RAM ASSAULT WEAPON CLUTCH

In oval track racing, the ability to quickly accelerate and decelerate the engine will lead to improved lap times.  The engine should be able to help slow the car into the turn, and accelerate quickly coming out of the turn in order keep the engine in its optimum power band.  The further you are able to drive into the turn and have the car slow sufficiently, and the faster you are able to have the engine accelerate into the optimum RPM range coming out of the turn, the lower the lap time will be.

Oval track cars use small diameter lightweight clutch assemblies.  The lighter the overall weight of the clutch, and the closer the weight of the clutch is to the center line of the crankshaft, the faster the engine will be able to accelerate and decelerate.

Oval track multiple disc clutches use diaphragm covers to provide the clamping pressure.  Due to the reduced diameters of these clutches, multiple clutch discs are required to prevent clutch slippage.  The number of discs needed will be determined by the load on the clutch, and the diameter of the assembly.  Normally, an oval track clutch uses two to three discs.

Because oval track diaphragms are smaller in diameter, the pedal effort to disengage the clutch will be higher.  Oval track clutches are frequently used in conjunction with hydraulic release systems in order to relieve the pedal effort and simplify the release mechanism.  Rounded or angular contact release bearing faces will also reduce the pedal effort required.

Because of the very low flywheel mass, oval track clutches have poor street drivability from a standing start and are not recommended for use in street vehicles.  

The most sophisticated drag racing clutch systems allow controlled slippage to get maximum horsepower to the ground with minimum tire spin.  These clutch systems are fully adjustable for spring and centrifugal pressure, as well as wear so that the system can be tuned for the car to meet the track conditions.

Adjusting the clutch system requires the use of a data recording computer.  Engine, transmission, and driveshaft RPM graph overlays indicate the amount of clutch slippage and tire spin on both the launch and subsequent gear changes that allow us to make proper tuning decisions.  Without a data recorder, any tuning decision is merely a guess.

The most important consideration is how the car will react to the clutch settings when the car is launched.  On the launch, you want to achieve optimum wheel speed without excessive tire spin or clutch slippage.  To accomplish this, the launch RPM needs to be significantly lower than the shift RPM to provide some initial slip while allowing the clutch to gradually applying additional clamp pressure as engine accelerates to the shift point.

Tech Tip

Launch RPM

Since centrifugal assist is RPM sensitive, increasing or decreasing the launch RPM will increase or decrease the clutch pressure at launch without any actual adjustments being made.  Additionally, increasing the launch RPM increases the inertia applied to the rear tires from the stored energy of the clutch system.

Once the vehicle leaves the line successfully, the performance of the clutch can be evaluated on subsequent gear changes.  If the vehicle tends to have no slip on the gear change, spins, or if the tires shake,  reducing the counterweight will provide additional clutch slippage reducing the spin or shake.  Without the data recorder, it is extremely difficult to determine exactly what is happening at these points in the run. Examining each successive gear change, we should see the slip time reduced until there is very little clutch slippage in high gear. 

Clutch tuning is managing the clutch system to obtain the desired performance.  In drag racing, this is achieving maximum vehicle acceleration with little regard to clutch system wear.  Clutch tuning in a street application may be obtaining maximum service life.

TUNING A CLUTCH FOR A GIVEN APPLICATION

A Long Style assembly may be tuned through two adjustments – spring and centrifugal pressure. 

Centrifugal pressure is not a linear function and increases substantially at higher engine RPM.  A vehicle that requires the clutch to be disengaged to shift will be limited in the amount of centrifugal clamp pressure that can be utilized.  Linkage components will encounter higher stresses as the centrifugal clamp pressure is increased, making disengaging the clutch more difficult.

Applications using clutchless transmissions have no limitations on centrifugal clamp pressure.  Use of centrifugal clamp pressure allows the use of less initial clamp pressure at low engine RPM, allowing the vehicle to achieve maximum traction and acceleration leaving the starting line.  As the vehicle accelerates and the engine RPM increases, the centrifugal pressure increases and will allow the clutch to generate sufficient clamp pressure to handle the load of successive gear changes.

