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Car Building and Tuning Guides!

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Car Building and Tuning Guides!

Post by AndyT306 on Sat Oct 20, 2012 4:38 pm


Reconstructing a build from a replay is easy once you know what to look for.

What you need to do is go into photo mode and have a good look round and in the car.

Roll cage:A full race roll cage is easy to spot,look dirctly through the front widscreen and out the back you cant miss it.
Street and sport roll cages are almost impossible to detect.

Weight reduction:Again look inside the car,if the passenger seat is missing the full race weight reduction is being used.
Street and sport reductions are harder to spot but there are visual clues that can help identify the parts but off the top of my head i cant remember what they are.

Tyres:Race compound tyres are the easiest to spot as they have no tread and no markings on the sidewall.
Street tyres i belive have subtle differences to stock tyres and are hard to spot.
Sport tyres have different tread and sidewall markings to street.

Rims:The make and size of the rims should be fairly easy to spot.

Engine:When in replay mode press up on the d-pad to bring up the telemetry,under 'general' youll be able to see what horsepower and torque is being ran (you can also see what gears are being used for corners and even work out ratios when you get good)
Check to see if any boost is being used and at what level.
Generaly if there is boost it will be provided by a 'centrifugal supercharger',There are audible clues to look out for to work out if this is the case.

A centrifugal supercharger will have a disstictive whine to the engine note that stays contstant through the revs.

When a turbo is being used youll here the air being sucked into the intake,youll also hear the turbo when they come off the throttle.

A twin screw supercharger has an even higher pitched note than the centrifugal supercharger that grows in volume as the revs increase.

On low boost cars you may need to go close to the back of the car to be able to hear these sounds.

The actual engine parts are tricky to work out but if you know the hp/torque as a baseline then after a bit of trial and error you should be able to get there.

Intercoolers and the oil and cooling give a good boost to torque and power for the PI gained but at a cost to weight.

Cams give alot of HP but little in the way of torque,The street upgrade gives a good amount of hp for reletively small PI increase

Upgrading the supercharger probably gives the best Hp/torque-PI ratio out of all the engine part,A small gain in weight is the downside but only with the full race upgrade.
Upgrading turbos causes the PI to shoot up and does not really have many benifits on low PI cars.

Exhaust upgrades give reasonable amounts of HP an torque aswell as reduction in weight but the PI cost can be high.

Dissplacement and the pistons give the best increases in torque but can be costly in PI.

All the others give average increases to torque and power.

Aero: Forza aero will be easy to spot.
Other aftermarket bodyparts should be easily identifiable,These reduce drag and make the car slightly quicker in a striaght line.

Differential:These are a almost always used.

Brakes:As you upgrade the brakes the calipers and disks change size and shape,not easy to spot but with practice you can learn the difference.

Any othe parts that arent mentioned here are nearly impossible to work out,however once you know what parts ARE being used you can use your judgement and trial and error to find what best works for you.
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Re: Car Building and Tuning Guides!

Post by AndyT306 on Sat Oct 20, 2012 4:38 pm




This is the tuning guide I wrote a bit ago. If you have anything to add to it let me know and I'll be glad to do so.


--VVV Worm

First Rule: Fix the end of the car that has the problem. If it's the rear sliding around don't go adjusting the front. Start with adjusting the back end to stabilize it first. You can then go up or down in unison to stiffen or soften the car.

Tires and Temperature

Getting the correct temperature/pressure combination is like fitting together a big puzzle. Stiffer spring = more temperature and stiffer roll bars = higher temperature and on and on. It all fits together and the tires are the only thing we have direct control of. It's often the most overlooked part of tuning yet it is the most important.

I'll use the recommendations that I have gathered from Pirelli for temperature as some manufacturers differ +/- 10° F. The optimal grip for a tire will differ by what compound you put onto the vehicle. The softer the tire the lower the temperature needed for optimal grip.

GT cars 27.5 to 31.9 psi hot.

Touring cars 29 to 36.2 psi hot.

Temperature range can be from 180° to 210° F. A softer tire will lose grip more rapidly once it gets past 210° than a harder compound.

How does tire temperature relate to pressures?

Higer pressure = Lower temperature
Lower pressure = Higher temperature

This works in small amounts. On FWD cars in the game you can have a really high PSI in the rear and it will slide all over the place.

Adjustments Decrease Understeer Decrease Oversteer
Front Tire Pressure Higher Lower
Rear Tire Pressure Lower Higher

Uniform rules no matter what

Give the tires 3 laps to warm up before looking at telemetry.

