Lightweight wheels have many dynamic advantages and carbon-fibre is key to ultimate performance...

Carbon-fibre-reinforced polymer (CFRP) is an extremely strong, rigid and light composite material with many engineering applications. For once, I allowed my emotional side to overshadow my technical mind as I marvelled at the beauty of one of Blackstone Tek’s (BST) carbon-fibre wheels at its facility in Gauteng. Yes, BST is proudly South African and not European as its involvement with Ducati and Arial Motor Company (to name a few) would suggest. Local motorcycle racing legend Brad Anassis and managing director Gary Turner took me through the process that leads to these works of art. We’ll also highlight why lightweight hoops carry such a dynamic advantage over the standard alloy units while they appear so visually enticing.

WHEELS

COMPARED

Size OEM unit mass BST mass Price
BST motorcycle front rim (superbike) 17 inches aluminium from 2,8 kg BST GP TEK: 1,8 kg from  R18 000
BST motorcycle rear rim (superbike) 17 inches aluminium from 3,8 kg BST GP TEK: 2,3 kg from  R20 000
BST car wheel 19 inches aluminium from 10,5 kg  BST: 7,2 kg R230 000 per set

 

The process

When the research and development team at BST receives a request for a new wheel design (or application), the first step is to employ computer-aided design (CAD) to come up with the new concept in the software domain. Finite-element analysis is used to determine the wall thickness needed as well as the optimal orientation of the carbon-fibre weave to support the loads to which the wheel would be subjected. When the design meets all the performance requirements, a mould is produced to build the prototype wheel. The time from the initial request to the final product is usually around six months.

The material

A single carbon-fibre strand is between 0,004 and 0,010 mm in diameter and consists of carbon molecules in microscopic crystals that are mostly aligned parallel with the longitudinal axis of the fibre. This structure endows the fibre with incredible tensile strength qualities (up to 10 times stronger than steel). Thousands of carbon fibres can be twisted together to form a yarn which is used in the weaving process to create the signature carbon-fibre cloth.

Strands on their own are not suitable for the production of wheels. For this, a composite material called CFRP is used. BST believes that pre-impregnated (with epoxy) carbon cloth yields better results compared with the resin-transfer method (RTM) where the matrix material (mostly epoxy) is injected only after the carbon-fibre cloth is in the mould.

The first cut

Pre-impregnated CFRP rolls must be stored at -20 degrees Celsius to prevent curing and this walk-in freezer is located adjacent to the room where the computer numerical control (CNC) laser cutter trims between 130 to 180 pieces that form each wheel depending on size and type. Each piece is labelled and numbered for traceability throughout the process. The pieces are bagged and taken to the appropriate dust-free room where the individual parts are accurately laid out in the mould by a single person. Motorcycle wheels are created as a single, monocoque unit before the curing process takes place in the autoclave.

Baking

The curing process happens in the oven (called an autoclave) where the pressure is controlled to six bar and the temperature to 130 degrees Celsius. Each wheel is cured inside a plastic bag under vacuum. Gary Turner has developed a special technique (part of BST’s intellectual property) to keep the dimensional stability during baking.

Finishing

After weighing and visual inspection, it is time for final machining. The wheels are produced with some excess material which is cut away on a lathe to produce the exact dimensions needed. Where a metal hub is used, this is installed first to provide the lathe with an accurate datum from which to work. These hubs are manufactured in-house from aircraft-grade (6082 T6), billet aluminium-alloy.

To achieve the smooth surface finish, the wheels are inserted into a shaker with aggregate doing the work before hand-finishing ensures a perfect result. Lastly, the wheel is painted with a clear lacquer to produce that shine and to show off the carbon-fibre weave. Colour options are available but the transparency to the beautiful weave remains.

Testing

To make sure there are no voids, foreign objects or wall-thickness variations, motorcycle rims are CT scanned before being shipped. As part of JWL and DOT E compliancy testing, the wheels are subjected to impact loads (388 kg front and 420 kg rear), cornering loads and bending loads. The very last test involves a bending moment of 514 N.m on the front and  719 N.m on the rear rim of a motorcycle for a million cycles.

Performance

According to Turner, in terms of bang for buck, there is no better performance enhancement for your vehicle than lightweight wheels. Independent motorcycle tests have proved that by simply replacing the stock wheels of a superbike with lighter rims, lap times are reduced by around 1,5 to 3 seconds – depending on track length – thereby confirming the advantage of lightweight wheels. “Although the performance potential of these carbon-fibre wheels is impressive, we believe many clients will make the purchasing decision purely on their exotic appearance,” Turner says.

Inertia

Newton’s second law states the acceleration of an object is directly proportional to the magnitude of the net force in the same direction. This law can also relate to the mass of the object. Simply put, if you push a vehicle weighing 1 000 kg, the acceleration would be doubled when pushing a vehicle weighing 500 kg.

In a similar way, the mass of a wheel influences the rate of rotational acceleration when torque is applied. More accurately, how the mass is distributed around the axle affects the inertia of the wheel. The further the bulk of the mass is away from the axle, the higher the inertia. For a thin cylinder or disc (resembling a wheel) of homogenous material, the inertia is calculated by:

I = 1/2 m r2

Where I is the inertia [kg.m2]; m is the mass of the cylinder [kg]; r is the radius of the cylinder [m].

The carbon-fibre wheel is therefore not only much lighter than a standard wheel, but it also carries the mass closer to the hub and therefore lowering its inertia. Hence, the carbon-fibre wheel takes much less power to accelerate or decelerate than the standard wheel. This improves vehicle acceleration and braking. According to Turner, calculations show carbon-fibre wheels take 3 kW less power to accelerate to 200 km/h. The 3 kW can now be used to accelerate the bike and improve performance.

Unsprung mass

Unsprung mass is the mass influenced directly by road surface undulations without a spring-damper interface supporting the mass, as in the case of the vehicle body. This includes tyres, brake assemblies, hubs and, of course, the wheel.

Any mass in motion has momentum and inertia to overcome when a directional change is needed. For example, take a wheel travelling up a bump compressing the suspension of the vehicle. In an ideal scenario, the wheel weighs nothing and easily follows the bump down the other side with the spring-damper system pushing it against the surface at all times, never losing contact.

When the wheel’s mass is increased (and/or the bump’s sharpness intensifies), the momentum of the wheel moving up over the bump prevents the suspension pushing the wheel down in time on the downward slope of the bump and the wheel loses contact with the surface for a brief moment. The more unsprung mass, the greater this effect, leading to handling issues on all types of vehicles. It is clear: a lighter wheel is beneficial.