In the third and final instalment of our ‘Back to School’ handling guide, found in Issue No. 209, our resident expert Gareth Court takes a close look at shock absorbers: what’s good, what’s bad, how they work, and, most importantly, how to make them work for you. If you missed the first two instalments of this feature, make sure you pick up Issues No. 207 and No. 208, which deal with alignment, to gain the ultimate understanding of your car’s handling systems.
Mono or twin?
Let’s look at the types of shock absorbers and what their internal components are.
Shock absorbers can be simplified into two main types: twin-tube and mono-tube. This simply describes the number of working cylinders inside the shock absorber.
We’ll start with twin-tube shock absorbers, as they are the most widely used and commercially available. A ‘twin-tube shock absorber’ consists of an inner working cylinder, an outer reservoir which normally forms the outer shock body as well, a piston rod and piston, a foot valve, a rod guide, and finally a top seal.
A ‘mono-tube shock absorber’ is just that — a shock with only one working cylinder that usually forms the outer body of the shock. It consists of very similar components to the twin-tube shock. It has one working cylinder, a piston rod and piston, a floating piston, a rod guide, and a seal.
Both styles have their ups and downs. The twin-tube shock is very resistant to damage because of the outer reservoir. It is cheap to manufacture, but because of the two tubes, bore size is limited. Mono-tube shocks, on the other hand, have fewer components but a larger piston and more oil. They are also susceptible to damage, and when the body of the shock is dented, it’s all over.
A lot of people think an inverted mono-tube shock has a large piston rod; what they are actually seeing, though, is the body of the shock, because the shock is upside down.
Both shock absorbers exploit exactly the same hydraulic principles but in slightly different ways.
When the piston rod is compressed into the body of a shock absorber, two things happen. Because oil is not compressible, it has no choice but to flow through the small holes in the piston, which is where some of the hydraulic resistance is generated.
The piston rod and piston assembly also possess a physical volume, which will displace the oil inside the shock absorber. This amount of displacement is proportional to the amount of piston rod inside the working cylinder. At full compression, the displacement is at its maximum, and at full extension it is at its minimum.
A good example is if you fill a bucket of water right to the very top, then put your arm in the bucket. Straight away you will notice that the bucket overflows because of the volume of your arm. When you take your arm out, you will notice the level of the water has lowered quite a bit. Yet, if you put your arm back in the bucket, the water will rise to the top but not overflow. This same principle applies to the shock absorber. If you fill it to the top and seal it off, when you try to push the piston rod into it, you won’t be able to. So, therefore, inside the shock, we need an area where there is compressible media to allow for piston-rod displacement. There are different ways of doing this. The two main compressible medias are gasses — one is the air we breathe, and the other is nitrogen.
Air is OK and has been used for many many years, however, it does posses one big problematic property: it is highly affected by heat, and seeing as we are converting kinetic energy to heat energy, this is not ideal. It is also prone to cavitation (mixing with the oil). Nitrogen, on the other hand, is an inert gas and heat has a minimal effect on it, so its reactions inside the shock absorber are more consistent.
In a twin-tube shock absorber, the gas is above the oil in the outer reservoir of the shock. It is kept well away from the pistons. If it were to mix with the oil and then pass through the valves, it would create a loss of resistance. In a mono-tube shock, the gas is completely separated from the oil by a floating piston. This set-up is ideal, as it almost completely eliminates cavitation.
Now, seeing as we are talking about the gas inside shock absorbers, we thought we would dispel some common myths.
First, the big one: yes, gas shock absorbers still have oil in them. The only difference between a hydraulic shock and a gas shock is what we described earlier in this article. Both use oil to provide hydraulic resistance.
No, a gas shock absorber will not hold the vehicle up — the pressure of the nitrogen inside all modern shocks will never be enough to support the weight of the vehicle.
Gas shock absorbers are not firmer nor harder riding because of the gas. The harder ride is because most aftermarket gas shock absorbers are usually uprated in damping force for performance benefits, not because of the gas or gas pressure.
The only thing that has an impact on the resistive force of the shock absorber is the weight or thickness of the oil and design of the valves in the piston or pistons.
