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Donpost's Tuning Guide for Forza 4

July 16, 2013

Donpost's Tuning Guide for Forza 4

Under the bonnet

Donpost's Tuning Guide for Forza 4

Please note that this is donpost's way to tune cars. It is not the only/best way of tuning in Forza and is intended as a guide to help those who wish to learn how to tune. All of what is written here works with FM5 too  as it has the same telemetry system - however as donpost hasn't had a chance to get his hands on the new physics engine it is possible some things are now more or less important than they were in FM4.

This tuning guide uses the Renalt Clio from the Clio Cup as an example. If you wish to follow along with the guide in this car to get a feel for tuning click here for the build.

Initial setup

Tyres – 32 front and back
Gears - leave alone
Camber – 0.0 front and back
Toe – 0.0 front and back
Caster – 7.0
Anti-roll bars – 20/20
Springs – leave alone
Ride height - minimum front and rear
Dampers – 6/6/3/3
Aero – If you have any downforce adjustable parts on the car take them off before starting to tune. I'll let you know when to add them back on!
Diff – 25/25
Brakes – 50 for balance. Brake force is personal preference.

 

Suspension – Springs

Springs have three functions. They smooth out the tyre’s passage over bumps in the road, they stop the car from rolling and pitching about under braking/cornering/acceleration, and they must be stiff enough not to make the car feel sloppy or even worse run out of travel and hit the bump stops. Body roll will be dealt with by the anti-roll bars. Pitching isn’t normally an issue. So that just leaves the two things to worry about; making them stiff enough to give your car good response and body control, and soft enough to absorb bumps. Like most tuning options it’s a compromise.

It’s best to think of springs in Forza in terms of frequency rather than stiffness. Spring frequency is a function of spring stiffness and the weight they have to support – the car. Two different cars with the same spring frequencies will have the same trade off of response vs. bump absorption.

As a rule, I keep the ratio of front to rear stiffness the same as the car’s weight distribution. The only reason to deviate from this would be to tune the balance of the car, but the roll bars do exactly the same job and better so we’ll be using them instead.

So that just leaves the overall spring stiffness. I’ve attached a spreadsheet that will help you determine the correct spring rates to use. Fill in the green cells with information about your car. In the yellow cell you need to choose a spring frequency. Higher = stiffer, lower = softer.  2.5Hz is a good place to start.

https://docs.google.com/spreadsheet/ccc?key=0AkcH9BAmz-4XdEg0VG9ScmNVeEJ3X0gzZko5VXdvblE#gid=0

IF YOU ARE GOING TO BE USING AERO, NOW IS THE TIME TO PUT IT BACK ON FOR A MOMENT!

Take your car around your track of choice and keep the suspension telemetry screen on. Watch out for any of the springs turning red – indicating that they have hit the bump stops. If it only happens once or twice for just a flash, or they never go red then you are ok. If it is happening a lot then you are too soft and need to increase the frequency. We could alternatively raise the ride height – but in almost every circumstance in Forza 4 you are best off having the ride height as low as possible.

There is one other problem to keep an eye out for and that is bodywork scraping the ground. This doesn't show up on the suspension telemetry screen, you have to listen and look for it as you drive. Scraping/banging bodywork off of the road means that for that moment, some of the weight of the car is supported by the bodywork and not the tyres. Bodywork isn't very grippy (suprisingly), so you momentarily lose grip. The solution is the same as above, either stiffen the springs or raise the ride height. Nurburgring GP is an example of a track that can cause this problem because its so bumpy.

The more downforce you have, the higher the frequency/stiffness you will need as the downforce pushes the car down and therefore there is less travel in the suspension to absorb bumps.

Now unless the car feels really stiff or really soft no further adjustments need to be made. REMOVE YOUR AERO AGAIN!

Dampers

PLEASE NOTE: Dampers in racing games in general are rarely effective in changing handling due to the limits of the simulation. What follows below is a very longwinded way if coming up with some settings that might be optimal - but you can generally get away with just setting bump to something between 1.0 and 2.0, and rebound to 8.0 or higher.

