The Citroen C3 is the fourth World Rally Car from the French manufacturer (after the Xsara, C4 and DS3), and it is the first built to the new 2017 technical regulations. Work on the new car began in April 2015 and was stepped up once the programme was given the go ahead by senior management at the PSA Group. On 19 November 2015, when the sporting future of Citroën was announced, the C3 WRC already existed… virtually speaking, at least, on the CAD workstations.
On 11 April 2016, Kris Meeke drove the car on its first outing at the Versailles-Satory track. The car that made that first test run was very similar to that of its predecessors in terms of the overall structure and layout. The production body shell has been cut to accommodate the roll cage, the transmission tunnel and the subframe supporting the chassis and suspension systems.
The C3 WRC is Citroën’s first World Rally Car to be based on a five-door bodywork. The rear doors have been taken out, but this configuration nonetheless required substantial work on the car’s layout and ergonomics in order to find the optimum position of the crew, taking into account factors such as weight distribution, visibility and passive safety.
On paper at least the C3 should be the fastest ever Citroen works rally car due to new regulations which allow for wider cars with extreme aerodynamic solutions the like of which have not really been seen since the days of Group B. For example, the weight-to-power ratio of the Citroën Racing engineers’ latest creation is 3.1kg/bhp, compared with 3.8 for its predecessor. Since improved efficiency is not only a matter of figures, the dramatic technological changes can also be seen in the wider wings, the substantial aerodynamic features and the four-wheel drive that now comes with a centrally-controlled differential.
The aerodynamics of the C3 WRC are immediately apparent looking at the car. Two packages were developed, gravel and asphalt using both CFD and wind tunnel testing at 40% scale. The centrepiece of the car’s aerodynamic performance, the bumper incorporates a splitter and dive planes, which generate downforce and reduce understeer. The lower part is different on the tarmac and gravel versions.
“The experience acquired with the Citroën C-Elysée WTCC has meant we didn’t have to start from scratch with this car. But it was just the starting point. We worked in iterations. We tested solutions using CFD (Computational Fluid Dynamics) and then in the wind tunnel with a 40% mock-up. The results led us to devise new shapes, which we tested again and so on. At the same time, we had to try parts on the car, to ensure they would last on the roughest surfaces. It’s a never-ending process, really. With more time, we would undoubtedly have made further progress and found even more performance!” Laurent Fregosi, Technical Director
There has been a power increase for 2017 as well though the 1600cc Global Race Engine concept continues. As with its two previous iterations, the Citroën Racing engine has been built using a machined, aluminium cylinder block. This finely crafted piece of metalwork must meet strict rules as regards minimum weight and the height of its centre of gravity.
(an earlier iteration of the Citroen GRE)
The increase in performance expected in 2017 can be explained by one main factor: the bigger turbo restrictor, which has increased from 33mm to 36mm. The power has been boosted by around 20% to 380bhp. However, the turbo pressure limit of 2.5 bars means that the torque remains relatively stable at about 400Nm. Since a 36mm restrictor was already in use on the WTCC engine, the Citroën Racing engine specialists were able to get to grips quickly and confidently with the increased output and internal load. This head start has been used to study the slightest details in even greater depth. Alongside the chemical engineers from Total, in-depth work on reducing friction has helped to improve the engine’s output and efficiency
In terms of details, the 2017 regulations have added freedom in a number of areas. The most visible of these is the increase in maximum width, which has risen to 1,875mm (+55mm), providing more stable handling and new aerodynamic options. A key component as regards traction and the feeling of the drivers is suspension, and this area has undergone some substantial changes. Designed and built by Citroën Racing, the spring-shock absorbers are now titled to increase travel. Among the major innovations introduced on the C3 WRC, the suspension geometry will be different between the car’s tarmac and gravel versions.
The four-wheel drive has also undergone a major change, with the return of centrally-controlled hydraulic differential. This system – used on the Xsara and C4 WRC – allows the front and rear axles to rotate at different speeds. By controlling the hydraulic pressure in the central clutch, it is therefore possible to transfer torque from one axle to the other in order to offset understeer and reduce any slippage.
Looking at the evolution of the C3 WRC there has been a lot of change to the overall design since it made its first test run.
During the early test runs many parts of the bodywork were not finalised, including the whole front end of the car. Note the twin front brake ducts below the main radiator duct (which helpfully tells us that this was test 2). Another interesting area is on the thin grille of the car where the combustion air enters via a duct on the left hand side, there is also an additional cooling slot on on the same side of the car just ahead of the windscreen.
At the next test Citroen’s engineers reworked the combustion inlet as well as the cooing exits on the bonnet, the latter now being larger with pronounced edges.
By test 4 the car had finally gained its front lights and proper grille (again with a minor change to the ducting in that area).
For its eighth full test the C3 WRC underwent a major aerodynamic update, with the front end of the car totally reshaped and LMP style front dive planes added. The brake ducts were reworked as was the main duct for combustion air in the grille of the car, now being smaller and rectangular in shape. Two additional ducts were introduced in the hole left by the adoption of smaller headlights. The left hand side duct being much larger than the right. Its purpose is not yet fully clear.
Curiously at the 9th test the same basic package was used but with some minor changes, note in the image (above) that the upper dive plane on the drivers left has been removed and replaced by a smaller version, while the cooling duct at the rear of the bonnet on the same side has been covered over.
In the official launch of the car another iteration of front bodywork was shown with a slightly reshaped main radiator duct as well as revised brake ducts, note how the bodywork around those ducts appears to be interchangeable.
A final iteration ahead of the cars competitive debut was seen shortly before the Rallye Monte Carlo. The front splitter has been removed as have the lower most of the two front dive planes.
It was a similar though perhaps less extreme story in other areas of the car too, notably the rear brake ducts which are located on the sides of the car just ahead of the rear wheels. At the early tests the ducts were a distinctive slit like shape (above)
The fifth test of the car, on loose gravel appeared to show one of the weaknesses of this design, it quickly fills up and gets blocked (above).
As part of the major aerodynamic update for test 8, a new rear brake duct was introduced, mounted slight higher off the floor of the car and probably less prone to being filled with debris.
This duct has remained on the car since including at its launch (above), note the wire mesh to prevent the build up of debris.
At the rear of the car there are two major aerodynamic devices, the most obvious of these is the large rear wing. A relatively convention WRC style wing was used in all of the early test runs of the C3 (above) but as part of the test 8 upgrade a new version was introduced (below)
The second major aerodynamic device on the C3 WRC is a large diffuser. Initially the designs used in testing were quite simple (below)
But with the big aerodynamic update in test 8 came a far more complex design with four large strakes (below)
A clearer look at the diffuser could be had at the launch of the car and it is apparent that it is had two decks, perhaps the WRC equivalent of the infamous F1 ‘double-diffuser’
Finally to the side of, and above the diffuser the bodywork at the rear of the C3 underwent a major upgrade.
The intitial testing bodywork featured a large rectangular cut out behind the rear wheels (above) used on both grave and asphalt. What looks like a cutout behind the rear wheel is simply a solid black panel.
At the final tests before the launch of the C3 the bodywork was revised with the trailing edge of the front wheel arch reshaped and the duct behind the rear wheel arch removed and replaced by a much smaller opening lower down.
Finally an additional duct was opened up near the top of the rear wheel arch.
At the launch of the car the new layout was clear to see and interesting the front wheel arch (enlarged below) had again been re-worked with a small aerodynamic element added at its upper edge and a opening above the tyre appearing. The small bump in the floor extension accommodates the stands used in the service park.
