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Suspension Tuning & Optimization: Choosing and Tuning Anti-Roll Bars

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Choosing and Tuning Anti-Roll Bars

09.09

00:00 - As we learned in previous modules, the anti roll bar is one of the main tools we have at our disposal for adjusting the amount of body roll and the lateral load transfer distribution.
00:10 In this module, we'll discuss some of the practicalities of selecting and tuning our anti roll bars.
00:16 As we've already discussed, while the way the anti roll bar works to increase our roll stiffness is distinctly different to how the main springs work.
00:25 They can somewhat used interchangeably to tune the lateral load transfer distrbution and therefore the balance.
00:32 The main reason we often end up tuning balance with anti roll bars is that they're usually simply a lot quicker to make changes with compared to springs.
00:40 One of the downsides of increasing the roll stiffness with an anti roll bar is that we reduce the amount of suspension independence.
00:47 This means that on rougher tracks, we'd be less inclined to use anti roll bars for stiffness increases.
00:53 When it comes to selecting anti roll bars for a car, there are a few different approaces possible, depending on our situation.
00:59 The first is to calculate the required stiffness we need which can be done using some similar methods to what we went through in the previous spring rates module.
01:07 The main barrier to this for most people, apart from the extra effort involved in going through the calculations is that almost no aftermarket anti roll bar manufacturers specify the stiffness values of their products in the same way that they're given for springs.
01:22 This means if you do want to make those calculations, unless you can convince the manufacturer to give you the stiffness information, then your only options are to test for stiffness for yourself or to make some stiffness calculations based on some simplifying assumptions.
01:38 You do have the option of at least testing your own anti roll bars using the method we went through in a previous module.
01:44 Then the manufacturer can give you a percentage difference in stiffness relative to the OE bars you can apply this to your measurements and get yourself in the ballpark of the stiffness number.
01:54 If you don't have the ability to test your anti roll bars yourself, the manufacturer can't give you stiffness values but you still want to make lateral load transfer calculations then your only option is to make some stiffness calculations of your own.
02:06 We've included the equations you'll need to estimate the stiffness of a U bar type anti roll bar in the calculation sheet you can download from below this video.
02:15 In many cases, these are an estimate at best as few factory anti roll bars form a proper U shape, having extra bends which will affect the actual torsional stiffness.
02:25 At this point, we'll have the information we need to start calculating our lateral load transfer distribution, something we'll go through in more detail in the final module of this section of the course.
02:36 If you won't be going through the process of carrying out calculations and you're looking to upgrade your anti roll bars, then your next best option is to go with the recommendations of the bar manufacturer.
02:46 They'll generally sell them in matched pairs of upgraded anti roll bars that are intended to work together to keep you in the balance window for your chassis.
02:54 For something like a circuit racing application, we generally always benefit from an increase in roll stiffness, contributed by the anti roll bars because the OE stiffness will be too light for circuit racing.
03:05 As you start using better quality motorsport tyres, this will become more and more necessary as the cornering forces increase.
03:12 OE bars also don't give us any way to adjust our anti roll stiffness distribution as they'll generally only have one link attachment point at each end of the bar..
03:22 Aftermarket anti roll bars on the other hand will generally have multiple holes at each end of the bar to attach the links to, allowing us to modify the effective stiffness.
03:30 With the OE bars, the only tuning option we have is to disconnect one or both of them.
03:36 Which is generally too extreme of an adjustment to be much use as it'll tend to skew our anti roll stiffness balance too much.
03:43 Assuming the car was somewhat close to the setup window in the first place.
03:47 When it comes to tuning anti roll bar stiffness on track, we want to start conservative from a balance perspective.
03:53 This is especially true if we're running on brand new anti roll bars for the first time.
03:57 To begin with, we want to shift the balance towards understeer to keep the car safe and stable, gradually tuning in the other direction which is where we search for grip and balance.
04:07 If we want to begin in the understeer direction, we know that we want to shift our lateral load transfer distribution towards the front axle.
