Suspension Secrets M3ATER Build – Part 2 – Hateful Weight

If you haven’t already heard, we’ve undertaken an ambitious project, here at Suspension Secrets – to make a BMW 330Ci (E46) faster around a circuit than a clubsport spec E46 M3, for half the price!

In this build update, we’ll take a more in-depth look at the weight saving measures that M3ATER will undergo in order to get it close to our ambitious target of a 1000kg kerb weight.

This means confronting the enemy of all race car designers, something that has given them sleepless nights: weight. A heavy racing car is not a competitive racing car. A heavy car requires more power to move it, bigger brakes to slow it down, and stiffer suspension to manage the weight as it transfers around the chassis.

This is why we’ll be scrutinising every inch of M3ATER to work out how weight can be saved, and where a change in weight will be most influential on the dynamics of the car. Now, it would be easy to get M3ATER under 1000kg if money was no object. The chassis could be redesigned and acid dipped, all other components made from carbon fibre, and the wheels and uprights could be made from magnesium, but we are building this car to a strict budget to keep it achievable at home!

Why reduce the weight?

Unfortunately, weight is inherent to cars. Steel has frequently been the material of choice for building cars, as it gives the cars strength and consistency whilst remaining as one of the cheaper metals of choice. Modern safety standards have also necessitated that modern cars be designed, understandably, with occupant safety as a priority. This however, means adding weight. 

In terms of a performance car, weight is undesirable. Weight and the transferring of weight during cornering, accelerating or braking, can put a high amount of strain on a car and the tyre. In response, areas such as suspension, tyres and brakes are often upgraded or reinforced to deal with this added weight. These reinforced items are often heavier too, so the whole process snowballs, and weight increases. Further to this, the increased weight transfer also acts directly on the tyre which can often lead to overwhelming the contact patch past what it is capable of handling, which will cause a loss in traction and result in understeer or oversteer, which ultimately means a slower lap time. 

A reduction in weight, however, makes a car more responsive when accelerating, braking and cornering. A lighter car will turn more easily due to it having less inertia in the chassis, and this is especially apparent during changes of direction, where the weight of a car is transferred from being loaded on one side of the suspension to the other in a short time. The lower the weight, the less inertia is present, and the weight being transferred is contained and managed far more quickly and with less impact on the dynamics of the car.

A lighter car will also be more efficient. It will require less engine power, and subsequently less fuel to get up to speed, it will not overheat brakes as easily, as the brakes are worked less trying to slow down a lighter car, and tyres will not be worn anywhere near as much as they would be by a heavier car. 

How are we going to lighten M3ATER?

As mentioned earlier, we will scrutinise every aspect of M3ATER to work out where unnecessary weight can be saved, but still within reason to maintain an aspect of safety and stay within budget.

There are a few obvious areas of improvement, which include major body panels and parts of the interior, but if you are really aiming to shed weight, then you need to be a little bit crafty.

In Part 1 of this build, we mentioned the M3ATER’s interior had already been mostly removed. This is quite a simple first stage, and is easier than it looks. As this had already been done, we looked to other areas of improvement. The most obvious to us was the body panels that make the ‘spine’ of the car. This includes the bonnet, roof and boot, which we will be replacing with single skin carbon fibre items. We chose single skin carbon fibre due to its very light weight and high strength to maintain chassis rigidity in areas such as the roof. 

We have chosen to replace the bonnet, roof and boot with carbon fibre as our budget means that we are limited to a select amount of panels to replace. Therefore we chose the panels at the highest points on the car which will allow us to lower our centre of gravity position (CoG) substantially. To accompany this we will also be replacing all windows except the windscreen with plastic windows to further reduce the centre of gravity height.

As well as the overall mass figure being reduced, the distribution of mass reduction is also very important as we could make the car 1000kg by replacing the whole floor pan with carbon fibre in theory, however, this would mean that all the mass is high up in the chassis in the roof, windows, doors and panels. This high CoG would increase our roll moment substantially and would cause the car to lean and wallow in corners like a pendulum, which would make the car handle very badly. That’s why it is very important to focus attention on the areas that you choose to strip mass from and spend some money on select parts to ensure that the CoG and therefore your roll moment is reduced as much as possible, which will produce a much more controllable and direct chassis out on track. 

Whilst our interior was stripped, there was still more weight to be removed. We looked to remove anything that wasn’t essential. There were redundant brackets, the metal rear parcel shelf, the dreaded chassis sound deadening sheets and the preexisting interior fittings such as the foot plates, seats and seat mounts.

The Bimarco Grip seats that came with the car are light, so were not an issue, but the metal plates they were mounted too were extremely heavy, so they were removed. The flat floor plates were also heavy and the bottom of the footwells had been filled with expanding foam and pieces of wood, in order to get the plates to sit flat. These plates and the foam all had to be removed. Then we chiseled away all of the sound deadening from the transmission tunnel, the floor and the rest of the chassis. It may not seem like much, but the weight of this lead-derived material really adds up!

We then removed the rear side windows, and had the front and rear windscreens removed by a professional. We then removed the drop glass from the driver’s and passenger’s doors. Glass is not only heavy, but also placed high up in a car. We are replacing all the windows except the front windscreen with race-spec plexiglass, which is far lighter and helps to bring down that ever important CoG.

M3ATER also had some track-day spec door cards fitted instead of the moulded BMW original items. Whilst they looked smart, they were made from thick, heavy steel. We removed them and will replace them with far lighter fibreglass door cards.

It would be possible to buy entirely fibreglass doors, but these are costly, and would require a full roll cage which adds weight. 

In the interest of safety, we are adding a half roll cage which will add some weight, but it will help add some structural rigidity into the chassis in a mid-mounted location. After all of this has been done, all that is left to remove any excess, but obscure weight that might be left in the car.

Inside the front bumper of M3ATER were two redundant horn klaxons. They may not weigh much, but they were removed. The rear view mirror will be replaced with a lighter version, and the dashboard will be gutted in order to access and remove the heater blower motor. We also plan to remove a windscreen wiper, the inner wheel arch liners and to replace the heavy stock BMW battery. We also removed the spare wheel well, which will be repurposed for something more… interesting, later down the line.

With all this in mind we hope to hit our target of 1000kg, but even if we can’t, we know that weight has been removed from various important areas, which will help M3ATER’s performance.

How will we get on? Who knows! Just be sure to keep checking in with our weekly build updates here at Suspension Secrets!