Street Legal expects you to bring your own automotive knowledge.
This guide is a quick refresher of main concepts relevant for the game.

Everything here is information you may already know from real life or other simulation games, unless specified otherwise. Most SL1-specific details are up to you to discover in-game.

As much as we've tried to keep things short, there is still a lot of text; Remember to use the panel on the right or Find (Ctrl + F) for quick navigation!

Before we can talk about handling, you need to understand what these mean;

When turning:
Understeer means your car doesn't rotate enough (wants to keep going straight);
Oversteer means your car rotates too much (wants to spin around).
Oversteer is caused by rear tyres losing grip, understeer is caused by front tyres losing grip.

Tyres are critical for almost every aspect of your car, like: Turning, braking, acceleration, stability, top speed, maintenance frequency & costs, plus more!

Needless to say: choose wisely, or prepare for pain.

Bigger tyres (width and/or radius) greatly improve grip, but also increase rolling resistance.

Different rubber compounds offer different levels of grip and longevity.

Tyre size (outer circumference) affects top speed and effective wheel torque similarly to final drive ratio.
More on that later.

Wheels:
8x19 would mean 8" wide, 19" diameter.

Tyres:
225/40 would mean 225mm wide, of 40% profile, meaning the sidewall height is 40% the width of the tyre (225mm x 0.4 = 90mm in this case).

Between the front & rear, one end will usually experience more load, thus wear quicker than the other;
By swapping worn tyres around, you can help even out the wear, extending the tyres' lifespan and saving cash.

Alternatively:
You can fit tyres of different grip levels on the front and rear, to promote understeer or oversteer, and/or to give the driven wheels more grip for faster launches.

Brakes use friction to turn a car's kinetic energy (speed) into heat.

Racing brakes will be more responsive/grabby than street brakes, though the fastest your car can stop generally depends almost entirely on your tyres, as once your brakes have the power to bring the tyres to the limit of their grip, further brake upgrades are no longer beneficial, at least in just a single stop;

Brakes are generally upgraded not for raw power, but to delay or eliminate fade caused by overheating from repeated use, increased surface area and/or upgraded materials improving heat dissipation / resistance.

Street Legal does not simulate brake thermals, but brake wear is greatly exaggerated for purposes of fun, and higher-end brakes do offer much better durability, so your main reason for upgrading them is similar to real life: Sustained braking performance.

To stop as quickly as possible, brake strength can be balanced between the front & rear such that front & rear tyres reach their limits of grip at the same time, where neither end is prone to locking up earlier than the other, though it can be safer to allow the front to lock up first, as this increases stability, albeit at complete loss of steering.
Rear bias is prone to causing oversteer (similar to pulling the handbrake), which can be fun!

In Street Legal 1, you can adjust bias by installing different sizes/types of brakes on each end.
In reality, you'd do this by adjusting a brake proportioning valve.

All suspension in Street Legal 1 is simulated as a symmetrical double wishbone setup, meaning the wheels will simply travel straight up / down with suspension compression.

Repairing damaged arms will do you no good, if the suspension mounts on the chassis are screwed up.

Springs allow the suspension to move independently of the car's body.
Spring stiffness is referred to as spring rate.

Spring rate allows you to adjust how much or how little suspension movement you get, movement reducing as rate increases.

Bumpy roads:
Increased movement will help tyres remain in contact with the ground, avoiding loss of grip.

Smooth roads:
Reducing movement and thus body roll will help keep the relatively flat bottom of each tyre in full contact with the ground during turns, maximizing grip.

The ride height you get is determined by both the length and rate of the springs.

Spring rate determines how much the weight of the car is able to compress the spring;

For example, a 14" 23000 N/m spring will offer much less ride height than a 13" 56000 N/m spring, unless your car is absurdly lightweight, barely compressing the softer spring.

High ride height will help avoid damage from bumps, jumps and curbs.

Low ride height will reduce body sway by lowering the center of mass.

As the weight of the car changes how much a spring compresses, heavier cars require higher spring rates to achieve similar handling characteristics to lighter cars with lower rates.
Also note your car's front-rear weight distribution, especially after major modifications like engine swaps, as you may want different springs on each end.

A 14" 23000 N/m spring has a loadless (rest) length of 14", and exerts 23000 newtons of force per meter of compression;
Spring force increases linearly with distance of compression, so this number is useful even if the spring is not a meter long.

23000 newtons are equivalent to 2345.3 kilograms of force.
At 4" (~10cm) of compression, this spring would be putting up ~234.5 kgf, enough to hold a (~234.5kg x 4 wheels ~=) 938kg car.