Dampers affect handling in two ways. The first is their main job, which is to absorb the energy from the spring. Without dampers every time you hit a bump your car would start to oscillate and keep one going. The other thing dampers do is fine tune the handling of the car when weight is moving around. So the entry and exit of corners basically.

Tuning dampers by feel is virtually impossible I think. Personally I can’t feel the difference in damper settings unless I tune them to extreme settings then you can tell something is up. So my philosophy is to use a bit of maths and telemetry to set the dampers up for their main job of absorbing spring energy, then fine tune from there.

Bit of boring stuff to come but it’s important so bear with me. Look at this graph showing movement of the spring after feeling some force, like a bump:

The blue line represents no damping.  See how the spring just keeps oscillating back and forth. Not good for handling.

The green line is under-damping. There is some damping so the energy of the spring is absorbed and eventually comes to rest.

The red line is critical damping. This is the defined as the amount of damping that is required to get the spring back to its settled position as quickly as possible.

The light blue line is over-damping. Here the dampers are so stiff that the spring doesn’t even oscillate once, it just slowly drifts back to being settled.

Tuning articles for the real world say the optimal amount of damping is about 65% of critical. They also say bump damping should be 50% of rebound damping. If we can find the critical amount of damping we can set up our dampers we can use formulas to work out the correct damping for any spring stiffness (on a per-car basis, this has been confirmed by me spending many tedious hours staring at telemetry!). For each new car or if you change the weight distribution or overall weight of a car through changing the build you need to re-run this test.

To do this you need to take your car to Benchmark Layout E and drive to the flat bit of road that cuts across the yard. I’ll describe the technique to you first then tell you how to use it to find the correct damper settings.

Bring up the suspension telemetry. Drive the car forwards and get up to the top of first gear say. Then slam on the brakes full and hold them on. What you will see in the suspension screen is that as you brake, the springs at the front end of the car will compress, and the springs at the rear end will extend. Then at the moment you stop the weight goes back to the natural weight distribution of the car, and the green bars will return back to the centre line and eventually settle there. You can also do this test by reversing up to speed and again braking. This time the front springs will extend and the rear springs will compress before again settling once the car comes to a stop.

Go into your damper settings and set all the dampers to 12.0. When you perform the test under these settings you will notice the springs are over damped as at the moment your car comes to a stop the green bars will slowly move back to 0.0 and settle there without overshooting.

Now set all your dampers to 1.0 and do the test again. This time you will see the green bars return to the middle, overshoot, then return to the middle again, then overshoot again etc until eventually the springs settle in the middle. This is under damping.

We are looking for the smallest damper settings that do not result in any overshoot. To prepare your car for this reduce both springs to the minimum level of stiffness, and raise the ride height at both ends of the car to the max, whilst keeping them equal. So for example in a car with max ride heights of 6.0/5.9 – set the ride height to 5.9/5.9. We do this because it makes it easier to spot an overshoot. Set all of your dampers to an initial value of 4.5.

Concentrate on one end of the car at a time. Perform the test technique both in forward and reverse multiple times. If the spring overshoots, increase the damper settings in both rebound and bump at that end of the car. If it doesn’t overshoot, decrease the dampers. Keep testing until you find the smallest damper value that DOESN’T result in an overshoot. When you are close to the sweet spot the overshoot can be very hard to detect. Look for the spring flicking from -0.00 to 0.00. In our Clio’s I got settings of 5.0 front and 4.1 rear at spring stiffness’s of 132.6 front and back – see if you can arrive at the same values.  Here’s a short clip demonstrating:

http://www.youtube.com/watch?v=MDcTZfi7RWY&feature=youtube_gdata

Now you have the settings put your ride height back to minimum and put the spring stiffness back to what it was – don’t forget to do this!

