sample chapter
How to Build a
High-Performance
Mazda Miata MX-5
sample chapter
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•
•
•
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Modify and tune your Miata for maximum performance
Add power with a turbo or supercharger
Improve cornering ability with suspension upgrades
Reduce stopping distance with big brake kits
Bolt in V-8 power with an engine swap
Keith Tanner
Contents
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Introduction
.............................................................. .5
CHAPTER 1
The Philosophy of Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
CHAPTER 2
Engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
CHAPTER 3
Intake and Exhaust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
CHAPTER 4
Forced Induction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
CHAPTER 5
Fuel and Engine Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
CHAPTER 6
Clutch, Transmission, and Rear Ends . . . . . . . . . . . . . . . . . . . . . . 77
CHAPTER 7
Handling Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
CHAPTER 8
Suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .100
CHAPTER 9
Suspension Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110
CHAPTER 10
The Braking System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .125
CHAPTER 11
Wheels and Tires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .142
CHAPTER 12
Body and Chassis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .154
CHAPTER 13
Safety Gear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .166
CHAPTER 14
Engine Swaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .177
Chapter 7
Handling Theory
T
his chapter is an overview of what handling is and some of
the factors affecting it. This should help you understand
why various changes will affect your Miata the way they do. It’s
not meant to be an all-encompassing treatise on the subject.
If you want to dig deep into handling, there are a number of
excellent volumes on the market. Disclaimer aside, let’s dive in.
Oversteer vs. understeer
Two of the basic terms to understand when talking about
handling are oversteer and understeer. The fun way to explain
it is that oversteer is when you go off the road tail first;
understeer is when you go off the road nose first. On a more
mundane level, oversteer is when the back end is sliding, and
understeer is when the front is sliding.
Here’s the more technical definition. It’s when you’re
in a corner and your tires are slipping slightly, even if you’re
not actually sliding. Why? Because tires are flexible, and the
movement of the sidewall and the rubber at the surface of
the road allows them to move sideways slightly as they deal
with cornering forces. The difference between where the
tires are pointing and where they’re going is called the slip
angle. Obviously, when the car is sliding sideways, it has a
large slip angle; but there’s always some slip present due to tire
deformation, even at very slow cornering speeds.
Understeer is when the front of the car has a bigger slip
angle than the rear. In other words, the car’s not turning as
fast as it should. Oversteer is the opposite, with the larger slip
angle at the back. Oversteer is a big favorite of Hollywood and
the Dukes of Hazzard because it looks dramatic, but it’s not
really the fast way around a corner when delivered in large
quantities. Understeer is the default setup for most production
cars because it’s safer and easier for most drivers to control. Lots
of understeer isn’t the fast (or fun) way either. Big slip angles
scrub off speed as the tire is dragged across the surface.
When a car is well balanced and both ends slip at about
the same angle, it’s referred to as being “neutral.” And that’s
a pretty good description of the Miata, most of the time. It’s
one reason why it’s such an easy car to drive quickly, because
it doesn’t have a lot of bad habits. The car will oversteer or
understeer depending on what the driver does, of course.
Let’s walk through some of the factors involved.
Handling Theory
Here’s an extreme example of understeer. The front wheels are turned all the way,
but the car’s simply sliding forward. It’s a pretty helpless feeling for a driver, and this
car did end up sliding off the road. Against the driver’s instincts, the best thing to do
would have been to apply less steering angle so the wheels would have a chance
to grip.
Before you can talk about handling, you have to define it. Just what is handling? To many people, it’s simply the amount of grip a car can generate. That’s easy to understand,
but it’s only part of the equation. All aspects of a car’s dynamics fall under “handling,” whether it’s straight-line stability or the amount of steering feel when you first turn the
wheel. To me, the most critical aspect is how the car behaves at the limit of grip. It needs to be communicative, responsive, and forgiving. Luckily, the Miata handles nicely
from the factory, but that doesn’t mean it can’t be improved.
