Bigger is better
Spray coming off the tyre spreads out the further it travels. So the closer the mudguard sits to the tyre (within reason – see Safety, below), the better it protects. The most effective mudguards are long ones that fit to the frame and fork, shrouding a large arc of each wheel. The width of the mudguard is important too. It must be wider than the tyre. If the guard sits close to the tyre, around 10mm wider overall is enough. If the guard is a long way from the tyre, wider still is better.
Mudguards can be jammed by mud, sticks, or small stones picked up by the tyres. This is why mountain bikers don’t use frame-fitting mudguards: there’s not enough clearance to use them safely off-road.
However, mudguards can jam on road too. A bit of road grit is all it takes. If that happens to the rear wheel, you’ll skid. If it’s the front wheel, the mudguard can fold up behind the fork and you’ll go over the handlebar. It’s important that front mudguard stays can snap or pop free, so that a jammed mudguard is released and won’t concertina behind the fork. To stop jams in the first place, avoid fixing the mudguard too close to the tyre. Aim for 10mm or more of space above the tyre.
Fitting & removal
Full-length, frame-fitting mudguards attach to: eyelets on the frame and fork; the chainstay and seatstay braces; and the fork crown. If your bike doesn’t have eyelets at the dropouts, you can use P-clips fastened around the fork legs and seatstays instead. If there’s nowhere for a bolt to go in the fork crown, seatstay brace or chainstay brace, you can bore holes in the mudguard with a bradawl and attach it with cable ties.
Fitting full-length mudguards isn’t very complicated but does take a while. If your bike has disc brakes, you may need to bend one of the stays to clear the brake calliper. If the mudguard shares a set of eyelets with a pannier rack, use longer bolts and put the rack struts closest to the frame with the mudguard stays on the outside. Note that while you can take frame-fitting mudguards on and off, you won’t want to do so too often.
Partial mudguards tend to fit at the rear to the seatpost or saddle rails, and at the front to the down tube, the fork crown, or the fork brace (the arch on suspension forks). You don’t need any special frame fittings and the guards can usually be removed quickly. The downside is limited coverage – which you may be prepared to accept if your commuter doubles as a racer.