular rate which cancels their larger moment of inertia.

Also, most people are interested in reducing the rotating mass because part of the rotational energy goes to braking. So it's more bad to have lots of rotating mass than it's to have little rotating mass.

Also wheel size doesn't enter into the friction coefficient.

Example #3: I want to upgrade the derailer from 24 to 27. I know it's possible, thanks to Bicycle Stack Exchange, I got good guidance for the upgrade and got lucky with the fact that my cycle allowed me to upgrade the derailer. If my cycle didn't have space for a 27-speed crank then it would not have been possible for me to upgrade. Let's say I bought a 21-speed bicycle because it's cheaper then I mostly wouldn't have had any space for a 27-speed crank or even a 24. I would have been stuck with my 21-speed until I buy a new cycle.

The rear derailleur isn't speed specific in the speeds of interest.

(Of course, some more recent designs have different cable pull ratios but even then it isn't only the speed count that tells the cable pull ratio. For example, Tiagra 10-speed 4700 uses the same cable pull ratio as Shimano 11-speed road groupsets. The older 10-speed groupsets have a different cable pull ratio. But this doesn't interest you as you are interested in 8 and 9 speed components which have the same cable pull ratio apart from old Dura-Ace 8-speed.)

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