dampers and transient load transfer - The Technical Forum ...

Posted 19 August - 10:20

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Good observations have been made in this thread. I would like to summarize these if I may &, perhaps, add to them on occasion.

Dampers are multi-functional. They control the dissipation of disturbance energy arising from road & inertial inputs, they affect vehicle transient response to discrete inputs, they provide driver "feel", and they affect the rate of heat input to tyres. They also affect contact patch load variations and platform disturbance whilst disturbance energy is being dissipated. The first affects "grip" directly, whilst the second affect down-force variations for an "aero" vehicle. Not all of these variables can be "optimized" at the same time. It follows, therefore, that the "best" damper settings will depend to a greater or less extent upon the vehicle, its tyres, its driver & the track.

So, setting up dampers for a race vehicle is a complex task, & is one of the responsibilities of a race engineer. Tools are available to him include "hardware in the loop" tests, mathematical modelling, track tests (on-track measurements and driver feedback) & (last but not least) his experience. I think it fair to state that none of these can achieve an optimum set-up in isolation. It is worth noting, in passing, that "single corner" calculations are unlikely to be productive, except in very specific (& rarely encountered) circumstances. Other simple "rules of thumb" can be helpful, but for minor "tuning", or for "hiding" a persistent problem when track testing. Changes of this nature rarely improve actual vehicle performance, although they may help a driver to make better use of what is available.

Mathematical modelling is the most versatile single tool because it can yield "sensitivity" information at different points during a manoeuvre (essentially, identifying the most efficient changes that will achieve a desired effect), but modelling is unlikely to be useful without hardware in the loop tests, comprising tyre tests, kinematics & compliance tests & multi-post rig tests. The last are, perhaps, the most useful, because they can also yield sensitivity information, but also reveal some important dynamic properties directly & can, at least, achieve a "generic" optimal track set-up, no matter what the starting point (a biased view, I admit, but one that is based on experience).

It can be assumed, I think, that "optimal" overall damping coefficients exist for any given vehicle, spring selection & tyre set-up. Their values will depend upon vehicle & tyre properties, and the definition of optimum. The relationship is complex, & a multi-post rig test is a good (&, perhaps, the best) way to optimize overall damping coefficients. Linear dampers will dissipate disturbance energy most efficiently (provided the vehicle itself is linear), but are unlikely to satisfy other requirements noted above. The "style" of damping is important for these.

It is, perhaps worth noting that the damping "coefficient" describes the energy dissipated per cycle, whilst the damping "style" describes the "shape" of the load-velocity trajectory - where, within a cycle, the energy is dissipated. In general terms, a "digressive" style is one where the damping coefficient reduces with increasing peak velocity, whilst a "progressive" style is one where the damping coefficient increases with increasing peak velocity. There are other properties that are, or can be, important in a damper, including internal compliance and dependencies on temperature, acceleration, position, & time (frequency), but I will ignore these for now.

It is possible to make some generalized comments about damping style, assuming that the overall damping coefficients remain unchanged:

- Moving damping from the rebound to the compression half-cycles will improve minimum contact patch loads (& hence "grip"), but will increase platform disturbance, and the vehicle will tend to increase average ride height on-track. The last should allow a reduced static ride height, which may result in a nett performance benefit.

- Excessive platform disturbance can be controlled by introducing "blow-off", limiting the maximum compressive damper loads (in effect giving up mechanical control if favour of aero control).

- Moving damping from the compression to the rebound half cycles will reduce minimum contact patch loads (& hence "grip"), will reduce platform disturbance, and will tend to reduce average ride height on-track. The last may be a benefit when a regulated minimum static ride height applies.

- Some damper architectures lend themselves to rebound-biased damping, but a modern 4-way adjustable damper is likely to be tolerant of a large range of damping styles.

- A digressive damping style for dampers attached to the steered axle will decrease steering time constant, may introduce a noticeable transient under-steer, and will increase the rate of heat input to the tyres.

- A digressive damping style for dampers attached to the driven axle may compromise traction, may introduce a noticeable transient over-steer, and will increase the rate of heat input to the tyres. The transient over-steer may reduce the magnitude of steering inputs (& hence may reduce turn in speed loss) on the entry to high speed corners.

- A progressive low speed characteristic at either axle will usually improve contact patch load variations for small inputs, but will reduce platform stability and will reduce the rate of heat input to the tyres (not necessarily beneficial for slicks).

- Digressive front damper compression & digressive rear damper rebound can help to reduce vehicle pitch velocity under braking (noting that driver inputs are essentially low velocity).

