July 2003 #2
WELCOME
Mark Ortiz
Automotive is a chassis consulting service primarily serving oval track and
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RIGHT REAR SPRING STIFFEST?
I frequently
watch the pre-race shows for NASCAR events and listen carefully when they are
speaking of setups. Two times now in the past year they have spoken about
the stiffest spring on the whole car being the right rear. One crew chief
asked a former driver if he ever thought he would see the day that was the case.
The driver said he had won many a race with the right rear being the softest
spring, and that has been the case with my experience as well. They spoke
further to say that the trend now was for the younger former open wheel drivers
to run off the right rear tire. Physics to my knowledge haven’t
changed. Are they running extremely heavy front bars and super light
springs, or what gives? I thought the jounce bumper trend had been made
illegal. Are their ways of thinking something we local drivers could
apply to our cars for an outside-the-box thinking advantage?
At the risk
of disillusioning my legions of admirers, first let me confess that I do not
have throngs of Winston Cup crew chiefs and engineers climbing over each other
to tell me their setups. I’ve worked with just a few, and a few get this
newsletter. However, maybe I can be of some help, and if I say anything wrong,
maybe I’ll get straightened out.
Traditionally,
the most common approach to spring splits has been to run the front end
right-stiff and the rear end left-stiff. This usually makes the right front the
stiffest spring and the right rear the softest. I
never really understood that approach, and I have generally advised either
running right-stiff or left-stiff at both ends, except perhaps in cases where
the rear end lifts under power – a condition normally only seen in dirt cars.
Two main
considerations determine whether we run the car right-stiff or left-stiff:
cornering balance when forward and rearward accelerations accompany cornering,
and adapting the car to the banking angle of the track. Taking the former of
these first, when a car has different amounts of pitch resistance on the right
and left sides, and the tires are accelerating the car forward or rearward, the
diagonal percentage changes. If the car is right-stiff, that tends to make the
diagonal percentage increase in rearward acceleration (braking) and decrease in
forward acceleration. That tightens entry and loosens exit. If the car is left-stiff, that does the opposite: frees up entry, tightens
exit.
Regardless
of banking angle, stiffer right springs add roll resistance. However, if the
track is banked enough so that the left springs compress (I don’t mean more
than the right ones, just compress rather than extend), we actually increase
the roll resistance with softer springs, on the left side only. This means that
with soft left springs, we can make the left wheels more compliant over bumps,
and improve camber control by reducing roll, at the same time. This is in
contrast to our usual dilemma of having to stiffen up the suspension to control
roll and camber change, at the expense of roadholding.
On a short
track, with tight turns, especially on dirt, it is most common to have trouble
getting the car to turn readily enough on entry, and to have trouble hooking up
the rear tires on exit. If this is a problem, that argues against right-stiff
springing. But on high-speed tracks, it is common to want to tighten entry. The
driver is going in as deep as possible, and braking hard while cornering, from
speeds as high as 190 or even 200 mph. It may take as long as ten seconds to
complete the turn, and the car is following a very large-radius path. We don’t
want to achieve large yaw accelerations then. The need is for a car that
doesn’t want to come around when braking and cornering at the same time.
On top of
that, the front ends on stock cars generally lose anti-dive rapidly as the
suspension compresses, because the side view projected control arm is shorter
for the lower control arm than for the upper. This means that when the car is
rolled to the right, anti-dive asymmetry de-wedges the car when braking, unless
the right front has a lot more anti-dive than the left front at static
position. Right-stiff springing helps compensate for this.
It is
possible to tighten entry by using lots of front brake. However, if the turns
require significant braking, using lots of front brake overworks the front
brakes. As the front brakes go away, the brake bias shifts
toward the rear again. Also, we really would like to set the brake bias
for shortest stops, which means we want the rears to do around 30% of the work.
If we run more than 70% front brake, we hurt the car’s ability to stop quickly
for pit stops, and to brake well when avoiding
accidents on the track. Therefore, it’s best to get the desired entry balance
with the suspension.
What about
exit balance? On a short track, we are often fighting to get the car tight
enough, to control wheelspin. However, even on a
short track it is possible to get exit too tight, and have a power push. At
high speed and with steep banking, it is harder to get wheelspin.
Cup cars do have stout motors, but they also have ample tire loading from the
banking and the aero, and a lot less torque multiplication from the gears than
they would on a short track. It is therefore not uncommon for the driver to
report a “push-loose” condition. That means the car is basically tight
power-on, so the driver feeds in more power trying to get it loose, but this
doesn’t happen until power is sufficient to really cause wheelspin,
whereupon the car goes wheelspin-loose. The driver
then has a hard time finding a stable point where the car has good balance. In
this situation, if the car is freer power-on, the driver can find good balance
at moderate throttle.
We see an
analogous situation in road racing or short-track racing in classes like
Formula Ford or pavement mini-stock, where the car has good grip and modest
power. Such cars often want a freer setup than more powerful cars would, to get
proper exit balance.
