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Fair enough. I buy it more for bike helmets, though, since there's a good chance that helmet hit road or metal.

And in an accident a car seat has all the force transferred through just 2-3 points of contact with the seat belt or isofix points.

I would replace my kids seats after any accident with airbags or pretensioners firing.

The damage on a bike helmet that has hit the road or metal in an accident will be very obvious in even the most cursory visual inspection.