90
The exact same corner, but this time there’s lots of oversteer. The rear of the car is
sliding more than the front. It’s the Hollywood way around a corner and a lot of fun,
but it’s actually quite slow. At least the car stayed on the road in this case.
A neutral handling balance. The slip angle of all four wheels is about the same. The driver can adjust the direction of the car with the throttle.
91
TIRES
WEIGHT TRANSFER
One of the most important concepts to understand in
vehicle dynamics is weight transfer. Every time you ask your
car to do something—whether it’s go around a corner, slow
down, accelerate, or some combination of the three—it will
shift its weight around. This is called weight transfer, and
it’s key to understanding how your Miata’s dynamics work.
Chances are you already know some of this subconsciously,
the same way you can figure out how a ball is flying and
catch it without knowing calculus. There’s going to be some
math in here, but it’s fairly simple. We’re dealing with high
school physics.
Whenever you ask your car to change its direction or
speed, the change comes from the tires. They’re exerting a
force on the car, and they’re doing all of their work at ground
level. But the car has inertia and wants to keep doing what
it’s doing. In a corner, you feel this as centrifugal force. The
inertia applies a force through the center of gravity (CoG),
which is the center of mass of the car. The tires are exerting
their force either at ground level (braking and acceleration)
or through the roll center (in cornering), which is fairly close
to ground level. Since the CoG is above ground level, these
unbalanced forces create a torque that shifts some of the
weight around between the four tires.
Time for some numbers: First, let’s talk about g and
cornering speeds. The symbol g is the acceleration due to
gravity (32 feet per second per second, or 9.8 meters per
second per second), and weight is defined as mass x g. Since
we’re not leaving the surface of the earth, most people will
treat weight and mass as the same thing. When we’re talking
about car acceleration, whether it’s forward, backward (i.e.,
braking), or sideways, we like to use the same unit of g. A
car that’s cornering at 1 g is accelerating sideways at 32 feet
per second per second. This requires a cornering force equal
to the weight of the car. A car that’s accelerating in a straight
line at 1 g is accelerating just as fast as if it had driven off a
cliff. Happily, the theoretical maximum acceleration in any
direction is the same as the µ of the tires, assuming all four
are being used equally. Street cars that can corner at 1 g are
pretty unusual, but well modified Miatas can manage to hit
that mark. The higher the level of lateral acceleration, the
faster we’re going around a corner.
CoG
centrifugal force
Here are a couple of imaginary tires to compare. The
shape of the curve can tell us how easy the tires will be to
drive. The street tire gives much more consistent grip as
the slip angle increases. This will make it more gradual as
it breaks loose, making the tire easier to drive. The race
tire offers much higher grip but has a steeper drop-off on
each side of peak traction. It’ll be faster, but if you start
to slide it could bite as μ falls away. The car will be less
forgiving. Also, note how the peak grip comes with a little
bit of slip in both cases.
cornering force
roll center
The tires are trying to drag the car around the corner, and
their grip is the cornering force. The centrifugal force is
the car trying to keep doing what it’s doing. You can see
how the two forces affect the car at different places. You
can think of the distance between the two points as a
lever. Physicists refer to this as a “torque.”
Take braking, for example. When you apply the brakes, the
tires apply a force toward the back of the car. The car’s
inertia provides a force toward the front of the car, applied
through the CoG. This shifts weight forward, putting
more downward force on the front wheels and unloading
the rears. Any kid who’s ever left big skid marks on the
ground with a bicycle understands this.
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93
HANDLINGTHEORY
HANDLINGTHEORY
Let’s start with tires. Tires grip the road. The amount of
grip available from a certain tire varies, depending on the
slip angle, the tire temperature, the inflation pressure, the
amount of vertical load on it, and the road surface. When
you take away all the variables, it comes down to the tire’s
coefficient of friction, also known as µ. If a tire has a µ of
1.00 under a given set of conditions, then it can deliver 500
pounds of cornering, braking, or acceleration force when it
has a load of 500 pounds on it. If µ is 0.80, then it can only
deliver 500 x 0.80 or 400 pounds of force with that same
500-pound load. This is easy enough to understand, and
it tells us two things. First, the more weight on a tire, the
more cornering force it can generate. Second, the higher µ
is the better.
