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.

[NickRally]

The only thing that would give you the optimum grip for each wheel in all conditions (short of using one electrical motor per wheel) is a transmission consisting of one clutch pack per wheel with continuously variable engagement, but as we established earlier in the thread, we do not believe such systems are reliable enough, and at the same time they are illegal in WRC.

[stefanvv]

That's probably the best way to go, and where the hybrids will come along.

[wrc2017]

Very interesting. So your theory explains what some of us expected. Citroen has designed torque split thats bias towards tarmac, and hoped it they could 'make' it work on gravel via hydraulics, leaving it unpredicable?
Also on the pictures is the negitive chamber on full droop what you would expect?

[GravelBen]

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.

I only have experience of such systems in road cars rather than competition, but that sounds quite similar to the Subaru DCCD system first used in the Type-RA homologation Imprezas in the 90s.

They had 36F:64R mechanical torque split, with variable locking centre diff (capable of full lockup but only recommended for gravel/snow etc as it would put a lot of strain on the drivetrain on tarmac). In the road cars its driver controlled with a small dial so you tend to set it and leave it rather than adjusting mid corner, but electronic controllers were used for competition (and later introduced to the road cars too).

The ones I've driven were far more neutral, balanced and understeer-resistant than normal 50:50 viscous LSD Subarus, but also snappier when they let go and I know they caught out a few people who expected idiot-proof predictability. IIRC they also had a fairly agressive rear LSD which may have contributed to that as well. Heaps of fun anyway.

I disagree about locked centre diffs being unpredictable on gravel though, at least in my (enthusiastic but non-competition) experience - yes the torque split is effectively determined by traction available, but being locked the front and rear axles rotate at the same speed so you won't get either axle overspeeding relative to the other. One of my vehicles has both open centre diff and locked centre diff 4wd modes, and on rough inconsistent gravel it behaves much more predictably when locked. Does require a more agressive corner entry to avoid understeer though I guess.

[traxx]

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.

Mud flap ?

[NickRally]

Wrc2017 – wheel cambers: yes, the positive cambers in full droop are inevitable consequence of the type of suspension used, but there are some differences between the teams, which I would like to analyse in pictures when I get the time.

GravelBen – thanks for the additional notes. I should also clarify that what I meant is the car might feel inconsistent when switching from open diff with massive rear torque bias to locked diff, rather than a constantly locked centre diff being inconsistent, which is what we had last year. With centre spool (locked diff), the rest of the car would be setup to deal with it and also the driver would adapt to such arrangement.

traxx – mud flaps or mud guards (depending on your preference), which Citroen have been using in gravel rallies. Not only this, but while Toyota and Hyundai have a diffuser ahead of the rear wheels, Citroen used to have another mud guard there acting like a negative gurney, sort of lift creator rather than downforce.

[GravelBen]

GravelBen – thanks for the additional notes. I should also clarify that what I meant is the car might feel inconsistent when switching from open diff with massive rear torque bias to locked diff, rather than a constantly locked centre diff being inconsistent, which is what we had last year. With centre spool (locked diff), the rest of the car would be setup to deal with it and also the driver would adapt to such arrangement.

Ah I get your meaning now, agree with that.

[Steve Boyd]

traxx – mud flaps or mud guards (depending on your preference), which Citroen have been using in gravel rallies. Not only this, but while Toyota and Hyundai have a diffuser ahead of the rear wheels, Citroen used to have another mud guard there acting like a negative gurney, sort of lift creator rather than downforce.

Mud flaps in accordance with FIA Appendix J - Article 252 - 7.7 (http://www.fia.com/file/51626/downlo...token=bM0OZdyu) are required on gravel rallies. They aren't aerodynamic appendages added at the discretion of the vehicle builders.

[NickRally]

Mud flaps in accordance with FIA Appendix J - Article 252 - 7.7 (http://www.fia.com/file/51626/downlo...token=bM0OZdyu) are required on gravel rallies. They aren't aerodynamic appendages added at the discretion of the vehicle builders.

Correct. I was just pointing out to the fact that they are testing without them, no doubt in pursuit of finding some answers.