Rebellion R13 Technical Analysis

Rebellion Racing are no strangers to the WEC LMP1 privateer title, having won it from 2012 to 2016. The question is can their Rebellion R13 take them to overall LMP1 victory – beating Toyota’s hybrid TS050 at this year’s Le Mans?

Gibson GL458 Engine

The Rebellion R13 is powered by the Gibson GL458 4.5L V8 engine and runs the ORECA chassis. Although, unlike previous seasons, ORECA are heavily involved in the design and running of the car as with only one LMP1-hybrid (Toyota) competing in the 2018/19 season and with two Le Mans 24 hour races – the Rebellion R13 is ORECA’s best shot at claiming an overall Le Mans win.

‘We wanted to run a naturally aspirated engine and decided to go with the Gibson because the external dimensions of LMP1 engine are the same as the old LMP2 engine,’ highlights Bart Hayden, Team Manager at Rebellion Racing. ‘This meant that the installation was pretty straightforward because the only difference is the internals of the engine. Also, we were given the green light on this project quite late in 2017, so in the timescale we had, it was much easier to get this engine in than anything else.’

The Gibson GL458 is based on the LMP2 engine and has therefore had to increase in capacity from 4.2L to 4.5L. Consequently, this has changed the heat rejection figures and so Rebellion, along with ORECA have had to work on the cooling. ‘It needs more cooling. The car looks from outside [to be the same] philosophy as the ORECA 07, but the airflow structure is quite different, because the car in detail is different. We had to work towards improving the cooling and this has been a challenge,’ explains David Floury, Technical Director at ORECA. ‘The front of the car has quite different aero and that impacts the flow to the radiators. The flow from the front to the back is quite different and that is why in many details you see differences between the R13 and the 07.’

Rebellion R13 Aerodynamics

Luckily, ORECA were able to focus the majority of their attention on the aerodynamics of the car because of the proven concept of the Gibson engine, which supplied the entire LMP2 grid in 2017 as well as the 2018/19 season. ‘Because their engine and its performance in the ORECA 07 chassis is well known, ORECA didn’t have too much guessing to do in terms of whether the figures supplied by Gibson were what you would actually see on the track,’ highlights Hayden. ‘The engine was a known quantity for them which meant that they could focus more on the aerodynamic performance of the car.’

Like most teams, Rebellion arrived at Spa for the 1st round of the 2018/19 season with the low downforce package to focus development on the first of two Le Mans 24 hour races. By the third round at Silverstone in 2018, the Rebellion R13 was running the high downforce package. ‘There is so much downforce in this car, much more than the LMP2. Even in the Le Mans [low downforce] aero configuration, we’ve got more downforce than the LMP2 has in high downforce configuration,’ concludes Hayden.

Rebellion R13 Chassis

The Rebellion R13 uses the same tub as that used in both the R-One and the LMP2 customer chassis and the design is now more than five years old. However, ORECA’s late decision to take on the project in 2017 compromised some of the performance upgrades that ideally would have been introduced. ‘We would have liked to have redesigned the tub, but considering the timescale to do the project from the start this was not an option,’ highlights Floury. ‘We had six months from the day we started the project to the day the car hit the track, so in this timescale we did not have time to redesign the chassis. Had we had 12 months extra for sure we would have considered it. This original chassis was designed to LMP1 2014 regulations, and it is still the monocoque regulation that is valid for LMP1 non-hybrid and LMP2.’

Rebellion R13 Weight

Floury estimated that 10-15% weight saving could have been achieved on the Rebellion R13, however the short timescales meant that the design team had to save weight on the rest of the car instead. One of these areas was within the engine and the collaboration between ORECA and Gibson resulted in reducing the weight of the base engine unit. Another of these areas was the Xtrac gearbox which is again a similar concept to that raced on the LMP2 in 2017, but the details have been optimised to minimise weight, whilst improving efficiency.

Rebellion R13 Suspension

To meet the target times around Le Mans as specified by the ACO and FIA, weight distribution, suspension design and steering concept have all had to be redesigned. The splitter, floor, and flow to the tightly regulated rear diffuser have also seen major developments too. ‘Nothing is carried over from the LMP2 car,’ says Floury. ‘The steering is different, the suspension geometry is different because we don’t use the same tyres, and it is not the same characteristics in terms of weight, cornering speed and so on, or in terms of engine characteristics. Weight distribution is different, and from the LMP2 we had to save a lot of weight, so if you keep everything the same then you don’t hit the target. We carry quite a lot of ballast in LMP2, but we still had a lot of weight to save for LMP1. The target was not only a matter of hitting the minimum weight, but hitting the weight with ballast.’

Despite the different suspension geometry and uprights, the overall concept is similar to the 2017 LMP2 version, with a torsion bar at the front and springs on the rear, running PKM dampers, which are the same as LMP2.

Rebellion RB13 Brakes

During the 2017 24 hours of Le Mans, several LMP2 teams had to change their brakes during the race due to wear. ‘We knew it was marginal going into the race, so we wanted to make sure that in this year’s car with the increased speeds that we weren’t going to face that, so we were keen to see a good mass of carbon material in the brakes,’ explains Hayden. ‘Cooling on the brakes was a bit of a challenge in the ORECA 07 and 05, so we wanted a brake pad that was a step up in terms of cooling as well. The discs are vaned all the way through so cooling comes in through the centre of the hub and blows through the disc. We’ve got two options for the cooling and aero around the brake discs. If you’re looking for aero performance you use one and if you want to try and get heat radiated from the brakes [to the tyres] you use the other.’

The optimum temperature at the point where the driver starts to apply the brakes is around 400°C and that temperature rises to peaks of around 800-900°C. ‘What you’re looking for is that temperature when you first hit the pedal, if it’s too cold then you don’t get the bite, if it’s too hot then you start wearing them and when they get too hot they wear very quickly,’ highlights Hayden. ‘If the brakes are working in the right window then there’s no wear at all. The drivers will lose temperature if they’re following a safety car or if there is a full course yellow, but normally [the brakes] warm up fairly easily, within the 1st lap. We also have a lot of tuning in terms of blanking panels for the air that feeds to cool them so there’s quite a lot we can do.’

Rebellion R13 Tyres

Running the low downforce configuration, particularly at Spa could lead to lower tyre grip, consequent slippage and therefore wear and degradation. However, this doesn’t seem to have been an issue so far with the Rebellion R13. ‘We haven’t really seen much tyre degradation at all, we’re happy with it in terms of suiting to our car,’ says Hayden. ‘We’re expecting to triple stint the tyres.’

‘The main motivator for Rebellion to come back into LMP1 is not to win the championship, it is to win Le Mans,’ says team owner Bart Hayden. ‘You have a car that looks similar to the ORECA 07 LMP2 car, but it generates more downforce for less drag, weighs 100kg less, has got 60 to 70bhp more than the LMP2, in Le Mans trim, so it should be pretty handy, but I am not sure that it is handy enough to keep up with the Toyota.’ The result at last year’s Le Mans race proved Hayden right, with both Rebellion’s finishing behind the Toyota’s. However, that was the first Le Mans in the 2018/19 season – Rebellion have a second shot at claiming that all important Le Mans win.