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.
