Why is my CBR600's mudguard cracked?

I have a Honda CBR600 Sport that I have owned for eight years. After my second outing this year I noticed a series of cracks around the lower mounting point of one side on the front mudguard. I fitted a new one, but on the next outing both sides on the lower mounting points where cracked.

The forks seem to be OK with no oil leaks and are on the factory settings, while the bearings and tyres are in good nick. On the two occasions the guards cracked I noticed I had ridden over some deep rumble strips. Could these have been the cause and would stiffening the front suspension help. At £75 a time it’s taking the fun out of my riding
Wayne1960, MCN forums
 
If you had a fender extender fitted that could stress the guard beyond its design limit, but for it to happen twice in quick succession I’d say it’s more likely some distortion in the front-end set-up or a very rough road surface. Have the wheel or forks been out of the bike recently? For a tyre change or head bearings perhaps? You could also double-check the suspension settings are on standard. If you have too much preload or too much compression, the forks won’t move easily enough on smaller bumps like rumble strips. It is also possible to put too much rebound damping on, which means over such ripples the forks can’t recover position quickly enough, and pack down, losing travel until you get to a bottomed-out situation.

Have a good look at what order the forks yokes, clamps, bolts and wheel spindles and bolts are supposed to be tightened in when they have been disturbed. It is possible to do this in the wrong order and pull the bottom of fork legs too close together, or twist the yokes slightly in relation to each other, and so put things out of line. What then happens is the forks bind as they are compressed, and don’t use full travel. Over a period of time the fork bushes wear unevenly and travel increases, and the twisting or flexing inwards and outwards increases, so the forks are being damaged/prematurely worn out, also this can cause the mudguard to flex and crack or break. These things don’t show up easily on a road bike with short suspension travel, but they are certainly more common on off roaders with up to a foot of suspension travel.