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Shock Testing: Rebound and Compression in the Real World
Shock Therapy: Choosing the right shocks for your Mustang
Have you ever noticed when searching the online blogs that there are those subjects in our hobby where everyone has an opinion, and yet they don’t know why their opinion exists? Which oil, which tires, which paint, you name it—everyone knows exactly what you should and should not be using. Their opinion is generally based on what they read 20 years ago, and by golly they will not be swayed. I don’t know about you, but it drives me crazy. In God we trust—all others must have data.
So we were in the middle of a full installation of a Street or Track tubular coilover suspension when they informed us that not only does their system improve performance (which we expected), they also claim their system improves ride quality (which we didn't expect). We are from the old school, and that means improved handling comes at a price—the ride quality goes down, not up. The classic example is the 600 lb-in coil springs Shelby installed in the early cars. Once you put those in, it will never ride better than a 600-pound coil spring. So we went back to Street or Track and asked if we could put their claims to the test, to which they didn't flinch one bit.
We decided to open up the test to several different options for your car based on current availability and popularity. As in all product lines, different shocks are designed to perform differently, and not all drivers like the same “feel” or ride quality. Some drivers like the feel of a soft, comfortable ride and some like the assurance of a firmer ride. So “ride quality” is a personal choice rather than a firm definition. We hope to help you understand what some of the different shocks can do and how they relate to each other, and to see how the shocks used in the Street or Track kit fall in the line with other shocks on the market.
For some of you this is going to be a review, but some don’t actually know what the shock is doing in the suspension system, so let’s go over the basics.
The shock absorber is used to dissipate the energy from the springs in the car. Left un-dampened, the spring would undulate back and forth like one of those kid rides at the park. This is caused by kinetic energy. To dampen the spring, the shock absorber has to convert kinetic energy into another form of energy: heat. This heat is then dissipated out of the shock through the hydraulic fluid in the shock. The shock achieves this by pushing hydraulic fluid through a valve when the shock is pushed in (compression), or when it is pulled out (rebound). It is how the shock manufacturers control the compression and rebound that gives the shock absorber its ride characteristics.
The way the shock valve works defines the characteristic of the shock. Although there are several ways the valves can work, it is basically restricting the flow of hydraulic fluid from the upper and lower parts of the shock. Basic physics—the smaller the valve, the harder it is to shove fluid through it, and more friction is turned into heat.
Compression and Rebound
Compression of the shock occurs when the suspension is driven up by a bump, a pothole, or any road variation. The more compression the shock has, the more resistance to movement, and the firmer the suspension is against the road. Like the 600 lb-in springs Shelby used, it takes more force to compress the suspension, and the more control you have. With rebound, the firmer the rebound, the slower the shock is going to allow the spring to return to ride height, which also affects the feel of the shock.
Some of you drag racers may remember when 90/10 shocks were popular on a drag car. The drag shock had a 10 percent resistance on rebound, which meant the car’s nose came up immediately when you hit the gas, shifting the center of gravity backward toward the rear wheels. The 90 percent compression meant the front end came down very slowly, which kept the center of gravity toward the back of the car.
So what does this mean for ride quality? The more compression and rebound, the less the damper is going to allow movement of the spring, and the less movement, the more stable the car is in harsh conditions. The tradeoff is a less desirable ride for city driving. Going back to team Shelby, they chose a Koni adjustable shock back in the day—not because the Ford shock was bad, but the OEM shock was chosen to please a wider range of drivers, whereas the Koni was used in a high-performance vehicle to please a narrow range of drivers.
Most shocks are designed exactly the opposite of the 90/10 drag shock; you want an acceptable amount of compression and more force on rebound. The more compression, the harsher the bumps, but the harder it works in the turn. The more rebound, the slower the shock will return to ride height, and the more it will smooth out the ride; in a turn, it will help reduce some of the roll on the inside wheel.
One final thought: The shock is directly related to the other suspension components. The 600 lb-in coil spring was a quick fix to an existing problem. Today, new performance cars use a lighter spring with an adjustable shock or strut. That way if you don’t like it you can always put it back the way it was. Once the heavy spring is installed it won’t ride any softer. If you are thinking of a performance upgrade for your vintage ride, look for a system that has truly been engineered to work together, not just a bunch of off-the-shelf parts put together as a “handling” kit.
Oil vs. Gas Shocks
There is a lot of misinformation on the Internet, and some of it comes from the manufacturers themselves. Here are the basic facts about oil and gas shocks. All shocks are a hydraulic-style shock; some of them use oil and some of them use gas. Oil shocks are what were available to Ford in the 1960s. Almost every shock you will buy today is a gas shock. When a shock compresses, it loses volume, so there is a small area filled with nitrogen gas or air that can compress or expand as the hydraulic fluid requires.
