The “i” of the Beholder
by Blake Desaulniers —
BMW’s i3 leads the lightweighting race.
The pace of change inevitably catches most people off guard. There is a tendency to think of change as a constant, a linear function. In reality, it accelerates over time. Rather than a straight line, change looks more like a series of “s” curves, one after another.
In the automotive world, two massive trends have been shaping — and will continue to shape — the industry. Those trends are automation and energy efficiency. They are accelerating the arrival of new vehicle designs using advanced sensor technology and new materials.
Carbon fibre changes the way vehicles are designed, manufactured, and repaired.
Just when we thought we had a handle on change, adapting to aluminum and super steels, another, more fundamentally disruptive material has slipped into the mass production market. Just as the rapid adoption of collision avoidance systems and fully autonomous vehicles have caught many in the automotive world flat-footed, so there are few in the industry who have anticipated the accelerating advance of carbon fibre. This material is set to invade the market sooner than people had estimated only a few years ago.
BMW Is First
Use of carbon fibre fundamentally changes the way vehicles are designed, manufactured, and repaired. Perhaps the best example of that is BMW’s i3, which uses a carbon tub, or “life module,” produced mainly from carbon fibre reinforced plastic (CFRP), to form the passenger compartment and exterior panels of the car.
The use of carbon derives from the need to “lightweight” vehicles going forward in order to meet energy-use regulations. A few years ago, the Obama administration in the U.S. imposed new Corporate Average Fuel Economy (CAFE) regulations. The new rules boosted the auto industry’s mandatory overall fleet average for passenger cars from 25.3 miles per gallon (mpg) in 2010 to 43 mpg by 2025. This was, by far, the biggest CAFE increase in decades.
Lightweighting is not a distant event. It is already here. BMW’s i3 gives us a picture of how much can be accomplished in a real, road-ready vehicle, not ten years from now, but today. Although still on the pricey end of the scale at $50,000, the i3 is a groundbreaker in new vehicle design. The Environmental Protection Agency (EPA) rated the i3 at 124 mpg. The car weighs 20 percent less than the Nissan electric Leaf.
The i3 has been built very light and very green. When it comes to embodied emissions — the sum of all emissions generated during production — the downside of carbon fibre is that it is mostly unrecyclable. BMW is minimizing this by powering its carbon factory in Moses Lake, Washington, with clean hydro and wind energy.
What’s more, BMW has now developed innovative processes to recycle carbon fibres, a process that will yield benefits for the entire automotive industry. Recycled materials are used in the roof of new BMW i models and in the rear seat of the BMW i3. In total, 10 percent of the CFRP materials used in the BMW i vehicles is recycled material.
Shell’s Project M
While the i3 is an electric vehicle and wins green points for it, electric still owns only a tiny market share overall. Yet much can be achieved even in vehicles with internal combustion engines in terms of fuel efficiency. Shell has been showing off just how much effect lightweighting can have on a gasoline-powered vehicle with its own Concept Car. The three-seat Project M vehicle weighs only 550 kilograms (1,212 pounds) and uses one-third less energy over its entire lifetime compared to a typical gasoline-powered city car. You could build the Shell Concept Car and drive it for more than 100,000 kilometres and still use less energy than it takes to manufacture a typical SUV and deliver it to a showroom. The Project M car has been designed to be compact and lightweight but also roomy inside. It fits three people and luggage comfortably, yet, at 2.7-by-1.5 metres, it would fit on top of a ping-pong table.
Lightweighting is not a distant event. It is already here.
Shell is not aiming to bring the Project M car into production, but the car does show what can be achieved with an internal combustion engine and lightweighting.
BMW’s Carbon Core
As for BMW, the company is not confining its use of carbon fibre to electric vehicles. In 2016, the flagship 7 Series received a makeover that included carbon fibre frame components. BMW has called the technique its “carbon core.”
In lightweighting the new 7 Series, BMW focused its efforts on lowering the car’s centre of gravity. The designers have kept heavier materials and components as low as possible and reduced mass up high. Carbon fibre is used in the B and C columns, in the roof bows, along the centre tunnel, on the package tray, and in a nine-foot arc running from the base of the A pillar to the rear of the car along the roofline.
The use of CFRP has resulted in a 285-pound net weight reduction over the previous 7 Series. Equally as important, the carbon makes for an extremely rigid shell around the passenger compartment.
Most notably, BMW says its carbon fibre production process has advanced to the point where integrating the material on an industrial scale is now feasible.
Getting back to the i3, what is truly remarkable is that BMW essentially reinvented the design and production process for a mass=market, consumer vehicle. The i3 is worth understanding because it points the way to the very near future.
Unlike other electric vehicles, BMW did not simply take an existing platform and adapt it to electric drive. Instead, it designed the car from the ground-up as an electric vehicle and as an exercise in lightweighting.
The car has two main components — the “life module” passenger compartment and the “drive module,” comprised of the battery, suspension, and drive components.
The life module is a carbon fibre construction. Its high strength protects passengers, while its light weight offsets the i3’s battery weight. Since carbon fibre composite is much stiffer and stronger than steel, it was easier to eliminate the B pillar in the i3. The front doors open normally, while the rear doors are hinged at the back. These “coach doors” make it easier to load child car seats, groceries, and more, and contribute to a sense of spaciousness and accessibility.