The friction material used on the clutch disc will effect how much clamp pressure is required to hold the vehicle.  Friction materials with higher coefficients of friction require less clamp pressure.  Other factors to consider are wear and engagement characteristics.  

By increasing the number of clutch discs in the assembly, the clamp pressure needed will decrease, the clutch will engage smoother, clutch diameters can be decreased, and overall service life will increase.  Reducing the diameter of the clutch will also allow the engine to accelerate and decelerate quicker.  This is the principle behind using small diameter clutches in oval track racing. 

Street/strip vehicles can benefit from the ability to adjust the clamp pressure for maximum performance at the race track.  We would advise a customer to use an adjustable pressure plate that has a range capable of accommodating both street and strip use.  For street driving, we would recommend the clutch be adjusted to maximum pressure to accommodate the lower RPM and higher torque load.  At the race track, the pressure may be reduced to allow maximum acceleration with minimal tire spin, and also because the clutch is operating at higher RPM and more centrifugal clamp pressure is applied.

Making your clutch selection is like trying on a suit.  It’s important to be sure the clutch system is going to fit the vehicle needs in terms of how the unit performs either at the track, on the street, or both.

The number one factor in determining clutch selection is load.  The following are the three most important load factors.

VEHICLE WEIGHT

The mass or weight of the vehicle is an important factor in clutch selection.  The flywheel weight has a great deal to do with creating enough inertia to get a vehicle rolling.  Many older muscle cars are very heavy and have engines that produce higher torque numbers at lower engine speeds.  This creates a high load situation on the clutch that can result in slippage if an inadequate clutch system is selected.  The higher the vehicle weight, the higher the load on the clutch and the more clamp pressure is needed to move the vehicle.

ENGINE RPM OPERATING RANGE

The higher the operating RPM range of the engine, the more centrifugal clamp pressure is applied by the pressure plate.  High torque low RPM applications will require higher clamp pressures, lower torque higher RPM applications will require lighter clamp pressures.

TRANSMISSION GEARING

Transmission gearing selection drastically effects the amount of clamp pressure required to move the vehicle, as well as the amount of centrifugal clamp pressure required to accelerate the vehicle through subsequent gear changes.  By manipulating the transmission ratios, optimum gearing can be selected to match the load on the clutch system to maximize performance.

REAR END GEARING

High rear end gearing (lower numerically) puts heavier load on the clutch system.  If you have ever tried to let out the clutch from a stop in second gear, you likely experienced difficulty getting the vehicle rolling without ‘bogging’ the engine.  This same effect occurs when too high of rear gearing is used.  This situation also requires the clutch to be slipped more on takeoff to make a smooth transition, resulting in accelerated wear to the clutch.

TIRE SELECTION

Tires with more traction or roll out, such as DOT or conventional slicks, allow the vehicle to hook up harder and are less likely to spin than street tires.  When a tire spins, the load on the driveline is decreased, but when the tire is makes good traction, the load on the clutch is increased substantially and more clamp pressure is required in the pressure plate.

In addition to these load factors, there is ENGINE HORSEPOWER and POWER ADDERS.  These factors are secondary; Increased horsepower will require additional clamp pressure.  However, this is usually the result of a change in one or more of the load factors listed above.  

All clutch systems require a release mechanism to operate the clutch.  There are several types of systems that have been used over the years by the various manufacturers.

Billet steel flywheel

Billet aluminum flywheel with steel insert

The flywheel is primarily an inertia device.  As the flywheel spins it stores energy or inertia that helps move the mass of the vehicle as you engage the clutch.

Factory flywheels are designed to apply the optimum amount of stored energy to provide good drivability for the vehicle.  Vehicles with smaller engines have relatively heavy flywheels due to the extra inertia needed for a smooth transition to engagement.  Reducing the weight of the flywheel, while increasing performance, could reduce the drivability of the vehicle.

Under racing conditions, the flywheel weight can be used to control the inertia applied to the drivetrain.  For instance, if a vehicle tends to ‘bog’ upon engagement of the clutch, increasing the flywheel weight will increase the inertia needed to launch the vehicle smoothly.  Too much flywheel weight may cause excessive inertia to be applied, causing the tires to spin.  Reducing flywheel weight under this condition will reduce the inertia applied to the vehicle and allow smoother acceleration.

