The Monocoque

The Monocoque is not new, however it is expensive and though there are other far reaching benefits, the trade off against using a body-on-frame chassis is volume. If you have the production volume, it's a win.

Lancia is credited with producing the first monocoque unibody in The Lambda in 1923 developing some 13,000 vehicles in 9 series, some running concurrently over a 9 year period.
André Citroën utilised a monocoque unibody for the mass production of the Traction Avant (pictured above) which produced 760,000 vehicles between 1934 and 1957 (with right hand drive manufactured in Slough) and as a result saw a reduction of 70 kg of steel required per car.

The Austin A30, the unsung hero?
Well, in its 5 year production between 1951 and 1956 when it was superseded by the Austin A35 a total of 223,264 A30's had been built, so the public were certainly enamoured of the car, but it's lesser known claim to fame is the fact that it was the UK's first monocoque unibody car.

1954 Austin A30 Saloon 800cc

Ladder Chassis Frame - BOF

Ladder Chassis Frame - BOF Fig.1 Conventional Ladder Chassis

In the beginning

All vehicles, cars, vans, and trucks used to use a “body-on-frame” chassis which was something akin to a ladder with 5 or more rungs, sometimes opting for a cross piece or diagonal cross brace taking the place of some rungs and thus known as the Ladder Chassis frame.

The frame was constructed to be extremely strong, rigid and heavy to withstand all the stresses a vehicle encounters, such as braking, collision impact, centrifugal forces on bends, the weight of the vehicle chassis itself, the vehicle body, occupants, cargo load and indeed overload.

The chassis was the first stage of the vehicle construction.

In assembly the Ladder chassis frame would include the axles, wheels, suspension, radiator, engine, clutch, gearbox, drive shaft, exhaust, petrol tank.

In the vehicle design process, companies would try to establish a chassis frame size that would be cross compatible with vehicle engine and body variations i.e. 2 door, 4 door, and even different vehicle models.

Once the ladder chassis frame components were complete, then the vehicle body (Coachwork) would be assembled on top, hence the name body on frame (BOF).

This design is still in use in most full-size pickups and SUVs and virtually all medium and heavy-duty work trucks.

Ford Transit models are built on a Ford uniladder frame, the emphasis being on heavy duty loads, towing and durability.

Fig. 2 Ladder Chassis with mounted engine, gearbox, brake shaft, suspension

The Ford Model T production time dropped by 87%

Henry Ford did not create the first vehicle assembly line in 1913, these were well established, but the following year he did create the first moving chassis assembly line.

The Ford Model T vehicle Body-on-Frame Chassis would move along to each workstation initially pulled by ropes, and subsequently a moving chain mechanism, where the assembly workers would incrementally add the sequential components to each chassis whilst their workstation would be constantly replenished with their respective components.
By adopting this method a completed car would roll off the production line every 90 minutes down from a previous productivity of 12 hours per vehicle.

Model T sales would reach 15 million over the next 14 years.

The production process was constantly evaluated and was an evolving process year on year, one example being that the petrol tanks arriving at the assembly line were pre-filled with just one gallon of petrol prior to being fitted, so that the fully assembled car could be started and driven off the end of the assembly line.

Core Benefits of Traditional Body on Frame (BOF):

The inherent strengths of the traditional body on frame chassis are:

Off-road driving: The design of this frame allows for more resistance to uneven terrain.

Greater Towing and cargo capability: The body on frame allows for more flexibility and power to get through challenging terrain because it’s built with a solid steel frame.

Overall rugged durability: It is far more durable at handling stresses and heavy load.

Repairs: The body or coachwork is easy to repair as panels can be removed and replaced without having to do too much work compared to the monocoque unibody which is, apart from doors, boot and bonnet a single shell.

Dent the body and you hope that it is on a flat surface that can be “pulled out” or a door where you can work inside out, but if it is on a corner or is a sharp angled dent it is more difficult to repair, if possible at all.

What is a monocoque unibody?

The term monocoque is in fact misleading. Semi-monocoque is more pertinent as a true monocoque is one where the skin takes virtually all of the load: Think egg.

However, the “chassis-less” design as seen in Fig. 3, 4, and 5 below is an all in one rigid body, but with a stressed skin bonded to it, of which the skin itself also contributes to the load-bearing of the running gear mounted directly to it.

There is no rigid frame below as per a conventional chassis and thus the very complex design of the shell and outer skin serves to counter stresses and loads.

Benefits

Safety

The monocoque rigid frame is designed to protect the occupants in the event of a crash with crumple zones front and rear which absorb and dissipate the impact energy during a collision.

What is a monocoque unibody? Fig. 3 The Monocoque is used for over of 90% of cars today

Automation

Automation Fig. 4 Monocoque Assembly line

Roll-Over

Because the Unibody monocoque frame is self supporting and does not sit on a chassis (as per the conventional body on frame chassis) in addition to a more rigid structure it has a much lower centre of gravity which reduces the risk of the car rolling over in an accident.

Fuel efficiency

The unibody is lighter than a body-on-frame vehicle, using less fuel than a heavier car, has better acceleration, braking, and much better handling and may require a less powerful engine than a BOF vehicle to achieve the same performance.

