A rubber mud flap fixed to the front mudguard is the most effective. Solid plastic fender-extender type things cause problems if you ride on gravel. I ripped my mudguard mounts off after a stone got lodged in the tread and caught on/jammed in the fender-extender. They basically sit too close to the tyre. Rubber mud flaps are flexible and angle away from the tyre so don't trap stones. You can also select/make your own length width and level of protection.

Most people I know who have tried "huggers" at the rear have found them little or no help at all at keeping things clean, and give the same sort of issues as above on gravel/off road.

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