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How to fix understeer in ACC

Following on from our article about how to fix oversteer in ACC, today we bring you part two of this series, offering hints and tips on fixing understeer. Like the oversteer article before it, we will use the same framework to explore what options are available in ACC to minimise understeer in a setup. 

You will not be surprised that a lot of it will be about doing the opposite of what we suggested in the previous article, so let’s get to it. 

What is understeer?

“Understeer is where a car rotates slower than the racing driver wants and results in the front of the car sliding across the surface of the track.”

Seems fairly straight forward, but how do we use the setup parameters in ACC to solve this problem?

In order to solve our understeer problems we will focus on some key setup areas of the car:

In this article we will focus on the broader rules of setup as opposed to exploring car specifics. In most cases, it is always better to have an oversteering car than an understeering one. This is because a driver can much more easily influence how much oversteer they want the car to have corner to corner, whereas there is nothing you can do to reduce heavy understeer, other than to over slow the car a lot and wait for the front end of the car to grip up again. 

Ride height and Wing level

In ACC ride height is very important. Due to GT3 cars being very aero sensitive, you want to run the car as low as you can on the front to keep the centre of gravity as low as possible, without the risk of stalling the floor of the car or bottoming the car out over kerbs or big compressions. 

Because of this, once you have settled on a good front ride height, focus then shifts to your rear ride height and balancing that with the wing level you are currently running on the car. When you raise the rear of a car in ACC you are shifting the centre of pressure forwards and therefore dialling more oversteer into the setup. This is because you are increasing the difference between the front and rear ride heights and this promotes an effect where the floor of the car isn’t being sealed as effectively as before. Whilst the area of low pressure is greater within the main diffuser area, the cars downforce production and operating window become peakier. 

This then brings us onto the wing level you choose to run. Adding wing level to a car in ACC has the opposite effect of raising the rear ride height. By adding downforce to the rear of the car you are taking the centre of the pressure away from the front of the car, adding more force over the rear wheels in the form of aerodynamic load and therefore creating a more understeer biased car.

To summarise

If you are experiencing understeer there are a couple of things you can do. To gain more rotation at slow speed it is generally best to raise the rear ride height up to the point where you feel the car is starting to be nervous on the transition from braking to turn in. That is generally the moment the floor is starting to feel the effects of not being sealed properly at low speed.

In higher speed corners when aero loads increase, you normally experience understeer if your wing levels are too high as the centre of pressure is too far rearwards. At this point the wing becomes a tweaking tool where you get the car into a window where it maintains good minimum apex speed whilst not being too safe and stable. 

  • Understeer in slow corners: Raise rear ride height
  • Understeer in high fast corners: Lower rear wing 
  • Understeer over the whole lap: Raise rear ride height and lower rear wing 

Anti-roll bars

Generally, if you have a setup where the spring rates and dampers are to your liking, anti-roll bars can become a key component in shifting car balance to become more neutral front and rear. 

One thing to remember about anti-roll bars, they do not directly generate or take away grip from a car, they simply alter how the load of the car is being distributed across the tyres during the cornering phase. The aim of anti-roll bars is to allow a driver to reach the cars optimum grip potential. Any deviation towards understeer or oversteer takes the car away from that. 

With that in mind, how would anti-roll bars create understeer and what do we need to do? 

By softening the anti-roll bars you are making the car more susceptible to roll when the car is loaded up in the corner. This makes the car slightly less responsive and therefore in most instances safer and more stable. If you find your car in ACC is nervous through the entry and mid-corner phase, there are a couple of things you can do with the bars to make it more neutral. 

Softening the front ARB will mean the front of the car will roll more during the cornering phase. Because this, therefore, decreases the roll stiffness and the weight transfer from one side to the other is controlled far slower building the load up in the tyres progressively on the front, the car will want to oversteer more. 

Alternatively, stiffening the rear ARB will do the opposite. In this instance, it will mean the rear of the car will roll far less during  the cornering phase. Because this, therefore, increases the roll stiffness and the weight transfer from one side to the other is increased, the car will want to oversteer more. 

Spring Rates 

When it comes to rectifying understeer through spring rate changes, we need to be careful, as spring rates not only affect kerb handling/bumps but also control how a car pitches through acceleration and braking phases. Spring rates are an integral part of your setup, therefore any big changes will have knock-on effects through the whole lap. 

With GT3 cars being so aerodynamically sensitive, your spring rates are directly linked to your ride height. Due to you wanting to run the ride height as low as possible, compromises need to be made with your spring rates to ensure the car doesn’t bottom out under pitching loads and compressions. This generally means you want to run them pretty stiff as a rule of thumb. 

