Wednesday, September 24, 2014

Space jump or car setup?

I was asked by a reader to write a little more about setups and what to do in certain instances when a car does something awkward.

“I know you've talked before about car setup, but could you please right something a bit more general. What I mean is that you could write saying things like: "if the car's oversteering, then you can make changes to the damper rebump", "if you have understeer, maybe you could have a change in the diffs preload".”

Ok, let me take a crack at it…

Setup is crucial if you want to get the most out of your car, we all know that. But it’s not only about theoretical performance (engineer’s calculation of optimal setup), it’s also about which setup gives you confidence to push more! It’s important to understand that the best performing teams have understood they need to find a compromise between the two. 

Do you remember the “energy drink” space jumper guy? Well, guys like him drive rally cars as well; I bet you’ve seen them! They are fearless, can push it to their maximum, driving whatever, and sometimes (more like often) don’t make it far into rallies because there's just too many hazards. I will focus on the other type of driver, who needs to feel good with his car to perform at his best.

Back in 2004 we were running with a 2002 Focus WRC and I had some confidence issue with taking jumps flat out. The problem was that I knew if I jumped much there was a very high risk that the car would nose-dive into the ground. Luckily I got some advice from François Duval, he was driving a factory Ford at the time. One day he told me that tightening the rebound on the rear dampers would help with the nose-dive issue. Problem solved, thanks François. You’re gonna ask me why a loose rebound? Easy: For maximum traction! If the rebound is loose, the spring will extend easily when the wheel wants to lift off the ground. This means the wheel will keep in better contact with the ground, hence more traction. The down side to that is when you are about to jump and the wheel starts to get light, the spring extends freely and catapults the back of the car in the air, resulting in a nose dive! Nowadays things have changed a bit because there is a second rebound adjustment, I believe first invented by DONERRE sometime around 2004-2005. They are specialized in off-road dampers. The system is called "DETRA" for détente rapide, or "fast rebound". 

courtesy of Donerre

The damper can sense the difference between sudden loss of contact with the road and “normal” movement. It will release the rebound control and let the spring extend quickly as soon as the wheel is off the ground. As a WRC driver you have to deal with that adjustment as well. 

I’d like to add that the Swedish Rally was a good example of a loose rebound rally. The reason being that stages are very smooth, slippery, fast and flowing, with few jumps. You need as much traction as possible and making the damper rebound very loose is necessary to allow the wheels to follow the terrain. As soon as you start driving on rougher roads, for example Argentina, you need to tighten up the rebound. Why? Because of the jump issue for one, and especially to prevent the wheel from falling into every rough hole. A colorful way of putting it is you want your wheels to fly over the really rough stuff. I know what you think so I’m just gonna say it: yes, obviously, this only works when you drive really fast over the rough stuff. With the advance of technology, huge travel and bullet-proof strength of today's WRC car dampers, world rally car engineers don’t mind so much the rough stuff. The result is wheels follow the terrain nicely, especially thanks to the "fast rebound" system.

I can speak more about my experience with dampers in another post if you’d like. The Reiger shocks we had on the Fords were really interesting pieces of tech.

Back to our eggs… Somebody once asked me what to do with a problem he had while driving his rally car. He had what I believe is a rather common issue: under steer going in the corner and too much over steer coming out. He felt like he was losing time especially in tighter corners. There are various things you can look at and sometimes many things need to be looked at. I’m going to explore one specific and very common source of problems: 

Pre-load on a mechanical rear diff

I had conversations about that subject with my team back in the days when I was driving a Gr.A Celica and the Corolla WRC. I remember that some of the rear diffs out there had outrageous pre-load settings and some people were driving with 80N of pre-load. Let me be very clear about what this amount of pre-load would feel like in terms of driving, as one of my mechanics used to say:

“Having a locomotive pushing against your arse each time you lift off gas.”

He was a Finn, by the way.

You’d feel it really upon turning into corners. For those of you who don’t know about pre-load : It’s what keeps your diff on a determined amount of tightness when off-throttle. Why would you want that? Simply because it will give the rear some stability under braking and prevent the wheels from locking too early, as well as give you a better feeling and confidence. It’s important but having too much pre-load for a given characteristic of stage will mean under steer on corner entry.

Why on earth would some of these diffs have such high pre-load settings? Word on the stages was Didier Auriol had been driving with that stuff but I don’t know if it’s true. Our Celicas were unaffected; they were built by Toyota Team Sweden (TTS)*. The Corolla which came from Cologne (TTE), on the other hand, needed some adjustment… If I remember correctly the pre-load setting I was driving with was somewhere in the 10-30N for the rear. As a comparison the Markko Märtin maps we got, later in 2005 for the 2004 Focus WRC, were generally with the same pre-load philosophy.

I can see you coming with the next question…

“Ok Antony that’s really nice and all but where does the over-steer come from?”

It comes from the botched up corner entry. Our unlucky driver has much work to do, dealing with that effing locomotive. He gives huge steering inputs and throws the car in, with lots of excessive momentum being created. All that momentum will result in excessive over-steer on the way out. His car is out of balance. The basic rule of thumb is you can have more pre-load on fast roads, less on twisty roads. How much exactly is up to the driver and his technical team to find out. I'd say not far from zero on twisty gravel, and double that amount of fast gravel is a good point to start from. 

*TTS was a satellite team of TTE. Based in Sweden and run by Leif Asterhag. They ran cars for Thomas Radstrom, Markko Märtin and Marcus Grönholm among others. Their cars originated from TTE but they had their own setups and "developments" on them.


  1. very nice post! in Reigers this rebound loosening system is called RCV. could you write also something about your experience with mechanical front diffs?

    1. ok, I am glad you liked. Sure, I'll write about front diffs as well.

  2. Fantastic!!! Thank you very much!

  3. In my national championship we had a growing driver with the exact problem. He switched from N4 to S2000. He was driving with heart though not too aggressive. Corner entries were fine, like a train but exits were looking like FWD production car. He was trying different setups but found it hard to adapt his input to that completely different chassis.

  4. as a club level rally competitor i found this very informative Antony. Thanks a lot!!

  5. Once again, a great read Anthony. Could you put some thoughts down in regards to reading tire wear? Not so much about compound, more of the understanding on what corrections can be made for different wear patterns.

    1. Hi Brian and thanks, happy you liked it. I am working on a post which will encompass this subject.

  6. Nice article,and here some questions.If argentina was without jumps would it be better to use looser rebound?you didint expained(or at least i didint understand)how the pre-load on the rear diff causes understeer on the strart of the corner?

    1. I am happy you liked the post.

      Argentina is also rough with all sorts of rocks and holes. So in my opinion even without jumps you'd need to be careful about loosening the rebound. But in my opinion, yes, you could try if you have very good shock absorbers which can withstand the bumps.

      The understeer is caused because when you turn, the rear inside wheel wants to keep turning at the same speed as the outside wheel (due to the relative diff lock from preload). To avoid this "pushing the car forward" effect the inside wheel needs to be allowed to turn slower than the outside wheel.

      I hope you find this answer useful!