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Suspension Tuning & Optimization: Bump Stops

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Bump Stops

08.04

00:00 - In road cars, the primary purpose of a bump stop is to prevent damage to components when the suspension is fully compressed.
00:07 Bump stops can be arranged in a few different ways but the most common is when they're mounted concentrically with the damper as a part of a coilover unit.
00:16 Without bump stops, the suspension travel will be limited by the mechanical travel of the suspension which would usually be metal to metal contact.
00:24 The problem with this is that it puts a large shock loading through the suspension and is likely to damage part of the suspension or even the chassis itself.
00:32 If full suspension compression is reached with metal to metal contact, not only are we likely to cause damage, but because the effective spring rate of that corner of the car becomes almost infinite, we're also likely to lose grip on that corner of the car which can cause some nasty handling behaviours and even a complete loss of control.
00:49 By using bump stops, we can lessen the shock loading at full compression by providing a progressive force as the suspension is fully compressed.
00:57 In motorsport, we use bump stops for the same reasons as in road cars but we can also make use of them as a tuning tool in their own right.
01:05 In a similar way to fitting different springs or antiroll bars.
01:09 When the suspension is operating and the bump stops aren't engaged, the loads on each corner of the car are reacted through the springs dampers and antiroll bars as the car negotiates the course.
01:20 If the suspension compresses enough that the bump stops begin compressing, then they act as another spring in parallel, adding to the total effective spring rate at that corner of the car.
01:30 Bump stops are commonly made from a polymer based material and are available in all sorts of different materials, lengths and geometries.
01:38 All of these properties can be modified in order to achieve a different stiffness of a bump stop.
01:44 Here we see some typical examples of bump stops and the kind of force response we might expect to see as they're compressed.
01:50 In particular, the external shape can have a big influence on the rate of force buildup.
01:55 It's not uncommon for people to modify these by changing the heights, cutting out sections and stacking them in different combinations to get the stiffness response we're looking for.
02:05 The different ways in which we build up bump stops is about tuning how progressive the rate increase is.
02:11 In many cases we don't want the initial engagement to add a large step in stiffness.
02:16 So the initial rate is low.
02:18 Then as it becomes more compressed the stiffness can progressively ramp up.
02:22 One of the interesting properties of a polymer based bump stop is that they are visco elastic.
02:27 This simply means that some energy is lost as heat as they're cycled.
02:32 If we look at the force response curve as we cycle a polymer bump stop, we can see that the force changes depending on wheter we're compressing versus extending the bump stop.
02:42 The area between these curves represents the amount of energy lost to heat.
02:46 Another term for this is hysteresis.
02:48 This is essentially a similar effect to what's happening inside a damper during operation which we'll discuss in the next section of the course.
02:56 But for now, just understand that we're adding some damping to the coilover when we engage a polymer bump stop.
03:02 A downside of using a polymer is that over time, their behaviour changes as they break down and deteriorate.
03:09 This causes changes both in terms of their compression stiffness and their damping behaviour.
03:15 Which means that polymer bump stops are considered a consumable item.
03:17 Temperature also affects the way polymer responds which is something that needs to be considered when we're tuning them precisely.
03:24 Another type of bump stop is actually more correctly termed a bump spring.
03:28 These aren't as common but act in a similar way to bump stops.
03:32 They're essentially a small coil spring that replaces the bump stop.
03:35 This allows them to be precisely tuned because being made from steel, they can be manufactured to closer tolerances and their response can be more consistent, particularly in relation to temperature.
03:46 Steel also has very low damping properties, meaning that the energy required to compress a spring, is largely recovered as it extends.
03:53 Little energy is lost as heat.
03:56 We can see this by comparing the force response of a polymer bump stop versus a bump spring in a plot.
04:02 While some energy is lost in hysteresis of the polymer bump stop, the force of the bump spring is the same on compression versus rebound.
04:10 The amount of energy lost to heat is represented by the area within the compression and rebound curves of the polymer bump stop.
04:17 The larger this area, the more energy is converted to heat, meaning the larger the hysteresis.
04:23 As the bump spring has essentially zero area between the compression and rebound, this indicates almost no energy loss.
04:31 In some cases, metal matrix type bump stops are used.
04:34 Here, the bump stop is made of a matrix of very small diameter wire.
04:39 One of the main properties of this style of bump stop is that their race can increase very aggressively compared to polymer which is helpful when we're looking to limit travel very precisely.
04:50 Something that's very useful for aerodynamic reasons.
04:53 Another big advantage of this style of bump stop is that they tend not to break down like a polymer does, giving more consistent performance over time.
05:00 When we talk about using bump stops to tune the mechanical behaviour of the car, this is usually to reduce the amount of suspension travel under a certain condition.
05:09 For example we might want to limit rear suspension travel under acceleration of a rear wheel drive car in order to prevent the rear wheels gaining too much rear camber.
05:18 Which would otherwise reduce our available drive force from the tyres.
05:22 By using a bump stop that engages at a precise point, we can also progressively increase the spring rate of the rear suspension as it compresses and that allows us to run a softer main spring than we would otherwise be able to.
05:35 The same would be true of limiting the amount the front drops during braking.
05:39 For example, there may be a minimum ride height that the front splitter needs to stay above the ground during braking in order to keep it working effectively.
05:46 We could use bump stops in this situation to control the minimum front ride height without having to stiffen up the front springs.
05:53 Probably the most common reason for tuning bump stops is to control the ride height of a car that has a lot of downforce.
06:00 Because the aerodynamic forces acting on the car increase with the square of road speed, at high speeds the right height will quickly reduce.
06:09 For cars with powerful underfloors, the proximity and angle of the floor with respect to the road surface has extreme effects on both the amount of downforce and how it's distributed.
06:20 Again, we can use bump stops to tune the height and angle of the chassis at high speeds, making sure we keep it within bounds that we require.
06:28 Using bump stops in this way, gives us an opportunity to run a setup more conducive to good mechanical grip while also controlling our aerodynamic platform.
06:37 As part of tuning our bump stops we can also vary the gap between them and the damper body which is generally referred to as tuning the packer gaps.
06:45 Once we're happy with the stiffness of the bump stop, this allows us to fine tune when it engages by adding and removing solid packers.
06:53 These packers can be easily slid in and clicked over the damper shaft which means we don't have to disassemble the entire damper unit in the pits every time we want to make an adjustment.
07:03 One thing we do need to be careful of when tuning the gaps is engaging the bump stops mid corner.
07:08 If this happens too aggressively, it can upset the car by making the effective spring rate increase quickly.
07:14 Making use of data acquisition by using damper pots is the best way to monitor this.
07:20 As a side note, although data analysis is outside the scope of this suspension course, we do offer dedicated motorsport data courses at both the fundamental and advanced levels.
07:29 I'd highly recommend checking them out if you want to get the most out of your racecar and your time behind the wheel.
07:35 In summary, bump stops primarily exist to prevent damage to the suspension when it becomes fully compressed.
07:41 But they also offer tuning opportunities through different materials and designs that can provide extremely different behaviours that are going to affect how the car behaves under heavy suspension compression.