To get our final settings we need another spreadsheet:

https://docs.google.com/spreadsheet/ccc?key=0AkcH9BAmz-4XdGZzWXZmZGgxMTFYT1lsOFZrV3BQN1E&usp=sharing

Fill in the green cells. Enter the same spring amounts in the purple cells that you put in the green cells (for now). The yellow cells should be completed with the test data, i.e. the minimum spring stiffness and the tested damper value for that stiffness. The values in the red cells are the ones you need to use.

As for the second part of a dampers job: adjusting to fine tune entry and exit of a corner, I have NEVER been able to notice any difference when trying. I know exactly what adjustments I should be making, but they never have a noticeable effect; not without making big adjustments and that just messes everything up. So my advice is don’t bother doing anything else with the dampers. The diff makes much more of a difference in those situations anyway. If anyone wants to give it a go I’ll let you know what to try and you can tell me if you notice a difference; maybe I just have fists of ham!

We will change the damper base settings again before we are done so wait until after that until trying anything out.

Tyre pressure

The above diagram shows us the effect of tuning tyre pressure; it is to maximise the size of the contact patch where the tyre meets the road and therefore maximise grip.

In real life, what you would do is run some laps then compare the temperature of the middle of the tyre to the average of the temperatures at the inside and outside. If the tyre is correctly inflated, the middle temperature will be smack bang in the middle of the two outer temperatures. If the tyre is overinflated the middle will be too hot, and if the tyre is underinflated the middle will be too cold.

Unfortunately in Forza this doesn’t work. You can run the tyre at the lowest pressure or the highest pressure, and the middle tyre temperature will always be slap bang in the middle. WTF Forza? So we’re back to trial and error again to find the optimum pressure.

The good news is that the Stig’s of the Forza community have already done the work for us. If you watch the fastest replays on the leader boards in circuit racing, they always have tyre pressures of roughly 35 front and back.

Tyres heat up when racing and the pressure increases, so you can’t just set the tyre pressures to 35/35 and forget about it, you have to get on track! Take your car down and run a few laps, checking the tyre pressures on telemetry as you cross the start/finish line. It usually takes about 3 laps or so before the tyres get up to a stable temperature. When they have stabilised, note what kinds of adjustments you need. So for example of you see tyre pressures of 36.4 front and 34.5 rear, you need to go into the tuning screen and reduce the front pressures by 1.5 and increase the rear pressure by 0.5. Run a few more laps to let the tyre pressures settle again and double check they are now running at 35/35. It’s easiest to do this in test drive mode so you can tune on the fly.

Toe  

The best way to describe what toe is is with this picture:

NOTE: IN FORZA’S TUNING SCREEN THE “+” AND “-“ ARE SWAPPED. 

Toe out makes that end of the car unstable, which means it can dart from side to side and be harder to keep straight. Bumps will throw you to one side more. Toe in does the opposite and makes that end of the car want to go in a straight line.

Too much toe in either direction can cause more drag when running in a straight line, and less grip at that end of the car in mid-corner.

You can try to tune this by feel - there is no telemetry method. I usually have 0.1 toe out at the front on all my cars, because the instability makes your initial turn in to a corner just a touch quicker, which I think justifies the small loss in grip. On a FWD or AWD I have 0.1 toe out on the rear for the same reasons. On a RWD I keep the toe at the rear at 0.0 because I don't want to encourage the back end to step out.

Camber and Caster  

I'm not going to go into much detail I'll just tell you how I do it. The only real effect of caster in Forza is to add negative camber to your front wheels when turning. I always have this turned up to 7.0 because it means you can run 0.1 or so less camber on your front wheels.

The aim of the game is to have the temperatures across the tyres as even as possible. The correct setting varies from track to track slightly. Go to your test track of choice in test drive mode and run some laps. Choose a reference point - I usually use the start finish line. Go to the tyre temperature screen on your telemetry and keep lapping until they are the same each time you pass your reference point. At this point your tyres are fully heated up so you can start to make adjustments.

The first thing to know is that you won’t be able to make the temperatures over across the tyres on both sides of the car, so we have to pick a side to tune for. Choose the side with the hottest tyres. If you go to the tyres telemetry you can see a single figure for the temperature of each tyre. Remember to wait until you have got the tyres up to temperature before looking.