A heavier car would further increase compression, further reducing ride height.

Coil springs are by their nature very efficient, losing very little energy in travel.
This means that once your car's body begins to bounce or sway, it'll want to continue repeating this motion unless the energy is absorbed somewhere.

Shock absorbers (shocks) turn kinetic energy into heat through the fluid friction produced by force-pumping an oil or gas through a small gap.
This is called damping.

The higher the speed of the suspension travel, the more damping the shock will provide.

Stiffer springs need to be paired with shocks of higher damping to avoid introduction of bouncing.

Beyond preventing repetitive motions, the suspension travel speed limiting nature of shocks works alongside spring rate in reducing body roll in cornering, and the chances of bottoming out with jumps & bumps.

Just like spring rate, increased damping will reduce suspension compliance.

Shocks may have equal or different specifications between bump (compression) and rebound (extension) damping;
For example, the front shocks of a drag racing build may feature low rebound damping, allowing the springs to quickly extend with minimal restriction, assisting a wheelie, then followed up by high bump damping, ready for the front of the car to slam back down!

A 3200 N/m/s shock will put up 3200 newtons (326.3 kgf) of resistance for every meter per second of suspension travel speed.

Swaybars, also known as anti-roll bars are torsion bars that link left & right suspension together, reducing body roll without sacrificing overall suspension compliance as much as a set of stiffer shocks & springs would.

Swaybars are great for balancing understeer vs oversteer - see next topic for more info!

Swaybars are optional to install.

A 5000 N/m swaybar resists suspension height difference at 5000 newtons (508.9 kgf) per meter.

As mentioned previously, more compliant suspension will offer more grip on uneven ground.
We can use this to advantage in balancing suspension for understeer or oversteer;

For example, if you soften the front suspension, the entire body of the car will be allowed to roll more. As roll increases, the stiffer (less-compliant) rear suspension will lose grip first, causing oversteer.
The same idea applies if you soften the rear suspension, except you'll increase understeer, as the front will lose grip first instead.

In short, the softer end of the car will place itself in advantageous position over the stiffer end by increasing body roll at both ends.

This is why you see FWD cars lifting their inner rear wheel off the ground in turns - their suspension is often designed to eliminate as much understeer as possible.

As this is all about body roll, the obvious part to adjust or even entirely remove is the swaybar, though stiffness of shocks & springs will also contribute.

You can strip body parts to reduce weight, improving handling and (low speed) acceleration.
There are compromises, however:

Without body parts, air will catch more severely on the chassis, greatly increasing air resistance at high speeds.
Air resistance is called drag.

The higher the drag, the lower the speed at which acceleration becomes faster with high weight + good aero, instead of low weight + bad aero.
The less drag you have, the less power you need to reach a given top speed, as the vast majority of your power goes into shoving air out of the way at high speeds.

The amount of drag an object suffers is expressed as CdA (Coefficient (of) drag x Area).

Area refers to an object's frontal area, being the surface area of a 2D projection along the direction of travel.
Drag coefficient meanwhile describes how efficiently a given shape cuts through the air, regardless of size.

For example, a cube will have a coefficient of ~1.05, while a sphere has ~0.47, meaning a sphere of the same frontal area will cut through the air roughly twice as easily as the cube.
0.25 to 0.4 is typical for modern cars.

Cars are usually marketed by drag coefficient alone, entirely disregarding frontal area, which can be misleading.

Wings will typically 'fly' within undisturbed airflow, generating downforce, but usually increasing drag. The low pressure zone that forms under the wing is what creates the most downforce.

The purpose of a spoiler meanwhile is to 'spoil' the attachment of air to the surface of the car, reducing drag via cleaner air separation. Spoilers don't generate downforce anything like wings do, though the cleaner air separation usually does reduce lift.
Street Legal 1 does not simulate downforce nor lift, but drag reduction will be observed with all spoilers & a few wings. Downforce / lift info is included here to clarify differences.

Damaged bodywork will produce more drag due to increased panel gaps and rougher surfaces.
Non-secured hoods may suddenly pop open at speed due to a low pressure area that can form at the front of the hood.

Repairing bent chassis is a big task, replacing bodywork is easy.
Bodywork will help spread the load of an impact, reducing chance / amount of chassis damage sustained.

Low-hanging bodywork may however catch on curbs & terrain imperfections, leading to an overall increase in damage.

Bodywork hitting the ground can be desirable too, acting as bump stops that protect oil pan or transmission from taking the impact instead.

When set ablaze, a mixture of oxygen & liquid fuel can create great pressures, propelling stuff forward.