The drive module is 100 percent aluminum. It houses the battery, suspension, structural and crash systems, and electric drive train. It was designed with a low, centrally positioned mass that contributes to its classic BMW handling.
Carbon Fibre Can Be Repaired
As more vehicles in the BMW line, as well as cars from Audi, Mercedes, Volkswagen, and others, start to incorporate more CFRP, repairers will have yet another new bag of tricks to master.
There is good news, though. Carbon fibre repair is not, as widely rumoured, black magic. While carbon fibre tubs are new to production vehicles, they have been used in F1 racing cars since the early 1990s and in aircraft structures since the 1970s. Repair techniques have been proven by large outfits such as Boeing and have been validated by the Federal Aviation Administration (FAA). Thousands of repairs are performed on aircrafts every year. They are, for the most part, no more difficult to perform than welded repairs on steel components.
The i3 is worth understanding because it points the way to the very near future.
When carbon is damaged, the first decision is whether to repair or replace. Some automakers insist that when a body panel is damaged beyond the point of cosmetic repair, it should be replaced. If a tub sustains significant damage, the vehicle may be written off. However, in less severe cases, structural components can be repaired.
Basic carbon fibre repair follows a predictable set of steps. Those steps include the following:
- Remove the body panels to get at the damaged area.
- Inspect the damage. Tap testing using a small hammer or even a coin can reveal the extent of hidden damage. An intact area will emit a sharp sound. A damaged area will make a duller thud. The damaged area can then be marked using a felt tip pen. Other methods include using ultrasound or thermography on thicker laminates or for larger, more complex areas of damage.
- Clean the damaged area thoroughly using clean rags and isopropyl alcohol. Remove the damaged plies (layers) using a die grinder with 80-grit or finer discs.
- Scarf sand back from the middle of the damage on a shallow angle of half an inch per ply.
- Clean the scarfed area, and remove all of the dust and grit. Using a Sharpie pen, trace the outlines of the visible edges of each ply in the scarfed area, ply by ply, on a thin sheet of clear plastic. You will use these outlines as tracing templates for cutting out the repair plies.
- Place a large enough piece of carbon fibre cloth between two pieces of plastic film. Using the tracing templates, cut out a large enough piece of the cloth for each replacement ply. Note that repairs should be made using the same type of carbon material as the original.
- Mix epoxy repair resin. Use only the type of resin specified by the original equipment manufacturer (OEM). The mix ratio for the resin is critical. If you are off by more than 2 percent, then the resin will never set at full strength — so, no mixing by eye.
- Saturate the repair cloth and place a piece of ply-traced clear plastic over the top. Use a squeegee to work the resin into the cloth, making sure there are no air bubbles. Check the back side of the cloth to ensure perfect saturation with no dry spots or air bubbles. Cut the cloth around the tracing outline and leave the two pieces of plastic film in place until you are ready to apply it to the repair area.
- Using a small paintbrush or acid brush, apply an interface coat of resin over the entire scarfed area. Starting with the smallest piece of repair cloth, remove the bottom layer of film and apply the cloth to the scarfed area. Remove the top layer of film and proceed with the next layer.
- When applying the plies, make sure to orient the grain to match the grain of the original structural plies. After applying the largest piece of cloth, gently squeegee out any air bubbles and verify that all of the plastic film has been removed.
- Curing some resins can be done at room temperature, although this can take days in a cool area. Most of the strong, structural wet lay-up repair resins require an elevated temperature—typically 140 to 200 degrees Fahrenheit for a few hours. Heat can be applied with a blower into a foil tent or with a heat blanket. However, resins will develop their best strength if they are vacuum bagged prior to cure.
- The purpose of the vacuum bag is to pull out the thousands of tiny air bubbles from the uncured resin, which can be trapped in the weave of the cloth and between plies. The vacuum bag also squeezes the plies together and presses the repair against the original structure, helping to ensure a good bond. There are thin layers that need to go inside the vacuum bag and over the repair to “bleed” excess resin out of the repair plies, “breathe” the air out, and serve as “release” layers to keep the bleeder and breather layers from sticking to the patch. The vacuum bag itself is a nylon film which goes on top of the other layers and is typically attached to the structure with a sticky, thick, vacuum bag sealant tape.
- Debag and tap test the cured repair. Smooth the surface with a fine abrasive pad, and then sit back and admire your handiwork.
While there are a lot of steps, each one individually is not particularly difficult or time-consuming. However, the details matter and make a real difference in the strength of the repair. The only real way to learn carbon fibre repair technique is to get good, hands-on training. Carbon fibre repair is quickly becoming a “must have” skill in collision repair.
Vehicles such as BMW’s i3 may hold only a tiny market share today. However, this car represents the first mass-produced product designed from the ground-up with lightweighting and extensive use of carbon as primary factors in all aspects of the car’s production, from design through manufacturing and ownership. For that reason alone, it is well worth understanding the nature of the vehicle and the repair processes used to return it to BMW specs after a collision. Carbon fibre is coming — maybe sooner than we all expect.
Blake Desaulniers is a digital media content producer, writer, photographer, videographer, and car guy based in Vancouver, B.C. He can be found on the web at blakedesaulniers.com
This article originally appeared in the Spring 2017 issue of Collision Quarterly.