I got the world-leading rally suspension manufacturer (2017 World Rally Champions with Ford / M-Sport) to build a bespoke suspension set for my road car, that I have now installed and tuned.
The set is based on four Reiger 5062 dampers, three-way adjustable with remote canister. The front has the canisters attached to a pipe, while the rear canisters are attached to the base of the damper through a CNC machined holder.
The front springs are "over" the strut and damper, in a so-called "coilover" configuration, like the OEM strut, by the way: the defining feature is not that the coiled spring is installed over the strut, this is the most common layout of all McPherson suspensions, but that the lower spring platform is adjustable in height, allowing to control the front ride height of the car.
The rear springs sit in the OE location, on the suspension arm, and there is no height adjustment, short of replacing the springs.
Everything is superlative about this set. The build quality, the robustness, this is not your typical aftermarket mass-produced bi-tube dampers.
The general "formula" for this suspension is directly derived from motorsports, and it happens to be completely opposite to all aftermarket approaches I came across (and to which I strongly disagree with).
Let me start by defining a few things:
As the shaft movement gets faster and faster, the damper resists more and more. The law is expressed in mm/s (millimeters per second) of shaft speed and usually charted on the X-axis, and in N (Newtons), usually charted on the Y-axis. Compression is when the damper is contracted (the wheel moves up with regards to the car's chassis) and Rebound is when the damper extends (the wheel moves down with regards to the car's chassis).
Below is a (random) example of damping laws. Observe the compression and rebound laws are independent. Also, observe there is a "knee" on the law curves (more on this later).
Before going further, we need to consider the "knee" that can be seen in the above example laws.
By reading the above chart, and the compression (upper, blue) curve, in particular, one can see that the curve rises sharply from zero to about 48 on the Y axis, between 0 and about 40 mm/s on the X axis, then there is an inflection point ("knee") and the slope of the curve becomes less pronounced: going further on the speed scale keeps producing more damping force, but at a slower rate than at low shaft speeds.
This introduces the notion of low-speed and high-speed damping, where "speed" represents the damper shaft's speed, and "low" and "high" are defined by the hydraulic valves as designed by damper manufacturer.
Typically, the low-speed range ends between about 50 mm/s and 150 mm/s, depending on the damper construction.
The low-speed compression setting is very important for the "feel" of the car. The body movements that fall into the low-speed range are typically those induced by driver input, such as steering input (roll), and squat during acceleration or braking, as well as the movements induced by small and smooth surface bumps, taken at low vehicle speed.
Low-speed compression greatly affects the comfort or "ride quality" of the car, and it also affects the grip (softer = more grip) and the steering response (stiffer = quicker response) so choose your poison: with stiff low-speed compression at the front, you can have a car that reacts swiftly to steering input but understeers at the limit, or a car that reacts softly to the steering, but has more ultimate front grip.
Unfortunately, you cannot have both. Usually, the more experienced the driver, the softer the low-speed settings, all other things being equal. You can readily see that on WRC pre-event test videos where cars driven by the top drivers in the world can be seen taking huge amounts of roll when driven slowly in zig-zag to warm the tires. At racing speed, the body motions are much faster and the damping prevents most of those movements.
The realm of high-speed compression is entered by road bumps, even small ones provided that the car speed is high enough: consider a 1 cm bump over 1 meter - that's not even a bump, actually. If you take this "bump" at 10m/s (36 km/h) then the car takes 0.1s to travel one meter horizontally, and that means that the wheel will move up 1 cm during that time, which gives a shaft speed of 100 mm/s. For most dampers, this is still in the low-speed realm.
Now take the same "non-bump" (less than a half inch vertically over one yard horizontally, for readers not (yet) accustomed to SI) at 180 km/h (50 m/s). It now takes 0.02s to travel one meter, and the wheel still moves 10mm vertically, resulting in a shaft speed of 500 mm/s, way off the chart in our above example and clearly very deep into the high-speed realm. And remember: we are only talking about a 10mm bump over 1 meter, a "bump" barely noticeable to the eye.
The bottom line is the car speed has a huge effect on the damper shaft speeds, and thus the expected car speeds and expected bumps, not just the corner weights, governs the choice of springs and the range of compression damping forces that are expected to be absorbed.
When the car travels at high speed, the high-speed damping is predominant. When the car travels at low speeds, both the low-speed damping and the high-speed damping are important.
