TranslationThis section describes how to make the car go fast. Going fast on the straight way is easy. All it takes is a fast motor and well charged batteries. Cornering is where race can be won or lost. Inorder for the car to corner, two independent things must happen. There has to be translation and rotation. Translation is where cornering grip comes into play. The higher g force a car can hold, the faster the translation around the corner. Tranlation alone is useless, the car also must rotate around. The two actions are completely independently of each other. One action does not affect the other. Rotation is like a flywheel, it takes some force to get it spinning. Once it spins, it will continue to spin until it is stopped. The force it takes to rotate the car around happens in the very beginning of the turn. Once the car has started to rotated, it continues thru the turn without any help until the end of the turn when rotation needs to be stopped. Stopping the car from rotating at the end of the turn is the easy part. The car is already going straight, and all the traction on the front wheels can be used to stop the car from rotating. This is assuming the rear wheels is busy trying to accelerate the car. Starting the car rotating in the beginning of the turn is the more difficult part. The theory of traction circle states that a certain amount of traction is avalible to do work on the car in any direction. If some traction is used in braking, then less can be used for cornerning without spinning out. Or if car is cornering, then the same tire won't have any traction to do acceleration without slipping.
Translation
+
Rotation
=
Cornering
Three things need to happen in a corner. If seen in slow motion, the first part of the corner is start of rotation. Rotation actually take some time to do. By the time the car is in full rotation, it is already deep into the corner. During rotation, the front wheels are doing all the work tryint to rotate the car. The rear wheels are not yet generating full traction. Middle part of the corner is pure lateral grip. This is when the car is rotating at the same rate as its travel thru the corner, and all 4 wheels are generating side traction. Last part of the corner is acceleration out. Here the rear wheels are doing the work accelerating the car. Because during acceleration, weight of the car is on the back wheels, the front will be loosing traction. As a result, the car will stop rotating as it exit the corner.
Braking
Do you want to go fast? If you want to go fast, the first thing to do is to
hit the brake and slow down. That is right, slow down to go fast. Brake is something
that many people don't want to use, and for good reasons. The first time you
try the brakes on the Mini-z, either one of two things will happen, and both
will convince you that the brakes on the Mini-z are useless, and was a mistake
to add that feature on the car. When you hit the brakes on a corner, the car
will either spin out if you brake too late, or slow to a crawl thru the corner
if you brake too early. Either case doesn't seem like a fast way to drive around
the corner. One way to correct that is to reduce the braking end point on the
radio. 40% braking is a good start. See the section on end
point adjustments. By reducing the end point, you have better control of
braking because braking is not so sensitive. You can't do that on the stock
KT-5 transmitter, so you just have to be careful. Another thing you can do is
to set the throttle trim so that there is braking when you let go of the throttle.
If you set it right, it will give you just enough braking thru the turn when
you let go of the throttle. To go into reverse, just squeeze the throttle slightly
to find neutral, then the car will back-up. On more expensive radio, there is
a button called throttle preset that you can press, which will bring the throttle
to neutral for a mement, then the car will go into reverse.
The reason braking is so important is because of a thing called trail braking. Simply put, you are not utilizing all four wheels in a corner if you are not braking. Someone who is utilizing all four wheels will be faster than someone who is not. In the beginning of the corner before the car is in full rotation, the rear wheels are not doing its full job while the front is overloaded. Solution is to brake a little. Two things happen when you brake in a corner. There is weight transfer to the front, so the front wheels can bite down on the track. Second thing is the rear will loose some traction, and start to slide out. In both case, it is helping the car to rotate. The correct way to trail brake is to apply hard braking before the turn so you approach the turn at a reasonable speed. Usually less than a fraction of a second of braking is enough unless the corner is at the end of a long straigh way. Continue to brake hard at the beginning of the turn, and ease up on braking as the car is in full corner mode.
Acceleration
You can start acceleration when you sense that the car is in a steady corner
and has pass the apex or even before the apex. If the braking and cornering
is done well, the car will be at a steady cornering attitude. Gradually apply
throttle. As you accelerate, weight is transfered to the rear, so it still have
traction eventhough you are under heavy acceleration. The front on the other
hand will loose traction, but that is ok, because is better to understeer coming
out of a corner than it is to spin out. Understeering also allows the car to
gradually stop rotating and straighten out and get ready for the straight. A
rear motor typicall is better in acceleration than mid motor. Rear motor place
all the weight in the back of the car for traction during acceleration.
Turn In
To get sharp turn in, set the springs soft, have high center of gravity, or
reduce side to side damping. Changing the batteriy position so they are closer
to the center of the chassis is another way to increase turn in. Use of MR015
instead of MR020 will increase turn in. The reason soft springs and soft damper
increase turn in is because of increased body roll. Body roll allows the mass
of the body to continue going straight for a split second while the car is turning.
Therefore, more traction can go to rotating the car instead of generating corner
force. High CG and soft springs aid turn in can be seen clearly on a mini-z
Overland. The mini-z trucks have soft springs and high center of gravity. When
it turns, you can clearly see it lean. However, the turning is very consistent
and quite sharp. This is because during turn in, instead of generating cornering
forces, the car leans over. The time it takes for the car to lean over, gives
the front wheel more time to give the car the rotation.
Mid Turn
In mid turn, over and understeer is controlled by stiffness of the front spring
relative to rear spring. Stiffer front than back cause understeer. Stiffer rear
than front cause oversteer. Camber in theory can also affect mid turn traction.
