Chainguard vs front mudguard to protect the chain from spray

Question

My local bicycle mechanic advised me to get mudguards (such as the ones in Image 1) to extend the life of my drivetrain and protect it from spray in the winter. If I have £20 to spend on either a chainguard (see image 2) or a front fender, would I be better off with a chainguard? Would this be lighter and provide better protection for the drivetrain vs front-wheel-spray? Image 2 shows how a chainguard on a bike (3-9 derailleur) could block both spray which would be stopped by a front fender (green lines) and spray which would slip past underneath a front fender (blue lines).
Are there any chainguards which would divert mud flowing along the underside of the down tube, so it drips to the ground before it can flow onwards and build up around the axle for the pedals? (Otherwise I guess I could accomplish this by DIY-fixing a mudflap to hang down from the down tube.)
NB, I would still use a rear mudguard regardless.

juhist · Answer

Chainguard is not there to protect chain from dirt spray from tires, unless it is a completely enclosed chainguard (which is not feasible on derailleur bicycles). A chainguard is there to protect your pants from chain oil.
Should you install a chainguard, then? Yes! It protects your pants from chain oil.
But to protect your chain from dirt spray, you'll need fenders, too.

DavidW · Answer

The rear fender will act mostly to keep spray off of you, as opposed to the drivetrain;  the spray hitting the bottom bracket and chainrings will be coming off the front wheel.  I would argue that it's more important to have a front fender than a rear one.
Sheldon Brown has a pretty good page on fenders; it explain how full fenders are much better than anything else.  Your front fender should definitely have a mudflap on it.
Note that while the bike's frame itself (downtube or seat tube) will catch a fair bit of the spray from the wheels (in the absence of fenders), the rounded shape of the tube means that this basically just spreads the spray out in a wider pattern. :)  Also, since you steer with the front, it won't line up with the tube all the time, which is a problem with "fenders" that clip onto the frame.  And from personal experience, in a rain heavy enough to leave standing water on the road if your front tire has any tread to speak of (like a touring tire, not a slick) it can move an amazing amount of water into your shoes in a very short time! :)

Chainguard vs front mudguard to protect the chain from spray

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