The key goal of high-speed compression damping is to prevent the car from bottoming out (hitting the end of the suspension travel, and hit the bump stop) as this transfers raw energy to the chassis and greatly unsettles the car, potentially sending it up in the air as the chassis flection is going to give that energy back very quickly and without any rebound damping, which brings us to the next sections...
Without a damper, the car would go up and, as it picks up vertical speed as the spring extends to recover its length, the body will overshoot its static ride height, carried over by inertia, and raise higher than expected. Sure enough, gravity will stop the ascent and the body of the car will go down again, picking up vertical speed downwards and overshooting the static ride height in the other direction, etc, etc, in an oscillating and slightly decreasing movement sometimes called "ringing", and it will take a very long time to settle (many seconds!).
The rebound damping slows down the spring expansion by applying an opposing force proportional to the expansion speed (as governed by the rebound damping law) which helps the car settle to its normal ride height much more quickly.
Let's consider the following synthetic case, in an attempt to make things clearer: let's assume that the car is sitting on a platter, and the platter is held 10cm above the ground, held by chains attached to the four corners and tied to a hook above the car. Observe that the car is at its normal ride height on the platter.
Now, suddenly drop the platter with the car on it. The car and its supporting platter will quickly pick up vertical speed and reach the ground, which will stop the platter. From there, the car's body will continue its downward motion, only slowed down by the suspension.
The springs will compress, and the amount of compression will depend on the spring rate and the compression damping rate. Say, for the sake of the example, that the car goes half-way down the wheel travel available between the static height and the compression bump stop, whatever that distance is.
What happens next, now that the car briefly stops at its lowest point after the vertical energy has been absorbed by the springs and compression damping, is the springs will give the energy back and sent the car's body back up.
One observation is that some of the energy has been lost already, turned into heat in the hydraulic damper's oil, as the compression damping slowed down the spring's compression by absorbing energy.
The car's body starts its upwards motion, pushed up by the springs. Two things can happen:
The illustration below shows the ringing movement occurring when under-damped. One can see the oscillation will last several seconds when very under-damped, or for about one second, with one overshoot, one undershoot, and back to zero in about one second, then a possible critical damping curve recovering without overshoot in about 0.6s, and an under-damped one recovering without overshoot but in nearly 2 seconds.
Those numbers are just relatively realistic examples, the actual times depending on the spring rates, essentially, where stiffer springs make things happen faster (but also more violently and in a way that is harder to control).
You do not want to operate the car over-damped in rebound, as this prevents the springs to "push" the tires down the road surface and (greatly) reduces grip.
The rebound damping also controls the roll and pitch to some extent (more rebound damping, less roll, less pitch) as the wheels (from the opposite side) stay "up" a little longer during body movements, and the unsprung masses partially counteract those movements.
More rebound damping also affects the high-speed feel of the car, making it feel more "planted" and "stable" by reducing the upwards vertical movements, in particular in high-speed curves (say above 160 km/h or 100mph, to give an idea of the speed range affected.)
Unfortunately, all this comes at the expense of grip, in fact, while the (low-speed) compression damping mostly controls the "feel", one can say that the rebound damping mostly controls the grip, both lateral and longitudinal, where stiffer rebound sharply reduces said grip.
On the other hand, too soft a suspension makes the car more grippy but less sharp, and in some extreme situations, the lack of sharp steering response can lead the driver to slow down as he/she can not anticipate properly to steer the car down in really twisty, technical and narrow roads.
Needless to say, suspension settings are always a compromise, there is no completely right solution, although you can quite easily get to something completely wrong. In general, the suspension settings are too stiff, sometimes way too stiff, in most amateur circles, and I'm not blaming anyone for that: I've been there too.
Assuming you have a decent 3-way setup with spring rate appropriate for the car's weight, its ride height, the anticipated conditions, and the dampers adjustability range is just right, considering the base setup is still set by the valving inside the damper, and the adjustability is some sort of plus/minus around that base setup.
The first thing is to open all the clicks on all the settings. Damper clicks are always expressed in clicks open from the fully closed setting. That is, if you read 10 clicks, that means to close the setting all the way, then open until the first click is felt and call this "zero", then open 10 clicks from there.
My suspension has 17 clicks of low-speed compression, 15 clicks of high-speed compression, and 36 clicks of rebound compression. Just for your consideration, 36 x 17 x 15 = 9,180 possible damper settings on this set. Needless to say, there is no way you can find anything good by chance, and some sort of method is required.
