Tuesday, April 7, 2015

Comparing differentials

I have received an email from a fellow motorsport enthusiast from USA and he asked many questions!

Ok here we go:

"Would it be possible for you to compare the differentials from the current WRC cars to those active differentials used previously?"

It’s visible that today's cars behave differently than the fully active cars of 2005. As I understand the current cars have no CD (center differential) at all. The gear box splits the torque front to rear by 50:50. Torque goes via an output shaft towards the RD (rear differential). There is a FD (front differential) as well and I understand it’s located inside the gearbox or right next to it. Both diffs are mechanical, plate systems. You also refer to the viscous limited slip differential system in your e-mail. As far as I remember some of the old group A cars like the Celica st205 or the Mazda 323 had viscous CDs.

Concerning current machines I can give you my opinion based on my experience and knowledge of the various Gr.A and WRC cars I have driven:

The behaviors of today's FD and RD are as close as mechanically possible to what the active diff philosophy was in 2005 (see differential extravaganza). However, the understeer effect created by the front/rear lock is dealt with a combination of suspension, geometry and weight distribution adjustments. In my opinion you'd basically want to have a looser rear end.

Engineers have set-up the car’s suspension, geometry and weight distribution to have more pronounced weight transfer towards front and outside wheels upon braking and cornering, allowing the rear inside wheel to lift its weight as much as possible off the ground. This would help the inside wheel spin easily and relieve from some of the push effect. 

Anyway, the understeer will be pretty apparent in tight corners. The driver will end up driving more sideways than it used to be with activ diffs, to get the car to turn. 

In my opinion the shorter wheel base of these cars makes it easier to deal with this issue. 

“Has this aided some driver technique at the expense of other drivers?”

Surely. Also I am confident that my driving style would have been more suited for passive cars, for example. The driving style required for these cars also augments the danger of hitting objects with the rear end. Drivers with inaccurate note systems who rely a lot on last moment corrections, are more prone to accidents. On snow the sideways, balls-out, drivers will excel as the risks are diminished by aiding snow walls.  

“How does one drive these new cars on tarmac and loose compared to the cars fitted with active differentials?”

Nowadays on gravel you need to be aggressive with your driving style. That means really come into corners with speed and throw the car sideways into them. On snow, with spikes, it’s even more primordial. Active cars (when adjusted right) allowed smoother driving, steering rather than sliding into corners, and powering out effectively with straight pointing wheels. I believe Loeb really understood this principle and that’s why he drove the Xsara WRC faster than anybody else. Sainz, McRae and Duval were all trying to drive the Xsara like it had been for them with passive cars (Gr.A cars). It was their nature and they could not adjust to this “let the car do the work” philosophy as well as Loeb; who was really lucky in that respect because he was nurtured by Citroën into the FWD Saxo and Xsara kit cars which were the perfect learning tools for stepping into a 4WD WRC fully active machine. Markko Märtin had also understood that less is more with the 03 spec Focus.

“I expect on the loose, this differential was always locked”

It was hardly always locked. The CD adjustment was enormously important and absolutely crucial to get right. It took thousands of kilometers of testing and development to get this diff to behave in the optimal fashion. I described it in differential extravaganza. Going through a Corsican stage with a locked center diff would have meant a loss of 2 sec/km or more.  

“For Tarmac has the lack of an active diff been much of a reduction in performance?”

Yes, absolutely. But the newer cars have gained performance elsewhere to compensate.

“What has become the most preferred passive differential used front and rear on the WRC and R5 rally cars? Have these returned to mechanical design, plates or viscous designs? If you are not aware of the choice at the WRC level, which would be your theoretical choice for your driving style?”

Plate mechanical systems are used nowadays for the FD and RD. I am unaware of details used by current teams. For my RD on a Finnish type gravel stage (for example) I would start out with 75% diff lock, aggressive ramps and a slight amount of pre-load. For the FD maybe 45% diff lock with semi aggressive ramps and a slight amount of pre-load. It’s difficult for me to quantify because the numbers I am used to speak about are completely different from today’s, nevertheless I would be able to answer this question upon testing any car in real life.

4 comments:

  1. I am not sure if you are going to read this comment or not but I will post it anyway... Could you please explain how modern (2017) active center differential work?... Thank you...

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  2. This comment has been removed by a blog administrator.

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  3. Arnold. It a great opportunity for all of us to share ideas and informations with you.
    What do you mean 75%diff lock? I can understand preload and ramp angles, but diff lock? How do you measure this?

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    1. It means the percentage of torque applied to the wheel that has more grip, or that wants to turn slower. 100% diff lock is like a solid connection between both wheels, like a Go kart's rear axle. Imagine our Go Kart turns and I could put a diff on the rear... 75% diff lock would mean the inside wheel will be allowed to turn slower and this will help my Go-Kart turn better, improve my tire wear, etc. You need to ask an engineer how to measure this. All I know is 75% is the sweet spot where the compromise between traction and handling is good.

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