Project K9 Gets “Shocked” With A Bilstein Modified Shock Package

Our project car needed some serious chassis setup help.  The previous owner adopted a policy of “all things are created equal” and had mounted a different type shock on each corner of the car. Without doing some serious and time consuming testing to verify if the existing setup was really the best way to go, we decided to call one of the most revered names in modified racing and get his input. Rex Merritt gladly listened to us and gave us some direction in getting a decent starting point and guidance in tuning our chassis to the dirt.

Because the tracks in our area start off pretty choppy and end up with polished corners by the end of the night, Merritt recommended that we opt for a monotube type shock.  In the interest of full disclosure, Merritt works as a shock and spring specialist for Bilstein Shock Absorbers, so a person may get the impression that he is biased on shock manufacturers, yet looking at the number of wins that Merritt has amassed in his career, he obviously knows enough to help us get pointed in the right direction.

About Rex Merritt

Rex has over 500 career feature wins in over 30 years of racing. Accomplishments include over 36 Track Championships including the 2008 Lucas Oil Speedway Modified Crown, and Championships in IMCA Modifieds, NASCAR Modifieds, Dirt Late Models, and Asphalt Late Models. Rex is a 3 time “World Series of Asphalt” Winner in New Smyrna Beach Florida, IMCA Asphalt Supernationals Champion, 3-Time Winternationals Champion, 3-time IMCA Supernationals “Race of Champions” Winner, and IMCA Iron Man Award Winner. Rex has been featured in many motorsports magazines and has been featured on ESPN, SPEED Channel, and Prime Network Racing television shows. This talented veteran knows what works on the track which is what led him to become a Shock and Spring Specialist.

What does our project car need?

Dirt track racing has become very competitive and one of the few ways to get an advantage over the rest of the field is a good chassis setup and chassis tuning program. Merritt explained that; “Your choice in shock is pretty much dictated by the track’s surface and condition.”

After a session of Merritt asking a series of questions like: what our track’s soil was like, if the surface was flat or banked, and how rough the track started out and ended up, we got down to the finer points of shock selection.

“What are the shock rules in the class your are competing in?” Merritt asked. “Shock selection can be impacted by the rules of the class.  For instance, the IMCA Modified rules require a steel bodied, non-adjustable designed shock.” He added.  A spec type shock rule cuts down on the options that racers have when selecting a shock package.

Some sanctioning bodies also have shock claimer rules. The thought process behind these type of rules is to keep a better funded team from buying exotic equipment and outclassing the budget racers in the division.

We were fortunate.  Our local track’s Sport Mod rules require a Steel bodied, non-adjustable shock, but did not have a claimer rule.  So we could afford to select a high end shock package without fear of losing our shocks on the tech pad under a claim.

The local track rules simply state: “One steel, nonadjustable, non-rebuildable, sealed, unaltered shock per wheel. No bumpers or stops. No threaded body, front coil-over, air, or remote reservoir shocks. No Schrader or bladder type valve allowed. Front half may be shielded.”

Merrit strongly encouraged us to go with Bilstein’s Dirt Modified set (Part #F4-SE7-G696-M0).  This kit includes 10 of Bilstein’s SLS series shocks and instructions for dialing in your chassis at any track.  Originally the kit was designed for 4 bar dirt Modified race cars but works well on Stock Car/Hobby Stocks, Dirt Modifieds and Dirt Late Models.  Merritt explained; “The SLS series with it’s monotube design allows for dampening rates that are almost twice the rate of twin tube shocks.  This makes a big difference on rough tracks. People say that the monotube offers less protection from dent damage, but Bilstein’s tube wall thickness is strong enough that it takes considerable force to damage the shock tube. If you dent a Bilstein shock tube, you probably have bigger problems to worry about than limited shock travel.”

