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Every vehicle which takes to the track can roll, it only takes a touch of wheels or hitting a small kerb during a spin and over it goes. For that reason every Greenpower car must be fitted with a rollbar capable of protecting the driver. This is a critical part of the car and must be done correctly, before you even start designing it is essential that you carefully read section T10 of the regulations in detail:
T10 - Roll Bars
T10.1. The vehicle must have front and rear roll bars offering protection in accordance with the diagrams shown here – the helmeted head of all drivers must be at least 50 mm below the line A-B as shown. See Fig 2.
T10.2. Roll bars must be firmly secured to the chassis of the vehicle using mechanical fixings or welding. Roll bar to chassis mountings and points on the chassis to which roll bars connect must be suitably strong and where necessary reinforced to prevent failure in the event of a roll over incident. Gluing/bonding of roll bars to chassis with no mechanical fixings or welding is not permitted.
T10.3. One central triangulated brace or two side triangulated braces must connect the rear roll bar to the chassis. These braces must attach to the chassis of the vehicle at one end, to not more than 200 mm from
the top of the roll bar at the other, must be capable of taking loading in all directions and must be rigidly mounted. Advance Notice - 2020 Regulations - T10.3 will be extended to include: The angle between roll bar and brace(s) must be at least 25 degrees.
T10.4. All rear roll bars & braces must be produced from circular section steel, with minimum outside diameter of, main hoop - 25mm, braces - 19mm and minimum wall thickness of 1.5mm. Safety note - Greenpower reserves the right to drill a 4mm diameter hole in any roll bar for the purpose of inspection. Teams should avoid
drilling roll bars as it weakens the structure.
T10.5. Non-structural bodywork along with front and rear wheels must not be regarded as part of the roll protection. The top 150 mm of the roll bar must not have any fairing or other aerodynamic aid. See Fig 2.
T10.6. The rear roll bar and bracing structure must extend down into the car
to at least the level of the driver’s shoulder strap mounting points.
And then read them again!
Front roll bars. Most cars will have a strong structure around the front axle and steering assembly, often this doubles as the front roll bar. Note, the front roll bar does not need to protrude from the bodywork, but it is useful to mark on the bodywork where it is so that the scrutineers know where to place one end of the head clearance gauge.
Rear roll bar. Few schools will have facilities to bend 25 mm x 1.5 mm steel tubing accurately and to a standard acceptable for a roll bar. It needs to be formed with minimal distortion to the cross-section and the best way to achieve this is to find a fabricator with an NC bender, there are plenty advertising online if you can't find one locally. A typical 180 degree bend with 1 metre legs should cost under £30 including the material, not a lot to pay for piece of mind.
The roll bar must be fastened to the chassis either mechanically or by welding, the latter is only possible if you have a steel chassis and if you do then it is the preferred method. If you're not confident in your welding then take it to a professional.
For wood, composite or aluminium cars the only acceptable methods of attaching the roll bar are bolting and clamping. DO NOT drill though the roll bar anywhere other than the lowest points of attachment, instead you should weld on brackets and bolt through those. It is good practice to attach the roll bar to the car in as many places as possible, preferably tying together intrinsically strong parts of the structure such as safety harness anchor points, axle subframes, bulkheads, motor mount, etc.
Clamping is a very effective method of mounting the roll bar. U-bolts are easy to use and provide extremely high clamping forces, use a minimum of 8 mm diameter and ensure the distance between the legs of the U-bolt closely matches the diameter of the roll bar tubing. Safety railing clamps are very handy to use for mounting roll bars and the triangulating braces (T10.3), they are manufactured to a high standard and sized to fit steel pipe to BS1387, nominally 20 mm pipe has an outside diameter of 26.9 mm and 2.3 mm wall thickness so exceeds the minimum spec required for a roll bar. If you intend to use these clamps then it would be beneficial to have your roll bar fabricated from the correct tubing although it is possible to sleeve the clamps for 25 mm tubing.
A line drawn between the tops of both roll bars must clear the drivers helmet by at least 50 mm, and remember it has to be tested with your tallest driver in the car. However it is also important not to make the rear roll bar too tall as any forces acting on the top of it during an accident will have the unwanted benefit of greater leverage and put higher stresses on the chassis and mounting points; you will also be adding unnecessary weight and aerodynamic drag.
- Gary Coulden Smith