Once you know what side you are tuning for, go back to the heat telemetry. Check on the temperature profile across the front and rear tyre every time you pass a reference point on the track (I use the start/finish line). If the outside is hotter than the inside, add more negative camber to the wheels. If the inside is hotter than the outside reduce the amount of negative camber. After each adjustment you may need to run a few laps before the temperatures settle down again. Eventually you will settle on the camber setting that gives even tyre temperatures at both ends of the car as you pass your reference point. You are done.

If you look at the wheels on the other side of the car, the inside will be way hotter than the outside. This is normal it’s just the trade-off we make. This setting will be appropriate for all tracks except for maybe ovals where you will want to add more camber. Take your car around the oval and watch telemetry to find the correct setting.

Anti-roll bars

I’m going to put this link here because if you want to read up on vehicle dynamics it’s an excellent article.

http://www.turnfast.com/tech_handling/handling

Anti roll bars have two jobs. Job number one is to make the car roll less for the same amount of lateral force. The second is to fine tune the balance between oversteer and understeer.

Roll resistance

Anti-roll bars work by make the springs at their end of the car act like they are stiffer when moving in opposite directions. A stiffer spring moves less for the same amount of force. When a car is cornering the inside spring wants to extend and the outside spring wants to compress, so the anti-roll bar makes them act stiffer. That means the outside spring will compress less, and the inside spring will extend less, making the car corner flatter. It also has the effect of making the weight move across from the inside wheel to the outside wheel faster, meaning the car responds quicker to your inputs.

The overall roll stiffness of your car is rear roll bar stiffness plus front roll bar stiffness.  Adding more stiffness gives you better response (good for chicanes), and less body roll.  However, it also has disadvantages. Remember we wanted the springs as soft as possible to absorb bumps? Well if you have a stiff roll bar and hit a bump that only affects one side of the car the spring that hits the bump will act like it is stiffer than it is, losing grip. This situation happens when you drive over a kerb on the apex of a corner for example.

In Forza, you can tell if your roll bar is too soft if the car is slow to change direction in chicanes. It’s too stiff if you find yourself bouncing over kerbs. I will only change the stiffness from my initial setting of 40 (20 front and 20 back) if I encounter one of these problems. In the Clio on Nurburgring GP Short the initial setting is stiff enough to allow me to easily negotiate the chicane at the end of the back straight, and all its soft enough to drive over the kerbs without upsetting the car too much.

I’ll usually adjust the overall roll stiffness of my car so that I need no more than 0.9 or 1.0 camber at either end of the car. 

Balance

When you corner, weight transfers from the inside wheels of the car to the outside wheels. The amount that transfers is down to: Weight of car, track width of car, height of centre of gravity and nothing else. The only one we can affect by tuning is the height of the centre of gravity, via ride height which we have set to minimum.

What we can do is affect at what end of the car the weight transfers. Tyre pairs produce the most grip when the weight on each wheel is even. By making more of the weight move across at the back, and less move across at the front, we can remove grip from the rear wheel pair and add it to the front wheel pair. This increases oversteer/decreases understeer.

Understeer and Oversteer 

Weight is transferred in proportion to how stiff each end of the car is. The stiffer end of the car transfers most of the weight.

To tune the balance of the car head to Maple Valley Full. This track involves a lot of chicanes which are especially good for tuning the roll bars. Remember that anti-roll bars will affect the balance of the car in any situation where the car is experiencing lateral (sideways) acceleration, basically everywhere except for when you are driving in a straight line. Keep running the track until you find a good balance all around.

If you understeer too much, soften the front roll bar and stiffen the rear roll bar the same amount (so the overall stiffness we just worked on is retained). If you get oversteer do the opposite. Keep playing until you find a nice neutral balance.