You should never have more than 20° between the inner portion of the tire and the outer with the inside always being hotter.

You should never have more than 25° between the front and rear of the car


Keep in mind you want no more than 20° difference between the inside and outside of the tire with the inner 1/3 always being hotter (10°-15° are good numbers). Make sure you look when actual load is on the tires going through a corner and the tires have had proper time to heat up (3 laps minimum).

You always want negative camber

Negative = More inside
Positive = More outside

There is a sweet spot in this setting that usually doesn't change unless you move the Caster around too much once everything is set. You also always want more camber in the front than the rear. Rear camber isn't as necessary as those tires don't turn and I want them with a touch of negative camber when throttle is applied on exit.

Front toe favors a positive number (more responsive and better turn in) between 1° and 5°. Anything above 3° will cause excessive drag in the straights and gets really bad above 160mph.
Rear toe favors a negative number and becomes more responsive through turns at 1°. However anything over 2° and the back end loses a lot of stability in acceleration and braking. On bumpy tracks any amount of rear toe will cause the car to lose stability and you will have to make corrections even in a straight line costing you time.

EDIT: To clarify you shouldn't use more than .3 degrees in either direction unless the circumstance is extreme.

Allows the tires to roll with the chassis giving a larger contact patch through a turn. It also helps to balance the temperature of the tires on tracks with predominant left/right turns.

A larger number here will also help absorb bumps on the track while decreasing stability as too much caster causes the tires to wander a bit.

I personally like a really high Caster (5.5-7.0) in the lower classes but on tracks with longer sweeping turns less caster is better.. Most cars aren't sensitive to caster changes.

Adjustments Decrease Understeer Decrease Oversteer
Front Wheel Camber More Negative More Positive
Rear Wheel Camber More Positive More Negative
Front Wheel Caster More Positive More Negative

Anti Roll Bars

Anti roll bars control how much side to side roll a car has. In theory you would want to minimize body roll as much as possible. This is especially true in the higher classes but on occasion in FM3 some body roll helped things. I guess we will see in FM4. The end of this one is long winded but helps to understand it all.

Rear ARB soften = decrease oversteer or increase understeer
Rear ARB stiffen = decrease understeer or increase oversteer

Understanding what the ARB's do

"Limiting the lean of the body is good because it means that when you take a quick set into a turn, that the body isn't still moving sideways after the tires are at their limits. Otherwise you turn in quickly, the tires grip, then the body finally finishes leaning, when it stops, the tires lose grip."

"It limits camber changes. The camber directly impacts the angle at which the tire cross section meets the road and thus controls lateral grip. As the suspension compresses the camber angle generally changes relative to the chassis. With a normal McPherson strut that hasn't been lowered, the camber goes from positive to more negative as the lower A arm swings out straight, and then back to positive as it swings up. That swing up into positive camber is BAD. At that point the chassis is already leaned over so the tire may be starting to roll onto its sidewall. Changing the camber even more positive is just nasty. A big sway bar will prevent the body roll in the first place, and prevent the suspension compression on the outside which causes the positive camber change relative to the chassis."

"Here's where it gets really tricky: If decreasing the size of the rear bar doesn't help enough, the next thing you do is increase the size of the front bar. When the outside front compresses in a corner, it causes the inside front to compress and may actually lift that tire completely off the ground. The car is now sitting on 3 tires and guess where the weight that was on the inside front goes? Outside front? Some of it. The rest goes to the inside rear where we need more grip. The total weight of the car hasn't changed. It's just been redistributed, and a sway bar at one end, actually transferred weight to the other end of the car. Increasing the front bar then translates into more motive grip at the rear, and thus more acceleration"

Springs & Ride Height

From speaking with Dan the spring tuning and adjustments have changed drastically from FM3 to FM4. Every car is now different based upon what it is in reality as opposed to just one generic suspension being stuck under every vehicle like Forza titles in the past. (his direct words were not all suspensions are the same and the same springs weights don't do the same thing on every single car)

Any change in the spring rate will change the ride height due to the rate of spring deflection. They also change the compression(bump) and deflection(rebound) ratings of the shocks.

I personally like to balance the weight of my vehicle with my shocks and leaving it set to the weight % of the vehicle. Aero also affects the weight on the car and needs to be figured in. Some tracks require a softer spring while others require stiffer.
If you use Xtreme Skills formula it figures everything in for you to actually balance the springs. You can then multiply things by 1.1 to increase or .9 to decrease the spring rate and still keep the % correct and balanced. In most cases I like to soften the springs until the telemetry starts to blink red on the spring screen and multiplying it by 5% or 1.05.

