ISOFIX, top tether and support leg requirements in R129

The ISOFIX system

ISOFIX connects a seat to the vehicle through two low anchorages — 6 mm round bars on the vehicle — and two matching attachments on the seat, combined with a means to limit pitch rotation §2.5, §2.11. All vehicle anchorages are approved under UN R14 or R145.

An i-Size seat must use both ISOFIX attachments and an anti-rotation device, whether it faces forward or rearward §6.1.2.1. For i-Size the anti-rotation device is either a top tether or a support leg §2.12; specific vehicle seats may also use lower tethers or other means.

ISOFIX attachments

• The part engaging the anchorage must stay within the dimensional envelope of Figure 3(b) §6.3.3.2.
• Latching indication: the seat must clearly show that both attachments are fully latched — audible, tactile or visual, and visible in all normal lighting if visual §6.3.3.3.
• Release needs either two consecutive actions (the first held while doing the second) or an opening force of at least 50 N §6.7.5.2.
• Attachments and indicators must survive 2 000 ± 5 open/close cycles before the dynamic test §6.7.5.1.

Top tether

Support leg

A support leg is permanently attached, adjustable in length and transmits load to the vehicle floor §2.15. For i-Size seats its geometry is tightly defined so it lands on the floor zone that i-Size vehicles reinforce §6.3.5:

• Dimension volume: the leg must sit within a box 200 mm wide, between 585 mm and 695 mm forward of the ISOFIX origin; rigid, non-adjustable parts may not extend more than 285 mm below the origin §6.3.5.1.
• Foot adjustability: the foot must reach the whole height range from 285 mm to 540 mm below the origin, in steps of no more than 20 mm §6.3.5.2.
• Foot size: at least 2 500 mm² contact surface, at least 30 mm in each direction, edge radius at least 3.2 mm §6.3.5.3.
• Checked with a jig based on the matching child-restraint fixture, or by computer simulation §6.3.5.4.

In frontal tests the leg is set to its maximum adjustment compatible with the trolley floor; the Technical Service picks the worst case for rear impact §7.1.3.6.3.

Misuse tests

If a user could forget the anti-rotation device, the seat is also tested without it — unless a mechanism or a visual and audible warning prevents that misuse §6.6.4.1.6. A +10 % tolerance on head excursion applies to those misuse tests §6.6.4.4.1.1.1.

Lower tethers (specific vehicle belted seats)

Rear-facing specific vehicle belted seats may use lower tethers to the seat track or floor: straps of at least 900 mm, a no-slack indicator (or an automatically locking retractor) and the same hook geometry as the top tether §6.3.6. Generic brackets supplied by the seat maker must withstand 5 000 ± 100 N and come with fitting instructions for qualified personnel §6.3.7–6.3.8.