"In this Special Series, I'm taking another step towards the M5 CSL conversion. I document the development, features and conversion process of the E60 M5 CSL carbon roof panel.

In this entry, I look back at the hidden signature component of the M5 CSL: the carbon roof. I document the development steps to creating the E60 M5 CSL carbon roof panel."
- Matt

When tradition > tradition. 

In the entry "The BMW M5 CSL: the V10 that never was.", I documented the known specifications of the M5 CSL prototype created by BMW M to celebrate the 25th anniversary of the BMW M5. 

Amongst many outrageous specifications was a hidden signature component of modern BMW M cars that started with the E46 M3 CSL: the carbon roof panel. 

We have come to perceive carbon fiber panels as signature aesthetics of BMW M cars over their competitor and as a differentiator to non-M BMWs, yet BMW M chose to paint, or wrap this panel. 

Their motive was to celebrate the longer standing tradition of the M5 with a unique white and tri-color livery. 

To this day, I question the motive to cover the carbon weave. Was it a one-off with dubious weave aesthetics - or did they truly want it white? We will probably never know.   

We have to go by their word here: there's carbon hiding underneath. 

Its sole benefit: weight!

BMW M's decision to cover the weave ultimately reduced the benefits of the carbon roof conversion to its lighter weight, and lower center of gravity. 

Unfortunately, BMW M did not specify the weight reduction of the carbon roof conversion. Instead, they quoted the weight of the M5 CSL to an estimated 3,900 lbs. This is roughly 250 lbs less than a standard E60 M5 with the SMG3 and sunroof. 

The CSL added weight with the DCT transmission and its cooler. It removed a lot with the Recaro Pole Position bucket seats and rear seat delete, some with the M5 forged wheels. 

Extrapolating from the E92, we know the sunroof cars were 44 lbs heavier than an equivalent carbon roof model. With the bigger size of the E60, there's probably over 50 lbs in it. 

They also kept the shark fin: the antenna for the SOS system if memory serves me right. 

Rewinding the clock. 

My end game with the Papi Express is to develop the blueprint to a CSL V10 conversion. After bringing to market the 1 piece carbon plenum variants, bringing up the 5.0 beyond the CSL's quoted power figures with the S85-EVO program, the next two core components were the DCT and carbon roof conversions. 

The first present a significant technical challenge that is currently being worked on, whereas the carbon roof was purely a financial and market challenge. Having gone through a carbon roof conversion with the E46 M3, I was in tune with the core challenges of such an endeavour: cost, and availability of the headliner. 

On a technical level, the E60 had been available in non sunroof specifications for non-Ms and European market M5s. Commonly referred to as "slick top", this implied headliners had been made without sunroof openings.

In early 2024, I sourced a brand new slicktop metal roof panel from BMW Germany and had it delivered to a German manufacturer for mold development. 

Unfortunately, these panels are in limited supply and have been sitting for years in BMW storage leading to surface imperfections. 

This brand new panel had numerous dings that required paintless dent repair to avoid having dings in the mold, and the production carbon roof panel. 

Using the repaired panel, the first mold was created using resin. As the market viability of this product is limited, a complete positive mold was not created. As such, this will be available in limited quantities. 

Tests were then performed to evaluate various pre-preg carbon fiber resins, finding the ideal balance between stiffness, and weight. 

A few panels had to be scrapped throughout this testing process to finally settle on the adequate resin composition. The panel below was considered the first pre-production panel meeting target specifications.  

After excess resin was cut, the raw carbon roof was sent to the paint shop to perform clear coat testing. 

This is another critical step in the development as the wrong combination of preparation, gel coat and clear coat could easily lead to bubbling issues as I experienced with the Karbonius E46 M3 carbon roof. 

The clear coat was also wet sanded and polished to remove visual imperfections and dust specs from the spraying process.

This was exciting, we had a roof panel - but that was only half the conversion. 

Up next: I document my quest and failures to source slicktop headliners, and the solution I ultimately landed on. 

Both get delivered to me and I unbox the kit, reviewing it with my own eyes.