What I do is as follow: as said above, I start by opening all three ways all the way: first close down, then count the respective number of clicks, opening all settings to the max. With any adjustable damper, never open more than the maximum stated number of clicks: always start fully closed, and open exactly the number of clicks allowed for your particular damper and setting.
The car should feel soft now, perhaps a little spongy, but should nevertheless be perfectly driveable if the base setup (spring rates and damper valving) was gotten right.
Proceed with one setting at a time.
The first thing that I adjust is the rebound setting. I find a "suitable bump" - which can be a (long) speed hump, and drive over it, paying close attention to what the car does when recovering from the bump. The speed should be so that you generate some decent spring compression, but do not drive too fast as to bottom-up the suspension (remember the compression settings are all the way open).
The speed I pick depends on the bump, something around 30 km/h (20 mph) to begin with, and I pay close attention to how the car gets back up to its ride height, and in particular, if it goes too high then down again right after, ie. I try to get a good feel of the magnitude and number of oscillations. There should be some if your base setup is right.
I use the cruise-control to get a repeatable speed and go over the bump both ways a few times. Then I close the rebound (by the same amount front and rear, usually 2 clicks at a time) and try again and again until the oscillation stops, i.e. the car recovers from the bump without overshooting its ride height, or maybe just a tiny little bit.
On one occasion I used a sports camera with a suction cup (a Garmin VIRB Ultra 30 in 480p and 300fps slow motion), looking at the top of the fender, to actually see the wheel's vertical movement.
This proved moderately useful in the field as the movie runs so slowly when you watch it at "normal" speed, some 12x slower, that it is very challenging to seek and find the point of interest on the tiny camera. It's probably easier on a computer but you first have to download a huge video, and this takes way too much time.
When I think I got the rebound about right, in this case, it was about 20 clicks open, on a 36 clicks range. I increase the speed slightly (by 5 km/h) and get a feel of how deep the car plunges (or how high the wheel climbs, which is the same. The front dampers have a rubber band, like a O-ring, that gets pushed by the strut and which marks the maximum damper travel so I close the high-speed compression (and reset the rubber O-ring) by a couple of clicks and consider the "feel" on that bump at that speed, does it feel harsh, or soft, and what was the damper travel.
I increase the compression damping until I can pass the hump as a pretty decent speed (about 50km/h or 31 mph) with about 10mm if damper travel left. Of course, the actual speed highly depends on the bump, here I'm talking of the ones with a short ramp, a flat, then another ramp. The long, arched humps can be passed at twice that speed and are not the best choice for suspension settings.
Finally, I adjust the low-speed settings by closing bit by bit until I start feeling the surface changes and tiny surface imperfections, then back off a bit. As a rule of thumb for low-speed settings, if you rock the car by stepping with both feet on the doorstep, one hand on the top of the open door, and flex your knees trying to shake the car vertically, the suspension should move, you should be able to inflict 1-2 cm of vertical movement easily, depending on your weight. If the only movement you can generate is a slight flex of the tires, your low-speed compression settings are too stiff.
Back to our settings, from there, you should be in the ballpark where the car can be driven faster. One of the tests should include a high-speed curve, ideally with a compression in the middle, where great attention should be paid to the vertical movements and overall stability of the car. Close the rebound a click or two all around until the stability improves, but be careful to not over-do it. Close the high-speed compression one more click if the car seems to go down too low on the high-speed compressions.
Finally, adjust the low-speed compression setting by driving on twisty/bumpy stuff at moderate speed. You want a reasonably sharp turn-in while still maintaining a smooth ride at low car speeds. Things like expansion joints should be felt, but barely. In doubt, rock the car "by hand" as described above, and see if you can still generate at least some suspension movement. If not, back off a bit until you can again.
Consider that the suspension automatically gets stiffer as the car speed increases, as the same bump taken faster will cause the dampers to compress at a higher speed, which in turn generates more damping force. As such it is important to consider your suspension settings globally, over a range of speeds and road conditions.
The set is based on four Reiger 5062 dampers, three-way adjustable with remote canister. The front has the canisters attached to a pipe, while the rear canisters are attached to the base of the damper through a CNC machined holder.
The front springs are "over" the strut and damper, in a so-called "coilover" configuration, like the OEM strut, by the way: the defining feature is not that the coiled spring is installed over the strut, this is the most common layout of all McPherson suspensions, but that the lower spring platform is adjustable in height, allowing to control the front ride height of the car.