Note that the location of the drive wheels (front or back), and your ability to throttle control is going to be a big influence. For example if you drive a powerful rear wheel drive car and you are having trouble keeping the back end in line then you might set the roll bars up to give you understeer in the mid-corner in exchange for having a more neutral car on corner exit that can put down more power. Do not try to tune away corner entry problems with the roll-bars, there are other devices for that, just concentrate on getting a good mid-corner balance and a clean and quick corner exit.

Sometimes you will run out of adjustability in the roll bars. For example, in our Clios, I set the roll bars to 1 at the front and 40 at the rear, and was still understeering. If this happens, the first thing you should do is get back on Benchmark Layout E. Drive in the tightest circle you can at about 60mph and look at the friction telemetry. Here’s mine from the Clio:

See the traction circle of the inside rear wheel? It’s tiny, because that wheel has no weight on in any more. We are transferring the maximum amount of weight possible over the back wheels. There is nothing more we can do. You can test this yourself by stiffening the rear spring. Nothing changes, you still will understeer, and now every time your back end hits a bump it might let go.

However, in the VERY unlikely event that in your car there is still some grip left on that inside wheel we can continue to change the balance of the car. We do this by going back to the springs and adjusting their balance. Go back to the suspension spreadsheet and play with the balance figure in the purple cell. To make less oversteer/more understeer make the balance number larger. To make less understeer/more oversteer make the balance number smaller. Keep checking there is still some grip on the inside wheel to use for balance adjustment. If there was still traction left in the Clio we could make the balance number in the purple cell smaller, to say 58% from 60%.

There are four bad things about adjusting the springs like this though that you need to consider:

1) The bigger the difference in stiffness from one end of the car to the other, the less effective further increases in stiffness become because the car’s body twists. A roll cage helps with this.

2) ARBs only cost you grip when a bump only affects on side of the car (eg. A kerb). If you start stiffening the springs as well, you will lose grip at all times at that end of the car. You might think what’s the problem with that? Well you might achieve perfect mid corner balance, but also you might notice that when you drive over a bump the soft end of the car is fine whereas the stiff end gets loose.

3) If you change the balance of the springs you will inevitably be making one end softer. This might cause the suspension to bottom out (go red on telemetry). Increase the spring frequency to fix this.

4) If you change the spring stiffness you will need to retune the dampers. This is an easy task on the spreadsheet though. Change the spring stiffness in the green cells to the new spring stiffness on your car.

Given all those issues, if I had a car that couldn’t be balance on the roll bars alone I would either set them as best I could and get on with it, or rebuild the car so it has a better weight distribution or play with the tyre widths.

NOTE: You may have to redo camber and maybe even tyre pressures if you change the roll bar settings by a large amount.

Dampers – Part 2  

PLEASE NOTE: Dampers in racing games in general are rarely effective in changing handling due to the limits of the simulation. What follows below is a very longwinded way if coming up with some settings that might be optimal - but you can generally get away with just setting bump to something between 1.0 and 2.0, and rebound to 8.0 or higher.

Now we’ve sorted our roll bars we need to go back to the dampers. The roll bars effectively make the springs stiffer in roll, and stiffer springs mean more damping is needed. It’s time for another weird telemetry setup.

Go for a test drive at Sunset Peninsula Oval raceway and park your car up here at the first turn:

Go into tuning settings and raise your ride height as high as possible. Go back to the suspension telemetry. Look at the front springs and note down the difference between left and right. For example if the left is -0.9 and the right is 0.15 then the difference is 0.26. Nudge the car back or forward a little until the difference at the front and the back is in the sample ballpark – it doesn’t matter if they are difference by about 0.05 or so.

When you are in position make a note of the front and back differences. Now go back to tuning and reduce the roll bars to minimum. Make a note of the suspension differences again – this time they should be bigger.

Now here’s the complicated part. Add stiffness to your spring settings in the proportion of what your roll bar settings were. Keep doing this until you find the spring settings that give the same front and back suspension differences as when the roll bars were setup. Here’s an example:

Go back to the damper spreadsheet from earlier and put these spring figures in the purple cells. Take your damper settings from the sheet. Set your springs, ride height and roll bars back to what they were. Done!

