BMW’s Megacity is set to shake-up how the cars in the company’s range are constructed in the future. Here, Autocar takes a look at the different elements that make-up its radical construction.
Life and drive construction
The Megacity’s construction is split into two sections, dubbed ‘Lifedrive’. The Life section is the carbonfibre passenger cell and the Drive section the aluminium chassis.
whole monocoque is made from carbonfibre mouldings that are glued together. BMW has developed its own cutting-edge technology, using special heated moulds and sophisticated resin injection systems in order to mass produce the carbonfibre pieces.
Aside from the floor and bulkhead mouldings, most of the pieces that make up the passenger cell are relatively small. In order to keep repair cost low, the Megacity’s bonnet and wings will probably be made from injection-moulded plastics.
One of the great advantages of the way the MCV’s separate aluminium chassis has been designed is that it’s very simple (and inexpensive) to change for future, differently sized, MCV variants.
A longer wheelbase can be achieved by extending the twin aluminium extrusions that run front to back. A wider cabin can be accommodated by extending the outriggers, which support the monocoque under the sill. It seems likely that future MCVs will include more luxurious long-wheelbase versions and possibly even a seven-seater.
The MCV’s chassis is made up of aluminium castings and extrusions, which offers tremendous rigidity and greatly aids ride and handling. The chassis’ front end acts as subframe for mounting the front suspension and steering. It’s also the crash structure for the car.
This lab crash test picture (below) shows that the aluminium structure has managed to absorb nearly all of the energy pulse, minimising damage to the carbon monocoque. It will be easier and less expensive to repair the aluminium front structure than the monocoque.
The battery pack is mounted in the chassis, under the cabin floor. Because it is very heavy, the battery pack is best mounted low down and centrally. This also helps protect the pack from impact damage in an accident. The use of an extruded chassis makes it far easier to package square batteries than in a conventional pressed steel structure.
Rear doors that stretch
The final production MCV will not have a B-pillar but instead Mini Clubman-style rear-hinged doors. Because of the stiffness of the monocoque and the stiffness of the aluminium chassis, this kind of construction is made much easier. It’s also ideal for building a longer version of the MCV because the front door can remain the same and only the rear door has to be stretched.
Since painting directly onto carbonfibre results in a rather odd ‘fabric’ pattern finish, BMW is said to be experimenting with other exterior finishes. One possibility is very thin polished aluminium pressings, which will be bonded to the door skins.
Electric motor goes here The rear of the chassis is a cast aluminium ‘box’ which will house the MCV’s electric motor. Although the first MCV, due in 2013, will be a purely electrically driven model, BMW sources admit that a range-extender version with a 1.0-litre, two-cylinder engine is possible. Using a range-extender powerplant would allow the battery pack to be smaller, saving weight and cost.
The motor would likely be mounted in the nose of the car, driving a small generator. Range-extender transmissions are probably most likely in future luxury BMWs. Using the battery pack as a power buffer means that high performance can be achieved with a very small engine.
Transmission The MCV’s electric motor and single speed transmission are mounted at the rear and drive the back wheels.
Crash-friendly carbon monocoque
Tests have shown that in side impacts the MCV’s monocoque suffers exceptionally low levels of intrusion, partly thanks to the structural foam in the sill’s void.
Even though the damage is significant, BMW claims this monocoque could be repaired. Ultimately, this construction method (combined with electric or range-extender drivetrains) could be used for vehicles such as the 2020 7-series or X5.
It would be an ideal way of providing proper luxury cars with the CO2 emissions of today’s superminis. Using the same aluminium chassis for a wide range of larger cars would also save a great deal of development money. Only the unstressed carbonfibre passenger cell, seating positions and exterior styling would differ.