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Good Handling Is Fun: Inside the Ups & Downs of Dampers

Topic #10 from the "Good Handling Is Fun" series of MM newsletters.

Welcome to the tenth installment of our Good Handling Is Fun series! Read on for more expert advice on transforming your Mustang into a high-performance handling machine. This week's topic is all about dampers, one of the key elements to creating a good-handling Mustang. (If you missed the previous episodes, they're posted on our website in the FAQs & Tech Tips section.)

Please note that what we're talking about here are dampers—not dampeners. Often misspelled and mispronounced as "dampener," "damper" is the correct term for today's subject, high performance struts and shocks for your Mustang. For the record, a dampener is something that makes something else damp, i.e., moist. If your dampers turn into dampeners you need new dampers.

OK, it's a damper. Why is it called that?
"Damper" comes from the primary function of a shock or strut, which is to control spring oscillation by damping, i.e., calming it down or reducing it. After an initial compression or extension, a spring will vibrate for quite some time unless something damps the oscillation. A damped oscillation is one that fades away to zero over time. A damper, in the case of a late model Mustang in the form of a strut or shock, damps the spring's natural tendency to oscillate. When the strut or shock is well matched to the spring it controls, the spring oscillation will damp to zero very quickly, usually in only one complete oscillation cycle. If your Mustang had no dampers it would bounce uncontrollably while driving down the road, as hitting a succession of bumps would add new spring oscillations on top of the slowly fading oscillations of previous bumps. Not a sexy look, in our opinion, and downright dangerous.

What's the difference between a shock and a strut?
While a shock is primarily a damper, it may also have secondary functions. For example, a shock may also be used to limit suspension droop travel, to limit suspension jounce (bump) travel, or to mount a spring (commonly called a coil-over design).

On the other hand, a strut has an additional function over that of a shock; it's an integral structural link that helps define the suspension geometry. If you remove a strut from the car's suspension, the wheel will flop about as the geometry of the suspension is no longer completely defined. This cannot happen when you remove a shock from the suspension. Struts are made quite robustly because they are structural links and must withstand high bending loads from cornering and braking.

All 1979 to present Mustangs left Ford's assembly line with a strut front suspension and shocks in the rear suspension.

How a damper damps
The suspension springs and the car's mass form a resonant system; think of a guitar. Once activated by a bump, like plucking a string, the spring (string) and mass (car or guitar body) will continue to vibrate for a long time unless the energy is more quickly absorbed by a damper. On a guitar, you can stop the vibration by putting a finger on the string you plucked. In a Mustang, the damper works something like that finger to stop the spring from oscillating.

Give me some details on this spring oscillation thing.
Here's the big picture of how a damper does its work: it converts kinetic energy (motion) into heat. In the case of a Mustang, the hydraulic dampers have a piston inside an oil-filled tube. The piston moves up and down as the suspension moves. The piston motion forces hydraulic oil through a number of small holes. As the oil moves it heats up from friction, which dissipates the kinetic energy.

There are many different damper designs. Some of the differences are large, such as monotube vs. twin-tube, gas charged or not, high-pressure gas vs. lower pressure, adjustable damping or fixed. Other differences are small, and include a myriad of little design differences affecting a damper's performance and effectiveness.

What else does a damper do?
Dampers help keep the tires' tread in contact with the pavement. As every Mustang (and driving) enthusiast knows, that's pretty much the most important thing! They control how quickly weight transfers when braking, turning into a corner, and while accelerating (whether out of a fun curve on your commute or during a drag strip launch).

Dampers produce a force based on how fast the shaft moves, so the amount of damping produced is somewhat proportional to the shaft velocity. The damper only produces a force when it's moving. Good dampers will alter the amount of force according to the shaft velocity. The specifics of the valving design and resulting damping at different velocities affect ride quality as well as performance.

How much damping do you need?
The exact amount can be difficult to quantify—and more difficult to achieve—especially over a range of shaft velocities. Too much damping reduces tire grip and makes ride quality worse. Too little damping leads to vehicle instability because the spring oscillations are uncontrolled. Since both overdamping and underdamping reduce tire grip, getting it right is quite a trick!

If you're up for some technical stuff, in general, the suspension needs more damping forces at low shaft velocities. More damping is required during turning, braking and accelerating, when the velocity of the shafts in the dampers is low (1"-3"/sec). When the car goes over sharp bumps, the shaft velocities are going to be much higher (10"-20"/sec). To maintain decent ride quality, higher velocities require lower damping forces.