How it can affect your car
So let’s take a look at how the piston or pistons function. The main piston does the bulk of the work in both twin-tube and mono-tube shock absorbers. When the piston rod is compressed, oil will flow from one side of the piston to the other. It flows through orifices in the piston. Some of these orifices are just little holes, and others are holes covered with spring steel shims that open at various rates of oil flow. In a twin-tube shock, oil will also flow through the foot valve into the reservoir tube; this foot valve behaves the same way as the main piston. In a mono-tube shock, the main piston is the only piston that restricts oil flow, so its valves do all the work.
There are three stages of damping — high speed, medium speed, and low speed. This doesn’t refer to how fast you are driving; it refers to how fast the piston is travelling inside the shock absorbers.
High speed is usually attributed to short, sharp impacts like hitting a pothole, while low and medium speed are where all the main work is done. These stages control the kinetic energy of the spring, but, due to the nature of hydraulic resistance, these two stages will have a large impact on body roll and vehicle stability. The majority of modern shock absorbers use ‘velocity-sensitive damping’ — this means the faster the piston velocity, the more resistance the shock absorber will provide. Because of velocity-sensing technology, the majority of modern shock absorbers are self-adjusting, in the sense that they use the different stages of the valving to suit different road surfaces. As we mentioned earlier, the extra benefits you get from velocity-sensitive damping are more than just controlling the kinetic energy of the spring. You also get improved resistance to body roll, bounce, dive, and squat.
This is where all the performance gains come from, and changing the shock valving to change certain aspects of vehicle handling is extremely beneficial. That is, when it is done correctly. Just like anything else within the suspension system, changes to the valving of the shocks can have massive consequences, both good and bad.
You can increase or decrease:
- body roll
- dive under brakes
- squat under acceleration
All these changes will have an impact on understeer, oversteer, how a car responds under braking and acceleration, spring control, and axle tramp.
A rear-wheel drive car that is set up for drag racing will typically have shocks valved to maximize weight transfer on launch from the front of the car to the rear. This is done by having front shocks with very little resistance when they extend (rebound), and a lot of resistance to compression (bump). That allows the front of the vehicle to effectively transfer weight from the front towards the rear where it is most needed. At the same time, the shocks in the rear are typically the opposite. They will have very little resistance on bump and lots on rebound; this way, when the vehicle launches and squats, it’ll do so with little resistance, and then the large amount of rebound resistance will try to hold the vehicle in this stance. This style of shock set-up can yield very good results in drag racing; it greatly assists in transferring weight, and most good shock builders can achieve this easily. That being said, it is only suited to drag racing and is no good on the street or circuit.
A circuit car is a completely different story; the valving of the shocks will have many other factors dictating the set-up for circuit racing. Things like the spring rates, sway bar rates, type of tyre, and even type of driver and track will have a huge impact on valve codes. As a good starting point, most of the commercially available shocks will be geared towards this style of racing, so will more than likely work well until it starts to get serious. The adjustability that is built into them will usually be enough to cater for different track conditions and driving style. A good general rule of thumb when setting up adjustable circuit-style shocks is that the stiffest part of the car will break traction first. The best way to describe this is with understeer or oversteer: if the front of the vehicle is too hard, it will understeer and if the rear is too hard it will oversteer. So, if the vehicle is oversteering, softening the rear will help, and the same for the front — if the vehicle is understeering, softening the shocks will help. In the wet, sometimes a slightly softer overall set-up is required.
In an extreme four-wheel-drive scenario, it is not so much about how hard or soft the shock is; it’s more about how much travel it has and how well the valves control the flow of oil. In this case, large bores with big pistons, big piston rods, and high oil capacity will be a minimum requirement.
In all these scenarios, the shock absorber is a highly beneficial tuning tool. But they are still just a small part of the set-up. Sway bars, tyres, springs, and chassis stiffness all need to be taken into account at the same time. It is a well-balanced entire suspension system, not just shock absorbers, that will yield the best results.
What do I need?
Adjustable shocks come in many different shapes and sizes. Some only allow you to adjust the rebound; some adjust both bump and rebound at the same time.Some are easy to adjust and some need to be removed from the vehicle. Some are upside down to change the amount of unsprung weight (inverted mono tube).
Talk to a suspension specialist before handing out your hard-earned cash. It pays to be honest with them about the intended use of the vehicle, as this way you will get something suited to what you want to do.