The shock absorber is an integral part to any performance suspension system. Softening the blows of harsh road conditions and adding some checks and balances to the otherwise frenetic spring action. What goes on inside these magical oil-filled tubes can be somewhat perplexing, but there’s nothing to fear.
I often equate [shock tuning] to drinking wine. I didn’t know that much about it but if they put two or three in front of you — you begin to refine your senses. And that’s what we’re trying to do with shocks. -Lee Grimes, Koni
While the springs support the weight of a car and provide resistance, shocks lend the balance. In the search for better lap times, cornering speed, feedback, and control; most of us gearheads inevitably turn to suspension tuning. At the risk of falling prey to internet-forum-special advice, we turned to some of the best grounded in the business, Koni.The primary function of the shock absorber is not to support the sprung mass of a car, but to dampen or subdue the oscillations of the springs as they track the profile of the road surface. Left to their own devices, undampened springs provide a bouncy and uncontrolled ride.
Lee Grimes is the Automotive Product Manager at Koni, and an avid road racer. Grimes came to the business side of the suspension world after already accruing a career of laps racing.
“When I joined the company I’d been racing sports cars for eight years. I didn’t claim to be a shock expert, but I felt like I had a pretty decent feel. I had to start learning what we were looking for. I often equate [shock tuning] to drinking wine. I didn’t know that much about it but if they put two or three in front of you — you begin to refine your senses. And that’s what we’re trying to do with shocks,” Grimes prefaced.
The road to understanding the racing shock absorber is a complex one, starting with an anatomy lesson. While there are a multitude of different designs, they all share some common key elements.
Moving from the outside in, we have the shock body which contains any oil or pressurized gases along with hard parts. The shock shaft protrudes from the body and is usually chrome-plated for a flawless smooth finish.
Passing through the only real opening to the interior of the shock, the shaft is located and guided by tight-fitting seals to keep fluids in, and contaminants out. Once inside the damper we find a divergence in designs depending on the type of shock, but all will include a viscous oil and a piston of sorts that allows for metering of oil passage from side to side.
Differentiating between the shock shaft, piston, and shims. Image source: Pirate4x4
Reaching from the interior and back out can be an assortment of accessories including adjustment screws, knobs, Schrader valves, and external reservoirs — items we will explore later.
Monotube Or Twin-Tube?
Basic shock architecture usually gets lumped into two different groups; the monotube and twin-tube. Both of these styles of damper have their own merits, and can be found on OEM and performance aftermarket applications.
Cutaway of a Koni twin-tube damper. Image source: Koni
Twin-tube shocks are less commonly understood than monotubes, and that fear of the unknown has given them a bad rap over the decades.
Twin-tubes feature two reservoir areas into which oil can be pushed or pulled. One of the benefits of a twin-tube design surfaces in lowered applications with limited wheel travel, like many road racecars. Because the twin-tube does not have the added internal structure of a divider piston, more stroke can be generated out of a shock body than a monotube of the same length.
When in a neutral position, most of the oil is located within the inner tube which is surrounded by a concentric outer tube; these two chambers are then separated by the metering valve that provides adjustment.
“There are basically two types of twin-tube shocks; there’s a hydraulic non-gas, and a twin-tube low-pressure gas shock, in that one you’ve got an inner cylinder that’s totally full of oil, a piston with the rebound valving right on it, and an outer cylinder that’s basically a reservoir,” Grimes articulated.
Cutaway of a Koni monotube damper. Image source: Koni
Monotube shocks are more common in the racing world than twin-tubes and come in their own variety of orientations, both traditional and inverted. These shocks have the public perception as the only option for performance ride control, but this can be a snap judgement as Grimes cleared up.
“This is one of the biggest misconceptions. There are three designs, and all three of them are viable and can be absolutely good. Most companies only make one or two of those designs, while each one has it’s own set of plusses and minuses,” he continued.