Different Damper Designs
Monotube dampers
This design has a single tube mostly filled with oil. A piston rides inside the tube, with the oil on each side. At one end of the tube, separated from the oil by a second piston, is a section containing a high-pressure gas.

The damping is accomplished by the piston moving up and down in the oil, driven by a rod extending outside of the exterior tube. Oil moves through the piston, allowing the piston and shaft to move inside the tube. The oil's movement is carefully controlled by various holes and valves. The piston design and its valves and orifices are what determine the damping characteristics of the strut or shock. Kind of an amazing engineering feat!

The high-pressure gas keeps the oil from cavitating (creating gas bubbles in the oil) in a not-wanted process called foaming. Foamed oil has a much lower viscosity than bubble-less oil and significantly reduces the damping action.

Twin tube dampers
This design has two tubes, one inside the other. The piston rides inside the inner tube. The space between the inner and outer tubes serves as a reservoir for the oil displaced when the shock shaft moves in and out of the damper body. This reservoir can also contain a separate low-pressure gas reservoir. A valve connects the inner cylinder to the outer (reservoir) cylinder.

Just as with a monotube damper, the piston has valves and holes to control oil flow. That determines the damper's characteristics, or how much it damps.

Monotube vs. Twin Tube

Click on the image for a larger version.

Which is more effective?
Ah, good question. Monotube dampers have better heat dissipation than twin tube dampers because the oil doesn't have to go through as much material and distance to radiate heat into the outside environment. As the damper does more work, the temperature of the oil increases, which lowers its viscosity, which reduces its damping ability. This gives monotube dampers an advantage in high-performance applications, where the damper is worked harder.

Monotube dampers also have larger pistons that allow more fluid to flow through the valve for a given amount of suspension travel. This allows the piston valves to create a damping curve with a better tradeoff between ride quality and handling.

Pro Tip: Dampers have a very large effect on transient handling and ride quality. In fact—and most people don't know this—they have far more effect on ride quality than springs do!

Two other factors to consider: rebound and compression
Compression
This is when the spring and damper compress, when your Mustang hits a bump, or on the outside of the car when the body rolls during cornering. The spring and corresponding damper compress, allowing the suspension to absorb the bump. As the unsprung mass (tire, wheel, and suspension) moves upward toward the chassis, the damper turns this kinetic energy into heat so the chassis doesn't oscillate in an uncontrolled way in response to the bump. Picture the old beaters you've seen in grocery store parking lots heaving and bouncing up and down as they hit speed bumps. Good dampers prevent that. The compression valving in the damper is designed to keep this oscillation under control.

Rebound
This is when the spring and damper extend in situations such as after compressing to absorb a bump, or when the tire drops into a depression, or on the side of the car toward the inside of the turn when the body rolls during cornering. The unsprung mass (suspension, tires, and wheels) is moving away from the chassis of the car. When the suspension is moving in this direction, the damper is damping the kinetic energy stored in the sprung mass, turning it into heat.

Either way, during compression and rebound, good dampers damp vertical chassis movement to keep spring oscillation to a minimum, keeping the tires on the road and you in control.

Adjustable Dampers
Adjustable dampers have a provision to allow adjusting their damping force level. A damper labeled "SA" has one adjustment (single adjustable). A damper labeled "DA" has two adjustments (double adjustable).

There are two types of SA adjustment. With some dampers the adjustment changes only the amount of rebound force (typical of Koni). With other SA dampers, the adjustment changes both rebound and compression forces at the same time (typical of Tokico). Note: these settings are paired, not independent. With DA dampers, one adjustment is typically for rebound forces and the second is for compression forces. With DA dampers the adjustments are independent of each other.

Adjustable dampers can offer an advantage by letting you tune the transient handling behavior of the car, like if you have a weekend autocross that you're driving to and from. If you know what you're doing, you'll want softer settings on your way to the event to avoid spilling your morning coffee, and more performance-oriented settings while on the course.

However, this type of tuning requires knowledge and time to learn when and how to adjust the dampers. You need to determine what handling behavior you want to change, and then know what you should adjust, at which end of the car, and in which direction. That requires a skill set that many enthusiasts don't need for how they enjoy their Mustangs. Often, fixed dampers are a better solution because they can't be misadjusted, and they just need to damp the springs you're using with them.