The purpose of that was to find out just how much stiffer the roll bars make the springs act, and use that info via the spreadsheet to find the correct settings.

NOTE: A lot of fast hot lappers like the bump to be even lower so that they can attack the kerbs without upsetting the car. If you can make the corner faster by cutting it like this you can shave lots of time off of your laps. If you want to make this adjustment to your car just take the bump settings from the spreadsheet and cut them in two. If you do this be careful you are not bottoming out the car, keep an eye on the suspension telemetry and look for the bar going red. Again, once or twice for a moment isn’t a problem, but if it’s happening a lot and/or upsetting the car, go back to the original bump settings from the spreadsheet.

Gearing  

This is easy. Get in your garage and note down the peak power and peak power RPM values (Press "x" to bring up the Car list, then hit "RB" until you get to the peak power section).

Get yourself down to Benchmark Layout A in test drive mode and accelerate up to top speed. Wait until you're on the straight and flat bit and bring up your telemetry. You want to keep adjusting the final drive ratio until you reach top speed in top gear at peak power RPM. Make sure the power at top speed is equal to your peak power.

For example in the Clio's peak power is 205 bhp @ 7200rpm. By adjusting the final drive to 4.39 you hit a top speed of 143 in 6th gear @ 7200 rpm which is 205bhp.

Now take your car to the track you are going to be racing on. If it's one of those rare tracks where you can actually make it up to close to top speed skip this paragraph. If not, you need to adjust final drive until you are hitting the top of top gear at the end of the longest straight (i.e. the fastest point on the track). Going back to our Clio's, I found a final drive of 5.45 was about right.

Next you adjust first gear to get the best launch. I just do this by feel as best I can. If someone can think of a tune this a bit more accurately I want to hear it!

So now first gear, final drive, and top gear are set in stone. Do not touch them again! All you need to do now is spread out the middle gears. For now you'll have to do this by eye using the graph in the bottom right of the gear tuning screen.

Try using a different amount of total gears. For example, in a 6 speed car call 5th gear "top gear". Move the ratio for the real top gear down to as low as it will go to get it out of the way, then repeat the above with the new top gear.

ON TRACKS WHERE YOU DON'T GET NEAR YOUR TOP SPEED USING FEWER GEARS CAN SOMETIMES BE MASSIVELY FASTER. I put that in caps because it’s so important. I went from 6 gears to 5 gears in the Clios and gained 0.5 seconds on a 1 minute lap. Huge gain.

That's it. Your gears are now tuned.

When you want to retune for a new track adjust top gear until you are hitting the top of top gear at the end of the longest straight again. Then just re-spread the middle gears again. DO NOT touch first gear or final drive!

Brakes

Brake pressure is a personal choice. As long as the pressure is high enough to lock your wheels up you are ok. I just leave it on 100% on all my cars so the brakes feel the same (I use pedals).

Brake balance is the distribution of brake pressure between the front and back wheels. The best way to tune this would be to watch your friction telemetry whilst pulling the brakes. If the front wheels lock up first then move the brake pressure rearward, and if the rear wheels lock first move it forward.

Unfortunately it's a little difficult to tell which wheel lock first unless the brake balance is well out, so I use a lazier method which is basically to run a benchmark and look at the braking distance. I just play with the balance until braking distance is minimised.

This will give you the best braking distance in a straight line. Don't be tempted to adjust your brake balance to tweak your car's handling when braking into a corner. There is a much better tuning device for that....

Differential: Deceleration

I'm going to start with decel. There's no point trying to describe the MEANING of the decel setting, becuase frankly I have trouble getting my head around it myself lol. Besides you don't need to know how it works to tune it. All you really need to know is decel affects the car under engine braking at the same time as cornering. This situation occurs on corner entry.

What I'm going to suggest you do here is run you car with a decel setting of 0% and again with a decel setting of 100%. What you should notice is that at 0% you will oversteer coming into certain corners (even with a pushy FWD car like our Clios). At 100% you won't oversteer into those corners, in fact it will be quite the opposite - you will tend to have a hard time turning it.