- Digressive front damper rebound & digressive rear damper compression can help to reduce vehicle pitch velocity under acceleration.

Apologies for the length of this post. I hope some of it may be useful.

Since APEX first released these custom suspension packages built around JRZ RS dampers, these have been the two most prominent questions proposed to our Product Experts. This is usually followed up with, &#;what spring rates, rear shock mounts and other accessories should I choose&#;.

As car enthusiasts we all share a passion for a specific marque, but often times that is where common ground ends. Even within our tight knit team of car enthusiasts here at APEX, it is clear that these cars are enjoyed in different ways, and we each have different goals, opinions, and expectations on how a car should or shouldn&#;t be modified. This sentiment has stuck with APEX over the last 10+ years, and has been a driving force behind offering the community a wide array of wheel fitments that specifically met the diverse performance and budgetary needs of our customers. Forcing limited &#;cookie cutter&#; wheel fitments onto a diverse audience meant compromises would have to be made along the way, and that was quickly something that APEX navigated away from. In more recent years, this theme continues with our suspension solutions.

APEX hand picked race proven suspension components from three industry leading manufactures to create fully customizable coilover packages, and with JRZ&#;s 60+ years of experience in development and production of Motorsport grade dampers, we knew the JRZ RS line would provide our customers with the right foundation for each configured kit.

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BACK TO THE BASICS

Before we dive into the details and differences of JRZ RS dampers available to us, let&#;s review some basic suspension principles at a high level. In order to make an informed purchase decision, it is important to have a general understanding of the different components that make up a coilover, as well as the job of supporting suspension components. This is best explained by Motorsport guru Jay O&#;connell featured in the SAFEisFAST series, and we&#;ve compiled cliff notes below. 1. Springs &#; Springs are the load bearing device essentially keeping the car off the ground &#; Spring rate is the measure of how stiff the springs are, or how much force it takes to deflect them a certain distance &#; There are many factors contributing to choosing the right springs rates, but the primary factor is that you need springs stiff enough to hold the car off the ground under the aerodynamic loads it sees &#; Ideally you would run the softest springs you can get away with. The softer the springs, generally the more grip you can get out of the tires &#; Just stiff enough for the aerodynamic load that your car sees &#; Higher spring rates where the speeds are high and there is more aerodynamic forces at play (track configuration and whether or not a car has aero) &#; Softer spring rates where the speeds are quite low and there is less aerodynamic forces at play (track configuration and whether or not a car has aero) 2. Dampers (Shock absorbers) &#; The dampers main job is to control the release of energy that is stored in the spring due to the weight on them. They are the most important component for controlling the mass and movement of the car so that you can get as much grip out of the tires as possible &#; When a car drives over uneven surfaces all the energy in the compressed spring wants to come out fast, so the dampers job is to control that release of energy so that the car does not have a big response by the bump. We want to minimize the drama! &#; Uncontrolled motion from worn out shock absorbers will take away from grip &#; upsetting the car 3. Damper Adjustments &#; Rebound &#; Controlling the energy release out of the springs when the wheel & tire move away from the body &#; shock is expanding &#; Compression/Bump &#; Controlling the movement of the wheel & tire moving up into the body/suspension &#; shock is contracting 4. Bump Stops &#; In place to protect the dampers from the suspension running out of the travel or bottoming out. With these in place, you can feel a little more comfortable running a lower ride height without stiffening the springs because the bumps will &#;catch the car&#; before they hit. With that said, you do not want to be riding on the bump stops, as this indicates the car is running out of travel.

5. Anti-roll Bar &#; The main function is to reduce chassis roll. If you can reduce chassis role, you can maintain the camber angles of the tire in relation to the ground to give you more grip throughout the lap. It&#;s a quick way to tune the balance of the car &#; Stiffening the front anti-roll bar you will get more understeer &#; Stiffening the rear anti-roll bar you will get more oversteer &#; If the car is understeering, the first thing to do is to soften the front anti-roll bar to get some grip back into the front tires. Conversely you could stiffen the rear anti-roll bars and take some grip away from the rear, but now you are just hurting the end that is already well hooked up. Generally you want to fix/tune the end of the car that has has the problem first. &#; With each understeer/oversteer condition, there are multiple ways to fix it. You typically start with anti-roll bars, however you can then get into spring rates, damper settings and rake and that is when things can get somewhat complicated. Adjustments to correct a problem may cause a new problem to rear its head.

