Cycle Commute Mystery...

I have another commuting mystery.

Dirt on the outside of my mudguards!

I seem to get inordinate cakings of dusty dirt on the outside of my mudguards. It makes no sense as to how it gets there. It can't be falling out of the air onto the top of the mudguard. The mudguard should retain any mud picked up off the road and wheech it round to either deposit in on the inside of the mudguard or back to the road. My clothing doesn't get the same treatment from road dirt either.

But no, I end up with a substantial coating of dust on top of the mudguard once the bike has dried out and I have no clue how it gets there.

That said, it does rather satisfyingly just brush off.