Tires are interesting things in that they can actually have
a coefficient of friction greater than 1.00. This may contradict
what your physics teacher told you in school, but it’s true. The
reason is that the tire can deform and actually interlock itself
with the rough pavement surface. If you’re cornering hard
enough, you’re actually tearing the rubber off the tire. That’s
why you can leave black marks in a corner without sliding.
As you may remember from physics class, the coefficient
of static friction is greater than the coefficient of sliding friction.
In other words, once a tire is doing more sliding than gripping,
its ability to generate traction drops off dramatically. Despite
what you see in movies, you don’t get maximum braking,
acceleration, or cornering traction by generating lots of tire
smoke. But a tire doesn’t immediately change from gripping to
sliding, it’s a gradual change as the slip angle increases.
Inertia force = (weight x acceleration)
g
Weight transfer = (inertia force x cg height)
wheelbase
Plugging in the numbers, we get an inertia force of
(2,500 x 0.5 g ÷ 1 g), or 1,250 pounds. The weight transfer
is (1,250 x 17 ÷ 91.7), or 232 pounds. This weight is being
shifted from the rear wheels to the front. This means that
instead of having 1,250 pounds pressing the front into the
ground and 1,250 pounds on the rear tires, we have 1,482
pounds on the front wheels and 1,018 on the rear.
Looking at this, a few things become apparent. First, the
only way to affect the amount of weight transfer at a given
acceleration is to lengthen the wheelbase, lower the CoG, or
make the car lighter. That’s it. If the center of gravity was 6
inches higher, then we’d get 314 pounds of weight transfer
instead of 232. You already understand how this works. Get
a friend and have them try to push you over sideways. If they
push you down low—say, at your knees—it’s easy to resist
because you still have your weight fairly evenly distributed.
Not so easy if they push you higher up. Similarly, if you spread
your feet apart and widen your track, you’ll have an easier
time staying up than if you have your feet close together.
Second, there’s no mention of spring rates. That’s because
body movement—roll, squat, and dive—is a result of weight
transfer, not a cause. In our 0.5-g stop above, the nose of the
car will drop because the front springs are having to support
an extra 232 pounds, while the back rises because there’s 232
pounds less compressing them. A car with solid suspension
will still get the same amount of total weight transfer, but it
will sit flat.
Actually, when the sprung weight of the car moves under
weight transfer, the CoG will move slightly as well and this
will affect the weight distribution. But this movement is
minimal and the resulting weight shift is so small that we can
ignore it.
The same sort of transfer happens in a corner. The physics
of the weight transfer are more complex and can be separated
in several components based on the location of the roll center,
but the total lateral weight transfer is calculated the same way,
based on the height of the CoG and the track width.
Now, let’s look at what happens in a corner. The basics are
the same, only this time it’s the track that affects our weight
transfer. Our NC has a track of 58.7 inches. In a 0.75-g turn,
this means we’ll have 543 pounds being transferred from
side to side. Our two outside wheels will be pressed into the
ground with 1,793 pounds of force while the insides have
only 707 between them.
Now here’s where it becomes important. The more weight
forcing a tire into the pavement, the more grip it generates.
But the relationship between grip and load is not a straight
line. As the load increases, the coefficient of friction falls off.
A street tire with a cf of 0.75 when loaded with 625 pounds
might only have a cf of 0.72 when it’s loaded up with 900
pounds and .78 when it’s only seeing 350. This is important.
Really, really important.
at rest
2500 lbs total weight
1250 lbs
0.5g deceleration
232 lbs transferred
at rest
2500 lbs total weight
1250 lbs
0.75g cornering
543 lbs transferred
1793 lbs
The same sort of transfer happens in a corner. The physics of the weight transfer are more complex and can be separated in several components based on the location of the
roll center, but the total lateral weight transfer is calculated the same way, based on the height of the CoG and the track width.
