Suspension Tuning & Optimization: Springs
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Springs
08.06
| 00:00 | - In this section of the course, we're going to take a close look at some key components of a vehicle's suspension to make sure that we've got a really clear idea of what they are, how they work and how they can be used to tune the handling of a racecar. |
| 00:13 | Let's start with the obvious one first, springs which are a central part of most suspension systems we'll come into contact with. |
| 00:21 | The main purpose of a spring is to support the weight of the vehicle and control how far the suspension deflects in heave, roll and pitch as we complete a lap or set a time. |
| 00:32 | In a simple suspension system, each corner of the car is supported by a single spring and that means that each is only responsible for providing a resistive force to deflection to its own corner of the car. |
| 00:45 | By far the most common type found in a vehicle is the coil spring, mostly used as part of a coilover setup. |
| 00:51 | This is when the spring is mounted concentrically with a damper which has the advantage of taking up less total space and means less mounting hard points are required on the chassis. |
| 01:01 | Generally, the spring will be mounted on an adjustable height collar which can be used to adjust the ride height, corner weight and accommodate different lengths of springs. |
| 01:10 | Coil springs can also be mounted separately from the damper though this is much less common, especially in sports focused vehicles. |
| 01:18 | Less common again is the torsion bar which is just a different type of spring. |
| 01:22 | This is where a bar is fixed to the chassis at one end while the other is rotated by the suspension. |
| 01:29 | The end result is the same as using a coil spring but it can allow for more flexibility in packaging. |
| 01:35 | These are usually found in single seaters and prototypes, particularly in the front where space is always at a premium. |
| 01:42 | We should also mention the humble leaf spring. |
| 01:45 | These are generally only found in classis cars and are usually used with a live rear axle. |
| 01:50 | While they do have the advantage of forming one of the locating links of the axle, these simple springs are quite antiquated and have plenty of drawbacks so you're unlikely to come into contact with them unless you're involved with a classic car. |
| 02:03 | The last type of spring I want to talk about is the helper spring. |
| 02:06 | These are very lightweight coils with the singular purpose of keeping the main spring securely located in the event that its free length is less than the gap between the spring platforms with the suspension at full droop. |
| 02:19 | This prevents the spring from damaging the delicate thread adjusters and also the possibility fo the spring not seating properly on the perch if it was to come off the seat at an angle. |
| 02:29 | Helper springs don't affect the overall rate of the suspension as they are generally so soft that as soon as the vehicle's weight is on them they compress completely and become solid. |
| 02:40 | So with the different types of springs covered, let's take a look at how they work and their role within the suspension system. |
| 02:47 | Springs are elastic elements by definition, meaning that during normal use when they're either compressed or extended they return to their original position once the force is removed. |
| 02:58 | The key thing to understand here is that springs produce a force in response to being displaced from their original position. |
| 03:06 | The vast majority of springs we tend to come into contact with in motorsport are of the coil variety and linear. |
| 03:12 | These produce a force proportional to their displacement, meaning if we apply a force of X, it gives us a displacement of Y. |
| 03:21 | And if we apply a force of 2x, we get a total displacement of 2y. |
| 03:27 | Which is what we see here in this example. |
| 03:29 | The stiffness of the spring determines the slope of this line. |
| 03:33 | The stiffer the spring the more force is required to compress it. |
| 03:37 | This is determined by the material stiffness, the pitch and diameter the spring is run with and the diameter of the spring wire. |
| 03:44 | Other parameters we're usually also interested are the inside diameter of the spring and its free length but these are more about choosing a spring that fits our application rather than being about performance. |
| 03:57 | To give an example with some real values, let's say we have a simple linear spring that's rated at 500 pounds per inch. |
| 04:03 | That means if we apply 500 pounds of force to the top of the spring, it'll compress one inch. |
| 04:07 | If we then add an additional 250 pounds of force, it'll compress an additional half an inch. |
| 04:14 | This means we have a total of 750 pounds of force and 1.5 inches of compression. |
| 04:20 | Depending on where you are in the world, you may be used to using different units. |
| 04:23 | In motorsport we most commonly see pounds per inch in the US and newtons per millimetre for the rest of the world. |
| 04:31 | In the aftermarket industry, KG per millimetre is also popular. |
| 04:35 | Regardless the units are always a force divided by a given distance and conversion is easy. |
| 04:41 | There are plenty of free online calculators out there that'll do this for you if you're not sure. |
| 04:45 | You may have also come across non linear springs. |
| 04:48 | Sometimes known as progressive springs. |
| 04:50 | But they're not that common. |
| 04:53 | As the name suggests, the force is not proportional to the displacement. |
| 04:56 | As we see here in this example, the force rises in a progressive way as we compress the spring. |
| 05:02 | As the displacement increases, the spring rate gets higher. |
| 05:06 | This has the effect of stiffening the suspension the more it's compressed and is one way to achieve a rising rate suspension. |
| 05:13 | The variation in rate is usually achieved by winding the spring at a different pitch at one end to the other. |
| 05:20 | The coils at the closer end are more compliant so they're the first to compress. |
| 05:24 | This is the part of the spring with the lower initial spring rate. |
| 05:27 | As they compress more, these tighter coils begin to come into contact with each other until they're solid. |
| 05:33 | And this means that the stiffer coils then start to deflect. |
| 05:37 | As more of the softer part of the spring becomes solid, the spring rate gradually increases. |
| 05:43 | How aggressively the rate increases depends on the way the coil is wound. |
| 05:47 | Eventually whether a spring is linear or progressive, it's going to fully compress, which we refer to as coil bind. |
| 05:55 | This is where the spring is compressed so much that all the coils are sitting on top of each other which leaves to essentially a solid stack. |
| 06:02 | Coil bind is something we never want to see in our suspension system and we always need to check the rest of the suspension will bottom out before the spring becomes coil bound. |
| 06:13 | Otherwise, the effective rate becomes essentially infinite which can be damaging to the suspension and will cause an instant loss of grip. |
| 06:20 | The final concept we need to discuss here and one that's often overlooked is pre load. |
| 06:25 | This is when the spring has a compression force applied to it while the damper is at full extension. |
| 06:31 | This will occur if we have a coilover type suspension and we wind the spring collar up enough that the damper has been topped out. |
| 06:37 | As you continue to wind up the spring platform, the spring will come pre loaded. |
| 06:42 | The reality is that we'll often end up with pre load in order to achieve our target ride height. |
| 06:48 | Adding pre load will increase the car's static ride height but it's important to understand that for a linear spring, this doesn't increase the stiffness. |
| 06:57 | The spring rate stays the same. |
| 07:00 | For a pre loaded linear spring, once the initial pre load force has been overcome, the force required to compress the spring a given amount will be identical to a non pre loaded spring. |
| 07:10 | Which is what we see here in this example. |
| 07:13 | For any practical combination of car weight and spring rate, the static weight of the car will overcome the initial pre load, meaning the suspension stiffness is not affected. |
| 07:23 | In summary, springs are an important part of a suspension system. |
| 07:27 | The 3 main types you'll see in the automotive world are coil, torsion bar and leaf. |
| 07:32 | With coil being by far the most common in modern vehicles. |
| 07:35 | The spring's main purpose is to support the weight of the car and control the amount of deflection of the suspension at each corner. |
| 07:43 | Linear springs are the most common examples we're likely to come into contact with. |
| 07:47 | But progressive springs are also available and useful for some applications. |
| 07:51 | The rate of a spring defines the stiffness which is usually the primary thing we're interested in and remember, despite what you may have been told, adding pre load to a linear spring doesn't change its stiffness. |
