Technical Analysis: WRC Cars 2017

[stefanvv]

Finally I have some time to sit down and write again. I was planning to pick up the subject of suspension kinematics characteristics (i.e. camber change with bump and so on), but now that there was news on Citroen using homologation jokers to update their car and while the sporting side is being discussed in the Citroen thread, I thought I should start a similar one on the technical side here.First the “easy” one on aero – I noticed that while testing for Poland, C3 was run with and without lower closing panels behind the rear wheels. Short of these being knocked off (something I tend to doubt in this instance), I can only assume Citroen was interested in finding out the effect of these panels on rear downforce in a fast gravel rally. Also note the missing mud flaps in either case, which is another thing that would affect rear aero.Now on a more challenging subject – differentials and I should start by saying that I wrote the text below in the morning, but I can now see that another forum member has posted some numbers regarding the Citroen differentials in the WRT thread. Anyway, here is what I wrote:In the press release it was said that Citroen have changed the rear differential rail (I assume ramp angles) and front/rear torque split. I will start with a very wild guess (and this is from someone that has zero experience of vehicles with centre diffs) and say that they have gone towards a more equal split of torque between front and rear axles with the newly homologated parts.In my simplistic view of the world, I originally thought that a bigger mechanical split between the front and rear axles on the asymmetric diff coupled with the traditional hydraulic locking, would provide for wide ranging opportunity for varying the torque split - for example let’s say we design in an extreme mechanical split of 20/80 front/rear: when the diff is open that is what we would get, 20/80 front/rear, but as we start applying hydraulic pressure and locking the differential, in theory we can get to 50/50 split. This is where things get complicated though – as we apply locking, the actual torque split (rather than the theoretical one) would more and more depend on the grip each axle has rather than our wishes. In the extreme, with completely locked diff and the front axle in the air, all the torque would go towards the rear despite the locked diff. This make me thing that having a large mechanical front/rear split would be more beneficial on tarmac where you would naturally exploit the handling benefits of rear wheel drive car in tight turns, while being also able to lock the diff momentarily when you need to induce some stability in the car. At the same time this arrangement would be a bit more challenging to make it work on gravel where the grip on each wheel would vary to much greater extent than tarmac (it would seem to me) and so when we lock the diff to induce stability, this could actually make the car wildly switching from under to over steer and make it feel very inconsistent.Funnily enough, with my lack of experience in four wheel drive competition cars, that is exactly the kind of arrangement I would have designed – a large mechanical split, which can then be varied by hydraulic pressure locking, but now that I went through that theory, it appears very flawed to me. It now seems to me that a bit more balanced mechanical split coupled with hydraulic locking would provide a more consistent platform. This is not to say that 50/50 mechanical split is the way to go, but that if you stray wildly from 50/50 mechanical split, you might be in a danger zone. Once again, the disclaimer here is that all this comes from someone with zero experience with centre diffs in race conditions.Going back to the rear diff update and assuming it is the mechanical diff ramp angles that they have changed, this can of course bring its own benefits when it comes to making the car more consistent, with more locking on overrun (braking) providing a lot of benefit in stabilising the corner entries (at least in circuit racing) and then having not too much and not too little lock on throttle dependent on power to grip ratio.Anyway, that is my take on the situation, other opinions welcome, especially if someone has experience from going testing different mechanical splits centre diffs

I'm not a technician in this respect, but isn't an active center diff supposed to give You optimum performance in various corners (in some degree). There used to be fixed ratio diffs already in 90's I believe.