In case of an internal combustion engine, the "stuff" is a reciprocating piston connected to a crankshaft.

In case of a car, this crankshaft is connected to the car's driven wheels through a series of gears & rotating shafts.

To avoid a much longer explanation, let's just say the piston does these 4 strokes:
[DN] Suck an air + fuel mixture into the cylinder.
[UP] Squeeze the mixture into a small volume (combustion chamber), compressing it.
[DN] Surf a Bang as the mixture is ignited by the spark plug.
[UP] Blow the waste gasses out the exhaust port.
Repeat.

Each stroke takes 180 degrees of crankshaft rotation to complete.
2 revolutions per 4-stroke combustion cycle total.

Before getting into details about specific engine components, let's clear up what we're trying to make in the first place.

Torque is a measure of twisting force.
At 300 lb-ft (pound-feet) of torque, an engine is putting out an equivalent twisting force to hanging 300 pounds of weight on the end of a 1 foot long bar, or 1 pound of weight on a (weightless) 300 foot long bar.

This means that a fat dude hanging from a foot long wrench could put out as much twisting force as some car engines are capable of.

Though our burger-loving friend can put up such an overwhelming force using just a simple tool, the engine will win in a race every single time, for what matters is not only twisting force, but also twisting speed!

A horsepower (HP) is a unit of power delivered, equal to 745.7 watts.

To twist a 1 foot lever with 300 pounds of force at 5000 revolutions per minute (RPM), 285.6 HP worth of power are needed.
Or, to put it another way: If an engine is delivering 300 lb-ft of torque at a speed of 5000 RPM, it will be putting out 285.6 HP worth of power.

We know this because horsepower can be calculated from torque and rotation speed:
HP = (torque X RPM) / 5252.
(Torque in pound-feet).

Because of the division by 5252, torque is always equal to HP at 5252 RPM (RPM component of the formula cancels out).

A power graph like this shows how much horsepower and torque your engine puts out at a given RPM.
As you can see, torque peaks early in the RPM range.
This coincides with peak volumetric efficiency of your engine - where each combustion stroke harvests maximum amount of energy from the air + fuel mixture, creating maximum instantaneous twisting force - torque.
This is where you get maximum fuel efficiency.

As RPM continues to increase, torque starts to reduce, as the engine has to fight harder against various source of friction, flow resistances & other challenges of high RPM operation.
Still, power output continues to climb, as the volume of air ingested and amount of combustion strokes completed within a given amount of time also increases.

Peak power is where the car accelerates the fastest.

Beyond peak power, increase in combustion strokes can no longer make up for increase in resistances, and the power begins to fall off.

As far as the engine is concerned, horsepower is ultimately what defines how quickly your car accelerates, not torque.

However, listing an engine's peak torque and HP gives a good general idea of how it delivers its power, without need to see the power graph.
The higher the torque / HP ratio, the lower the RPM where you can expect the engine to make peak power;

A lot of torque and low HP implies peak power at very low RPM, desirable for low speed, high efficiency engines: Busses, trucks, generators.

Torque higher than or similar to HP is typical for street and sports car engines.

Race engines often deliver peak power at very high RPM, making HP greatly exceed torque.

Crankshaft translates rotational motion into linear motion and vice versa.
It is effectively a series of levers (crankpins), each with a bearing connected to a connecting rod, in turn connected via a wrist pin to a piston.
The distance of the crankpins from the center of axis of rotation multiplied by 2 would normally determine the stroke length of the pistons, though in Street Legal 1, this important parameter is determined by your choice of engine block instead.

Big hunk of metal, onto which all the important engine parts are attached.

Diameter of the cylinder holes bored into the block determines the size of the pistons that must be installed.

In Street Legal 1, the pistons and connecting rods come with the block.

Displacement is the volume between the pistons' top dead center (TDC, all the way up) and bottom dead center (BDC, all the way down) positions, multiplied by the amount of pistons.

Displacement volume of each piston is calculated from the engine's bore and stroke.

Bigger displacement means that more air + fuel mixture can be ingested per cycle, thus more power can be made.

A 152 cui engine block has a displacement of 152 cubic inches, or ~2.5 liters.

Cylinder head is mounted above the cylinders, and contains a camshaft, a rotating metal shaft with a series of cam lobes, controlling the valves of the engine.

A drive belt connects the camshaft to the crankshaft, for power and sync.
The camshaft rotates once for every two rotations of the crankshaft.

Intake valves open for the intake stroke, allowing the air + fuel mixture into the cylinder.

Exhaust valves open for the exhaust stroke.

All valves remain closed during the compression & combustion strokes.