Shock manufacturers have discovered that by using nitrogen gas, it helps keeps the aeration of the hydraulic fluid down. When hydraulic fluid heats up rapidly, it can cause bubbles in the fluid and this will affect the performance of the fluid. Nitrogen gas is inert and does not heat or expand like atmospheric air, and it keeps the bubbling from occurring. (Racers use it in their tires because the pressure doesn't change.)
Even though most of your choices today are gas-charged shocks, the manufacturers can use the gas to “stiffen” the shocks by injecting more gas and increasing the pressure in the shock. Gases and liquids can only be compressed so much, and by increasing the pressure of the gas, they can prevent the oil from filling and compressing the gas any further, thus “stiffening” the shock with the gas, rather than by changing the valving.
The way the industry tests shocks is with a shock dynamometer that measures compression and rebound. It does this by measuring the force required to move the shock piston at different velocities.
We went to Advanced Racing Suspensions (ARS) to do our shock runs on their dynamometer. ARS builds custom racing shocks for almost every racing venue you can think of, and its customers frequently win. We wanted an expert, unbiased opinion on our shock selections.
What the charts show us is how much force is required to move the shock at a given velocity. When the shock is at the very top or bottom of the cycle, it is still and has no velocity. Like an engine piston, the movement up or down will increase the shock velocity. The faster the shock moves, the more force is required to push the hydraulic fluid through the piston valve to fill the chamber. The lower line on the chart is the velocity in inches per second, up to 6 inches. The side bar is the amount of force required to move the piston in pounds. Compression is noted on the top side of the chart and rebound is on the bottom.
Corey Fillip with ARS confirmed that what you are looking for in a good street/track setup is a shock with mild/good compression and lots of rebound. Let’s take a look at our set of test shocks.
We selected four different shocks to test: A Monro-Matic Plus purchased from the local auto parts store to serve as our “baseline” shock, as it is probably closest to the OEM shock originally installed on our car; The Monro-Matic “Heavy Duty,” which was a used shock with about 2,000 miles of use, as we wanted to see how it performed after a couple years of use; a brand-new KYB Gas-a-Just shock that we keep in stock because we use and recommend them on my customer cars; and the Bilstein coilover used in the Street or Track tubular coilover conversion kit we are installing on our 1968 test car. A wide variety of shocks, and we expected a wide variety of results.
The Monro-Matic Plus
The Monro-Matic (PN 33059) was chosen because we felt it would probably be very close to the OEM ride and feel, and this is what was sold at the local auto parts store. In the test, it ran just about where we thought it would be, and this shock should be considered if you want a smooth, comfortable ride in your classic Mustang. The compression and rebound are good and are just right for a nice, smooth ride.
We decided to test this one for a couple of reasons. We wanted to see what years of driving and sitting in the winter does to a shock, and as for the “Heavy Duty” moniker—was that simply a selling point or did it actually perform better than the “Plus” shock? We did notice that the piston rod itself was slightly bigger than the Plus shock, but we don’t know if that directly related to performance. We also need to note that we don’t believe Monroe lists the heavy-duty version (PN 20767) in its current lineup, but they may still be available. We were pleasantly surprised that the HD Monroe performed stronger in both compression and rebound, even after four years of use. The HD would give you a slightly firmer ride than the Plus version. Chalk one up for Monroe for both good ride quality and a well-built product.
Your author has recommended this shock for years to my customers, and I wanted to see how it would perform on the dyno. This is a mono-tube shock and is much stiffer than the Monroe shocks, and the baseline force that it has on the spring is triple that of the Monroe shock (27 lb versus 9 lb). The KYB scored the highest in compression and slightly higher than the twin-tube shocks in rebound, which means it is going to have a very firm ride. This is a good choice to improve the performance of either a stock spring or a firmer front spring in the original configuration.
Why we were here in the first place: to test the Bilstein coilover shock included in the Street or Track tubular suspension system. What we were looking for was good compression and lots of rebound to get the best combination for handling and ride quality. And that is exactly what we found. In the words of Corey Fillip from Advanced Racing Suspensions, “This is the shock I would pick if I were building a street/track car.” We’ll take the shock expert’s word for it.
So which shock is right for your car? It really depends on the type of driving you expect to do with your Mustang. The good news is that we feel that all four of the shocks did as we expected them to do, and depending on what you plan to do with your car, you can make an educated choice from these shocks.
Street or Track also sells a Bilstein shock designed to work with the original suspension in street, sport, and race valving. So if you are not ready to step up to the full coilover setup, a Bilstein is available for your car in its original configuration. Street or Track has done its homework on their coilover system and have several different valving solutions for whatever your driving needs are. We’d like to thank Greg Roe of Valparaiso, Indiana, for allowing us to rip into his ’68 fastback for this article.