When child seats are fitted with the adult seat belt, it must be the webbing of the seat belt holding the seat in place, not the seat belt buckle. If any part of the plastic seat belt buckle casing is on or over the plastic frame of the seat, it is an unsafe fit, which is called buckle crunch.
Buckle crunch typically happens in vehicle's that have the female buckle on a long stalk, causing it to protrude out of the vehicle seat.
However, buckle crunch can happen on buckles with a shorter stalk, if the wrong car seat is used.
Why is it dangerous?
Buckle crunch is dangerous, because the plastic casing of the seat belt buckle is bent on or over the plastic of the child seat. This puts pressure on the casing, and in a collision, the casing could shatter - causing the car seat to not be safely restrained.
The seat on the left is the Maxi Cosi Tobi, which is a forward facing group 1 seat that often avoids buckle crunch. The seat on the right is the Axkid Minikid, which is a rear facing group 1,2 car seat that also often avoids buckle crunch.
If your vehicle has ISOFIX, it is useful to use it, as this removes the need for the adult seat belt. It is important to visit a retailer to buy an ISOFIX seat, as not all ISOFIX seats fit all cars. ISOFIX child restraints are normally classed as 'Semi Universal', and have a vehicle compatibility list.
What if your current child car seat has buckle crunch?
If you have checked your child’s car seat and found it has buckle crunch (example above), try fitting it in a different seating position in the car. Occasionally the seat belt buckle length differs, and moving the seat to a different position solves the problem.
You can also check your model of child seat, on some seats there may be several ways to secure it, with one of those methods avoiding buckle crunch.
As an example, the Britax First Class has an alternative buckle routing:
Do all types car seats suffer with buckle crunch?
No, only seats which use harness to restrain the child, and occasionally an impact shield, can have buckle crunch. It is dangerous because the seat belt is holding the child seat in place, and the child seat is restraining the child with a harness. In a collision, this puts a lot of pressure and force on the child seat, seat belt and buckle.
High back boosters and booster cushions do not get buckle crunch, because the booster is simply a 'belt positioning' device, which lifts the child up so the adult belt can fit them safely.
It is not dangerous for the buckle to sit over the frame of a high back booster - although you do need to make sure the buckle doesn't sit under and behind the frame of the seat, as this gives an unnatural belt route.
Examples of 'buckle crunch'