This is the small dome-shaped space above each piston into which the air + fuel mixture is squeezed during the compression stroke.

The volume of the combustion chamber relative to the total (displacement + combustion chamber) volume of each cylinder determines an engine's compression ratio.

Higher compression ratio improves an engine's efficiency, and thus power output.
Compression too high increases chance of knock - premature, uncontrolled ignition of the mixture caused by localized heat & pressure somewhere in the cylinder, instead of the spark plug.
Knock is especially likely at high RPM.

Knock can quickly destroy an engine.

A 57 ccm cylinder head has a combustion chamber of 57 cubic centimeters above each piston.
With a 122 cui (2.0 liter or ~2000 ccm) 4-cylinder engine, each piston displaces (2000 / 4 =) 500 ccm, making for a compression ratio of ((500 + 57) / 57 =) 9.77.

Collects waste exhaust gas from the engine and sends it to the exhaust pipe.

An exhaust manifold designed with pulse timing in mind can increase power compared to having no manifold installed at all.

Also helps removing heat from the engine bay.

Distributes air or air + fuel mixture evenly to all cylinders.

Carburetors and single point injectors add a mist of fuel into the air stream before it enters the intake manifold.

Port injectors do the same where the air leaves the intake manifold and enters the cylinder through an open intake valve, separately for each cylinder.

Catches particulate before it enters the engine.

Restrictive / clogged filters can reduce airflow, causing loss of power.

Free-flowing racing filters may compromise filtering, reducing engine lifespan.

Within a combustion engine, there is very little metal-to-metal contact, most moving components gliding on a thin film of oil. This lubrication is critical for long engine life.

Oil is pumped through oil channels all around the engine, lubricating (and cooling) various components, then returns back to the oil pan under the crankshaft at the bottom of the engine via gravity, ready to be picked up and circulated again.

Being at the very bottom of the engine, the oil pan is susceptible to damage from bottoming out.

A bashed-in oil pan will have reduced volume, raising the oil level potentially to the point where the crankshaft may start splashing in the oil.

This not only adds resistance, reducing power, but also causes cavitation (bubbling) in the oil, reducing effective lubrication.

An engine may shut itself off to prevent damage if there's no oil pressure.

The flywheel is a weighted metal disc attached to the crankshaft, storing rotational energy of the engine (inertia), and providing a large surface area for the clutch to engage with.

Continued operation of a combustion engine relies on inertia to keep up rotation between combustion strokes.

The added inertia also slows down how quickly the engine can change RPM, making it easier for the driver to control, at expense of responsiveness.

The outer rim of the flywheel has teeth for the starter motor to engage with.
A 15 pounds flywheel weighs 15 pounds. Amazing :D

A spring-loaded friction disc connecting the rotation of the flywheel with the input shaft of the transmission.

Can be disengaged by the driver pressing the clutch pedal, allowing the engine and drivetrain to rotate at different speeds.

A combustion engine works best at specific ranges of RPM.
If it were directly connected to the wheels, you'd need to be approaching 100 mph to hit mere 1000 RPM (depending on tyre circumference).

Gear reduction is used to trade rotational speed for torque, depending on ratio.

If a smaller input gear rotates twice for every rotation of a bigger output gear it's connected to, that makes for a gear ratio of 2, doubling the torque, but halving the speed.

A car transmission typically has 4-6 gears of different ratios, suited for different combinations of speed & RPM.

After the engine rotation goes through gear reduction in the transmission, it travels through a driveshaft to the differential.

A differential delivers equal torque to the wheels on both sides, while allowing them to rotate at different speeds. This is important as the outer wheel will need to travel more distance (thus rotate faster) than the inner wheel during a turn.

The differential's gear-based mechanism further adds gear reduction, similarly to the transmission.
The amount of this final gear reduction before the power reaches the wheels via halfshafs is called final drive ratio.

In Street Legal, only the transmission is a separate part you can interact with, powershafts and differential being fully simulated, but not visible. Your choice of transmission will determine your final drive.
Final drive is also influenced by your car's chassis, just like the chassis also determines the drive type.

Gear ratios may be spaced for wider or narrower ranges of RPM per gear.
For example, close ratio transmissions will keep RPM within a tight range, but require you to shift frequently to keep it there.

Some transmissions are only suited for cars with adequate engine torque and/or small enough wheels (tyre circumference) to launch without bogging down.

High torque in combination with high grip (where excessive torque can't be safely dumped as wheelspin) can cause drivetrain damage.

In Street Legal, drivetrain damage always means transmission damage.