What does this mean to our hypothetical Miata? With
625 pounds on each corner and a cornering force of 0.75,
each imaginary tire can generate 469 pounds of cornering
force for a total of 1,875 pounds and theoretically corner
at 0.75 g. But when we plug in our weight transfer, here’s
what happens:
Outside tires 1,793 lbs total x 0.72 = 1,291 lbs
Inside tires 707 lbs x 0.78 = 551 lbs
Our total maximum cornering force has dropped from
1,875 pounds to 1,842. Uh-oh. The outside tires are providing
more grip than before, but not enough to make up for the
grip lost by the inside. There’s not enough grip to support this
1018 lbs
1250 lbs
94
cornering speed, so the car is going to slide sideways until it
slows down enough to regain grip or it falls off the road.
And that’s the big lesson. Weight transfer will cost us
total grip. That’s bad for braking and cornering. The amount
of weight transfer is going to determine the ultimate limits of
the car, and by managing this transfer we can set up the car
for maximum performance.
If you run the same numbers with an NA or NB, you’ll
discover that the shorter 89.2-inch wheelbase and narrower
56-inch track mean more weight transfer. It’s a good thing
they’re a bit lighter to start with. A 2,300-pound NA will
transfer 219 pounds forward under our 0.5-g stop and 523
pounds sideways under a 0.75-g cornering load. It’s a smaller
amount but a larger percentage.
Vertical tire load versus coefficient of friction in our
theoretical tire.
1482 lbs
Here’s a diagram of what’s going on under braking.
707 lbs
1250 lbs
Tire load (pounds)
95
Handling Theory
Handling Theory
In order to figure out how much weight is transferred
in our braking example, we need to know the rate of
acceleration, the weight of the car, the height of the CoG,
and the wheelbase of the car. A Miata’s CoG is approximately
17 inches above the ground. This will change with lowered
cars, but it’s a good rule of thumb for now. Let’s use a car that
weighs 2,500 pounds with the weight evenly distributed over
all four wheels. This would be a fairly normal 2006 MX-5.
The wheelbase of the NC is 91.2 inches. If the driver brakes
hard enough to decelerate at 0.5g (a moderately hard stop on
the street), we get the following:
Real-World Tire Numbers
Tuning with Weight Transfer
T
he imaginary tires used in this chapter have idealized µ curves, so they provide better examples. What do the graphs look like with real tires? This
information is actually quite hard to find, but Toyo Tires has provided a couple of graphs that compare one of their high-performance street tires with
a Toyo race tire. A big thanks to Toyo for this!
When we compare slip versus µ, we see that both tires keep gaining grip as the
slip angle increases. The street tire is starting to taper off here and will probably
plateau somewhere around 9 degrees, but the race tire just keeps gaining.
Eventually it will peak, but where?
Race tire
We can do some interesting things with weight transfer in
a cornering car. While we can’t affect the total amount of
weight transferred without altering the geometry of the car,
we can move it between the front and rear wheels, and thus,
the relative traction at each end.
Roll stiffness is the car’s resistance to roll. You can adjust
the roll stiffness at each end of the car by playing with spring
rates or anti-roll bars. And here’s the trick: The end with the
higher roll stiffness will get a higher percentage of the total
weight transfer. If you increase the roll stiffness at the front,
you’ll get more weight transfer at the front and less at the rear.
This means the front will lose traction and the rear will gain
it, leading to more understeer or less oversteer. It’s important
to remember the total weight transfer doesn’t change,
though. Increasing the roll stiffness at both ends, but keeping
the proportion the same, will result in the same handling
balance. This is important. It’s the key to tuning your Miata’s
handling, especially because it’s a lot easier to adjust the roll
stiffness than it is to start playing around with your center of
gravity or your suspension geometry. We’ll get into how to
tune with weight transfer later.
