How can I avoid mud from splashing onto my back without a rear wheel fender? I enjoy cycling off-road. During rainy season, the cycling trails can get muddy. Most of the bikes from rental shops do not come with a rear mudguard. I tried to slow down and avoid puddles of water, but it seems inevitable that blotches of mud would stain the back of my shirt at the end of the day. Are there better methods of avoiding mud from splashing onto my back?

Use a seatpost-mounted rear mudguard, like the SKS X-tra Dry:

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For the front, you can also use a downtube mudguard like the SKS X-board:

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These are convenient for bikes which don't have proper fender mounts/fenders would interfere with clearance/fenders need to be easily removed or not [Most mountain bikes fit this thing]. They don't protect the drivetrain from crud thrown up from the wheels so they aren't great for improving drivetrain life on a road bike, but this is not a concern for your riding.

The other option is to wear clothes you don't mind getting dirty.