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Suspension Tuning & Optimization: Suspension Travel & Motion Ratio

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Suspension Travel & Motion Ratio

08.01

00:00 - Travel is a term that you'll often hear in motorsport and refers to how far a component of the suspension can move over the full extent of its movement range.
00:08 This travel can actually refer to any part of the suspension but one of the most important is the vertical wheel travel.
00:15 This simply refers to the vertical distance that the wheel can travel from full droop to full compression.
00:21 We're also often interested in the damper travel which is the amount of travel available in the axis of the damper shaft.
00:28 We can equally apply the term suspension travel to any other part of the suspension we might be interested in.
00:35 There are many factors that go into deciding how much suspension travel we need.
00:39 But two of the most important and interrelated ones are the required stiffness of the suspension we're working with and the roughness of the surface that the vehicle is running on.
00:49 Which are both topics we'll get to later in the course.
00:52 At one end of the spectrum, it's obvious that something like a stadium truck running on rough terrain that has to be able to take large jumps and land with sufficient compliance to not damage the vehicle or its occupants must have a large amount of available vertical wheel travel.
01:07 By allowing the suspension to absorb the energy of a landing over a large suspension travel, the accelerations involved and therefore the forces can be reduced.
01:16 At the other extreme, is a high downforce, single seater running on a smooth surface.
01:21 In this case, the smooth surface means that the suspension won't require much travel to follow the road profile and the energy input from the road surface will be relatively low.
01:32 For this case, the suspension geometry which is something we'll be looking at in detail later in the course, will also require the travel to be kept to a minimum to keep things like camber, caster and toe within suitable limits as the car negotiates the circuit.
01:46 This is because how these parameters change as the wheel moves through its travel is optimised for a suspension that's intended to have a relatively small amount of travel in the case of a car like this.
01:58 In the case of a car that's generating a lot of its downforce from its underfloor, a good proportion of its total performance will be related to how well we can keep the sprung mass within a certain window of pitch, roll and heave values.
02:11 This means that the movement of the sprung mass should be minimised which will necessarily result in less wheel travel.
02:18 At the other end of the spectrum, if we're running a car with less aero and we're looking for more mechanical grip, softer, more compliant suspension which also implies more vertical wheel travel is preferred.
02:30 How we bias the setup philosophy between these two extremes will depend on the configuration of our car, the type of racing we're participating in and the specific location we're running at.
02:40 Another important aspect of suspension travel is how much of the travel is available in compression versus rebound.
02:47 Which are terms we introduced in the previous module.
02:49 The reference or starting position is our static ride height.
02:53 The height of the car as it sits stationary on a level surface.
02:56 From this position, the available compression travel is the amount of vertical wheel travel from the static ride height to when the suspension will compress no more.
03:06 As you can probably guess, rebound is the opposite.
03:09 Where we take the load off the suspension and let is fully extend.
03:14 The ratio between these two parameters is important to understand.
03:17 In general, you want more suspension travel available on compression than you do in rebound.
03:23 A sensible starting point is to allow for approximately 2/3 of the available travel in compression and 1/3 in rebound.
03:30 This isn't a hard and fast rule and there are many times when a suitable suspension setup will be outside of these bounds.
03:37 But it's a great starting point.
03:39 We'll go through the practical application of this later in the course with a dedicated module in our practical skills section.
03:45 Having the majority of our suspension travel available on compression allows the suspension to absorb undulations in the road surface.
03:52 While the rebound travel is predominantly there to allow the tyres to remain in contact with the road as the chassis heaves, rolls and pitches without being unnecessarily lifted off the ground.
04:03 You'll no doubt hear the term motion ratio at some point when discussing suspension travel.
04:08 And that's because they're closely related.
04:10 Motion ratio describes how much one component moves relative to the other when they're linked together.
04:16 We're interested in the motion ratio between a number of different elements but the most common is between the wheel and spring and damper assembly.
04:24 That is, the vertical movement of the wheel relative to the change in damper length.
04:29 The most common definition of damper motion ratio is the amount of wheel movement divided by spring movement.
04:36 In the case of a coilover type setup, the same motion ratio applies to both the spring and the damper.
04:43 The kinematics of the suspension is going to determine the motion ratio.
04:46 And in almost all cases, this ratio will not be 1:1.
04:51 The value of the motion ratio will generally also vary as the suspension moves through its travel.
04:57 For many situations, it's sufficiently accurate to assume the motion ratio is constant.
05:02 However in cases where the motion ratio is changing a lot over the suspension travel, more precise measurements must be taken in order to allow a variable motion ratio to be calculated.
05:13 I know this sounds quite complicated but don't worry, we'll be covering the measurement of wheel to damper motion ratio in more detail in the practical skills section of this course.
05:22 On a related note, many of you will be familiar with the term rising rate suspension.
05:27 This is where we have the motion ratio changing as the wheel position changes.
05:31 In particular, it means that the effective spring rate at the wheel, is increasing as the result of the change in motion ratio between the wheel and the spring.
05:40 A rising rate suspension is preferred in race car applications because it allows the suspension to remain softer to absorb undulations in the road surface and then effectively stiffen as significant loads are applied to it.
05:52 When the car is braking, cornering and accelerating.
05:55 In case you were wondering, a falling rate design is also possible but this should be avoided.
06:01 The last thing I want to mention here is more of a word of caution.
06:04 When discussing something like spring rates with another driver, it's important to understand that if you're comparing non like cars, where the motion ratio between the wheel and the spring isn't identical, you're not making a direct comparison in spring rate at the wheels.
06:18 We care about the effective spring rate at the wheel, which is generally referred to as the wheel rate rather than the actual physical spring fitted to each car.
06:27 By using the motion ratio, we can calculate the wheel rate so we can make more relevant comparisons between cars with different suspension systems.
06:37 This concept applies equally to any other component with a motion ratio not equal to 1.
06:43 Including components like anti roll bars.
06:45 Let's take the case of a front axle of the car that has an average motion ratio between the wheel and spring of 1.25.
06:52 Meaning that if the wheel moves into compression by 1 inch, the spring will compress by 0.8 inches.
06:58 Dividing 1 by 0.8 gives us our motion ratio of 1.25.
07:03 Let's say that we have a spring fitted that has a rate of 500 pounds per inch.
07:08 To calculate the wheel rate from this, we divide the spring rate by the motion ratio squared.
07:14 This would give us an effective wheel rate of 320 pounds per inch.
07:19 Which is only around 65% of the original 500 pounds per inch spring rate.
07:24 In this module, we've discussed that the amount of suspension travel required will depend on the application and our travel will also be influenced by how much of the performance of the car comes from aerodynamics.
07:36 Full travel consists of both compression and rebound and we should be looking for a ratio of approximately 2:1 respectively.
07:44 Lastly, the motion ratio describes a relationship between vertical wheel travel, and the travel of other relevant suspension components.