Are soft
front springs and stiff anti-roll bars still favored since bump rubbers have
been outlawed? Yes. The springs can’t be as soft as when bump rubbers were
legal, but it is common to run the front end as soft in ride as is possible
without the rubbers. This soft ride rate makes the front end run lower through
the turns. That adds aero downforce. This will work
on a short track too, though the effect will be less pronounced. One might
think we could do the same thing by running a lower static ride height, but
stock car racing rules usually include a minimum static ground clearance.
In the
August 2001 newsletter, I addressed the subject of things that make spring
changes work backwards. I introduced the term critical angle to describe the
track banking angle at which the left spring neither compresses nor extends.
This angle is usually not identical for both ends of the car. At angles steeper
than critical, effects of left spring changes reverse, for the end of the car
in question.
Running soft
springs and a stiff bar reduces critical angle. The left spring will compress
rather than extend at surprisingly small banking angles. (Putting it another
way, with the soft ride rate, the entire front end will drop more on the
banking.) That means a softer left front spring tightens the car. This is
neither a disadvantage nor an advantage, just something to be aware of when
running such setups.
Regarding
the suggestion that the young drivers come from open-wheel racing and therefore
like to run a stock car with the right rear tire heavily loaded, I question
that.
First of
all, it may be true that some drivers are getting seat time in sprints and
midgets – Jeff Gordon and Tony Stewart for example – but the most common road
to Cup is through lower pavement stock car divisions: NASCAR Weekly Racing
Series, USAR Hooters Pro Cup, ARCA, ASA, Busch. It is
normal for young drivers to spend time in these series before transitioning to
Cup. And not all drivers who do well in sprints and midgets are able to run
well with a stock car. Some do, some don’t.
As you
correctly note, no laws of physics have been repealed. A stock car doesn’t have
60% rear and a huge right rear tire, or a big rear wing. If you make the right
rear carry a lot of load when cornering in a stock car, you get a loose car.
Driver preferences on car balance vary, but only within a narrow range. Nobody
likes a car that’s way loose at 150+ mph. Also, the car has to be fairly neutral
to avoid having a tire wear problem.
But you
can’t necessarily conclude that the right rear is more heavily loaded just
because the spring is stiffer. Other things being equal it would be, but other
things don’t have to be equal. If you combine a stiffer right rear spring with
more static diagonal, a bigger front anti-roll bar, or a lower Panhard bar, you can compensate for the effect of the
spring and load the tire about like you were loading it before.
Of these
different possibilities, the one that looks most appealing to me is running
more static diagonal. If we do that, then when grip is poor and lateral
acceleration is less, the springs have less effect on wheel loading and the
static loadings have more influence. That means the car loads the right rear
less, and runs tighter. When grip is good and lateral acceleration is greater,
the spring
affects wheel loadings more, so right rear loading is
greater and the car runs looser. This means that with more right rear spring
and more static diagonal, the car will have less tendency
to go loose on slick, as stock cars have traditionally done. With enough rear
roll resistance and static diagonal, a stock car can even be made to go tight
on slick. Between these extremes, we can find a setup whose balance varies
relatively little with changes in the condition of the track or the tires.
I have
clients successfully applying this reasoning on dirt and pavement short tracks,
although generally the added rear roll resistance is achieved without a
right-stiff spring split, except in cars that lift the rear under power.
INFLUENCE OF
PUSHROD ANGLE ON WHEEL RATE
I would like
to know how to calculate vertical stiffness and roll stiffness taking into
account the angle of the pushrods. I have seen lots of formulas but none
of them take into account the pushrods. Also, how do I calculate the
total roll of the car starting with the difference in movement between the
wheels?
Taking the
second question first, take the difference between the
right and left wheels, and divide by the track width. The quotient is the
tangent of the roll angle, so you find the angle that has that tangent. As an
equation:
qr = tan-1[(hl-hr)/T]
where:
qr = roll angle
hl = left ride height (average of front and rear)
hr = right ride height (average of front and rear)
T = track width (average of front and rear)
Now, as to
the angle of the push rods, you need that if you want to calculate stresses in
the pushrods. But to calculate wheel rate in ride or roll, you just need to
know the motion ratio from the spring to the wheel. Everything in between – the
pushrod, the rocker, the control arm – only matters to the extent that it
influences the motion ratio. The pushrod angle does affect the motion ratio,
but it is just one factor. For an existing car, the easiest method is to simply
measure how much the spring shortens or lengthens for an inch or centimeter of
wheel motion. For a car that’s in the design process, if you’re designing on a
computer, you can move the wheel and measure the spring length on the computer.
If you’re drawing manually, you basically do the same thing by hand and
estimate the motion ratio.
Once you
have the motion ratio, square it and multiply by the spring rate, and you have
the wheel rate in ride. Then do the same for the anti-roll bar, which may have
a different motion ratio. Add the rate from the anti-roll bar to the wheel rate
in ride, and that’s the wheel rate in roll. Be sure that when figuring the rate
of the anti-roll bar, you calculate the pounds or Newtons
per inch or millimeter per wheel – that is, the force change per unit of
opposite motion on both wheels, not just one.