Thursday, May 5, 2011

Chapter 15: Differential extravaganza

Ok the title sounds like I'm about to give an advanced math course (differential equations etc).  Don't worry, I'm not, because I am neither good in math nor have the patience.  But it will be a bit of a heavy read, so don't say I didn't warn you!  

Active differentials were this thing that could either make your car undriveable, great, or something in between.  There was just so much to do.  It was like magic, really.  The car I drove in 2005 had a hydraulic system benefitting from components built by Moog, Inc. a company specialized in building high tech hydraulic equipment.  The system ran up to 100 Bar in pressure.  This high pressure system was extremely sophisticated and reacted in milliseconds to different inputs.  There were sensors all over, that could be used by the software to give exact, instant pressures to all three diffs, independently.

Mainly, there was a throttle sensor, sensitive to the pedal angle.  A brake pedal sensor, sensitive to the pressure exerted on it by your foot.  A wheel spin sensor on each wheel, a ground speed sensor, a steering angle sensor, gear sensor etc.

We experimented with lots of different options, during rallies mostly, as tests were too short to do any real work on it .  The car could be programmed with up to 3 different calibrations for each diff, plus a fully locked and a free mode.  You could cycle through them on the control board at any given time.  There were also buttons on the steering wheel.  I used one of them called the "option button", so that when I pushed it, a different set of diff maps was instantly loaded by the system.  This made life easy in the car as you did not have to take your eyes off the road and start fiddling down there with the 3 buttons... For example you could have a very stiff, aggressive map for wide fast roads and a looser map for narrow twisty sections, so you switched instantly as you turned off onto the other section of the stage.  On our car, when the option button was being used, a yellow light came on next to the gear indicator, so you always knew what you were on.

Drivers and engineers regularly discussed their diff maps and made frequent changes to them.  I had the habit of noting down every change I had asked for, on a regular basis, which eventually allowed me to review the latest map at home and plan how I would have it eventually modified on the next rally.

The main things I was looking for, as a driver, was stability and feeling for the car.  I needed to feel and know that if I hit the brakes at speed, the car would lock the diffs to help prevent wheel lock, give a stable hard braking and especially to give some feel for the grip.  Then, still under braking, I steered the car into corners and I needed those diffs to open up just enough to relieve the inherent understeer that locked diffs give.

When I approached the apex of the corner, I came on throttle and I needed a smooth transition from brake mode to acceleration mode, which did not upset the car, but applied strong pressure on the front diff to help pull the car into the apex, a relatively strong and specific pressure on the center diff to keep the car neutral, a smooth rising pressure on the rear diff for traction but not too much so as to prevent excessive oversteer.  All that depended on the speed of the corner.

If the settings were right, I could jump from brake right onto full throttle, and do it as an "ON/OFF" manner because that was my style.  The beauty about active diffs was you could adjust for ON/OFF style or progressive throttling style, or whatever.  The car obeyed.

Once at corner exit and the steering wheel came back straight, a stronger steady pressure was applied for maximum traction.  At higher speeds that pressure was fading because otherwise your car would start "looking" which meant any little irregularity on the road would pull you to it, making it difficult to keep the car steady.

All these varying pressures had to be adjusted to be smooth and transitional between the different modes, that were used, which were an "accel" mode for "on throttle", a "brake" mode when pushing the brake pedal and a "decel" mode when "lifting throttle", without braking.

These 3 main modes were intertwined with others like the "throttle delays" which were called accel TPS and decel TPS (TPS for throttle position sensor).  This was an adjustment you could specify so the modes would not change immediately at the slightest variation of the throttle pedal angle.  As drivers are often playing a bit with the pedal, I had some ~25-35% delays which meant the modes did not switch before my pedal angle changed by X% down from full throttle or up from zero throttle.  There was also a "steering angle" input which was basically in charge of reducing the diff pressures accordingly in corners.  You would specify the reduction in % of the base pressure.  Finally, the foot pressure input, on the brake pedal, activated the brake mode at a certain amount of foot pressure so as to prevent modes switching when I just tapped on it for minor speed adjustments.  Note that you could adjust the total throttle pedal travel, however you wanted.  In this car, I appreciated the pedal travel, which was rather short and suited my on/off style.

Basically, the 3 diffs could be directed in two main distinct ways, or with a combination of the both.

1.  From the TPS system which meant you adjusted your "on throttle/off throttle" only with a throttle position setting, varying constantly the pressures as the pedal moved, which was very basic because this did not take into consideration how much wheel spin difference there was from one wheel to the other.

2. The Accel and Decel modes, a system relying on the constant calculations of differences of spin between the wheels.  It was represented in the software as a graph of speed expressed in km/h (kph) versus wheel spin differences which was expressed in % of slip.  In this system the computer used a certain set of figures when you were accelerating and another set when decelerating, which you specified in the software on 2 distinct graph tables.

The latter system was mainly used for center diff and front diff while the TPS was often used on the rear.  I did not really like using the TPS on the front and center diffs.  I believe this would have been ok for a "smooth on the pedal" kind of driver.  For me, it made the car too loose and nervous to drive,

We had a slip versus speed mode on front and center diffs with a TPS mode on the rear.