04:14 This means that with the front anti roll bar, we would have on its highest stiffness setting and the rear on its lowest stiffness setting.
04:21 For the test runs, we want to keep them as structured and repeatable as possible to make each test fair.
04:27 It also helps to limit our driving to around 90% pace in most cases as it helps make the driving more consistent.
04:34 I suggest making up each test run of an out lap, two flying laps and a slow in lap, that's 4 laps total.
04:41 Before starting the test, carry out a warm up run of the same format, come in and then bleed your tyres to target.
04:48 Do your best to keep the time period between each run the same as well.
04:51 Depending on the amount of fuel your car consumes and the lap length, you may also want to consider topping up your fuel between each run or every few runs to increase the repeatability further.
05:02 On each run, we need to pay close attention to some particular behaviours.
05:06 Braking stability, mid corner balance, the amount of body roll you sense and traction on exit.
05:14 Purely from a balance perspective, the anti roll bar will be most effective in the mid corner.
05:19 So concentrate on this.
05:21 Stop after each run, take notes on these behaviours, make changes for the next run and continue.
05:28 Assuming both bars have 3 adjustment positions each, I suggest starting the first run with the front bar full stiff and the rear bar full soft.
05:36 The following run would see the rear bar increase in stiffness by 1 step to medium.
05:41 The one after that being full hard on the rear.
05:44 After that, we'll have an expectation and a feel for what the car is likely to feel like as we continue to make changes.
05:51 Start the next set of runs with the front and rear bars set to full soft, then sweep through the different adjustments on the rear bar in subsequent runs, leaving the front bar full soft.
06:02 Now that the rear bar has been swept across all its settings on both extremes of the front anti roll bar settings, we should now have a good understanding on the directions and combination of bar settings we want to use as our baseline.
06:13 This may or may not include testing the same rear bar sweep with the front anti roll bar in its medium setting.
06:20 Aside from changes in braking stability, cornering balance, body roll and traction, stiffer bar settings will generally result in a more direct connected feeling for the driver, giving the feeling of the car being a little crisper.
06:33 A downside of running with more roll stiffness with anti roll bars is that it can lead to higher tyre degradation.
06:39 This is the main reason why many teams run more anti roll stiffness in qualifying runs and then reduce it for the races.
06:46 Simply going through the process of sweeping the bar settings, thinking about the different behaviours of the car and noting this down, will teach you a lot about what to expect from bar changes and how you might use them to tune for different handling traits.
06:59 When it comes to tuning mid corner balance, assuming you have sufficient adjustability remaining, you can make use of adjustment at either end of the car.
07:07 For example, if you're suffering from mid corner understeer, you could either soften the front bar or stiffen the rear.
07:14 Alternatively you could have mid corner oversteer and you could stiffen the front bar or soften the rear.
07:19 You can use the amount of body roll to guide you on the decision of which of these routes you can take.
07:25 If you have mid corner understeer and the car also feels like it's rolling excessively then you'd tend to increase the rear anti roll stiffness rather than decrease the front.
07:34 This will simultaneously reduce the understeer and the amount of body roll.
07:39 If instead you were suffering from mid corner oversteer that the car felt like it wasn't rolling as much then you could look at softening the rear bar.
07:46 This should help reduce the oversteer while not leading to excessive body roll.
07:50 While tuning for balance, you also need to keep track of braking stability and traction.
07:55 While there are plenty of other aspects that affect both of these traits, it's worth paying attention to them.
08:01 Sure, you might make gains in balance, but if you hurt these areas performance, you might be worse off overall.
08:08 It's always a compromise.
08:09 In general, increasing the front or reducing the rear anti roll stiffness can help increase corner entry stability.
08:17 Similar traits apply for corner exit traction where increasing the front or reducing the rear anti roll bar stiffness can also help.
08:26 You can use these effects as part of your decisions when tuning for balance if you're feeling limited by either of these corner entry or exit behaviours.
08:34 If you're making use of lateral load transfer calculations and once you've done some on track testing, you can also start to build a picture of the lateral load transfer distribution window you and your car like to work in.
08:46 This is extremely powerful because now you have an objecting reference for tuning different aspects of the car while being able to control, understand and manipulate the lateral load transfer distribution.