Anatomy of Bilstein’s monotube shocks

For being an incredibly simple component, shocks are very effective at what they do. Selecting the right shock for your application is critical in getting the most out of your suspension. According to our expert, Rex Merritt, “Energy from the piston moving inside the shock turns into heat. When the oil gets too hot, it can allow bubbles to form and then your shock’s effectiveness starts to fade.” Merritt went on to explain,”Shocks must be engineered to dissipate that heat to ensure consistent performance.”

To keep shock fade and foaming from happening, Bilstein uses a high internal pressure system. A common analogy used to explain how internal pressure systems control boiling and foaming is an automobile’s cooling system. Have you ever noticed that a radiator doesn’t boil over until you open the cap? That’s primarily because the system is pressurized and that pressure increases the coolant’s boiling point. Shock absorbers work under this same principle. Shock fade is controlled by gas charge and tube design.

The primary components in the Bilstein shocks are similar to other shocks, consisting of mounts, shock tubes, pistons, piston rods, valves and oil. Where the differences occur is in the construction and design of the shock itself. A basic shock configuration consists of these parts:

  • Mounts – The shocks must mount firmly to your chassis or it serves no purpose. On dirt track modifieds, most sanctioning bodies require a rod end type mount. The bearing in the rod end allows for radial movement and temporary misalignment conditions.
  • Tube Cylinder – The tube or cylinder is the tough outer housing for the shock’s intricate inner workings. The size of the cylinder is one of the most important factors in the overall performance of the shock. A larger cylinder means a bigger piston and more oil, and when it comes to shock absorption, size matters.
  • Piston Rod – The piston rod is always at work when the vehicle is in motion. Energy from the springs transfers directly to the piston rod which pluges the piston into the oil to dampen the bounce.
  • Working Piston – The piston mounts at the end of the piston rod and creates the hydraulic dampening pressure that smoothes out the oscillation effect when the piston rams into the oil. The diameter of the piston matches the tube so that oil cannot flow around the piston and must go through the valve.
  • Valve – The valve controls the flow of oil from one area of the shock to another. Then the piston rams into the oil, the oil is forced through the valve in small amounts at a time. The movement of the oil is what dampens the springs energy. Valves that allow more fluid to pass through in compression or rebound are called split valve shocks.
  • Oil – Most manufacturers use a highly viscous oil that resists the piston’s motion and absorbs the kinetic energy caused by the piston plunging into the oil. This motion is transformed into heat and dissipated to other parts of the shock and eventually to the ambient atmosphere.

Bilstein's newer SLS series shocks are tailor made for the IMCA type Modifieds. 7 inch fronts and 9 inch rear steel body, non adjustable shocks.

Linear style valving

Bilstein’s Dirt track shocks are available in two basic valve design types: Linear or Digressive.  The SLS series uses linear style valving.  As described earlier, valving is all about flow.  The differences between the two styles of valves are listed below:

  • The linear valving has a high flow rate at low shaft speeds and therefore, lower resistance. As the shaft speed increases, the resistance increases.  The rate of the shock continues to increase as long as the speed increases.  On dirt tracks with a moderately decent surface, the linear piston shock works extremely well.
  • The digressive valve design has a low flow rate at low shaft speeds, providing a lot of resistance and control. As the shaft speed increases, the resistance rate increases to a designed level and then tapers off. If the shaft speed continues to increase, the resistance stays uniform above a certain shaft speed. The control characteristic works well for reducing excessively high amounts of resistance associated with abrupt increases in shaft speeds due to running over bumps and holes in rough racetracks.

Merritt explained that the “linear style valving is a good choice for dirt track modifieds because the track surface will usually pack down even if it starts out rough cut.  The linear valve shocks work well when the track is rough and really shines when the track surface gets packed, which is usually at the end of a race where you want to be the quickest car on the track.”

When we installed the front shocks on our project car, we mounted them upside side down for less unsprung weight. Checking for shock body clearance is important if you mount shocks this way.

How a shock reacts on the track

A racecar’s shocks go through two basic stages as it goes around the track.  The first stage is compression as the car enters the corner.  Shock compression is easy to wrap your head around because it’s simply the a measurement of how easy the shock is compressed.  Merritt told us, “If the shock has stiffer valving then it will be harder to compress.”