The types of corner this happens on are corners where you start to turn BEFORE you start most of your braking. On Hockenheim the entry to the last double right hander before the start/finish straight is one. The first turn at Suzuka Full and the first turn at Mugello are other examples. Every track has them.

Anyway - you should have really noticed the difference in your settings there. Even someone with fists of ham like me can feel it. The basic premise when tuning decel is that we want it as low as possible, without causing us to oversteer too badly coming into those certain corners. The reason why is that low decel also has the beneficial effect of causing the car to rotate a lot easier when off throttle (i.e. on engine braking). This behaviour is really useful in long sweeping turns. If you find yourself beginning to push wide, just let off the throttle a little bit and the decel setting will get your car to turn back onto the racing line.

Start at 0% and work your way up. Most of the time you will end up with a setting less than 20%. If you are taking the tune racing rather than hotlapping I would suggest upping it a couple of % over what you settled on. In races you tend to be a bit more aggresive than the smooth driving of hotlapping, and the oversteer on corner entry can come back. I learned this at Hockenheim in the Clio's - anyone following me would have seen me going sideways quite often at that last double right hander. In hotlap mode it was fine, but in the race....

This setting is so powerful you are well off spending a bit of time on it. I would say its probably THE most important tuning setting - but that's just my opinion!

Differential: Acceleration 

The accel setting is in effect whenever you are putting torque into the diff – this includes situations where you are using throttle to maintain the same speed like in mid-corner. 

The diff in Forza works by transferring torque from the wheel of least resistance to the wheel of most resitance. At 0% no torque is transferred and the wheels can spin at completely different speeds with impunity. At 100% as much torque is transferred to the wheel of most resistance to ensure that both wheels always turn at exactly the same speed. The wheel of most resistance is basically the one that is turning slower, and vice versa. From here on out I will refer to them by the “slower” and “faster” wheel for simplicity, although obviously if you have a 100% setting they will both be rotating at the same speed and you’ll have to go back to thinking in terms of most and least resistance. 

The diff has two main effects on the performance of your vehicle. It controls how much of the available traction you can use, and it controls the amount of throttle steer you get. 

This graph shows how higher accel settings allow you to use more of the available traction:

E.g. if you are cornering and only 30% of the weight on the rear tyres is on the inside tyre (therefore 70% on the outside tyre) and you have an accel setting of 0%, you will only be able to use 75% of the available traction. 

At 0% unless the wheels are perfectly evenly loaded there will be available grip remaining unused on the outside wheel when the inside wheel breaks traction – this makes the transition into wheelspin more manageable. At 25% there will only be unused grip when less than 30% of the rear weight is on the inside wheel. At 50% there will only be unused grip when there is less than 15% etc etc. The less unused grip there is at the point of wheelspin, the loss of grip when you spin the wheel(s) will be more dramatic and difficult to control. 

Looking at the diagram, if you never have less than 20% of the rear grip on the inside wheel then there is no difference between settings of 50% up to 100% in terms of being able to put down power. In reality during normal cornering the inside wheel will very rarely go below 20% (except for when you go over something bumpy like a kerb with the inside wheel). Therefore in terms of being able to use all available traction there are massive benefits up to say 50%, but the extra benefit (and also the extra difficulty in controlling the car when you do lose grip) from increasing beyond this isn’t much unless you are on a very bumpy track or have a tall heavy car that transfers huge amounts of weight when cornering. 

This diagram shows how a higher accel setting causes more rotation of the car (oversteer), even in a front wheel drive car, compared to taking the same corner at the same power with a lower accel setting.

99.9% of the time, when you are cornering the inside wheel will provide less resistance to the engine’s torque than the outside wheel (because it has less weight pushing down onto it). The result of this will be that the diff will want to send more torque to the outside wheel, which creates a rotational force in the direction of the corner. This helps to turn the car around the corner and is commonly known as “throttle steer” as the use of the throttle controls the strength of this rotating force. Whether or not this is a good thing depends on the balance of your car and your preference. 

