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Looking at http://decathlon.co.uk, I see a derailleur guard only on more affordable bicycles, e.g. this one.

enter image description here

This one does not have it, and there seems to be no obvious way to install one.

enter image description here

Are derailleur guards important? Why does the more expensive bicycle (with higher component replacement costs if something breaks) not have one? Is it correct that one cannot be installed on the one shown in the picture? If it can be installed, is it recommended?

One might always fall, or a parked bicycle may get knocked over.

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    Good observation. Notice the cheaper bike has a spoke protector too, and a wheel reflector. I'd bet its sold with a white front reflector too. Also the rear axle of the cheaper bike uses a nut, where the mid-range bike looks like a QR. Upshot, the cheap bike is at, or just above the BSO area.
    – Criggie ♦
    Oct 23, 2019 at 9:58
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    @Criggie They all have wheel reflectors, whether shown on the photo or not. It is a legal requirement for going on the road here.
    – Clueless
    Oct 23, 2019 at 10:04
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    @Criggie And it turns out the spoke protector (a.k.a. "the dork disk") is also a legal requirement to sell a bike in at least some countries/states, as I learned from a Youtube video.
    – Grigory Rechistov
    Oct 23, 2019 at 10:12
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    @Criggie I've been using spoke protectors for the last few years, after having my chain get in between the cassette and spokes three times while downshifting on steep climbs. A perfectly-adjusted-in-the-shop setup can still flex quite a bit under load, and it can be really hard to do an unloaded downshift when a hill goes from 9% to 13%. I'd rather have a dork disk than have to spend the money and time to rebuild the wheel.
    – Andrew Henle
    Oct 23, 2019 at 10:16
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    Practically speaking, bash guards are a PITA in situations where several bikes are parked close together -- they tend to get caught in the cables/spokes of the adjacent bike.
    – Daniel R Hicks
    Oct 23, 2019 at 17:30

6 Answers 6

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Derailleur guards are rarely found on more expensive bikes for several reasons:

  • Protection: These guards only protect from minor contacts. Forces that would bend a typical derailleur would also bend these guards. Also, more expensive bikes usually have exchangeable derailleur hangers that are supposed to be the first thing damaged in a crash (and, thus, prevent the more expensive derailleur from more damage).
  • Weight: if the bike is handled properly (as owners of more expensive bikes tend do), the little added protection against minor touches is generally seen as not necessary.
  • Style: These things are ugly!
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    The first bike in your question does not seem to have a removable hanger, so a little added protection might sense there. But I'm not 100% sure about that.
    – anderas
    Oct 23, 2019 at 9:31
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    There are better reasonings in the answers and comments, but in addition, the second bike is outfitted with a Shadow-type derailleur which tucks it more behind the cassette. Also, more expensive MTB class bikes are more likely to be used off road and the protection afforded by a derailleur protector is lost when considering the risk of snags. You're just asking for it with one of those.
    – Jeff
    Oct 23, 2019 at 11:39
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    The RD may be protected by the guard which then in turn applies the bending force side-ways on the rear triangle. The RD will still be OK after you've written off the frame. ;-)
    – Carel
    Oct 24, 2019 at 10:22
  • In support of @Jeff, I believe those are actually against the rules for races that I've been involved in coaching, for exactly the reason you state. Similarly, bar ends are forbidden, although I've always suspected there's a bit of "Ugh, those are SO 1990's" in that one. ;-) Kickstands are not allowed since they could extend and cause injury in a crash.
    – Andrew
    Aug 28, 2020 at 16:59
  • @Andrew I can’t find it right now (ugh) but I remember reading a medical journal article (peer-reviewed) that found a significant reduction in abdominal injuries to mountain bikers after barends fell out of fashion.
    – Paul H
    Jun 1 at 20:09
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Are derailleur guards important? Why does the more expensive bicycle (with higher component replacement costs if something breaks) not have one? Is it correct that one cannot be installed on the one shown in the picture? If it can be installed, is it recommended?

  • Are derailleur guards important?
    Often a guard is on a bike geared toward kids or less experienced riders. If you are likely to crash (or "ghost ride" your bike) then a guard may help. If you don't crash then you don't need a guard.

  • Why does the more expensive bicycle not have one?
    More expensive bikes are often marketed toward more experienced riders who - for the reasons listed in other answers - don't want the guard. It may be seen as not needed and added weight. Sometimes there is snobbery that prevents people from putting things like derailleur guards, kickstands, fenders, spoke protectors, reflectors, and other useful equipment on their bike.

  • It is possible to install a guard on the one shown in the picture?
    It's possible to install a guard on any bike - even if it does not have bolt points - by using clamps as one option. It's always easier to install a part that is actually made to go on a bike but there are creative ways of attaching things to bikes.

  • If it can be installed is it recommended?
    It depends on your situation. If you need a little added protection on your derailleur then I'd recommend it.