MOVING IN THE RIGHT DIRECTION &#; JRZ RS DAMPERS

Quite often aftermarket suspensions targeted at car enthusiasts are not actually improving the handling capabilities of your vehicle. There&#;s never a shortage of &#;lower grade&#; dampers or coilover packages on the market disguised in enticing packaging with fancy marketing, so it is important to know the genetic makeup of what you are buying. When I ask customers to identify some of the key features they would like out of their new suspension, height adjustability is commonly the first answer. The ability to adjust vehicle ride height is desirable for a few reasons beyond getting rid of that unsightly wheel gap, however this is a standard feature of any coilover design and you owe it to yourself to set the bar much higher.

Premium grade coilovers are designed around quality dampers built for a specific operating range of spring rates, and these two components are co-developed to ensure maximum control over the vehicle&#;s movement while it negotiates uneven road/track surfaces. The end goal is to reduce drama transmitted to the chassis, prevent loss of tire traction / maximize grip, and to keep things consistent and predictable for the driver. This is not to be confused with suspension that is overly stiff &#; a common misconception amongst car enthusiasts. Needless to say, not all coilover packages are created equal.

The JRZ RS-Line is hand built on the same production line as their ($8,000+) red motorsport-grade dampers. The build quality and attention to detail is second to none, and this is apparent from the moment you open the box and begin to handle the components. Every piece is anodized, leaving no uncoated corrosion-prone surfaces. Because the dampers are made by hand, each and every set is reviewed by a craftsman/craftswoman to ensure consistent quality, and personally signed off before making its way to you.

Performance & Comfort A unique compression blow off system is used to filter out large, high-velocity bumps you&#;d typically experience on rough streets or when attacking a berm on the race track using lower-end coilovers.

Longevity The shaft bearing has been upgraded for extended life between service intervals and to facilitate easier rebuilds. When you finally do need a refresh, JRZ has two partner service centers in the USA that can complete all necessary work.

CNC Machining The JRZ RS Line dampers use CNC machined internals that allow precise damping characteristics to be achieved. RS dampers are built on the same production line as the Motorsport line.

Strut Coating The McPherson struts from JRZ are made from high tensile steel to cope with the extreme side loads transferred from wheel to damper. JRZ applies a specialty black coating that is highly corrosion resistant, sourced from a cutting edge supplier.

Feel The Change Damper adjustments are highly intuitive compared to other brands. Each &#;click&#; of adjustment is felt with high precision not only by your fingers as you adjust rebound and compression settings, but also to the driver as the car is put through its paces. If you geek out over suspension like we do, this makes experimenting and dialing in your JRZ suspension a bit of fun.

THE GOLDEN QUESTION &#; TO WHAT DEGREE DO YOU WANT TO TUNE YOUR SUSPENSION?

Through conversing with a broad spectrum of customers, this is the major fork in the road. We can all agree that we would like our aftermarket dampers to significantly improve the handling characteristics of our vehicle, but to what degree are you willing and able to fine tune your dampers to optimize results given your vehicle setup, intended use, driving style etc.? Since dampers are the centerpiece to any high performing suspension package, it is important lay everything out on the table before making a decision. What are your long term goals for the car? Do you desire to compete in a form of class racing or time trials where the governing body imposes restrictions and guidelines? These are things to consider to ensure you are satisfied with your purchase in the long run.

I want to be clear that all JRZ RS dampers are very capable, and with the right supporting suspension components and chassis alignment they will dramatically improve the drivers ability to control the vehicle. The question is, how involved do you want to be? As we move through the different RS dampers available, each increases your ability to fine tune the suspension in a more granular way. The ability to externally modify damping characteristics is a desirable feature, particularly for those in continuous pursuit of improving lap times or beating out the competition. These enthusiasts and racers alike will go through the effort of adjusting rebound, compression and even external reservoir nitrogen pressure to compensate for tire wear, variations in track surfaces leading to poor or inconsistent levels of grip, and even in some instances driver errors.

RS ONE &#; Single Adjustable Dampers

The RS ONE is a single adjustable damper with 21 precise clicks of adjustment, and valving capable of handling a broad range of spring rates. Through the twist of one knob, JRZ strikes a balance of adjusting both rebound and compression together. Moving between comfortable street use, aggressive track mode, or anything in between has never been easier. The RS ONE design minimizes complexity, while still providing a significant increase in performance. If you are not the type of enthusiast to explore different spring rates, alignment settings or other chassis setup features on a continuous basis, and prefer to get a capable suspension as close to &#;set it and forget it&#; as possible, the RS ONE dampers are the perfect solution.