Sometimes when you are in a low torque situation, like an underpowered car or a slow mid corner when you have to use a slither of throttle, the outside wheel will be rotating faster as it is describing a longer path than the outside wheel. The low torque means that the inside wheel isn’t starting to spin faster. In this situation the torque is being transferred from the faster OUTSIDE wheel to the slower INSIDE wheel. This will cause a force that rotates the car away from the corner (understeer). I did some testing and it seemed this situation happens VERY rarely if at all in Forza 4. 

The strength of this effect is correlated to the diff’s accel setting. 0% means you will get no throttle steer. 100% will give you loads. The strength is also related to how big a difference there is between inside and outside tyre grip. When driving in a straight line where the grip on the rear wheels is even, you will get no throttle steer from the diff no matter what the setting. If you use the throttle in a hard cornering situation where the grip on the rear wheels is massively unbalanced you will get more throttle steer. 

As an illustration, imagine a RWD car with a diff setting of 100%. You drive around a corner and there is say 30% weight on the inside wheel, so you are getting a moderate amount of throttle steer. Then you reach the apex and decide to run the inside wheels over the kerb – nothing massive just a normal kerb. The bumps of the kerb cause the inside wheel to lose some of its grip. Now maybe only 5% of the grip of the rear tyres is on the inside. Suddenly you get a lot more torque steer and the car spins. 

Now imagine the same thing in an understeery FWD car. This time the extra torque steer is useful as it rotates your under-rotating (understeering) car around the corner for you! 

The conclusion is this; keep your accel setting high – at least above 50% but ideally above 70% This will mean you can use all the traction available all of the time. The only reason to use less than 100% in this respect is to reduce the effect of snap oversteer/understeer when you lose traction. 

Now adjust the setting for throttle steer. Signs of too much throttle steer are spinning out the car when riding kerbs, or too much oversteer when using the throttle mid-corner to exit WITHOUT spinning the wheels. In a RWD car spinning the wheels will cause too much oversteer no matter what you’re diff setting is! 

In a FWD car it’s VERY unlikely that you will have too much throttle steer because these cars tend to understeer under power due to weight being transferred away from the driven wheels. This means that in a FWD car an accel setting of 100% is usually the best. 

Aero

At the start I said if you have aero take it off. This is so we can tune the mechanical grip without the effect of aero distorting the results. If you are using aero now is the time to add it back on!

Bit of a weak area for me this. Just as a recap - aero adds grip without increasing the car's mass, meaning you can take corners faster. The faster you are going the more grip is added. There isn't really enough grip to make a difference unless your cornering speed is above 70mph.

The downside is that aero causes extra drag, making the car slower on the straights so straight away there is a balance to be struck. Secondly the balance of the aero front and rear affects the balance of the car in fast corners.

The first thing to do is to tune the balance of your aero front to back. To do this you'll need a long sweeping fast corner at at least 100mph. I can't really think of one - answers on a postcard pls lol. Maybe the first turn at Maple Valley again? Tweak the balance of the aero until you have neutral handling in all the fast corners on your track.

Now you have the hard part which is deciding how much aero to have overall. The only way to do this is by looking at your laptimes. If you adjust the total aero up or down make sure that you increase the front and rear in proportion, and double check the balance is still correct.

I've read people say that they either have no aero on their cars, or they do have it on them and then choose the maximum amount possible. This sounds good enough to me. If you can run consistent laptimes you might be able to see the difference in laptimes from having different amounts of aero, otherwise just stick with this mantra!

 

To discuss this tuning guide further, or ask any questions, please comment below!

Comments

Tue 16 Jul, 2013 21:27

I can't thank you enough Don for this amazing contribution. Just incredible!

Sun 23 Feb, 2014 01:12

I don't quite get your dampers formula spreadsheet yet, you say "The yellow cells should be completed with the test data, i.e. the minimum spring stiffness and the tested damper value for that stiffness.", how exactly I should get the tested number? From telemetry? But I don't see where to read them.

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