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    For an example of snobbery I would just link to the velominati website!
    – Rider_X
    Oct 23, 2019 at 19:13
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    Regarding the clamps: Wouldn't a guard fixed with these be very likely to simply twist out of the way in case of a crash? (Or need really strong clamps?)
    – anderas
    Oct 24, 2019 at 6:05
  • @anderas The "for sale" guards attach to the axle or quick release. I was imagining a home made something that would use clamps. A well made home made something with two clamps - one that attaches to the chain stay and one that attaches to the seat stay - could take as much impact as a factory version if built right.
    – David D
    Oct 24, 2019 at 13:28
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Not noted in the other, excellent answers:

Inexpensive bikes have a tendency to not be well cared for - they're purchased by people who either need inexpensive transportation or for kids (who are notorious, as a group, for not taking good care of expensive things*). More expensive bikes tend to be better cared for by riders who actually care about the bike they've purchased and would prefer not to have to replace unnecessarily damaged components.

When a casual rider of an inexpensive bike stops riding, he can often be seen simply dropping the bike to the ground and walking away*. If the bike happens to drop on the right side, the rear derailleur will hit the ground. Even from the relatively low height of a rear derailleur, damage can ensue. The derailleur protector can take significantly more hits than the derailleur itself can before damage build-up is enough to cause riding issues.

More serious riders of more expensive bikes rarely "drop" their bikes when they're done riding. The bike is carefully balanced against a wall/tree/rock/etc to ensure that the precious componentry and paint job are not damaged. If the bike is laid down flat, it will be laid on its left side precisely to avoid having either the front or rear derailleur come in contact with the ground.

*These statements based upon personal observation of my own children not heeding parental instructions to not drop the bike when finished riding, and personal observation of other people treating their BSOs this way. No offense intended to any casual rider who may happen to actually care for her bike.

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  • I have never seen anyone just drop their bike and walk away ...
    – Clueless
    Oct 25, 2019 at 16:04
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    @Clueless that's... amazing... Maybe in some cultures/countries people in general take better care of their bikes, but whether it was as a kid growing up in the western US or an adult in the midwest US, I've seen it time and time again. (Don't make me bring my kids into this again! >:/ )
    – FreeMan
    Oct 25, 2019 at 16:34
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One word KIDS, the little buggers love to jump off their bikes and let them crash to the floor, especially hard surfaces - these make perfect sense. I am about to buy these for both of my kids bikes. You can try to educate / plead with the ragamuffins or just pay the $10 and be done with it. Pick your battles ......

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Simple. They’re not AERO!!!!!!

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    While this answer is very short, it does contains a nugget of truth. A derailleur mech protector does add some frontal area, and weight and air resistance, and provides a protection function without adding any performance improvement. So its of little interest to those who want to go fast. Consider it similar to a spoke protector, routinely removed by many riders and never seen on a pro's bike, but also perform an important protection function. Conversely, a chain catcher can be found on a pro bike, despite serving similar protection functions.
    – Criggie ♦
    Oct 24, 2019 at 19:24
  • Welcome to Stack Exchange - please browse the tour to learn how the site works, and look at other answers to see the level of depth and detail required for a good answer. There are very few times where a 4 word reply is long enough. Please use edit to expand your answer and potentially reverse the downvotes.
    – Criggie ♦
    Oct 24, 2019 at 19:26
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    The other nugget of truth in this answer is that there's some pure snobbery involved in advanced riders not wanting derailleur guards.
    – Weiwen Ng
    Oct 24, 2019 at 20:22
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    Welcome to Bicycles SE. We're looking for answers with more detail. Please consider an edit, expanding your answer to explain why more affordable bicycles wouldn't have components that interfere with aerodynamics. A short, one-line answer like this is likely to get downvoted, flagged for moderator intervention, and possibly deleted.
    – jimchristie ♦
    Oct 25, 2019 at 15:45
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Code of Federal Regulations Title 16: Commercial Practices PART 1512—REQUIREMENTS FOR BICYCLES Subpart A—Regulations

§1512.9 Requirements for protective guards.

(a) Chain guard. Bicycles having a single front sprocket and a single rear sprocket shall have a chain guard that shall cover the top strand of the chain and at least 90° of the perimeter where the drive chain contacts the drive sprocket as shown in figure 7. The chain guard shall extend rearward to a point at least 8 cm (3.2 in.) forward of the centerline of the rear axle. The minimum width of the top area of the chain guard shall be twice the width of the chain in that portion forward of the rear wheel rim. The rear part of the top area may be tapered. The minimum width at the rear of the guard shall be one-half the chain width. Such chain guard shall prevent a rod of 9.4 mm ( 3⁄8 in.) diameter and 76 mm (3.0 in.) length from entrapment between the upper junction of the chain and the sprocket when introduced from the chain side of the bicycle in any direction within 45° from a line normal to the sprocket.

(b) Derailleur guard. Derailleurs shall be guarded to prevent the drive chain from interfering with or stopping the rotation of the wheel through improper adjustments or damage.

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    Which nation has this regulation ? Given "federal" I'm assuming USA. Is "Shall be/have" the same as "Must be/have" ?
    – Criggie ♦
    Aug 27, 2020 at 23:58
  • This regulation appears to be specific to bicycles without a derailleur, so completely inapplicable to this question.
    – DavidW
    Aug 28, 2020 at 2:21
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    Reads to me that the Derailleur guard mentioned here in section b sounds more like a dork disc rather than the frame around the rear derailleur? Not sure how one of those pictured in the op question would stop the chain interfering with the wheel rotation.
    – Hursey
    Jun 1 at 2:46

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