“In monotube shocks your rebound and compression valving are typically going to ride the piston through a single cylinder that’s totally full of oil. You have a separate piston known as the divider piston, and on the other side there is no oil but actually a nitrogen gas charged at a high pressure,” Grimes identified.
The purpose of gas pressure in a shock is a commonly misidentified feature, and one that often serves to misdiagnose a problem by novice racers. Just because a shock does not return to the extended length after being compressed by hand does not mean a lick of anything according to Grimes.
“One of the very common misnomers is that one way to gauge a shock is to push it down and watch the rate at which it self-extends. What you’re looking at is simply a side effect of the presence of gas,” Grimes assured.
Most all shocks are oil-filled. Image source: Race Tech
Koni treats each suspension project on a case-by-case basis, meaning they make applications of both twin and monotube construction with or without gas.
“We make a lot of non-gas shocks, but other big-name makers told everybody back in the ’70s that gas shocks are better than non-gas shocks. If your oil is a low enough quality its characteristics are going to include fade issues based on heat that it doesn’t generate, you need to get better oil not just put a band-aid fix on it.”
One of the very common misnomers is that one way to gauge a shock is to push it down and watch the rate at which it self-extends. What you’re looking at is simply a side-effect of the presence of gas. -Lee Grimes, Koni
While gas pressurization is not a requirement for a shock absorber to function properly, it does serve an expressly important function in severe duty applications like motorsports. Simply put, gas pressure prevents oil foaming under thermal stress, and therefore helps maintain a consistent viscosity.
“It’s the same basic theory of the tea kettle, if you try to heat or put pressure into a fluid that’s under overall pressure, you’re going to raise the boiling and cavitation point,” Grimes expressed.
What’s more important is how the manufacturer of a damper comes to the final design and execution. Gas pressure can be divided in the case of a monotube, held in solution with the oil in the case of an emulsion shock, or rather alarmingly in a plastic bag.
“Some companies contain the low-pressure gas in a plastic bag that they’d put in the outer cylinder, and it doesn’t take long for those bags to fatigue and break. One of Koni’s tricks on our twin-tube low-pressure is to put the low-pressure gas in with the oil itself,” Grimes explained.
Linear, Progressive, And Digressive Are Not Political Parties
The performance of a shock absorber is infinitely tailorable by the engineers, and then often includes a limited range of user adjustment for fine-tuning. In order to quantify the characteristics of a shock valved one way or another, suspension engineers use a shock dyno.
Looking at the dyno plots of different shock valving deigns is much like looking at an engine dyno graph. Image source: Moreno Brothers, Camaro5
These specialty dynamometers function similarly to an engine dyno, measuring the behavior of a shock over a given operating range and spitting out a curve. This curve can take three basic profiles; linear, progressive, and digressive.
“We have several different types of dynos here. A normal shock dyno isn’t going to be able to look at the same piston speeds at different frequencies. We use force-displacement and force-velocity mapping for development projects,” Grimes qualified. “One of the key things is to remember that a dyno is an excellent tool, it’s not an exact representation of what a car does on a track. A dyno can provide great information, but we don’t race dynos. We don’t race shaker rigs. We race people in a car, on a track.”
A linear rate damper will maintain a consistent-sloped damping force change throughout it’s stroke and across different shaft speeds, meaning it will change compression or rebound resistance in a fixed ratio.
Progressive-valved shocks are uncommon among road racing and tarmac applications. This profile builds resistance at an exponential rate, displaying like an upward sloping curve. In math terms these dampers “increase at an increasing rate.” Primarily off-road and rally cars will implement this style of damping to allow for compliancy over the high frequency bumps, but still provide that firm bump cushion upon a hard hit or landing.
Digressive profiles are where we start to see more race-bred technology. In this design an initial high damping force provides assertive weight-transfer control, but push that damper further into the stroke to absorb a curb or bump and the valving free’s up to an increasingly soft level of damping.
“Most performance dampers are going to be in a digressive arc, that means the shock is going to be building damping force at a pretty strong rate at subtle motion, but then at a point begins flattening out,” Grimes identified.
