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I live in an area with lots of debris, dodgy paving, and rain, and to make matters worse, I fall down sometimes (I know!). The main things that get damaged, of course, are the brake handles and the rear derailleur. The brakes aren't so bad since the cinch just needs to be scooted back into place, typically.

On the other hand, the rear derailleur is a bunch of fragile, lightweight dangly bits sticking further out than anything on the rear of the bike, and takes a special tool and time out to bend back into place.

The question is, is it worth buying a derailleur guard like this one for a road bike, or am I going to get noob status, ruin the frame instead, or weigh down the wonderfully light frame?

Given that a replacement rear derailleur (Tiagra) would run me 30-50USD and the guard would cost 5-10USD, it seems to be more effective to get a guard. On the other hand, this bicycle co-op raises a serious concern about the axle:

We have to admit that we are ambivalent about derailleur guards. While a derailleur guard is useful in theory, the fact that the wheel track nuts or QR secures it in place causes misgivings. A derailleur guard can't help but interrupt the interface between the frame and wheel nut. This means that the wheel nut will be less secure, and a less secure wheel is potentially more dangerous than a whacked rear derailleur. Having said that, derailleur guards are usually reserved for kids' bikes where the rider will probably not have the strength to loosen a wheel while riding.

(Don't tell me not to fall down so much; I'm a lost cause ;)

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I'm so trying not to say anything about not falling down :-) –  andy256 Dec 29 '13 at 0:28
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I'd say try it and see if it gives you problems. If the wheel is slipping you should notice that before it leads to a serious accident. –  Daniel R Hicks Dec 29 '13 at 0:29
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As far as I know, derailleur guards are more likely to leave more damage than protection in most crashes where they would come to play due to how they distribute the impact. I'd rather get the derailleur fixed or buy some cheap derailleurs to have on hand. If this is really bugging you, note that if you switch to an IGH/Single speed, theres no derailleur to be hit (but terrain and riding type may make this an unfavorable proposition). :) –  Batman Dec 29 '13 at 1:21
    
possible duplicate of Does a rear derailleur guard provide effective protection? –  mattnz Dec 29 '13 at 7:04
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@ColinNewell I'm not sure theres much more to say other than no derailleur means your derailleur can't be damaged. Of course, this may require a different frame / eccentric bb in some (most?) cases to avoid a chain tensioner (which causes the same problems as a derailleur). You're essentially solving the problem by removing the problematic component but possibly introducing others (like STI-style shifters for an IGH seem to be made by 1 company only). Fred's answer does mention the IGH option though, but the weight issue ("light frame") of the OP makes it sound not viable in hindsight. –  Batman Dec 29 '13 at 18:51
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What I would be concerned about is that derailleur guard creates a lever for bending the rear triangle of the frame in a fall. A frame is much more expensive than a derailleur. Kids bikes with derailleur guards are generally heavy steel bikes and kids don't weigh that much.

Bikes come with replaceable dropouts for exactly this reason. It's a sacrifice part to avoid damaging the more expensive frame. Road bikes aren't really designed to be crashed multiple times.

If you're really hard enough on a bike that this is an issue for you, I would seriously consider switching to an internally geared rear hub. The cheapest way to do that would likely be to buy a new bike. Plenty of nice commuter bikes out there now that will likely stand up to the abuse.

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My bike doesn't have a replaceable dropout, or a replaceable derailer hanger. All one die-cast bronze part. Which is why I'd consider the guard if I had a problem with the derailer getting hit -- better to bend the guard than the derailer hanger. –  Daniel R Hicks Dec 29 '13 at 21:06
    
Is replacing the jockey wheels or their cage easy to purchase for/cheap? I'm not really damaging the sturdier main body of the derailleur. –  Trevor Alexander Jan 6 at 10:31
    
You can get replacement wheels cheaply $20, not sure about the cage. But you can also get perfectly functional replacement rear derailers cheaply. bicyclebuys.com/item/1010062SS0/… –  Fred the Magic Wonder Dog Jan 6 at 15:35
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