Here's everything anyone ever wanted to know about wheel tethers, and, chances are, a whole helluva lot more.
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 separated by at least 90° radially with respect to the axis of the wheel and 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.
The following is from the appendix to the 2012 Technical Regulations.
Wheel restraint systems are important to improve protection to the drivers and the personnel
(spectators and officials) within the proximity of the race event. It has been shown that during an
accident a wheel may be ejected at velocities in excess of 150km/h (42m/s) relative to the car, which
corresponds to a linear kinetic energy of 17kJ for a 20kg wheel assembly.
This specification provides test methods, criteria and limits to assess the performance of wheel
restraint systems to ensure that the potential for wheel ejection is reduced.
During early development work, an advanced wheel restraint system was considered in two parts; an
energy absorbing unit and a connecting tether. However, the latest research has demonstrated that
an integrated tether can absorb the required energy without the need for a separate energy
absorbing unit. And, therefore, an integrated tether is the preferred solution. Other designs may be
acceptable, but the geometry and function must be approved by the FIA before submitting for
certification.
A definition of the key components is provided below.
2. DEFINITIONS
2.1 Wheel Assembly
Those parts, likely to include the wheel, tyre, upright, brake calliper and brake disk, that are
considered to be a single projectile during a wheel ejection event.
2.2 Wheel Restraint Cable (Tether)
Flexible load carrying element that connects the wheel assembly to the main structure of the car and
that provides the required strength and energy absorbing capability.
2.3 Energy Absorber
The energy absorbing capability of the tether. A separate energy absorbing element may be
permitted but must be approved by the FIA before submitting for certification.
2.4 Tether End Fitting
Feature at each end of the tether to facilitate attachment to the car and the wheel assembly. The
tether end fitting may include a bobbin if this represents the in‐car conditions.
The in‐board‐tether‐end‐fitting connects to the car chassis
The out‐board‐tether‐end‐fitting connects to the wheel assembly
2.5 Tether Attachment
Attachment between the tether end fitting and the main structure of the car that achieves the
strength and geometrical requirements defined by the Technical Regulations.
2.6 Tether Sliding Surface
Rigid structure that represents the local structure of the car over which the tether must slide if the
wheel is ejected in any direction normal to the axis of rotation of the rear wheels.
3. PERFORMANCE ASSESSMENT
3.1 Wheel Restraint Cable Test
The performance of the wheel restraint system shall be measured in accordance with the dynamic
tests defined in Appendix A.
3.1.1. One Wheel Restraint Cable (per wheel assembly)
During the tensile tests and tether sliding surface tests, the following performance shall be achieved
by all test samples;
The energy absorption shall not be less than 6kJ over the first 250mm of displacement.
The peak force shall not exceed 70kN (CFC 1000) over the first 250mm of displacement.
3.1.2. Two Wheel Restraint Cables (per wheel assembly)
During the tensile tests and tether sliding surface tests, the following performance shall be achieved
by all test samples;
The energy absorption shall not be less than 3kJ over the first 250mm of displacement.
The peak force shall not exceed 70kN (CFC 1000) over the first 250mm of displacement.
APPENDIX A: WHEEL RESTRAINT CABLE TEST PROCEDURE
A1. Apparatus
An appropriate test apparatus is shown in Figures A1 and A2.
The aim of the test is to dynamically load the tether in a tensile direction, in order to determine the
strength, elongation and energy absorbing characteristics. The tests shall be conducted using a
dynamic sled apparatus based on the Formula One frontal impact test. The mass of the sled shall be
780kg ± 7.8kg.
Two tether attachments shall be provided; one fitted to the sled and one fitted to a ground anchor
within a close proximity to the sled. The position of the sled tether attachment point relative to the
CoG of the sled shall be chosen to prevent excessive torque loadings to the sled. The position of the
ground anchor tether attachment point shall achieve the tether angle requirements defined in A1.1
and A1.2. The tether attachments shall reproduce the in‐car fixing method as defined by the
Technical Regulations. The tether manufacturer may provide a bobbin arrangement if this represents
the in‐car fixing method.