Mass Production

Volume production to a defined specification, producing a lighter, yet stronger car using less materials and nowadays with advanced computer aided design CAD which lowers the cost price and rate of production.

Downsides:

Repairs:

The monocoque is more expensive to repair than BOF as damaged areas are a composite part of the shell ( no replaceable panels) and crumple zones….well…..crumple.

Not Suited to Off Road

Very poor suitability to the rigours of off road driving.

Though many do have tow bars, they don't have the strength of a BOF for towing.

Monocoque Render

Fig. 5 Monocoque "Chassis-less" render showing frame, struts, support and crumple zone

First USA Monocoque Unibody:

The 1950 Nash Rambler was America's first compact car with a monocoque unibody and is recognised as creating the “small car" vehicle category in the American automobile market.

It was shrewdly launched as a very high spec 5 seat luxurious 2.6 litre economical convertible just under the the competitions base price, but boasting more accessories as “standard” than the competition had listed as options available at extra cost.

It was a success story and all the moreso because Nash stole a march on the competition dominated by the big three: General Motors, Ford, and Chrysler.

Get on the bandwagon: What's not to like?

Well quite a lot as it turned out…

Saving on materials and thus weight, delivering better fuel economy, safer cars and a growing domestic market ripe for mass production…it seemed the only way to go.

Reality Bites:

Monocoque unibodies whilst ideal for compact saloons and hatchbacks, were not so economically viable for the production of longer cars as they had to contend with two problems: Fashion and Noise….

The 2.6 litre Nash Rambler

First USA Monocoque Unibody: Longer than the Ford Anglia, but 31 inches shorter than the Ford Fairline

Refined Comfort of the Long Wheelbase Saloon

Refined Comfort of the Long Wheelbase Saloon Oldsmobile F Series with the Body on Frame Chassis

Noise, Vibration and, Harshness ( NVH)

Ride comfort in the american market was a key buying consideration and one of the key elements to master in switching to a monocoque was how to insulate against noise, vibration and harshness (NVH) as the unitary body tended to act as an echo chamber/sounding board and amplify noises that were transmitted through the independent suspension.

On the Body-on-Frame (BOF) chassis the suspension is beneath the rigid chassis eliminating a lot of NVH and then rubber blocks (see Fig. 2) in between the Frame chassis and the body on top acts as a damper to further isolate the engine from road shocks and the vehicle from engine vibrations.

The problem was that by the time the NVH had been eliminated on long wheelbase monocoque cars, the additional weight of insulation materials required made the car heavier than the BOF chassis vehicle thus negating fuel consumption benefits, though clearly mass production benefits enjoyed by smaller “compact cars” was still the ultimate goal.

Ford Fairline Skyliner

The weight of noise and vibration insulation materials compromised fuel efficiency on vehicles with a wheelbase of around 207 inches

Ford Monocoque Policy - No vehicle over 115 inches long.

The Ford Company decided that vehicles over a 115" - 2921 mm wheelbase length and exceeding 3200 lbs would continue to use a Body on Frame chassis…….well at least until 1965 when they adopted the perimeter frame chassis.

The modern Ford Focus length is between 4378 - 4397 mm - 173.11 inches and 1506kg
The 1959 Ford Anglia 105E 1000 was 3900 mm - 153.54 in and 737kg curb weight
The Austin A30 was 3460 mm - 136.22 inches with a curb weight of 685 kg
The 1955 Jaguar 2.4 litre was 4597 mm long - 181 inches and 1342-1395 kg curb weight.
The 1957 Ford Fairlane was 5276 mm long - 207.7 inches and curbweight: 1501.391 kg
The 1958 Ford Lincoln was 5,817 mm 229.0 inches and 2300 - 2660kg curb weight
The 1950 Nash Rambler was 4,470 mm - 176 inches and 1102 kg - America's first compact car with a monocoque unibody and recognised as creating the “small car segment” in the American automobile market.

Monocoque Unitary Chassis vs BOF Lifecycles

And what about the fashion “problem” you ask?

Ford did sell 15 million Model T's over 14 years ending in 1927.
However, in 1909 a new Ford Model T cost $850 which was the best value on the market, though not the cheapest, but by 1924 the price had dropped to $295 through constant assembly productivity innovation and cheaper vehicle component production to maintain and grow market share.

Companies who had invested in wholesale Monocoque Unibody design and production that were successful relied on leveraging every aspect of the business from production, design, sales and marketing as it was so expensive to change the design of a car once production had started.

In the 50's Computer aided design (CAD) didn't exist. The earliest most basic invention was in 1957.

Keeping your product trending, in the public eye and it's latest evolution is part and parcel of marketing.

Planned Obsolescence

Have you upgraded your mobile phone recently?

Planned obsolescence is a business sales strategy where products are planned and designed to wear out, or go out of fashion on purpose, within a scheduled timeframe with a view to shortening the replacement cycle….so then you buy the upgraded model.

Monocoque unibody production has the benefits of mass production: quicker, cheaper (less materials used ) per car, so lighter, and thus, more fuel efficient, but expensive to initiate and implement with a higher breakeven sales point per model for ROI.

Compare it to the body on frame companies or indeed companies whose pockets were deep enough to have monocoque unibody and BOF production lines who could introduce a new vehicle upgrade model every year, without even changing the chassis.