Xtreme Skills formula is: (do this for the front and rear) (I think this may be a bit too soft for FM4 and have been multiplying by 1.5-2.0)
Simplified (Car weight+ downforce at front-rear) / 100 x weight % at front-rear / ride height at front-rear x G-force = YOUR SPRING RATE

Also keep in mind that a softer suspension will create more load so if one side of the car is under more stress it MAY benefit from a stiffer setup.

Ride Height
Keep this as low as possible without the car bottoming out

Damping (shocks)

The extension when the tire is unloaded (force back out) = REBOUND
The compression of a shock (making it smaller) = BUMP

Under braking the front axle will compress while the rear will rebound. As the weight shifts the front axle will have a higher velocity than the rear which is good because the front should settle before the rear. If the front does settle too quickly the tires in the front will lock a little premature and the same for the rear. Brake bias only masks the problem and moves it to a different portion of the track.

Under acceleration the rear axle will compress while the front will rebound (think of a see-saw). On a RWD for example if the rear squats before the torque reaches peak it reduces the time the driver has to feather the throttle as the tires begin to break loose. Try to balance the compression against the required grip of the torque. FRONT REBOUND SHOULD BE FASTER THAN REAR COMPRESSION.

FWD is different in a sense. For acceleration increasing the front rebound will give more grip initially while increasing rear rebound increases a sustained level of grip.

Downforce and Braking

More at rear = more understeer and a stable back end
More at front = more oversteer and increased turn in.

More downforce = less speed in straights, more speed through cornering.

Don't forget that any downforce adjustment = a spring adjustment to compensate for the added weight. Especially at higher speeds.


Braking pressure is a preference of the driver in my experience.
Your brake bias controls understeer and oversteer while braking.

More front bias = more understeer and can bind the front tires up to where you can't trail brake if you prefer to.
More rear bias = more oversteer and can cause the rear tires to lock up faster than the front which is really bad. Will cause the car to spin. This can also be a preference as some like to go early in fast out and some find it easier to brake later and trail brake through the corner. Those two can create a full scale argument.

Differential Settings

Differential settings increase the amount of lock your inside wheel has while turning. At 100% both the tires turn at the same speed which on a rear differential creates oversteer (on FWD it eliminates torque steer). A higher differential will allow you to use the throttle to help turn around a corner but can cause mid-exit oversteer.

On deceleration when you lift off of the throttle one tire locks faster than the other. This can cause the back end to want to kick out over the locked tire in extreme settings (under 15%). A lower setting does help things and the further apart the accel and decel are on the same differential the more unstable the car seems to get in my experience.

There isn't a ton I can find on differential settings so sorry for being so brief here and it is pretty simple. A higher number creates oversteer and a lower number prevents it.



Straight Line Instability

* Too much rear wheel toe-out.
* Not enough rear downforce.
* Too much front downforce.
* Not enough rear toe-in (under hard acceleration) –presumably for RWD.
* Too much front toe (either in or out) – car darts over bumps.
* Front ARB is way too stiff – car darts over bumps.

Instability Under Brakes

* Front end darts or wanders – too much front brake bias.
* Car wants to spin – too much rear brake bias.


* Car feels heavy and unresponsive.
o Too much downforce.
* Car feels sloppy and is slow to take a set in corners.
o Too little shock.
o Too much body roll (not enough spring and/or ARB).
* Car responds too quickly, is twitchy, and slides easily.
o Too little downforce.
o Too much shock, too much spring, and/or too much ARB.
o Too much tire pressure.


* Corner entry understeer – car won’t turn in at all.
o Front
tires not wide enough.
o Too much front roll stiffness – ARBs and/or springs.
o Not enough front bump – shocks.
o Not enough front downforce.
o Too much dynamic camber on front wheels (not enough static negative camber).
* Corner entry understeer – car turns in initially then starts to push.
o Too much front toe-in.
o Rebound too stiff – shocks (not enough droop travel).
o Not enough front downforce.
o Bump not stiff enough – shocks.
* Corner entry understeer – car turns in and then darts.
o Insufficient front suspension travel in either or both directions – shocks.


* Corner exit oversteer – gets progressively worse from the time throttle is applied.
o Too much rear roll stiffness.
o Too much rear camber.
o Too little rear downforce.
o Too little rear toe-in.
o Not enough rear spring, shock, or ARB – allows car to roll over on outside rear wheel.
* Corner exit oversteer (sudden) - car takes its set then breaks loose.
o Not enough rear suspension travel (too much shock in either bump, rebound, or both).