Aluminum flywheels are used in drag racing high horsepower applications which require the clutch to slip as the vehicle leaves the line.  Steel flywheels are used primarily in street driven vehicles. 

Other load factors can effect flywheel selection, such as rear gearing or transmission gearing.  With the abundance of gearing choices available today, it is possible to use almost any flywheel if the proper selection of gears is made.  This was not always the case – in the 70’s when the gearing choices were not available, racers had no choice but to use the flywheel weight to control the vehicles on launch.  It is more efficient to use a light flywheel and proper gearing than to use tall gearing and a heavier flywheel.

Tech Tip

Flywheels and Drivability

Selecting the proper flywheel helps you achieve the drivability you desire for your vehicle.  Heavy street cars will benefit from a heavier flywheel to generate the inertia to get you moving.  An aluminum flywheel will not generate as much inertia to move the vehicle, and thus it would be necessary to slip the clutch more on takeoff. Some street vehicles may benefit from a lighter flywheel, but only if there is enough rear gear to help you transition the clutch smoothly without excessive slippage.

Tech Tip

How do I select a Proper Flywheel?

When we are asked for a flywheel recommendation, several factors are considered to make a proper recommendation.

What is the primary use of the vehicle? For street driven vehicles, a steel flywheel will provide easier engagement and longer clutch life.

What is rear end gearing? Lower (higher numerically) gears will make engaging the clutch easier, while higher gearing requires the clutch to be slipped more on takeoff for a smooth transition.

What is the weight of the vehicle? If it is over 2800 pounds, we will generally recommend steel. Remember that the flywheel’s job is- to help you get the car accelerating smoothly. If you have ever driven a vehicle and tried to pull off in second gear, you know you had to slip the clutch significantly more to get moving than if you started in low. This is the same effect you would notice if the flywheel is too light.

Typical sprung hub clutch disc

Typical solid hub clutch disc

Clutch Disc Construction

There are two types of clutch disc construction – solid hub and spring dampened hub.

Solid hub discs have the splined hub riveted directly to the disc carrier plate (the circular metal plate that carries the friction material).  This construction is typical of all out racing discs.

Sprung, or dampened hub discs, do not have the splined hub attached directly to the carrier plate.  The hub floats in a spring loaded assembly on the carrier.  When the hub is loaded as when the clutch is engaged, the springs help absorb the load rather than transfer it directly to the carrier plate.  This arrangement allows shock spikes from aggressive engagement to be dampened out to avoid possible damage to the drivetrain.  This type of construction is used in factory replacement discs.

Performance discs feature stronger damper springs to absorb higher loads.  Most RAM performance discs are equipped with RAM poly-coil springs, which are encapsulated in urethane providing a substantial increase in the spring rate over stock springs.  The stronger spring prevents over travel of the hub and subsequent damage to the disc.

Selecting the correct disc construction is simple.  Stock applications use the dampened hub.  From there, as loads increase, the rigidity of the hub should increase right up to solid hubs used for all out racing.

Tech Tip

Coefficient of Friction- A Simple Explanation

Let’s say we have a box sitting on a countertop. It takes 3 pounds of pressure to slide the box across the countertop. Now we place the box on a rubber mat. It takes 8 pounds of pressure to slide the box across the rubber mat. The rubber mat has more grip, or a higher coefficient of friction than the smooth countertop, thus taking more effort to slide the box.

In terms of the clutch disc material, a higher coefficient of friction mean thatit will take more load on the clutch to cause it to slip.

Friction Materials

As performance requirements increase, it is necessary to step up the friction characteristics and durability of the clutch facing materials.  We will review the full range of friction materials and their features.

ORGANIC – Organic material is common to all stock clutch discs, and offers the best drivability but has limitations as operating temperatures rise.  Under high loads accompanied by slippage, they fade because their coefficient of friction drops off.  In addition, at high RPM and high temperature they tend to fail structurally.

