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Suspension Tuning & Optimization: Suspension Joint Types

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Suspension Joint Types

06.15

00:00 - In this module, we're going to discuss the different types of pivoting connections used throughout a suspension system.
00:06 These are joints where we need to locate a certain component in a given area but also allow it to rotate.
00:13 This is what you'd find at the connections between the suspension arms and the chassis, uprights and steering rack.
00:19 One of the biggest challenges for designers in the OEM world is minimising noise, vibration and harshness.
00:25 Generally referred to as NVH.
00:28 Reducing NVH means the joints that locate elements like our dampers, springs and suspension arms, often include some form of rubber base or polyurethane bushing.
00:38 We'll refer to these as polymer type bushings going forward.
00:42 Using a polymer in places like this, reduces the transmission of vibrations and the harshness of shock loadings through the suspension.
00:49 Thereby partially isolating the chassis.
00:52 This is a great thing for road cars but in motorsport, the downside of a polymer bushing is that it usually allows a high degree of compliance.
01:00 This means the pivot point will deflect significantly when loads are applied.
01:05 This effect is magnified in competition where the loads are generally much higher than what they were originally designed for in road use.
01:13 The downside of this deflection is that our suspension geometry isn't as constant as we would like and internal forces are not reacted as directly as we'd want.
01:21 Compliance in the bushings that mount components like control arms, steering racks and dampers have negative effects on the suspension geometry and responsiveness of the chassis to driver inputs.
01:32 The exact amount of compliance will depend on the stiffness of the bushing.
01:36 This is often an area that high performance road cars have a lot of effort put into.
01:41 The bushing stiffness is chosen and tuned very precisely to balance comfort and performance.
01:46 A common upgrade for modifying road cars is to replace the factory bushings with aftermarket polyurethane items to reduce the compliance.
01:55 While in some cases, this can lead to a performance benefit, there's still a significant compromise here.
02:01 This is a solution that's most suitable for cars that are used both for daily driving and occasional competition use where some increase in the NVH isn't a problem.
02:09 One practical thing to note with many styles of polymer bushings is the position of the bushing when the mountings are tightened.
02:16 If there's a through bolt style of attachment used, this will generally mean the centre of the bushing will be rigidly attached to the component it's mounted to when the bolt is done up.
02:25 In this case, the component that's mounted by the bushing rotates in use and stress will build up inside the bushing.
02:32 This makes the position of the suspension when this style of bushing is tightened important.
02:37 Let's take the case where the suspension is in full droop.
02:40 If we tighten the bushing in this position, when we place the car back on its wheels, the bushing will already be under stress.
02:47 Over time, this stress can lead to premature failure of the bushing.
02:51 It's for this reason we should generally tighten this style of bushing when the car is sitting at its static ride height position to minimise stress to the bushings.
03:00 It's also worth noting that this style of bushing can contribute a noticeable increase in the effective suspension stiffness.
03:06 This is because the bushing will resist being twisted.
03:09 Another good reason to ensure they are tightened in their neutral position.
03:12 The most common solution used to upgrade road car style suspensions to make them more suitable for competition use is to upgrade the joints to use a spherical bearing.
03:22 This can be in the form of either a threaded rod end or a bearing that's integrated directly into a component.
03:29 This style of joint is far more rigid than any type of polymer bushing, it'll offer reduced compliance as well as crisper response when used in components like suspension arms and damper mounts.
03:40 The downsides include a lot more NVH as well as poor mileage before needing to be replaced.
03:46 This can be a poor choice for a car that does a lot of road driving as they'll require more regular maintenance and replacement when compared to a polymer bushing.
03:54 One thing to think about when upgrading your suspension joints to a rod end style spherical bearing is to avoid placing these in bending.
04:01 Unfortunately, this is something you'll see over and over again in the aftermarket world and in many situations this is purely down to laziness and cost savings.
04:11 So what exactly do we mean when we say bending? This is introduced into a component when the forces are not directly aligned with the part.
04:19 Take the case of a simple straight link.
04:22 Using threaded rod ends in a situation like this, is suitable as the spherical bearings can't support any torque.
04:29 That is to say, any torque applied to the spherical bearing simply causes the bearing to rotate rather than apply a bending force to the arm.
04:38 All the applied forces are aligned with the arm itself.
04:42 This is what's often referred to as a 2 force member.
04:45 In this example of a lower control arm that has a damper and anti roll bar connection mounted part way along it, this arm is subject to bending forces.
04:54 This comes from the applied forces from the components mounted part way along the arm that aren't aligned with the direction of force from the bearings.
05:02 This results in bending forces in this lower control arm.
05:05 The reason this is bad is the threaded sections of the road act as stress concentrations, making these parts of the arm susceptible to failure.
05:13 Using threaded rod ends where any bending is present should be avoided wherever possible.
05:18 In some situations, it's not always practical to use anything but a threaded road end in a suspension component that has some bending forces present.
05:26 In this case, the rod end will need to be upgraded in either size and/or material spec in order to perform reliably once the bending forces are considered.
05:35 Again, this situation should always be avoided where possible.
05:39 In summary, OEMs tend to use polymer bushings in their suspension pivots to reduce noise, vibration and harshness which keeps the ride more comfortable for its occupants.
05:50 A big downside of this style of joint is it allows significant compliance in operation.
05:55 Spherical bearings are often most appropriate for motorsport use.
05:58 Giving much higher compliance and more direct suspension response at the expense of poor NVH and expected life.