Why MM carries only certain dampers
There are many dampers available for late model Mustangs in a wide range of prices, quality, and features. Here at MM we're interested in improved performance, so we ignore the stock-replacement dampers, like those you'll find at your local auto parts store. Among performance-oriented dampers the old saying "you get what you pay for" holds true, but with some surprising exceptions: some manufacturers offer "high-performance" dampers that don't provide anywhere near the performance level advertised. Have no fear: you won't find those on the MM website.

That leads us to why we carry the dampers we do. Our primary requirement is excellent quality, of course, but we also look for whether there are versions suitable for street use and versions for track performance aimed at the amateur racing-on-a-budget enthusiast.

Two favorites, Koni and Bilstein, are excellent for street-driven Mustangs and those taken to open-track events. But many years ago we discovered their limitations for top-level amateur road racing. That moved us to develop our own line of MM Dampers based on Bilstein internal components. With our own dampers, we can offer several versions, each tailored to different uses and their own appropriate spring rates, plus additional features the big manufacturers don't offer.

Note: We also like Tokico. Their D-spec and Illumina series are excellent for street & strip Mustangs. However, they left the U.S. market and the remaining selection of new Tokico dampers is sadly scanty.

And now, back to why MM Dampers are awesome. Seriously.
Damper design isn't easy; if it was, then all manufacturers would be making great products. In reality, there are more sub-par dampers on the market than good ones.

We put a lot of development time into our dampers. Designing and building a good damper requires a ton of work, and they're by far the most difficult part of the suspension system to get right. If your dampers are bad, they can make an otherwise great suspension design handle and ride like an ox cart.

For a damper intended for street use, there's always a tradeoff between good ride quality and good handling. Small differences in the damper design can skew the results too far in one direction, or even ruin both ride quality and handling.

When developing damper valving, the MM engineering team tests prototypes on a damper dynamometer, commonly called a "shock dyno." This machine extends and compresses the damper at varying velocities, while measuring and recording the forces created by the damper. Once we're happy with the dyno behavior, the damper goes onto a Mustang for real-world ride and handling testing.

Here's a generalized example of how the results of a shock dyno test look:

Dyno Curve Sample

Click on the image for a larger version.

For on-car testing, we use a sophisticated Motec data logging system. This system has suspension travel sensors on all four corners, accelerometers for all three axes', a yaw sensor, and sensors for all the other basics such as throttle position, wheel speed, etc. We analyze the logged data to help design the next iteration of damper valving. Then it's back to the shock dyno. We repeat this process until the results meet our design goals for the ride and handling characteristics we're looking for.

Most MM dampers have fixed valving (check out the dampers listed for your Mustang to see our current offerings). This best suits the needs of most of our customers. Even for many in competitive road racing, the fixed valving provides the best results. Other than matching the MM series to your chosen spring rates, there's no need for any fiddling with damper tuning at the track (or on the street). That lets you concentrate on driving and improving your skills, rather than being a test driver.

You need a unique skill set to be a good test driver, including being able to evaluate the car's handling behavior to decide what changes to make to the dampers, if any. With adjustable dampers, decisions need to be made:
  • Can better handling be achieved by adjusting the dampers?
  • Should compression be increased or decreased?
  • Should rebound be increased or decreased?
  • Should those changes be made at the front, or the rear, or both?

That's a lot of decisions for an amateur racer. If you're up for it, MM does have very sophisticated high-end DA dampers for some applications, with more to come. At a more modest price level, we recommend Koni SA or DA dampers.

Matching dampers to springs
We've only been talking about dampers, but they're really just half of the important "springs and dampers" component of making a Mustang handle really well. Back in Episode 5 of this series we talked about springs in depth. We always recommend you choose the springs for your Mustang first—then get dampers to match. To help you make decisions we have a popular and handy Guide to Choosing Spring Rates and Dampers. The next step is contacting our Tech Associates for advice. For more advanced help, we have a Technical Services option to evaluate your situation and recommend springs and dampers just for your Mustang; choose the "Spring Rate" option.

Thanks for reading! Over the last 10 weeks we've presented the essentials of setting up a Mustang suspension that works really, really well. We hope you know now that elevating your Mustang's handling to being tremendous fun to drive in the twisties is a goal you can achieve.

If you've missed any of our popular Good Handling Is Fun newsletters or want to forward them to a friend, you can read them all on our FAQs & Tech Tips page. Meanwhile, MM Tech Associates are standing by to help you get going on your project list to achieve maximum fun in your Mustang! More to come soon from your favorite car nerds at MM.