BAC Mono – Under The Skin

We had a chat with the guys at BAC Mono who, if you don’t already know, create an single seater supercar called the BAC Mono. It is renowned for its handling capabilities and the pure driving pleasure that it delivers due to the lightweight and ultra-responsive chassis. We thought we would have a quick chat to find out a bit more about how the BAC Mono delivers this experience and the components and design aspects beneath the skin.

BAC Mono Car Build

Constructed from a chassis made from high tensile tubular steel and pre-preg carbon fibre body panels it is clear that the Mono has been designed with low mass in mind as the kerb weight of the entire car, without a driver and fluids, comes in at 580Kg; which is very light considering that a Vauxhall Corsa (considered a small car) weighs double that. But if you’re thinking that that’s not a fair comparison due to the Corsa having 5 seats, then let’s compare the mass against a X-Bow which is a track focused car aimed at a similar market. The X bow comes in with a kerb weight of 790Kg, a whole 210Kg heavier than the Mono.

BAC Mono in Production

Before we take a closer look at the suspension on this track weapon the engine and gearbox set up requires some appreciation as the Hewland FTR 6 speed sequential gearbox has a fully stressed casing with all the rear suspension components mounted directly to the gearbox. The inline naturally aspirated 4 cylinder is also semi stressed as it carries some of the rear load into the chassis. The main reason behind having a stressed gearbox is due to there being no need for a rear subframe or rear tubular chassis to house the suspension components. Instead, the arms and bell crank all mount to the gearbox which reduces overall car mass and particularly helps take some mass away from the rear end, helping to achieve the perfect balance that the car demonstrates. This is only one of many details that represent how much effort, time and expenditure has been put into ensuring that the Mono delivers on handling and performance out on track.

We couldn’t wait any longer until we started to delve deeper into the design of the suspension system to find out how a 580Kg car with minimal downforce manages to generate over 1.5G through the corners. For a start, both the front and rear suspension systems are pushrod activated twin high tensile steel wishbone systems. It would have been much cheaper to opt for the more standard set up of a double wishbone damper set up instead of using a bell crank so we asked why they decided to continue down the expensive route. The reply was:

“The Mono is the ultimate driving experience and as such no compromises were made to the technical specification. Pushrod suspension allows a 100% efficient roll bar as it converts the wheel movement into the suspension and then magnifies the wheel travel into the spring / damper assembly via the motion ratio on the rocker. This applies to front and rear”

The materials used for the bell cranks and uprights is billet 6082 T6 aluminium which is known for being very light and strong and is used heavily in the top levels of motorsport due to its incredible mass to strength ratio. Keeping the mass down in the suspension components is critical particularly with the upright as it helps to reduce the un-sprung mass of the car allowing the damper and anti-roll bars to have much greater control over the vertical movements of the wheel, keeping the tyre in contact with the ground for more time, increasing grip out on circuit to push harder and drive faster. Couple the use of the 6082 T6 alloy with the lightweight AP Racing Formula Car Calipers and cross drilled ventilated discs with billet machined centre hubs, it begins to draw a picture of how important keeping the un-sprung mass down was at the design stage.

Moving along the suspension components we get to the external reservoir dampers mounted to the bell cranks via solid bearings. The coilovers are made by a company called ZF Sachs Racing and they are made to BAC’s bespoke requirements as the Mono is so different than anything else there is nothing “off the shelf” for this car in terms suspension components.

“ZF Sachs are also renowned for their incredible development support which has helped cement the relationship with BAC”

The dampers are two way adjustable and offer a large range of damping adjustment so it can be soft and usable on the road but then when you arrive at the race circuit, they can be made firmer to handle the higher loads and speeds experienced when driving on track. The springs used on the dampers are made by Eibach (another very reputable brand in the spring world) and are once again fully bespoke springs made exactly to BAC’s requirements. The springs are rising rate so as the spring compresses it becomes progressively stiffer making the car more firm through the corner as it loads up but allowing for a much smoother and more stable driving experience on the road and circuit. Both the dampers and the anti-roll bar mount to the bell crank which then mounts to the upright. The bell crank is also adjustable allowing the pickup points to be moved, altering the wheel rates of the anti-roll bar and damper. This is a simple and effective way to adjust the effective stiffness of the coil spring or anti-roll bar at the wheel making track side adjustments fast and easy to execute to allow the Mono to be tuned to each driver’s style.

Looking at the car it has the characteristics and features of a fighter jet. We therefore wondered if the Mono generates much downforce and whether aerodynamics affected the design stages of the suspension and which aspect took priority. The short answer is that the Mono doesn’t generate much downforce at all. This is mainly due to the fact that downforce creates drag and drag reduces acceleration and top speed.

“In order for this car with a relatively small engine to do supercar speeds of over 170mph it had to cut through the air with minimal drag”

Therefore, aerodynamics did play a large part in the design of the suspension system as every component and packaging decision was made with drag reduction in mind. Each suspension arm on the car has a section profile that is optimised for aerodynamic performance, with a shape similar to that of a tear drop. Another benefit to choosing the more expensive and complex pushrod suspension system was that of aerodynamic gain. Using a push rod system with a bell crank allows the non-aerodynamic springs and dampers to all be mounted inside the bodywork panels and be replaced with a well-designed low drag pushrod instead which disturbs the air far less. An added benefit of having pushrod suspension is that the air through the front suspension is much cleaner allowing it to travel undisturbed through to the radiators to increase cooling efficiency as well. One compromise that had to be made when designing the shapes of the suspension components was that of pedestrian safety due to the fact that this track focused car had to also qualify as road legal. Therefore, the engineers were restricted to a minimum edge radius of 2.5mm on any external surface meaning that some optimum shapes were not attainable.

Finally we wanted a closer look into the design of the suspension system, in particularly the adjustments that are available in the system.  The rear suspension has both camber gain and bump steer in the system. The camber gain is beneficial as the outside wheel will require more negative camber through fast corners so as the load compresses the spring, negative camber increases, giving the tyre more contact patch as the body rolls outwards. The adjustable geometry settings available on the Mono are: toe, camber and ride height.

“Having these areas adjustable allows the customer to fine tune the setup of the car to their particular driving style and to each particular circuit it is driven on”

Furthermore, road and track surfaces differ hugely in grip levels so the car can also be optimised for that as it is used heavily in both environments.  The rear ride height is also 10mm higher than the front giving the car a rake angle. This angle is mainly for aerodynamic purposes to utilise the underbody aero of the car.

Looking at the car as a whole it gives the impression that every detail has been meticulously designed and every component of the car is only there to serve the pure purpose of driving pleasure and handling excellence. Now that we have delved deeper into the designs and decisions made, it seems that evidence has now been provided to confirm our initial thoughts and it really is no wonder that the BAC Mono is regarded as one of the best driver’s cars of all time.

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