I noted with experience that the TPS system had a tendency to wear the tires more than the speed versus slip system, which for me was important as you know from Chapter 8 that I liked to run softer tire compounds whenever there was the slightest opportunity.  As the slip versus speed system was constantly adjusting the diff lock if there was excessive wheel slip, the power was spread better over all 4 wheels, which in turn meant that the tire wear was evenly spread and you could push the limits as far as using soft compounds was concerned.  As a note on that aspect, I had noticed back in 2004 and 2003, when we were using diff maps that were much too loose, that I had huge differences of wear and I often came back to service with completely destroyed tires.  At the time I had not understood yet how to "read" tire wear and make conclusions from it and apply it to diff maps.  I understood this later with experience.    

On the slip versus speed system, the software basically allowed you to fill numbers on a 2 axis graph table with % slip vs kph scales.  These scales could be adjusted and were very important.  On my first maps, the scales were not adjusted well, which rendered whatever map was in there ineffective to a certain degree.  As an example, the standard setting I had in was a scale of slip % going from 0% to 100%, in 5% then 10% increments which made absolutely no sense because, as we learned with experience, most of the action was happening in the first 10% of slipping.  So a good slip scale started from 0% and went up to 10%, not 100%. 

It was hard for me to imagine, but a 5% difference in slip betwen 2 wheels was huge for a rally car.  So our initial customer maps, adjusted with scales going to 100%, were only really activating on the first line or two of the table.  All the rest (10,20,30%...) never came into play which rendered most of the map pressures useless and therefore the diff was basically running the same pressures all the time.

On tarmac, for example, the center diff accel mode had stronger pressures at lower speeds which gradually decreased in a very specific way as speed went up.  This part was the key to the tarmac map.  Drivers are usually on throttle from the apex onwards or even before and this part of the map determined the way you exited the corner. 

The center diff decel map had lower but needed pressures for keeping a steady car when going off and on throttle in long corners where you didn't go in brake mode.  You didn't want a zero pressure center diff in the middle of a corner as that would have spun your car out instantly.  In Corsica 2005, while in a 30+ km stage, my hydraulic pump was leaking.  I was gradually losing pressure and as the stage went on and the pump had more and more difficulty keeping the pressures up, I could feel how, in every other corner, the car was becoming more and more oversteering, until I really had to slow down the speed because I just could not keep the back under control anymore in corners. 

The brake mode was activating on all diffs, independently, when I pushed past a set amount of foot pressure on the pedal.  At that point I had strong pressures in the front and center diffs.  Extremely important for a driver's confidence, that one, as I explained in The Polish Affair.  

The ever important steering angle reduction on brakes was an awesome invention.  The pressures were smoothly reduced on all diffs, independently, depending on how much steering angle you gave.  This is what made the car turn!   

The rear diff was usually set with a TPS map hence throttle pedal angle.  There was a brake mode when you pushed on the brake pedal over a set amount of foot pressure.  It was a rather strong pressured TPS to make sure the car got maximum traction on the rear but in a rather progressive fashion so as not to have violent reactions.  There was also a left foot braking mode which I never used.  I did left foot brake but never at the same time as I gave gas.  I believe this mode was specially designed for drivers who pushed both pedals simultaneously.

Still on tarmac, the front diff was a slip versus speed map with low pressure across the decel graph, so as to make sure there would be absolutely no understeer when in long corners and coming off throttle for speed adjustments.  Very important, that one.  


  1. that`s why with active diffs you weren`t drivers, but something else. Let`s say something like operators.

  2. I really like to read all those tech details about WRC... It gives totally different perspective as to what the drivers needed to do to get the times right. Hope some can learn that motorsport is not just about the drivers :)It's everything else as well.

    I need to say it again that your writting is much appreciated!

  3. Martins (from Latvia)06 May, 2011 16:24

    Thanks for such a detailed explanation of tech stuff.

  4. This blog should be protected by unesco.. Thanks man for sharing your experiences! This is invaluable piece of knowledge for me!

  5. This chapter was very very useful! I was just about to ask what about "left foot braking" and you've answered at the end of the article :) Congrats for all you are writing in the blog and thank You!

    Just one question, what do you think about the simultaneously pushing brake and gas? Yes, you are not doing it but still it will be interesting for me to understand your opinion.

    Greetings from Bulgaria!

  6. In my opinion, doing simultaneous pushing is only useful on cars where you need the engine to keep revved and pulling. Like on the front wheel drive Golf I learned to drive with. on this car you braked while on throttle to get the back to come out, so the car turned better (on gravel and snow). This is a driving technique developed years ago by Nordic drivers.

    On WRC cars its good to do that only if you want to replace your discs and pads on every service.


  7. Hi Antony. Interesting stuff. What about if this should be used today. Imean we have faster digital computer aided gizmos. I think maybe you could have automatic changes without pushing buttons... In short: could it even be better today if allowed. Maybe RX use more sofisticated gizmos today?