As the body leans, camber keeps the tires perpendicular to the road surface.
There are different camber knuckles for the mini-z. Another way to change camber
is a castor bar by PN. It tilts the king pin towards the back. The result is
that the more steering you put in, the more camber the wheels have. In effect,
you have a variable camber. Great for corners that gets tighter and tighter.
However, how camber helps in a corner is only in theory. In practice, camber
does not seem to have much affect on turning. One thing mini-z tires differ
from a real car tire is that mini-z tires are solid rubber whereas full size
car tires are pneumatic. The real car tires distort to conform to the road.
The less the car tire has to distort to conform to the road, the more traction
you get. However, mini-z tires are solid, so they don't really distort to conform
to the road that much. What you really get is wear on the tire, and eventually,
wear cause the tire to conform to the track. As a result, no matter what camber
knuckle you are using, eventually, tire wear to conform to the road, and you
get maximum traction. Changing camber only changes handling of the car for a
few runs until the tire wear to conform to the track, then you get the handling
that you got before the camber change.
Two Schools of Thought
One set up method is to use stiffer springs and or anti-roll bar to give understeer in mid turn. Stiff springs affect mid corner more than turn in. That is because while the car is still going straight during turn in, both front springs are compressed evenly anyway. Anti-roll bar does not do anything. Only during mid corner once the car is settled in a corner where outside spring is compressed more than inside spring do spring stiffness play a role in traction. Using stiff springs and or sway bars, and soft front tires will give you good turn in. Once in mid corner, the stiff springs and sway bar will reduce front wheel traction, and prevent spinouts.
Another set up method is to use soft springs and or high center of gravity and hard front tires. Soft springs will allow the chassis to roll. This will let the car have good turn in. Hard front tires will prevent spin outs during mid corner since soft front springs gives more traction in the front, which may be excessive. Using body roll to enter turns will give consistent cornering performance since the front tires are not sliding around during turn in. However, using soft springs means more body roll, and longer transistion time before body settles into mid corner. For modified cars, soft spring may result in too long transistion times. Use hard springs with modified class, and soft springs with stock class.
Weight Distribution
Weight distribution plays an important part in handling. Batteries are the heaviest
part of the car, and plays a large role in weight distribution. On X-Mods, you
can move the battery housing forward and back for weight distribution, but on
mini-z, it is fixed. The only way to change weight distribution is to change
the chassis length and using different motor mounts. Either MM mount that place
the motor ahead of the rear axel, the HM mount that place the motor on top of
the axel or the RM mount that place the motor behind the rear axel.
Enzo
and Mclaren
A standard 98mm chassis like the Mclaren has very close to 50/50 weight distribution. The Enzo has a slightly heavier nose due to the larger over hang in front of the car. The Enzo is a popular car for the modified class. With the long wheelbase and wide track, it is very stable, and goes where it is pointed with no deviation. However, during acceleration out of corner, the Enzo and Mclaren can spin out on a slippery track.
94mm
Ferarri 360 GTC
This is where a 94mm chassis comes in. With the shorter chassis, the rear axel is moved close to the battery. Weight distribution is around 47/53 for a 94mm car with a mid motor mount. During acceleration out of corner, the rear wheels are planted to the ground. A 94mm wheel base with a RM mount that place the motor behind the axel has even more traction out of corner. However, with the extra weight hanging back there, car tends to continue to over steer after the initial turn-in. Steering is also much more sensitive. Braking into the corner can easily cause a spin out. Also because the the 94mm chassis requires a shortened motor mount for the MM motor mount, the motor is place higher on the chassis than the normal MM mount to save space. The resulting higher CG would also cause more weight to be transfered to the rear wheel during acceleration.
86mm
Porsche 934 and 935
At the extreme end of the scale is a 86mm wheelbase rear motor mount like the Porsche 934 and 935. Weight distribution is around 44/56. The 934 is very twitchy. The rear loves to come around when entering a corner, but has good traction out of the corner. The 935 has longer overhang on both ends of the car, so is slightly more stable due to higher moment of inertia. The large tail on the end of the car also puts some aero force on the rear, and is much better handling than the 934.
Choosing a chassis length then depends on the traction condition. On my home track where is kept very clean and flat, traction is very good. Is hard to break traction even under full acceleration out of the corner. The Enzo with its stability and even weight distribution is the hands down winner. It is the fastest car around the track. On a more slippery track at Kenon, the 94mm car is definitely faster than the Enzo. With the Enzo, a slight excess throttle pressure cause a spin out on the track. With the 94mm Lamborghini Jota, the rear is planted to the track. Both cars have +3mm offset in the rear for even comparison.
94mm Jota with +3mm
rear wheel offset.
Pan car bodies made by PN and Atomic use 98mm chassis, so weight distribution is about 50/50. However, due to the downforce generated by both those bodies, they have exceptional traction in and out of corner as well as good stability. They are probably the next best thing to AWD.
Real life example of weight distribution is comparison between the Corvette and Porsche 911. The Corvette with the V8 has much more power advantage over the Porsche. However, the Corvette has the engine in the front while the Porsche has the engine in the rear. In 0-60mph acceleration runs, they are both dead even because the Corvette just can't get the power to the ground like the Porsche which has more weight over the rear wheel. However, in high speed passing from 60 mph on up where power is more important, the Corvette easily beat the Porsche.
The 911 is know for its evil handling with all the weight in the back. Only an expert driver can handle the car effectively. Is common for rookie driver to spin out in a 911. This is similar to mini-z.