The rear springs sit in the OE location, on the suspension arm, and there is no height adjustment, short of replacing the springs.
Everything is superlative about this set. The build quality, the robustness, this is not your typical aftermarket mass-produced bi-tube dampers.
The general "formula" for this suspension is directly derived from motorsports, and it happens to be completely opposite to all aftermarket approaches I came across (and to which I strongly disagree with).
Let me start by defining a few things:
Damping law
The damping law is the relationship between the damper's shaft speed and the damper's resistance to the shaft movement. Some literature refers to the damping law as the "damping curve". If the damper's shaft moves very slowly, the damper typically offers little resistance against the movement.As the shaft movement gets faster and faster, the damper resists more and more. The law is expressed in mm/s (millimeters per second) of shaft speed and usually charted on the X-axis, and in N (Newtons), usually charted on the Y-axis. Compression is when the damper is contracted (the wheel moves up with regards to the car's chassis) and Rebound is when the damper extends (the wheel moves down with regards to the car's chassis).
Below is a (random) example of damping laws. Observe the compression and rebound laws are independent. Also, observe there is a "knee" on the law curves (more on this later).
N-way adjustable dampers
N-way adjustable dampers have N independently adjustable hydraulic circuits that can be controlled separately and which handles a different aspect of the damping laws.- Some dampers are not user-adjustable: the laws are set at the factory and cannot be changed.
- Most 1-way adjustable dampers (such as for example the common Koni aftermarket OE replacement dampers) control both the compression and rebound damping with a single adjuster and make the overall damping softer or stiffer. The adjustment is originally intended to compensate for aging, but it can be used to great effect to change how the car's feel. Some 1-way adjustable dampers only allow for rebound adjustments. Those are typically competition dampers meant to be adjusted to match a car's spring rates and alter the dynamic behavior of the car.
- 2-way adjustable dampers offer separate and independent compression and rebound law adjustments. Stiffening or softening the compression law has no effect on the rebound law and vice-versa.
Before going further, we need to consider the "knee" that can be seen in the above example laws.
By reading the above chart, and the compression (upper, blue) curve, in particular, one can see that the curve rises sharply from zero to about 48 on the Y axis, between 0 and about 40 mm/s on the X axis, then there is an inflection point ("knee") and the slope of the curve becomes less pronounced: going further on the speed scale keeps producing more damping force, but at a slower rate than at low shaft speeds.
This introduces the notion of low-speed and high-speed damping, where "speed" represents the damper shaft's speed, and "low" and "high" are defined by the hydraulic valves as designed by damper manufacturer.
Typically, the low-speed range ends between about 50 mm/s and 150 mm/s, depending on the damper construction.
- 3-way adjustable dampers have separate low-speed and high-speed compression adjusters, and a third separate rebound adjuster. This is a common configuration (and the one chosen for my car.)
- 4-way adjustable dampers have separate low-speed/high-speed adjusters on both the compression and rebound laws. These are the most sophisticated and usually found only on high-level motorsports.
Compression damping
The compression law governs the amount of resistance to compression offered by the damper as a function of the shaft speed.The low-speed compression setting is very important for the "feel" of the car. The body movements that fall into the low-speed range are typically those induced by driver input, such as steering input (roll), and squat during acceleration or braking, as well as the movements induced by small and smooth surface bumps, taken at low vehicle speed.
Low-speed compression greatly affects the comfort or "ride quality" of the car, and it also affects the grip (softer = more grip) and the steering response (stiffer = quicker response) so choose your poison: with stiff low-speed compression at the front, you can have a car that reacts swiftly to steering input but understeers at the limit, or a car that reacts softly to the steering, but has more ultimate front grip.
Unfortunately, you cannot have both. Usually, the more experienced the driver, the softer the low-speed settings, all other things being equal. You can readily see that on WRC pre-event test videos where cars driven by the top drivers in the world can be seen taking huge amounts of roll when driven slowly in zig-zag to warm the tires. At racing speed, the body motions are much faster and the damping prevents most of those movements.
As a side note, the modern "damping formulas" includes generous wheel travel (at least 60mm from static height to bump stop for tarmac, and at least twice as much (200+mm total travel) for gravel, with relatively soft and very long springs, and HUGE dampers capable of absorbing incredible amounts of energy without a sweat (large diameter, high-pressure monotubes with lots of hydraulic fluid.)