* Too much spring – overall
o Harsh and choppy ride, lack of tire compliance.
o Can’t put power down on corner exit - excessive wheelspin.
o Car slides too much.
* Too much spring – front
o Initial understeer.
o Front end breaks loose in corners.
o Front end breaks loose over bumps.
* Too much spring – rear
o Oversteer when power is applied on corner exit.
o Excessive wheelspin.
* Too little spring – overall
o Car bottoms out.
o Car feels like it’s floating.
o Sloppy response.
o Car is slow to take its set.
* Too little spring – front
o Front end hits ground under brakes.
o Too much body roll on corner entry.
o Initial understeer – car won’t point in.
* Too little spring – rear
o Too much squat under acceleration, and the resultant increase in negative camber.
o Car falls over on outside rear wheel causing power-on oversteer.

Anti-Roll Bars

* Too much ARB – overall
o Very sudden turning response and little feel.
o Car slides or skates instead of taking its set.
o May dart over one wheel bumps or diagonal bumps.
* Too much ARB – front
o Corner entry understeer that gets progressively worse.
o Steady state understeer in the middle of sweeping turns.
* Too much ARB – rear
o Corner exit oversteer when throttle is applied.
o Excessive sliding coming out of corners.
* Too little ARB – overall
o Car is sloppy and lacks response.
o Car is slow to transition, especially in chicanes and esses.
o Car rolls too much resulting in too much dynamic, positive camber and the resultant loss of cornering power due to decrease in tire traction.
* Too little ARB – front
o Car rolls over onto outside tire on corner entry then ‘washes out’ (understeers).
o Car lacks steering response and is slow to change direction.
* Too little ARB – rear
o Back end doesn’t want to rotate on corner exit under power (difficult to throttle steer).

Shock Absorbers

* Too much shock – overall
o Very sudden car with harsh ride, sliding, and wheel patter.
o Car crashes over road surface irregularities.
* Too much rebound adjustment
o Wheels do not return to road surface quickly after displacement.
o Inside wheel pulls off the road surface in a corner.
o Lack of tire compliance over bumps and surface undulations.
o Car may be jacked down in long corners.
* Too much bump adjustment
o Initial reaction to bumps and curbs is harsh.
o Initial chassis roll slow to develop.
o Car slides rather than sticks.
o Driven wheels hop when the power is put down.
* Too little shock – overall
o Car floats a lot in ride and oscillates after bumps (underdamped).
o Slow and sloppy response.
o Chassis rolls too quickly.
* Too little rebound adjustment
o Oscillates after bumps.
o Doesn’t put the power down well.
* Too little bump adjustment
o Initial bump reaction soft.
o Car dives and squats a lot under brakes and under power.
o Car rolls quickly and falls over outside tires – front tire on corner entry and rear tire on exit.

Wheel Alignment

* Front toe-in – too much
o Car darts over bumps, under the brakes, and during corner entry.
o Car won’t point into corners.
* Front toe-out – too much
o Car wanders under the brakes.
o Straight line instability especially over one wheel and diagonal bumps.
o Car may point into corners then refuse to take a set
o Understeer as a result of tire scrub in long corners.
* Rear toe-in – too much
o Rear feels light and unstable on corner entry.
o Car slides a bit in corners rather than rolling freely.
* Rear toe-in – too little
o Power on oversteer during corner exit.
* Rear toe-out – any
o Power oversteer during corner exit.
o Straight line instability.
* Front wheel caster – too much
o Excessive physical steering effort (probably non-applicable with the XBOX controller)
* Front wheel caster – too little
o Too little steering feel and feedback (also probably non-applicable with the XBOX controller)
* Camber – too much negative
o Inside of tire will be hotter than the rest and wear faster.
o Front tires – reduced braking capacity (dive)
o Rear tires – reduced acceleration capacity (squat)
* Camber – too much positive
o Outside of tire will be hotter than the rest and wear faster.
o Rear tires – corner exit oversteer and reduced tire traction.
o Rear tires - If extreme may cause corner entrance instability.
o Front tires – too much body roll and understeer after car turns in.


* Too much tire pressure
o Harsh ride.
o Excessive wheel patter, sliding and wheelspin.
o High temperature at center of tire.
* Too little tire pressure
o Soft and mushy response.
o High tire temperatures at inner and outer edges.
o Reduced contact patch


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Re: Car Building and Tuning Guides!

Post by AndyT306 on Sat Oct 20, 2012 4:38 pm


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Re: Car Building and Tuning Guides!

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