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Suspension Tuning & Optimization: Anti-Roll Bars

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Anti-Roll Bars

08.12

00:00 - Antiroll bars, also known as sway bars are used to reduce the relative movement of the suspension in roll.
00:06 They only provide a resistive force when there is a difference in the suspension position from one side of the car to the other.
00:13 And that's when the car is rolling.
00:16 This is an important distinction to understand, a main suspension spring resists one corner of the car from compressing.
00:23 This compression can occur from heave, roll or pitch but an anti roll bar only resists movement in roll.
00:31 When both wheels on one axle move by the same amount, the anti roll bar provides no resistive force.
00:38 Meaning that in heave and pitch, the antiroll bar remains unloaded.
00:43 An important and useful side effect of anti roll bars is that while they do reduce the amount of body roll, they're also increasing the amount of lateral load transfer from the inside to the outside wheel of each axle at the same time.
00:56 This is a very important characteristic to keep in mind and we'll go into how we make use of it to tune the vehicle for balance later in the course.
01:04 A downside of using anti roll bars is that they make an independent suspension behave more like a non independent suspension.
01:11 This is because we're making a physical connection from one side of the suspension to the other and if we didn't run an anti roll bar at all, each side of the suspension would be largely free to follow the road surface independently.
01:23 It's not a black and white situation because the stiffness of an anti roll bar determines the level of suspension independence.
01:30 A very soft antiroll bar will allow the suspension to behave very close to a true fully independent suspension while a very stiff anti roll bar will cause a more non independent suspension behaviour.
01:43 This means disturbances on one side of the car will affect the other more.
01:47 Thereby losing some of the advantages of having an independent suspension.
01:51 Another important effect an antiroll bar has on the suspension is that it tends to lift the inside wheel off the ground.
01:58 When the car rolls, the outside wheel will compress and the inside will extend.
02:03 As the antiroll bar connects both sides, the outer wheel compressing tends to take the inside wheel with it.
02:10 The relative stiffness of both the antiroll bar and the springs will affect how much the inside wheel will lift.
02:16 There are a number of different types of antiroll bar styles but in road cars by far the most common is the U bar type.
02:23 It gets its name from forming a U shape when looked at from above and in its simplest form, each end of the suspension is attached to one end of the bar and it pivots around two points that are attached to the chassis.
02:35 When one side of the suspension tries to move relative to the other, this has the effect of twisting the anti roll bar within its centre section.
02:43 Of course the ends will bend to some extent but the majority of the deflection is intended to take place in the central part of the bar in torsion.
02:50 With the U bar type anti roll bar, there are two ways we normally effect the amount of anti roll stiffness the bar provides.
02:57 Either we change the diameter of the bar or the lever arm length of the ends.
03:02 So how exactly does this work? For a round section in torsion, the stiffness is proportional to the diameter raised to the fourth power.
03:10 This means that a small change in the diameter of the antiroll bar will have a large effect on its torsional stiffness.
03:17 Let's take the example of a simple U shaped anti roll bar.
03:21 All other things being equal, if we have a solid anti roll bar of 20 mm diameter, then swap it out for an upgraded bar of 22 mm diameter, the new bar will be approximately 1.5 times the stiffness of the original bar for just 2 mm more diameter.
03:37 Take a look at the course resources section for our useful equations cheat sheet, which will help you work out everything you need to know about changing the antiroll bar in your car.
03:47 When it comes to motorsport specific equipment, while most stock antiroll bars are solid, in order to reduce manufacturing costs, motorsport specific bars are usually hollow.
03:57 This is simply to save weight because the material in the middle of the bar provides so little in terms of stiffness and torsion.
04:04 It can be removed with little effect.
04:06 Only a small increase in diameter is required to reclaim the stiffness lost by using a hollow bar.
04:13 As mentioned earlier, we can also increase the effective stiffness of an antiroll bar by changing the lever arm distance.
04:19 This is simply changing the mechanical advantage, AKA, the leverage that the suspension has over the antiroll bar.
04:26 When we attach one side of the suspension closer to the pivot point of the antiroll bar.
04:31 we're reducing the mechanical advantage the suspension has over it.
04:34 This has the effect of what would usually be called stiffening the antiroll bar which results in less roll of the chassis and more lateral load transfer at that end of the car.
04:45 An important distinction here is that we're not actually changing the stiffness of the antiroll bar at all.
04:51 We're simply changing the leverage the suspension has over it.
04:54 Even though this is technically incorrect, we'll be using this term throughout the course as that's how it's commonly used.
05:01 If we instead move the connection further away from the pivot point, the mechanical advantage increases.
05:07 This has the effect of softening the antiroll bar.
05:09 Which results in more roll of the chassis and less lateral load transfer.
05:14 In an adjustable antiroll bar you'll generally find multiple holes in the bar ends that allow you to change its effective stiffness by attaching the suspension at different points along the ends, increasing or decreasing the leverage.
05:28 WIth road car style antiroll bars, it's worth keeping in mind that they're almost never a pure U shape.
05:34 They're usually full of kinks and bends in order to avoid other parts of the chassis or suspension.
05:40 These do have the effect of changing the torsional stiffness and prevent the bar from acting in pure torsion.
05:46 However as long as the manufacture as the manufacturer has accounted for this, it's not a huge problem.
05:50 A common variation on the U bar style of an antiroll bar is what's usually referred to as a blade antiroll bar.
05:56 In this case, the ends of the U are replaced with a blade that can be rotated to change the stiffness of the antiroll bar.
06:03 The blade is machined in a way that gives different stiffness depending on how the blade is orientated.
06:09 When the blade is sitting up on its edge, this is the stiffest possible position.
06:13 By rotating the blade by 90°, this is the softest possible position.
06:18 By changing the angle the blade sits between these two extremes, we can make quick changes to the antiroll bar stiffness.
06:25 It's common to have a blade mounted at either one or both ends of the antiroll bar.
06:30 In the case of a blade antiroll bar, the blades themselves become an active part of the total stiffness.
06:36 When they are adjusted to the stiffest position, almost all of the deflection in the bar will occur in torsion within the body of the antiroll bar.
06:44 When they are in the softest position, the flex in the blade is significant enough to reduce the overall stiffness of the antiroll bar.
06:52 It's common for these types of antiroll bars to be connected to an adjustment mechanism that'll allow the driver to make adjustments to the antiroll bar stiffness while the car is on track.
07:02 This can be a significant advantage by allowing the balance to be changed without the car having to come into the pits for an adjustment.
07:08 This is particularly true during a race as the tyres wear and the fuel burns off.
07:13 There are many other types of antiroll bar out there, particularly in dedicated racecars.
07:18 These include T bars and shuttles.
07:20 Although the layout of these can be quite different from a U bar, the end result is more or less the same.
07:27 However, as U bars are by far the most common type of antiroll bar you're likely to come across in a road car style suspension, we won't be discussing these other styles.
07:36 Summarising this module, antiroll bars provide a resistive force between both sides of the suspension in roll only.
07:43 Not in pure heave or pitch.
07:46 Antiroll bars have the effect of reducing body roll and increasing lateral load transfer.
07:51 The more resistance an antiroll bar provides, the closer an independent suspension becomes to behaving like a non independent suspension.
08:00 U bars are by far the most common type of antiroll bar you're likely to come across in road cars and are normally tuned by changing the diameter of the bar or a length of the lever arm.