What features should I look for in a mudguard?
Mudguards / fenders work by intersecting an imaginary line between your wheel and you/the bike frame.

There are two main reasons for having them and they're similar but different.


Keeping your bike and rider DRY from water
Keeping your bike and rider CLEAN from mud/dirt


In addition mudguards might offer an increase in aerodynamics, and somewhere to hang more lights/reflectors.

So the best mudguards will look something like this:




The rear mudguard starts after the rear tyre.  So there is no straight line from the tyre to the rider or backpack.
The guard is close to the tyre, so spray can't flick off the sides of the tyre, evading the guard and hitting the rider under the thigh.  My ass-saver does none of this.
At the front of the rear mudguard, it does right down to the chainstays.  This somewhat-protects the bearings of the BB and your chain and chainrings from spray.
Up the front, the rear edge is quite low too, and this protects your face and front from upspray when turning.  It also shields the BB somewhat.
But in addition there's a mudflap behind the front guard.  This dangles and helps catch roadspray that would hit your chain/chainrings and BB housing.  Mine's made from a leather tongue from some worn shoes.
Finally, the front/leading edge of the front guard goes forward of the front wheel axle.  This stops water from being thrown forward or slightly upward and catching the wind, coming back either into your headtube bearings, your hands, or your chest and face.


The only thing missing is a flap on the rear guard - this is more for the benefit of someone riding behind you though.

A second downside, this style only works on rigid bikes.  If you have suspension then your guards can't be too close.



Notice the rear guard is fastened to the rear triangle, so as the suspension flexes, the guard stays in the same place.   The front guard is much shorter because it can't use stays for support, and is a very long way from the tyre.
The tyre/guard clearance has to be bigger if the bike is likely to be ridden in mud too, else there can be a buildup of mud enough to stop the wheel turning.  

The MTB guard has to be much wider too, because the tyre is wider.  Knobblies catch and throw a lot more water/mud/stones than smoother road tyres.

As such, this offers much less protection from water and mud.



Materials will probably be either metal or plastic.  There are wooden ones, but they're not common.   Metal ones can suffer from stress fractures due to vibration, so more stays work better.  Plastic ones are a bit more forgiving, but they crack slightly easier than metal and can embrittle with UV/age.

A final option is a home made hack.  All you need to do is intersect the flying droplets.  So search and you'll find fenders made from strips of plastic, old bottles, even tyres with the bead cut off.   If you have a rear rack, simply line it with flat plastic and you've got protection