TranslationDriving
Disclaimer: There are lots of theories on chassis handling and driving. This is one version of it. It may not be the complete story. The interaction between body, wheels and the road is so complicated probably nobody knows all the science behind it. Those who spend millions of dollars studying it probably won't be willing to divulge their secrets either.
This 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
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Rotation
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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.
Entry speed into a corner is important. If the car is coming in too hot, you will over run the corner and end up loosing time. Judge the maximum speed that the car can handle the corner, and slow down to that speed while coming into a corner.
One method of cornering for tight corner is trail braking. The basic principle is to maximize traction at all points of the corner. 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. 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. Gradually ease up on braking so no brake is applied in the middle of the corner when both front and rear wheels are sustaining maximum cornering loads.
Turn In
To get sharp turn in, set the springs soft, increase ride height to 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. Therefore, use of MR015 instead of MR020 will increase turn in. That is
because the batteries on the MR015 is closer to the centerline of the chassis
than MR020. 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 wheels are rotating into the turn under
the body. The fact that the majority of the mass of the car is still going straight
means the tires do not have to generate cornering force yet. This gives tire
extra time to cause the car to rotate. More traction can go to rotating the
car instead of generating corner force. By the time the mass of the car is starting
to change direction, the car already have angular momentum.
High CG and soft springs aid turn in can be seen 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 into the turn. 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 needed for cornering.
Down side of soft springs is that the car is less responsive. For modified class, the cars may be going too fast for the chassis to react to the quick changes, so soft springs may not work for modified class.
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.
Correcting for Under Steer and Over Steer
Most cars are set up to understeer. When braking into the corner, weigth transfer to the front and car will give in more traction in the front, and less traction in the rear. Braking reduce the amount of understeer. A understeering car is stable when braking into a corner. It will not spin out.
What about oversteering. If a car is set up to oversteer, then spin out usually occur in mid corner after the initial turn in when both front and rear of car are trying to hold maximum cornering force. Of course, spin out can also occur if too much brake is applied going into a corner. To correct for spin out, there is a point when just entering the corner where the throttle can be applied that will cause the car to understeer thru out the corner. That point is at around the turn in point as the car is just getting into the corner. With application of throttle, weigh transfer will shift back as car is accelerating, and will actually result in understeer in a car that normally oversteer. Takes several experimentation to find the point where accelerationn can be used to mask an oversteering car. The amount of steering can be controlled with the throttle. This could be useful in a section of the track where the car tend to spin out either due to change in track surface condition, or the track configuration somehow tend to induce oversteer.
Smooth Driving
Think of the tires as have a gram scale on them. When the traction force exceeds
a certain number, the tire will start to slide. Then you can think of stepping
onto a bathroom scale. If you jump on a bathroom scale, the needle will bounce
all over the place. That bouncing of the needle is not because the scale is
cheap, but because of spike force as the scale try to reach an equalibrium.
Same thing with driving. Anytime there is a change in direction, or acceleration,
the car try to reach an equalibrium. If steering or throttle input is jerky,
you will end up with spike force on the tires that cause them to slip. Throttle
has to be applied over a period of time rather than instantly. Brake also need
to be applied over a period of time. Steering should be smooth and gradual rather
than instant steering.
2WD mini-z unlike the AWD does not have brakes for the front wheel. Unfortuntaely, front wheels are where most of the braking take place in a normal car. By definition, a mini-z will be coming into a corner somewhat hot. The classical technique of braking before a corner may not work so well for a mini-z because braking is not its strength. It is better to use sliding to brake the car like in a dirt track or rally racing. The down side to rally type of driving is at this scale where things happen so fast, is difficult to slide the car and be consistent. Trick is to compromise between consistent smooth driving versus sliding.
Sometime the rules on braking and smooth driving goes out the window. It becomes
whatever works for you.