During the test, the entire kinetic energy of the sled shall be directed into the tether end fittings to
load the tether in tension. The tether shall move with the sled during the pre‐impact phase with the
in‐board tether end fitting engaged with the sled tether attachment. At the point of impact, the outboard
tether end fitting shall engage with the ground anchor tether attachment. As the tether is
loaded the sled will be decelerated. The motion of the sled shall be otherwise unrestrained until the
displacement of the sled has exceeded 500mm from the point of impact. After this time, the sled
may be arrested using crush tubes or any other appropriate device.
Two loading configurations are prescribed
A1.1 Tensile Test (0°)
During the tensile test, the tether shall be loaded between two points only; the sled attachment
point and the ground anchor attachment point. At the point of impact, the angle between the major
axis of the tether and the axis of the sled shall not exceed 20°.
A1.2 Tether Sliding Surface Test (90°)
During the tether sliding surface test, the tether shall be loaded at three points; the sled attachment
point, the tether sliding surface and the ground anchor attachment point. The tether sliding surface
shall be a solid steel cylinder with a diameter of 25mm and a length of at least 100mm. The major
axis shall be perpendicular to the axis of the tether. At the impact point, the distance between the inboard
end of the tether and the centre of the Tether Sliding Surface shall be 115mm ± 15mm. The
apparatus shall be configured such that the tether is flexed through 90° ± 5° around the tether sliding
surface. At the point of impact, the angle between the out‐board section of the tether and the axis of
the sled shall not exceed 20°.
A2. Test Samples
The test samples shall include the tether and the tether end fittings. The test samples shall have a
length of 600mm ± 15mm measured between the centres of the tether end fittings.
A3. Environmental Conditioning
The FIA may require that polymeric tethers are conditioned before testing as follows;
Temperature: 100°C for 24 hours
Moisture: Immersed in water 25°C for 48 hours
Ultra‐violet: 250mm from 125V xenon‐filled quartz lamp for 48hours
A4. Instrumentation
The apparatus shall be fitted with a single axis load cell to measure the force exerted at the outboard
tether end fitting along the direction of the tether. The sensitive axis of the load cell must be
aligned with the axis of the tether ± 5o at the point of impact. It is understood that during the impact
event, the angle of the tether will change as the tether extends. However, the sensitive axis of the[/code]
load cell shall be fixed at the point of impact position.
A method of measuring the velocity of the sled immediately before the point of impact shall be
provided. The sled shall be fitted with an accelerometer to measure the fore‐aft acceleration during
the impact event.
All instrumentation shall conform to SAE J211 (latest revision) with a channel frequency class (CFC) of
1000. The sampling frequency shall be at least 20,000Hz.
A5. Test Procedures
Test A5.1. Wheel Restraint Tensile Test
The test samples shall be fitted to the sled with the in accordance with the tensile test configuration
as described in A1. The impact velocity shall be at least 14m/s. The tests shall be conducted on two
test samples and the results shall be reported as defined in A6.
Test A5.2. Wheel Restraint Tether Sliding Surface Test
The test samples shall be fitted to the sled with the in accordance with the tether sliding surface test
configuration as described in A1. The impact velocity shall be at least 14m/s. The tests shall be
conducted on two test samples and the results shall be reported as defined in A6.
A6. Results
The results shall include:
(a) Length of test sample (mm)
(b) Diameter (or x‐sectional area) of test sample (mm or mm2)
(c) Mass of test sample (g) including end fittings
(d) Actual impact velocity (m/s)
(e) Acceleration‐time history of the sled CFC60 (g, ms)
(f) Velocity (1) time history of the sled (m/s, ms)
(g) Force‐time history for tether showing peak force CFC1000 (N, ms)
(h) Force‐displacement (2) history for tether CFC1000 (N, mm)
(i) Energy (3) absorbed over first 250mm
1. The velocity shall be calculated by single integration of acceleration
2. The displacement shall be calculated by double integration of acceleration
3. The energy shall be calculated by integration of force with respect to displacement
Figure A1. Test apparatus for 0° (tensile) tests on wheel restraint cables
Figure A2. Test apparatus for 90° (tether sliding surface) tests on wheel restraint cables