Incidentally, this is also the "formula" I choose for my car, as opposed to the typical aftermarket settings going for short, super-stiff springs and comparatively weak dampers: unpressurized bi-tube, which are not thermally stable and difficult to control with small pistons and very stiff valves. The only argument in favor of that approach is the low cost of manufacture.
The realm of high-speed compression is entered by road bumps, even small ones provided that the car speed is high enough: consider a 1 cm bump over 1 meter - that's not even a bump, actually. If you take this "bump" at 10m/s (36 km/h) then the car takes 0.1s to travel one meter horizontally, and that means that the wheel will move up 1 cm during that time, which gives a shaft speed of 100 mm/s. For most dampers, this is still in the low-speed realm.
Now take the same "non-bump" (less than a half inch vertically over one yard horizontally, for readers not (yet) accustomed to SI) at 180 km/h (50 m/s). It now takes 0.02s to travel one meter, and the wheel still moves 10mm vertically, resulting in a shaft speed of 500 mm/s, way off the chart in our above example and clearly very deep into the high-speed realm. And remember: we are only talking about a 10mm bump over 1 meter, a "bump" barely noticeable to the eye.
The bottom line is the car speed has a huge effect on the damper shaft speeds, and thus the expected car speeds and expected bumps, not just the corner weights, governs the choice of springs and the range of compression damping forces that are expected to be absorbed.
When the car travels at high speed, the high-speed damping is predominant. When the car travels at low speeds, both the low-speed damping and the high-speed damping are important.
The key goal of high-speed compression damping is to prevent the car from bottoming out (hitting the end of the suspension travel, and hit the bump stop) as this transfers raw energy to the chassis and greatly unsettles the car, potentially sending it up in the air as the chassis flection is going to give that energy back very quickly and without any rebound damping, which brings us to the next sections...
Rebound damping
The rebound damping is the reason why dampers were initially invented: without any damper, the car being suspended only by the springs, when hitting a bump the spring(s) would compress to absorb it. The amount of energy vs spring compression is expressed in N/mm (Newton per millimeter) or, in some regions, in pounds per inch of spring compression (lb-in). The energy is conserved by the spring (ignoring minor losses) which means that the spring will extend when the same force it was compressed.Without a damper, the car would go up and, as it picks up vertical speed as the spring extends to recover its length, the body will overshoot its static ride height, carried over by inertia, and raise higher than expected. Sure enough, gravity will stop the ascent and the body of the car will go down again, picking up vertical speed downwards and overshooting the static ride height in the other direction, etc, etc, in an oscillating and slightly decreasing movement sometimes called "ringing", and it will take a very long time to settle (many seconds!).
The rebound damping slows down the spring expansion by applying an opposing force proportional to the expansion speed (as governed by the rebound damping law) which helps the car settle to its normal ride height much more quickly.
Let's consider the following synthetic case, in an attempt to make things clearer: let's assume that the car is sitting on a platter, and the platter is held 10cm above the ground, held by chains attached to the four corners and tied to a hook above the car. Observe that the car is at its normal ride height on the platter.
Now, suddenly drop the platter with the car on it. The car and its supporting platter will quickly pick up vertical speed and reach the ground, which will stop the platter. From there, the car's body will continue its downward motion, only slowed down by the suspension.
The springs will compress, and the amount of compression will depend on the spring rate and the compression damping rate. Say, for the sake of the example, that the car goes half-way down the wheel travel available between the static height and the compression bump stop, whatever that distance is.
What happens next, now that the car briefly stops at its lowest point after the vertical energy has been absorbed by the springs and compression damping, is the springs will give the energy back and sent the car's body back up.
One observation is that some of the energy has been lost already, turned into heat in the hydraulic damper's oil, as the compression damping slowed down the spring's compression by absorbing energy.
The car's body starts its upwards motion, pushed up by the springs. Two things can happen:
- The car overshoots the ride height and goes back down (then back up, etc). The overshoot will be much smaller than without dampers already, first because some of the energy has been lost (converted to heat) by the dampers during compression, so the springs have deflected less than they would without any damping, and the rebound damping has slowed down the upward motion, although not enough as the car went too high, carried by the inertia of its mass in vertical upward motion. This situation is known as under-damping, or sub-critical damping, as there was not enough of it.
