Good rule of thumb is 10mm at least wider than your tyres.
Another rule is:
"Generally, fenders should be about 40% wider than your tires. This allows them to wrap around your tires without encroaching on the required clearances. This works well for tires up to 42 mm wide, which are best used with 58-60 mm-wide fenders."
Good rule of thumb is 10mm at least wider than your tyres.
Another rule is:
"Generally, fenders should be about 40% wider than your tires. This allows them to wrap around your tires without encroaching on the required clearances. This works well for tires up to 42 mm wide, which are best used with 58-60 mm-wide fenders."