Wheel tethers
Since 1998, F1 cars have had to fit wheel tethers connecting the wheels to the chassis. This rule was introduced to try to stop wheels coming free and bouncing around dangerously during an accident. Unfortunately, wheels still do come off the cars during crashes, tragically killing a marshall at the Italian GP in 2000.
The FIA have introduced an extra tether to each wheel for the 2001 season to try to stop the wheels coming off and causing injury to other drivers, marshals or spectators. The tether must attach to the chassis at one end, with the other end connecting to the wheel hub.
The wheel tethers are made by one of three companies in the UK, Future Fibres in London being one of them, and take the form of a rope. The tethers used in F1 are a derivative of high performance marine ropes, made especially for each car. They are made from a special polymer called polybenzoaoxide (PBO) which is often called Zylon. This Zylon material has a very high strength and stiffness characteristic (around 2700 MPa), more than carbon, but the advantage of Zylon is that it can be used as a pure fibre unlike carbon which has to be in composite form to gain its strength.
The drawback of Zylon is that he must be protected from light, so it is covered in a shrink wrapped protective cover. The tethers are designed to withstand about 5500 kg of load, but often they can break quite easily during an accident, especially if the cable gets twisted by the broken suspension members. The teams normally replace the tethers every two or three races to ensure that they can withstand the loads put on them during an accident.
Double wheel tethers for 2011
For safety reason a doubling of the wheel tethers has been regulated for season 2011. Each tether needs to pass through a different suspension member and have its own mounting points on the upright and the chassis. There’s not expected to be any performance impact with this. But the tethers are somewhat heavier, so they and the side intrusion panel are part of the reason for the greater minimum weight limit.
10.3.6 In order to help prevent a wheel becoming separated in the event of all suspension members connecting it to the car failing provision must be made to accommodate flexible tethers, each with a cross sectional area greater than 110mm². The sole purpose of the tethers is to prevent a wheel becoming separated from the car, they should perform no other function. The tethers and their attachments must also be designed in order to help prevent a wheel making contact with the driver's head during an accident. Each wheel must be fitted with two tethers each of which exceed the requirements of 3.1.1 of Test Procedure 03/07.
Each tether must have its own separate attachments at both ends which :
- are able to withstand a tensile force of 70kN in any direction within a cone of 45° (included angle) measured from the load line of the relevant suspension member ;
- on the survival cell or gearbox are separated by at least 100mm measured between the centres of the two attachment points ;
- on each wheel/upright assembly are located on opposite sides of the vertical and horizontal wheel centre lines and are separated by at least 100mm measured between the centres of the two attachment points ;
- are able to accommodate tether end fittings with a minimum inside diameter of 15mm.
Furthermore, no suspension member may contain more than one tether. Each tether must exceed 450mm in length and must utilise end fittings which result in a tether bend radius greater than 7.5mm.
