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84 votes
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What is the foam tube on the top of bmx frame for?

It's to ensure that young boys who ride BMX bikes will be able to have children when they grow up. Fine print: More accurately, it’s to assuage the worries of the parents that are buying the BMX for ...
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  • 28.4k
77 votes

What is the difference between brake cables and derailleur cables?

Cables: The main difference that I am aware of is the diameter of the cable. Most brake cables are 1.5 or 1.6mm in diameter. Most shift cables are 1.1 or 1.2mm, galvanized shifter cables are 1.3mm. I'...
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  • 9,051
59 votes

What is the foam tube on the top of bmx frame for?

It's to give the illusion of safety.
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57 votes

Random piece of plastic

This appears to be a disc brake spacer (block). Such a spacer may be inserted in the disc brake caliper between the brake pads. When the wheels are removed, and hence the brake rotor, an inadvertent ...
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  • 8,846
34 votes

Why is buying a bike as a whole cheaper than buying separate parts and building it?

Economics of scale. For instance, when a bike manufacturer buys thousands of groupsets directly from a component manufacturer, they get a significantly better per unit price than a retailer can. ...
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  • 29.2k
23 votes

What is this device attached to the seat-tube?

It appears to be a clamp-on pump peg that got moved down out of the way either intentionally or from coming loose.
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21 votes

Is it a good idea to buy second hand bike model that has been discontinued?

Generally it's absolutely fine to buy a relatively new bike, and not worry about component availability. Bicycles are not like cars that have specific components for each model, or at best some ...
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18 votes

Why is buying a bike as a whole cheaper than buying separate parts and building it?

In addition to economies of scale, you also have to consider the difference in price sensitivity and leverage between a manufacturer and a consumer. If Shimano told Trek that they'd start paying ...
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18 votes
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What are the most basic parts of a bicycle which cannot be upgraded over a period of time?

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 ...
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18 votes
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What is this U shaped metalic piece with 2 holes?

Looks like a derailleur guard It mounts to the rear drop out on the derailleur side. It protects the derailleur from some impact damage. EDIT: 06/19/2021 The comments seem to be running strongly ...
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  • 16.1k
17 votes

How are these plastic rings used on my Shimano disk brake mount adapter?

The stop rings go over the head of the bolt. The grooves on the ring line up with grooves on the bolt head and encourage it to keep from rattling loose. They are simply snapped or pushed over the ...
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  • 9,858
17 votes
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What is this spring between the fork and the frame?

The purpose of such a steering damper is to stop the front wheel from turning when using a (two-leg) kickstand or while pushing the bike. Not much use otherwise.
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  • 2,542
17 votes
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What are these tiny pieces of metal that came with my Giant TCR Advanced 2?

These are to protect your frame from the support bolt on Shimano front derailleurs (I don't know if SRAM have the same). You can see them on page page 7 of this manual.
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16 votes
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Are Shimano 10 speeds going away?

They're trying to sell you stuff. More expensive stuff (have you looked at 11 speed consumable (chain+cassette) prices vs 10 speed?). I would not bother upgrading. As groups go to higher and higher ...
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  • 45.5k
16 votes

Can a counterfeit Shimano chain be used on an authentic Shimano drivetrain?

You can normally use chains from other legitimate vendors, be it SRAM or KMC or something else. You do not need to use Shimano™® chains only. But no-one can tell you whether your chain is of ...
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15 votes

Can a new chain significantly improve the riding experience? If yes - what else can?

The one substitution that will probably give you the most bang for your buck is better tires. High-quality tires roll with much less rolling resistance, have better grip, weigh less, and roll over ...
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  • 23.8k
15 votes
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What is this stamped metal piece that fell out of a new hydraulic shifter?

Providing the 7900 is the same, I’m sure it is but someone more qualified can chip in if it’s different. How did it fall out ? Shimano have a proper tool for removing them. Info from here Fawkes
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  • 966
14 votes

Why are ball socket allen wrenches

The ball-end wrenches are really handy when there's an obstruction in front of the bolt, since they can be inserted at a slight angle. But since the contact area is smaller, you're slightly more ...
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14 votes

What are the most basic parts of a bicycle which cannot be upgraded over a period of time?

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 '...
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13 votes
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What is this device attached to the seat-tube?

Are you sure the upper part has not been sliding down such that we can really take a clue from the distance between the two parts? I suspect that the two parts of the fixture are meant to hold an air ...
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13 votes
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Why do folding bikes usually have MTB components?

Great question! I think the market drives this -- not necessarily just the folding bike market, but the component market. Mountain bikes have been the predominant type of bicycle for the past ...
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  • 28.4k
13 votes
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Can a counterfeit Shimano chain be used on an authentic Shimano drivetrain?

The main problem is quality, in terms of dimensions and variance. If you think its an 11 speed chain but someone has just repackaged some 10 speed chain, then there will be issues. Likewise, if it ...
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  • 96.9k
12 votes
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Bicycle Components: What's The Real Difference?

There's a couple of things here. First there are the physical properties of the groupsets. As you move up the groupsets, what you're buying into is essentially smoothness and lightness. But for a ...
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  • 9,593
12 votes
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Does it matter where you buy the bike?

It matters in many ways where you buy the bike from. If you're buying online, then the parts will most likely be the same, although you should be careful to read the specs. Some online (brick and ...
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  • 21.2k
12 votes

Why are foot-long pedals not widely used?

I'd wager the two biggest reasons you don't see foot-sized pedals are the increased rotational weight, and the difficulty you would have catching the pedal with your foot before it struck the ground ...
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12 votes
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Why is there a koozie on this bike's chainstay?

That is a "chain stay protector" and it is installed in, let us say, a non-traditional way. Typically it would be a couple inches forward of where it is. As the name suggests, the purpose of the pad ...
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  • 341
12 votes
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What is the name of this small component inside Shimano Tourney STI

This part is officially called "cable hook" and "cable hook unit" in Shimano docs. See attached image (I won't link it because the PDF is not hosted by Shimano) and this document (page 7).
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12 votes

What is a substitute inner tube for 27x2.125" tyres (in Prague)?

Tl;dr: use 700c or 29" 27x2.125" is very rare. 27x2⅛" would be more likely, and wouldn't quite be the same thing, but for tubes the answer would be the same. Generally though, 27"...
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  • 45.5k
11 votes
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Difference between road & mountain drivetrain

I think this is quite a broad question, so I'll highlight a few parts of the bike: Seatposts/saddles: Probably interchangable Forks: You wouldn't want to swap them, and chances are a beefy mountain ...
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  • 45.5k
11 votes
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What is the black piece above the bottom bracket on this bike?

It looks like a SWAT box (tube, co2, levers and multitool) see https://www.cyclingabout.com/2018-specialized-diverge-light-touring-bikes from https://www.topvelo.fr/actualite-materiel-velo-nouveau-...
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