FRC
The amount of weight transfer at each end of the car is
directly proportional to the roll stiffness. So a good way to
express the roll stiffness distribution is to look at how much
of the total roll stiffness is on each set of wheels. Usually this
is simply expressed as a percentage, such as a front roll couple
(FRC) of 58 percent. The higher the percentage for the front,
the more weight transfer will take place on the front wheels,
and the more the car will be biased toward understeer. Lower
FRCs mean more oversteer. Stock front roll couples tend to
be around 55 to 58 percent.
Handling Theory
Handling Theory
Street tire
Slip angle (degrees)
Race tire
Street tire
Tire load (pounds)
96
Here are the tires compared with different
loads. This is at a constant 5-degree slip
angle. It’s interesting to see the different
behavior between the two tires. The street
tire is much more tolerant of increased load
but falls off suddenly when it gets too high.
The race tire provides more traction overall
but punishes weight transfer much more
severely. This matches up well with their
intended uses, as a street car has a wider
range of operating weights depending on
passenger and cargo load and generally has
more weight transfer due to a higher center
of gravity.
Is there a situation where weight transfer is good? There sure is: in acceleration. Since the Miata is rear-wheel-drive, the front tires aren’t involved in trying to accelerate the
car. We want as much of the weight transferred to the rear wheels as possible. Luckily, physics is on our side here, as that’s where the weight goes. The harder the car can
accelerate, the more transfer there is and the more grip we get out of our rear tires. Drag racers know this, it’s why you see them jacked up to get a higher CG and thus higher
weight transfer. When a dragster has the front wheels off the ground, it’s reached 100 percent weight transfer.
97
Transient Behavior
If all we had to do was get the car balanced in a long sweeper
after it’s had time to settle down, chassis tuning would
be pretty easy. Unfortunately, there’s a whole lot more to
it than that. We also have to get the car working well in
transitions. Basically, when it’s changing what it’s doing.
Entering a corner, exiting a corner, changing from a right
to a left turn, a good handling car has to be able to cope
with them all quickly without getting out of shape. In this
sort of behavior, the weight isn’t just being shifted front to
back or side to side, it’s moving diagonally. When exiting a
corner, you’re accelerating both laterally and forward, and
weight is being transferred from the inside front wheel to
the outside rear.
The speed at which the weight is transferred comes into
play as well. The higher your roll stiffness, the faster your
weight transfer. Quick weight transfer will make the car more
responsive but can also make it twitchy and a bit harder to
control. Slow weight transfer is easier to manage, but the car
won’t be able to react as quickly.
The Effects of Body Roll
Now, we know that body roll isn’t going to affect our weight
transfer, but it does have other effects. First, it takes time.
This affects our transient behavior, as the car is not settled
while the body is still moving around.
Second, it affects the suspension geometry. Our tires
want to be flat and square on the pavement or even leaning
into the turn somewhat. But as the body of the car rolls, the
tires will be tilted outward into positive camber. This will
drop the amount of grip the tire is generating, and we don’t
want that. The Miata’s suspension is designed to lose little
camber in roll, but it still happens. We’ll discuss camber more
in Chapter 9.
So body roll is evil then. Well, not quite. Many drivers
will use the body roll as an input on how hard the car is
cornering, so we don’t want to eradicate it completely. That’s
pretty much impossible though. As long as the suspension is
allowed to move to absorb bumps, a Miata will always roll
somewhat.
So, there’s the theory. Now, what about the practice?
Handling Theory
Handling Theory
The NB in the background is in the middle of a corner
and is fairly settled. The NA in front is in the middle
of transitioning from turning left to turning right. This
particular corner is a good place to spin a car if it’s not set
up well because of the rapid transition required.
Autocrossing is a fantastic test of transient behavior. The
car never really has a chance to settle down. Alan Branch
Body roll in a stock NC and lots of it! Despite the NC’s well-designed suspension, you can see that the tires are starting to move into positive camber as the car leans, and that
costs us grip.
98
99
How to Build a
High-Performance
Mazda Miata MX-5
THE PHILOSOPHY OF
MODIFICATION
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ENGINE
INTAKE AND
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AND REAR ENDS
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SUSPENSION
TUNING
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WHEELS AND TIRES
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ENGINE SWAPS
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