- The car does not overshoot its static ride height, but instead recover its height and stops just there. This is at least somewhat better than under-damped, but we still don't know if the car recovered its height as fast as possible, just that it did not overshoot. The problem is if it takes five minutes for the car to recover the height, even if it eventually stops exactly at the right height, and does not overshoot, we may still be applying too much rebound damping, so the game is to apply as little rebound ramping as possible to suppress ringing, but no more. When the damping is set so it quasi-eliminates overshoot and ringing, the settings have reached critical damping. If the car recovers perfectly, but not as fast as it could recover, still without overshoot, with a little less rebound damping, the setting is said to be over-damped, or over-critical.
The illustration below shows the ringing movement occurring when under-damped. One can see the oscillation will last several seconds when very under-damped, or for about one second, with one overshoot, one undershoot, and back to zero in about one second, then a possible critical damping curve recovering without overshoot in about 0.6s, and an under-damped one recovering without overshoot but in nearly 2 seconds.
Those numbers are just relatively realistic examples, the actual times depending on the spring rates, essentially, where stiffer springs make things happen faster (but also more violently and in a way that is harder to control).
You do not want to operate the car over-damped in rebound, as this prevents the springs to "push" the tires down the road surface and (greatly) reduces grip.
The rebound damping also controls the roll and pitch to some extent (more rebound damping, less roll, less pitch) as the wheels (from the opposite side) stay "up" a little longer during body movements, and the unsprung masses partially counteract those movements.
More rebound damping also affects the high-speed feel of the car, making it feel more "planted" and "stable" by reducing the upwards vertical movements, in particular in high-speed curves (say above 160 km/h or 100mph, to give an idea of the speed range affected.)
Unfortunately, all this comes at the expense of grip, in fact, while the (low-speed) compression damping mostly controls the "feel", one can say that the rebound damping mostly controls the grip, both lateral and longitudinal, where stiffer rebound sharply reduces said grip.
How to adjust?
I'm going to start this section by stating that, in general, stiffening the suspension reduces (mechanical) grip and eventually makes the car slower.On the other hand, too soft a suspension makes the car more grippy but less sharp, and in some extreme situations, the lack of sharp steering response can lead the driver to slow down as he/she can not anticipate properly to steer the car down in really twisty, technical and narrow roads.
Needless to say, suspension settings are always a compromise, there is no completely right solution, although you can quite easily get to something completely wrong. In general, the suspension settings are too stiff, sometimes way too stiff, in most amateur circles, and I'm not blaming anyone for that: I've been there too.
Assuming you have a decent 3-way setup with spring rate appropriate for the car's weight, its ride height, the anticipated conditions, and the dampers adjustability range is just right, considering the base setup is still set by the valving inside the damper, and the adjustability is some sort of plus/minus around that base setup.
The first thing is to open all the clicks on all the settings. Damper clicks are always expressed in clicks open from the fully closed setting. That is, if you read 10 clicks, that means to close the setting all the way, then open until the first click is felt and call this "zero", then open 10 clicks from there.
My suspension has 17 clicks of low-speed compression, 15 clicks of high-speed compression, and 36 clicks of rebound compression. Just for your consideration, 36 x 17 x 15 = 9,180 possible damper settings on this set. Needless to say, there is no way you can find anything good by chance, and some sort of method is required.
What I do is as follow: as said above, I start by opening all three ways all the way: first close down, then count the respective number of clicks, opening all settings to the max. With any adjustable damper, never open more than the maximum stated number of clicks: always start fully closed, and open exactly the number of clicks allowed for your particular damper and setting.
The car should feel soft now, perhaps a little spongy, but should nevertheless be perfectly driveable if the base setup (spring rates and damper valving) was gotten right.
Proceed with one setting at a time.
The first thing that I adjust is the rebound setting. I find a "suitable bump" - which can be a (long) speed hump, and drive over it, paying close attention to what the car does when recovering from the bump. The speed should be so that you generate some decent spring compression, but do not drive too fast as to bottom-up the suspension (remember the compression settings are all the way open).
The speed I pick depends on the bump, something around 30 km/h (20 mph) to begin with, and I pay close attention to how the car gets back up to its ride height, and in particular, if it goes too high then down again right after, ie. I try to get a good feel of the magnitude and number of oscillations. There should be some if your base setup is right.
I use the cruise-control to get a repeatable speed and go over the bump both ways a few times. Then I close the rebound (by the same amount front and rear, usually 2 clicks at a time) and try again and again until the oscillation stops, i.e. the car recovers from the bump without overshooting its ride height, or maybe just a tiny little bit.