Let’s Get Theoretical
One of the most influential variables in shock design is size. The scale of parts used changes the behavior drastically if they are not considered as parts to a whole. For example, the surface area of the face of an internal piston (at the end of the shock shaft and swept through the oil) can be evaluated in much the same terms as resolution.
Piston design varies from manufacturer to manufacturer, and depending on which side of the piston corresponds to compression or rebound. Image source: Race Tech
A piston with greater surface area can detect and affect inputs with much greater urgency. The sensitivity is heightened and adjustments can be made with more precision. This can be taken to extremes, however, resulting in detrimental and harsh feedback.
“The people that make only monotubes will say our shock has a bigger piston, so it’s going to build more information because it has more opportunity over a given stroke with more surface area — however, that also depends on design,” Grimes added.
As we discussed above, gas pressure plays a role in most performance shocks, but there are some very small physics at play here. Gases are compressible fluids, while oil is not — well not on a normal scale.
“There’s a theory on hysteresis and the compressibility of oil. Oil doesn’t compress but there’s going to be some of that at the amounts, forces, and frequencies that we’re talking about,” Grimes alluded.
The easiest way to compare the fluid dynamics at play inside shock absorbers is to think about the plumbing of a turbocharger induction system. “The further you move that piston and compression valving away from the adjuster, the more low speed damping you lose,” Grimes warned. Think of this phenomenon like radio signal degradation over a distance. There is no direct connection between the piston and adjuster valve.
In applications using a remote reservoirs, the divider piston and nitrogen charge can be located further from the actual piston. Image source: Far North Racing
The concern over signal degradation can become more acute when you add external oil reservoirs to the mix. While external reservoirs are required to produce the desired performance in certain applications, they can be misused. As Grimes articulated previously, moving the fluid metering and adjustment further away from the actual force input (the piston) can degrade the responsiveness of the adjustment.
“If you’re going to have a reservoir shock, having a fat rod is going to wash a greater volume of oil down the line; at least you get more signal that way. There is a disadvantage though, there is what is known as ‘line-damping;’ by running it through a tube you’re going to get some restriction, and some inherent friction of pushing the oil,” Grimes warned us.
“A fat piston rod gives you more beam strength for a strut, plus it also helps give you a higher amount of displacement to flush more oil to a remote reservoir for double or triple adjustability.”
Additionally, one of the biggest errors surmised by users of external reservoirs is that they help dissipate heat more rapidly.
The greatest advantage of a reservoir shock is they look damn sexy. -Lee Grimes, Koni
“That’s a major misnomer, the oil is not part of a circulating system, so the oil that’s at the piston is always at the piston, and the oil that’s in the reservoir is always in the reservoir, it’s not like a radiator. If the brakes on your car are overheating do you put a bigger cup on your master cylinder?” Grimes explained.
While the additional oil may raise the thermal capacity of the damper — meaning that more energy can be put into the oil before it changes temperature, it does not aid much in cooling once the fluid is up to operating temperature.
In the end, the consensus is to leave reservoirs to applications where they are mandatory for proper function, like long stroke dampers and larger piston face areas.
“The greatest advantage of a reservoir shock is they look damn sexy,” Grimes summed up.
Adjustability And Smart Shocks
There are different internal designs for adjustment, this CAD representation shows one of Koni’s button-type units.
Adjustability in performance shock absorbers is one of the defining features that can set them apart from the stock units on your car. While the coarse tuning is done well before the damper makes it into consumer hands, in terms of valving, having some control over settings lets you fine tune.
With an initial damper profile set from the manufacturer, we are left to control compression and rebound at the fine-tuning level. On a road racing, off-road, or rally application this makes for many permutations of damping setups. Depending on the varying track conditions day-to-day or the driver preferences, you can influence the overall feel and response of the suspension.
There are shocks that make decisions all on their own, taking inputs from the road and responding accordingly in pre-programed fashion. Magnetic ride control and other high-tech on-the-fly adjustability is making its way into more high-end cars, but on the motorsports level the same technology exists in a more analogue format.