The other stage that a shock goes through is rebound.  Merritt explained rebound valving by saying, “Stiffer valving on the rebound side means that the shock is slower to return to its normal state once the shock has been compressed.”

A good shock set up will help with the car’s weight transfer in the corner and Merritt emphasized, “When the track has no grip, using the right shock combination will help with the weight transfer to find a little more traction.  Hopefully, you find traction before your competition does.”

Mount the rear shocks around 8" below the centerline of the axle housing and no closer that 4"-5" from the backface of the brake rotor. The ideal mounting angle is 15 degrees from vertical.

The common school of thought for a wet track that has no grip is to use right side shocks with very little compression so the weight is transfered quickly to the right tires in order to get as much traction as possible out of the right side tires.  Conversely, you do not want to run a lot of rebound on the left side when the track is wet because you want the weight to transfer to the right side quickly.

For IMCA type modifieds, the front shocks are 7-inch stroke and must be steel body and the rear shocks are usually 9-inch stroke and steel body shocks.  There are some suspension variations between manufacturers that occur, and when in doubt, call the chassis manufacturer for assistance.

For IMCA type modifieds that use a two link rear suspension, Bilstein recommends a starting setup where the left front has a rebound of 175 pounds of force and compression at 225 pounds of force (175/225).  The right front is suggested to be a neutral 225/225.  The rear shock setup for a two link modified would start at 175/175 on both rear shocks.  This allows the weight to smoothly transfer to the back under acceleration and to the front under braking.

For IMCA modifieds with a four bar rear suspension, Bilstein recommends a baseline starting place of 225/225 on the left and right front, and 175/325 on the left rear with a 175/175 on the right rear.  This setup will help keep the left rear pulling it’s share of the load with a four link rear end.

Top Ten Dirt Track Tricks For Modified Racecar Shocks:

1. For consistency, stick with just one brand of shocks and springs.

2. Analyze your handling problems correctly. You won’t help matters by fixing a misdiagnosed problem, so think through your problems before you make changes.

3. Mount the rear shocks around 8 inches below the centerline of the axle housing.  This will make weight transfer easier because the top mount will be located closer to the center of gravity of the car.

4. Mount rear shocks no closer than 4.5 inches from the backface of the the brake rotor.  This keeps heat away from the shock and allows for some movement room without contact when the chassis squats.

5. The ideal mounting angle for front and rear shocks is 15 degrees from vertical.  More or less will limit the amount of piston travel which reduces the effectiveness of the shock.

6. Ensure that the shock is at mid travel when the car is at ride height.  The shock should be long enough that it never reaches the end of it’s travel on either compression or rebound stroke.

7. Mount the shock upside down.  Moving the body of the shock to the chassis lowers the unsprung weight of the suspension plus it moves the body of the shock further away from the brake rotor.

8. Inspect your shocks before you go out on the track.  A dent in the shock tube could cause the shock to have limited travel.

9. Only make one adjustment at a time when track tuning.  If more than one adjustment is made, you’ll never be able to determine the effect of just one change on the handling.

10. Use shock travel indicators to check the chassis handling and if the suspension is doing what it is suppose to be doing.

Our Sport Mod project car will be upgraded with the Bilstein SLS shock package.

Our Project Car

We will be upgrading our K9 Sport Mod project car with a set of SLS Series shocks and doing some on track testing in the near future.  The SLS shocks from Bilstein are relatively new to the product line up and are offered in a 10 shock kit from the manufacturer.  On our project car, we will be using the baseline setups and making changes based on the Bilstein shock charts and trouble shooting that comes with the full 10 shock package.  We selected the Bilstein shock package because it allows us tremendous flexibility in making adjustments at the track.  The SLS series shock fits our track’s profile extremely well; medium rough to dry slick by the end of the night.


Bilstein USA
Phone: 1-858-386-5900

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