However, how stiff is very car-dependent and so we will keep this broad in spectrum. Softer spring rates will inevitably allow the car to pitch more forwards and backwards under big acceleration and braking phases, this is through the greater weight transfer the car is experiencing and this can lead to instability. 

Front and rear spring rates also play a greater role in differing parts of the corner. If your springs are too soft on the front for example, under big braking loads, the front will pitch too much and therefore the rear will become unloaded very fast. This will promote oversteer on the entry to a corner as grip at the rear has been taken away under the pitching motion. 

So how does this tie into rectifying understeer? 

In this instance, softening the front spring rates will increase that weight transfer to the front of the car and therefore decrease the understeer on the entry/ braking phase as the rear of the car becomes unloaded far quicker. 

When it comes to rear springs, running them too soft can also cause understeer. With a softer spring comes more load transfer and also more absorption of bumps and kerbs. This affects the car in a different part of the corner, most notably mid to exit. If you find the car you are driving in ACC too sluggish at the rear under power or on exit kerbs in higher speed corners, it will generally mean your rear spring rate is too soft. 

With less load transfer to the rear under power, there is less load pushing the rear of the car into the ground meaning a car that is more willing to rotate at speed 

With aero loads reduced in slow corners, mechanical grip plays a much bigger role in regards to car behaviour. This is why front end bite on a stiffer front spring won’t be as strong and that is why you will encounter more issues with stiffer rates as opposed to a softer spring which loads up the tyre more and allows for more weight transfer. 

To summarise

  • Understeer in fast corners: Stiffen rear springs 
  • Understeer in slow corners: Soften front springs 
  • Understeer under braking: Soften front springs/stiffen rear springs

Bump Stops

Bump stops can be used to reduce the compromises that need to be made with the spring rates. Think of the device as an additional cushion or shock absorber on top of the spring. In Assetto Corsa Competizione, not only can we determine how much travel the spring has before it hits the bump stops, we can also determine its stiffness through the rate. So how can we use these to help solve understeer? 

If you do not want to compromise on the spring rates in your setup, bump stops are a great way to control pitch and kerb handling. The way you can promote forward pitch under braking for example, to aid understeer with bump stops is increasing the range of the bump stop before the spring hits it. This is a great way of running stiffer springs to aid stability within the aero platform, yet not running into the bump stops too early so that the car oscillates and bounces back causing understeer which would lead to instability, as the car isn’t allowed to pitch over a certain amount. You are essentially giving the spring more room to breathe. 

This can also be applied to the rear of the car. In some cars like the Porsche, where there is great weight over the rear wheels thanks to the engine, the bump stops can be used to limit oscillations at the rear under hard acceleration as the car pitches rearwards. 

For most cars, you want to run a very high rear bump stop range at the rear, however it can create understeer as the rear spring receives the weight transfer under acceleration and unloads the front tyres too much. By limiting the spring travel on the rear by reducing the bump stop range, it can be a method of reducing exit understeer. 

To summarise

  • Understeer under braking: Increase front bump stop range and stiffen the rate 
  • Understeer in slow corners: Lower rear bump stop range 
  • Understeer in fast corners: Increase front bump stop range and stiffen the rate 

Rear Toe

Rear toe is a fantastic tool to use to fine-tune car behaviour once the rest of your alignment/suspension settings are to your liking. In Assetto Corsa Competizione, all cars run positive rear toe or what’s called toe-in. 

Essentially, rear toe affects how quickly the car rotates during a corner. Greater rear toe will cause the loaded outside tyre entering a corner to start with a slight slip angle, meaning cornering forces are going to build up much faster. The loaded tyre is also going to reach maximum rotation much faster for the drivers given steering input creating a more stable platform during the cornering phase. 

With the above in mind, if you are encountering turn in or mid-corner understeer, decreasing the amount of rear toe-in will help reduce it and take the car closer to an oversteer/neutral bias. In addition, this does come with the upside too of less drag, due to the cars rear tyres being pointed inwards far less than before, which leads to a positive impact in straight-line speed.

On shorter or twisty circuits, the benefit of lower rear toe can be most felt in initial rotation or quick direction changes. However it will have to be managed as you can quickly overheat rear tyres or cause blistering in hotter temperatures due to too much wheelspin. The key here is to reduce the rear toe enough so that you are on the verge of encountering that wheelspin but retain enough rotation to eradicate the understeer. 

To summarise 

  • Understeer > Decrease Rear toe > Increase straight-line speed

Differential

In ACC we have been given a simple differential with only one setting to adjust. Whereas in other sims we have diffs with preload, coast and power settings that can be adjusted, in Assetto Corsa Competizione we do not have to worry about that. 