Three of these types of intelligent damping include position sensitive, speed sensitive, and frequency sensitive.
“Position sensitive is often done with bleeds or bypass tubes that allow for a lot of free motion at a certain point. This can be done by positioning grooves in the middle of the sweet spot so the shock is getting a lot of blow-by around the piston,” Grimes illustrated.
Position sensitive shocks are a rare occurrence outside the off-road world, far more common in the road racing sphere are 2-way and 3-way adjustable shocks.
“If you’ve got a single-adjustable shock (rebound), that adjuster is going to be riding the piston and you make your adjustments there. If you have a compression-adjustable shock it only makes sense to put the adjuster on the piston,” Grimes continued. With 3-way adjustable you gain the added sensitivity of shaft speed; “You’ve got subtle motion control when the car is in a transitional situation and you’ve got body motion, but then when you start getting up into higher piston speeds where you start getting inputs and impacts from the road, you don’t want it really firm.” 3-way adjustability makes this possible.
Oil flow is shown on an FSD shock under compression.
The last of our high-tech damper categories is the Koni Frequency Selective Damping (FSD) technology.
“FSD is Frequency Selective Damping, not only is it looking at piston speed but also frequency or duration of a particular input. It’s easier to think of it as the reverse of a turbocharger blow-off valve — you build boost to a point and then release it, this is the opposite. It’s actually open until it closes based on frequency or duration of time,” Grimes explained.
Oil flow is shown on a FSD shock under rebound.
“We use it in Formula 1, there were a number of years we were working with McLaren, at Monza. The answer to get a good drive through the chicanes was to hit the berms, if you could forgive that high frequency impact of the berm the tire settles down more and gets planted, whereas if the tire is still shaking and you don’t have a solid footprint you have to wait a fraction of a second. That’s the kind of thing FSD help filter out.”
On what damping curve does this technology work? “FSD is entirely on the rebound side,” Grimes told us surprisingly.
Top 5 Things To Know About Racing Shocks With Lee Grimes Of Koni
Don’t be afraid, it’s not smoke, mirrors and magic pixy dust. Try something and then try something else to decide if it’s better or worse. If there’s no change try a lot!
You’re always going to be learning. Nobody is ever the absolute guru on the mountain top, it’s a ladder into the clouds. If you’re higher in understanding than someone below, you can certainly help them out but you’ll never know where the top of the ladder is, and if you think you do, you’re probably wrong.
Adjustable shocks are not that difficult, but understand what does what. What is it really affecting? Rebound controls the transitional or sprung-weight of the car — that’s your understeer/oversteer balance. Compression’s job is to control the unsprung weight of the car, it’s the tires’ ability to hold onto the ground. You want to use compression to maximize grip between the surface and the tire. Sticky tires and a relatively sticky surface? Add more compression until you find the edge of adhesion. Medium tire and bumpy surface? Back off the compression, because if you overstep the car is going to skate around. If you need more oversteer don’t change your compression because that affects your grip, you use rebound to affect your balance.
Fine tuning is always done at the shock, the tire is number one. The biggest way to make an improvement is the tire – if you put great suspension in a car with bias-ply recaps on there, you’re not going to do any good for yourself. You need to work all the way up stream. If you make all your shock adjustments and it’s not having an effect, maybe you’ve got a spring issue.
Be careful following other peoples’ advice. What may be a good setup for one person might be a different setup for another person. If the car and suspension setup gives the driver confidence, he’s going to get consistently what he needs lap after lap. Then he can push his limits.
What level of damper technology do you need? It’s temping to make your setup overly complicated in the name of flashy parts. Understand what you’re dealing with and build a setup systematically. Don’t be afraid to delve into the tuning side of things, but consult experienced racers or manufacturers for advice. After all, be thankful that the engineers did most of the work for you already with the initial design and construction of your dampers. Like making marks on a painter’s canvas, every adjustment you make will either add or subtract from the end performance.