Mnay enthusiasts want to fit aftermarket seats to their cars.
A problem arises when the original seat has the seatbelt buckle mounted on the seat, so that the seatbelt load goes through the slide rails to the car.
The easiest option is to keep the original slides, and mount the new seat to them. This way the seatbelt anchorage is unchanged.
If this is not possible, and the aftermarket seat has its own base and slide rails, this is where the problem arises.
There needs to be TEST DATA to show that the aftermarket rails are strong enough for the seatbelt load- and THERE NEVER IS ANY TEST DATA
There are two ways around this:
1 Fit a new Seatbelt Anchorage to the vehicle tunnel. This is done using standard Seatbelt Anchorage doubler plates of at least 3000 sq mm area (such as 80 x 50, or 100 x 40).
2 Bracketing the seatbelt buckle to the seat anchorage, using a piece of 40mm x 3 mm flat bar, usually about 100 to 150mm long, drilled for the 7/16 seatbelt bolt at one end, and drilled to fit the seat mounting bolt (usually M10) at the other.
It is a requirement that the Seatbelt is mounted with a 7/16 or M12 bolt. There are good reasons for this, and the seatbelt hardware is all made for this size of bolt.
The seat mounting bolt is whatever the vehicle manufacturer deemed appropriate- a M10 grade 8.8 bolt can take a shear load of 48 Kn, which is more than meets the load requirement for a seat and the seatbelt buckle loading.
The Manufacturers underfloor mounting plate for the seat is designed to take this load- the picture below shows that the seatbelt load in a standard seat is transmitted just to this one bolt- proving that this is true.
So we can rely on the Original, Factory seat mounting point, and the Original seat mounting bolt to take the seatbelt loads, the load just gets there by the new bar we have added.
NOTE- Remember to put the seatbelt load bar between the seat and the tunnel or floor.
S14 Seat showing Force Diagram
The LVV Standard for seats is 185-00. The LVV Standard for Seatbelts and anchorages is 175-00