On one occasion I used a sports camera with a suction cup (a Garmin VIRB Ultra 30 in 480p and 300fps slow motion), looking at the top of the fender, to actually see the wheel's vertical movement.
This proved moderately useful in the field as the movie runs so slowly when you watch it at "normal" speed, some 12x slower, that it is very challenging to seek and find the point of interest on the tiny camera. It's probably easier on a computer but you first have to download a huge video, and this takes way too much time.
When I think I got the rebound about right, in this case, it was about 20 clicks open, on a 36 clicks range. I increase the speed slightly (by 5 km/h) and get a feel of how deep the car plunges (or how high the wheel climbs, which is the same. The front dampers have a rubber band, like a O-ring, that gets pushed by the strut and which marks the maximum damper travel so I close the high-speed compression (and reset the rubber O-ring) by a couple of clicks and consider the "feel" on that bump at that speed, does it feel harsh, or soft, and what was the damper travel.
I increase the compression damping until I can pass the hump as a pretty decent speed (about 50km/h or 31 mph) with about 10mm if damper travel left. Of course, the actual speed highly depends on the bump, here I'm talking of the ones with a short ramp, a flat, then another ramp. The long, arched humps can be passed at twice that speed and are not the best choice for suspension settings.
Finally, I adjust the low-speed settings by closing bit by bit until I start feeling the surface changes and tiny surface imperfections, then back off a bit. As a rule of thumb for low-speed settings, if you rock the car by stepping with both feet on the doorstep, one hand on the top of the open door, and flex your knees trying to shake the car vertically, the suspension should move, you should be able to inflict 1-2 cm of vertical movement easily, depending on your weight. If the only movement you can generate is a slight flex of the tires, your low-speed compression settings are too stiff.
As a side note and as another rule of thumb, you need at least 60mm (2.4 in) of wheel travel from static height to bump stop to be able to find a good tarmac setup, springs and all, that works well for hard driving on typical back roads in developped countries. This figure comes from very experienced people.
Less wheel travel forces the use of stiffer springs, which in turn forces the use of more rebound damping force, which in turn reduces mechanical grip. There is no escape to that. Lower and softer is also a no-no (the car bottoms out, so is undrivable on the rough stuff) and a track setup, which can be slightly lower/stiffer, always keeping in mind you are dealing with a road car, is only good at track speeds.
Just to give an idea, the range of spring rates for a well known and very succesful WRC2 rally car (R5 class) ranges from 37 to 57 N/mm, knowing that the highest rate is only useable on silk-smooth tarmac like Rally Cataluña, a leg of the World Rally Championship where the roads are smoother than most tracks.
As an indication, 55 N springs (314 lb) are used on World Rally Cars during the Monza Rally show, which runs on the Monza Formula One track, and springs in the 40-50N range (228-285 lb) are used on most tarmac rallies. Meanwhile, an aftermarket supplier of springs and dampers for the Focus RS has 122 N (700lb) springs on their catalog and the "softer" coilover kit I came across for this has 70N / 400lb springs. Spot the flaw.
On the other hand, a well-known supplier of springs offers a lowering kit for the RS that uses 32 N/mm (183 lbs/in) springs at the front, lower yet 20% softer than the OE spring, a sure-kill recipe for bottoming out, at least in Normal suspension mode. That surely improves the ride though, but cars so equipped can not be driven hard on anything.
Back to our settings, from there, you should be in the ballpark where the car can be driven faster. One of the tests should include a high-speed curve, ideally with a compression in the middle, where great attention should be paid to the vertical movements and overall stability of the car. Close the rebound a click or two all around until the stability improves, but be careful to not over-do it. Close the high-speed compression one more click if the car seems to go down too low on the high-speed compressions.
Finally, adjust the low-speed compression setting by driving on twisty/bumpy stuff at moderate speed. You want a reasonably sharp turn-in while still maintaining a smooth ride at low car speeds. Things like expansion joints should be felt, but barely. In doubt, rock the car "by hand" as described above, and see if you can still generate at least some suspension movement. If not, back off a bit until you can again.
Consider that the suspension automatically gets stiffer as the car speed increases, as the same bump taken faster will cause the dampers to compress at a higher speed, which in turn generates more damping force. As such it is important to consider your suspension settings globally, over a range of speeds and road conditions.
Can you give an approximate price for this suspension setup which you recieved? Also what height difference did you get from the stock suspension?
ReplyDeleteapprox 5000€
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