Energy storage for onboard electronics.
Kept charged by the alternator, spun by the crankshaft via drive belt.

A 45 Ah battery can hold enough charge to provide 45 amps of current for 1 hour, or 1 amp for 45 hours.
At the nominal voltage of 12V typical for car batteries, such a battery holds (45 x 12 =) 540 Wh of energy.

As the battery discharges, the voltage will drop, reducing the power available to accessories, such as the starter motor.

A combustion engine cannot run if the RPM is too low.

The starter motor engages with the small teeth on the outer rim of the flywheel and spins up the engine to enough RPM to fire up, then disengages.

A 15 Nm starter, taking into account the expected flywheel radius of the engine type, will provide 15 newton meters of torque at the crankshaft.
Torque needs to be adequate to compress the air + fuel mixture and overcome other sources of friction.

A weak battery may reduce starter power to the point where it cannot start the engine.

In case of a starter failure or extreme weight reduction :D, an engine can also be spun up by pushing the car.

In Street Legal 1, a rev chip can be added to override the RPM limit of the ECU.
For some engines, this is necessary to access peak power at high RPM, and/or just to allow later gear changes.

Increasing the RPM limit increases risk of over-rev damage to the engine.

Note: The Electronic Control Unit (ECU) is not a separate part you can interact with in SL1; Its simulated parameters are automatically defined by your choice of engine parts.

Increasing an engine's displacement increases its power output because it increases how much air the engine can take in. The more air there is, the more fuel can be added, thus more power can be made.

An engine that 'breathes' in air purely using its pistons is called naturally aspirated.

Another way to increase how much air an engine works with is by externally forcing more air into it using a compressor.

An air compressor driven by a belt connected to the crankshaft. The direct connection to the crankshaft makes it very responsive to changes in RPM.
~
An air compressor just like a centrifugal supercharger, but driven using a turbine placed within the exhaust flow inside the exhaust manifold, rather than a belt.

More energy efficient, but less responsive than a supercharger.

Smaller turbos require less exhaust flow to spin up, so will already offer boost at lower RPM.

Bigger turbos will offer more max boost, but require more flow (RPM) to spin up.

By placing an air scoop within rushing air encountered at high speeds, intake air density can be increased by up to ~3%.

All types of forced induction are simulated in a similar simplified way: Increasing intake air density by a fixed amount depending on RPM, with varying optimal RPM ranges.

This means that the more responsive nature of superchargers is usually expressed by already offering boost lower in the RPM range, as spool rate of turbos is not accurately simulated, making those very responsive, too.

A 19.5 psi turbocharger will increase the engine's intake air pressure by 19.5 pounds per square inch at ideal operating RPM.
This is in addition to the 14.5 psi of atmospheric pressure.

A 2.0% compressor scoop can increase intake air pressure by up to 2% from atmospheric pressure of 14.5 psi.

To increase engine power, we want to mix oxygen with the fuel, and to shove as much of this mixture into the cylinder as possible.
A Nitrous Oxide System (NOS) can help with both by spraying N2O into the engine.

NOS can be activated via button or switch by the driver for a temporary power boost.

N2O is a chemical compound with two nitrogen atoms, and one oxygen atom.
Increasing oxygen amount within the engine means more fuel can be injected, and more power can be made.

Highly compressed N2O is stored in a pressure tank or few usually placed in the trunk or interior of the car. Said N2O is stored at enough pressure to turn from a gas into a liquid at ambient temperatures.

This not only means it can be stored within a much smaller volume than if it were a gas, but it also adds a cooling effect as it enters the engine;

N2O has a boiling point of -127°F (-88°C) at atmospheric pressure, meaning it vaporizes (fogs) as soon as it leaves its high pressure nozzle inside the engine.

When a substance changes phase, a lot of heat energy is released or absorbed.
In case of vaporization (boiling), a lot of heat is absorbed.

Said heat is absorbed from the air inside the engine, increasing its density, meaning more air mass can be shoved within the volume of the cylinder.

A wet system adds N2O to the air in the same place as where the fuel is added (at / near the carb or single point fuel injector).
Similarly to single point fuel injection compared to port injection, such a system is cheap, but difficult to accurately control. It also somewhat compromises the cooling effect compared to...

A port system adds N2O as the air leaves the intake manifold and enters the cylinder, similar to port fuel injection. This type of system offers the most accurate control.
The bigger the jets, the more N2O will enter the engine upon NOS activation.

A 24 pounds canister holds up to 24 pounds of liquid N2O.

In Street Legal, different cars allow different combinations of canisters to be installed.
As the canisters add weight, you may choose to only carry as much as you really need for racing.