With competitive price and timely delivery, teao sincerely hope to be your supplier and partner.

RS TWO &#; Double Adjustable Dampers

The RS TWO is a double adjustable non-remote damper which offers independent compression and rebound adjustment. Having both compression and rebound adjustability independent from one another allows users to make more precise inputs to find the balance they are looking for. The RS TWO has 21 precise clicks of rebound adjustment controlled by a knob positioned at the top of the strut, and 14 precise clicks of compression controlled by a knob positioned at the bottom of the strut.

Whether you are interested in getting slightly more involved in the suspension tuning process from the get go, or ensuring you have the option to get more granular with suspension adjustments as you become a more experienced driver, the RS TWO has you covered without the added complexity of external reservoirs.

RS PRO &#; Double Adjustable Dampers

The RS PRO is a double adjustable damper which incorporates championship winning external reservoirs for maximum control. Like the RS TWO, compression and rebound adjustments are made independently from one another, allowing enthusiasts to calibrate their suspension from compliant street mode to taking the checkered flag. The RS PRO has 21 precise clicks of rebound adjustment controlled by a knob positioned at the top of the strut, and 14 precise clicks of compression controlled by a knob located on the external reservoir.

If you are a suspension geek, serious track junky or competitive racer, the RS PRO is an extremely effective weapon. These dampers and external reservoirs are aimed at enthusiasts who will take the time to explore and document a variety of spring rates, alignment settings and other chassis setup features all in the name of pushing their cars and their skills to the limits. The value added from the external reservoirs is listed in the &#;remote reservoir&#; section below.

RS PRO 3 &#; Triple Adjustable Dampers

The RS PRO 3 is JRZ&#;s flagship triple adjustable damper for the RS line, which incorporates championship winning external reservoirs for maximum control. The RS PRO 3 has 21 precise clicks of rebound adjustment controlled by a knob positioned at the top of the strut. It also has 14 precise clicks of high speed compression and 8 precise clicks of low speed compression, both controlled by knobs located on the external reservoir.

Independently tuning high and low speed compression allows track junkies and competitive racers alike to get the absolute most out of their suspension. Double adjustable suspensions do not allow users to isolate and control high and low speed compression damping. Low speed compression controls the speed at which the suspension compresses while the car negotiates small bumps, and prevents wallowing. High speed compression controls the speed at which the the suspension compresses while the car negotiates quick or abrupt movements, like hopping berms or drastic changes in the road surface.

Although more adjustments can add a level of complexity to the end user, it is important to note that you can set low speed compression to full stiff, and the damper will now behave the same as the RS PRO double adjustable suspension. As you develop the car over time and become more familiar with fine tuning the suspension, you can then make adjustments to the low speed compression to find the perfect balance for your specific track surface(s). The RS PRO 3 is the ticket for hard core track enthusiast and competitive racers looking for any and every available edge over the competition. The value added from the external reservoirs is listed below.

Remote Reservoirs &#; Applicable to RS PRO & RS PRO 3 External reservoirs significantly increase performance capabilities by providing more fluid displacement and increased cooling capacity. Track enthusiasts and racers push their cars to the limits, and external reservoirs are one of the key ingredients to consistent damping performance from the starting line to the checkered flag. The nitrogen pressure within the external reservoir can also be adjusted between 150 &#; 350 PSI; a key feature for track and race enthusiasts looking to fine tune their suspension to the highest degree.

Lines to each reservoir are long enough to comfortably mount them under the hood and trunk in a cool location that&#;s easily accessible. For a painless installation, all PRO kits come with front quick disconnect lines.

Some enthusiasts within the BMW community have years of experience and seat time under their belt, and with good information available to them, they are able to digest it with relative ease and make good purchase decisions. For many others it can be information overload, and therefore we recommend having a one on one consultation with an APEX Product Specialist to further discuss personal goals, expectations and budget. We may not have all the answers, but working closely with JRZ, Ground Control and Hyperco allows our staff to deliver top notch customer support, pre and post purchase. Keep in mind, configuring a suspension package and installing it on your vehicle is only half the battle. To get the most out of your new purchase, you&#;ll want to know that experts are there to support your efforts along the way.

Next up we&#;ll be taking a closer look at our Ground Control adjustable camber plates and rear shock mounts, baseline E46 street and track alignment suggestions, as well as a deeper dive into our Hyperco linear race springs, helper springs and guides. Stay tuned!

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