In that regard, the differential can be explained quite easily. The setting we are adjusting in the setup menu is the preload and indicates how early or late the differential opens up and then locks again. A greater preload figure will force the rear wheels to rotate at the same speed for longer before the diff fully opens up and gives the car more rotation on corner entry. 

A lower figure will do the opposite, with the differential opening up earlier and allowing the rear wheels to rotate at different speeds far earlier into the corner. It will however lock up far later under power once you are at the exit of the corner. This means that traction is affected, as both rear wheels aren’t being forced to rotate at the same time as early as a higher figure selected in the setup menu. 

Now we know the difference between them, we can understand how it will affect understeer in different parts of the corner. In the entry to mid-phase of a corner, a higher diff setting will promote understeer as the diff is trying to unlock far later, whilst the car is also trying to deal with the braking and turn in phases. The knock-on effect is the car will naturally want to pull itself around the corner far less whilst requiring more steering input, creating an unresponsive front end

When it comes to latter parts of the corner, primarily on the exit when you are applying the throttle again, you may encounter understeer due to your differential setting being too low. Whilst with a high diff setting you may encounter slight wheelspin on the exit, it won’t promote understeer due to the wheelspin being caused by the inside rear tyre. 

With a lower differential setting the understeer is caused by both rear wheels not being forced to accelerate together earlier. This means on power, especially on worn tyres, a car with a diff setting too low can become unresponsive on the exit of corners.

To summarise

  • Understeer under braking/mid-corner: Lower differential preload 
  • Understeer under power on exit:  Raise differential preload

Front And Rear Damping

In Assetto Corsa Competizione, we have got 4-way adjustable damping available to us, broken up into rebound and bump in both slow and fast states. What dampers essentially do is help dissipate any vertical movement in the suspension and keep spring travel under control. This has a knock-on effect on helping keep the wheels on all 4 corners of the car in contact with the ground. Bump and rebound control both the compression and extension of the damper, they don’t directly control load levels received, but instead controls the speed of the load changes and ensure the suspension reacts in a stable manner. 

Tracks that have more bumps or quick transitions & changes of direction require more emphasis on dampers. The quicker the suspension has to react and move to the circuit’s demands, the more effect the dampers have. 

Because kerb handling and bump behaviour is very important on a lot of circuits in ACC, it makes sense to tune dampers to allow your car to take them effectively. However, going too far one way or another can lead to an unpredictable car. 

So let’s quickly highlight what can happen if you go too far on both bump and rebound:

  • Too much bump: Initial compression is very harsh > resistance to chassis roll
  • Too little bump: Car will dive a lot>great weight transfer > lots of body roll
  • Too much rebound: Tyres struggle to maintain contact on track surface > Inside wheels pulled away from the track surface
  • Too little rebound: Car will oscillate for a long time after hitting bumps > Car will struggle for traction on the exit of corners

With the above in mind, we can deduct a few things and help solve understeer utilising the dampers in a few scenarios. If you are encountering turn in understeer, it could be that the slow bump setting on the front is far too firm, allowing for not enough pitch/dive and weight transfer, meaning the front tyres want to scrub across the surface of the track. Therefore increasing the softening the front bump setting will help bring the car into a more neutral state. 

If you are struggling for rotation on the exit of a corner, you may need to increase your rear slow bump setting. This is because the initial compression under the longitudinal load of acceleration may be too much, leading to the car pitching rearwards and therefore the front tyres become unloaded far quicker. 

The same can also be applied to rebound. Increasing the rear rebound will allow the rear wheels to settle more slowly on the track and hold the rear tyres in the arches for a little  longer under heavy braking phases. This will eradicate understeer and bring the car closer to an oversteer bias. Front rebound can also help corner exit understeer. By lowering the speed at which the front tyre settles on the track after the load has transferred rearwards, will allow the front of the car to bite less, meaning the overall grip percentage front to rear increases.  

To summarise

  • Decreasing front bump > More oversteer 
  • Increasing rear bump > More oversteer
  • Decreasing front rebound > More oversteer
  • Increasing rear rebound > More oversteer

Summary 

Depending on how understeer biased the setup is will determine how many solutions you adopt to get the car into a window that is comfortable for you. 

Certain changes like spring rates and rear ride height will have a greater effect than tweaking tools like rear toe and the differential. That is true for cars too as some may require more changes and others less so. Use this article as a rough guide to understand the basics of rectifying understeer and use the tips to make a safe and predictable car a thing of the past for you in ACC.

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