RAM 300 series discs use organic facing material that is bonded to a steel backing plate that increases heat dissipation and provides excellent structural strength that precludes disintegration at high RPM.  This structural improvement does not compromise the excellent drivability of 300 series discs.

KEVLAR COMPOSITES – Kevlar material offers a higher coefficient of friction than organic material, but with some loss in drivability.  As the coefficient of friction goes up in the disc material, so will the aggressiveness of the material on engagement.  This means that some chatter can be expected with this material in low gear and reverse.  Kevlar is  compatibile with stock flywheels and pressure plates, making it a good upgrade choice for later model vehicles.  We do not recommend Kevlar for competition applications.

BRONZE METALLIC – Bronze metallic (paddle discs) is the most aggressive materials in terms of engagement.    It offers extended life using reduced static pressures, and a quick, clean engagement.  Bronze metallic is aggressive on the flywheel surface and should be used only with steel or nodular iron friction surfaces.  On the street, this material will cause chatter on engagement.

SINTERED IRON – Sintered iron is well known for its ability to withstand some slippage and not loose its coefficient of friction.  It is the material choice for high horsepower clutch applications for drag racing and truck pulling.  A street version of the sintered iron disc is the RAM 900 series, which utilizes a sprung hub.  These discs should be used only with RAM steel or aluminum flywheels or excessive wear to the friction surface will occur.  A key feature of the competition sintered iron material is that it is maintainable.  If the clutch is slipped excessively, the disc can be resurfaced and reused.

Tech Tip

Drivability vs. Performance

With disc choices ranging from organic to metallic to sintered iron, deciding which material to use is ultimately up to the end user, based on the type of performance and drivability is desirable.  When deciding, one must think about what is most important for the vehicle.  If on-the-track performance is the most important, select a disc material that is fairly aggressive.  But realize that making this decision will effect how smoothly the vehicle will operate under normal driving conditions.  If smooth and drivable is most important, select the disc based on this. Keep in mind though, that the life of the clutch disc under racing conditions will likely be reduced.

What’s a Pressure Plate?

The pressure plate applies pressure to the clutch disc to clamp it against the flywheel and engage the clutch.  Pressure may be generated by spring (static) pressure or centrifugal pressure.  Static pressure is constant, meaning that whether the engine is turned off or spinning 7000 RPM, the pressure never changes.  Centrifugal pressure is not constant.  It is a function of engine RPM.  Centrifugal pressure is generated by the clutch levers and increases to the square of the engine RPM.  It is less pronounced at lower engine speeds but very effective in the higher RPM range.

Diaphragm Pressure Plates

The diaphragm pressure plate utilizes a Bellville or conical spring to apply pressure to the pressure ring.  This type of pressure plate has multiple fingers that the release bearing presses against to disengage the clutch.  Diaphragm clutches rely completely on static pressure which is unaffected by engine RPM.

The Belleville spring allows the pressure plate to be released and engaged with a relatively light pedal effort compared with coil spring clutches.  This design is used almost exclusively in late model vehicles that have hydraulic or cable release mechanisms, due to the lighter effort required to engage and disengage the pressure plate.

The diaphragm clutch is excellent for use in street and heavy duty street applications where drivability and pedal effort are a major concern for the user.

Tech Tip

CENTRIFUGAL WEIGHTS IN DIAPHRAGM STREET/STRIP APPLICATIONS

Some aftermarket diaphragm pressure plates feature a centrifugal weighting system. Centrifugal assist is useful in drag racing and other high RPM applications to apply additional clamp load to the clutch disc. The drawback to centrifugal assist in highway performance applications is that the centrifugal assist is low until the engine reaches high RPM. The load on the clutch (the effort required to turn the driveshaft) is very high at low RPM and decreases at higher RPM. In most highway applications centrifugal assist is not effective in increasing holding power or performance because it fails to solve the high torque load at low RPM problem.

Borg and Beck Pressure Plates

The Borg & Beck pressure plate uses three levers to engage and disengage the clutch disc.  It is a coil spring design, where the pressure of the clutch is applied to the disc using coil springs similar to a valve spring.  By combining these springs at a specific installed height, different pressures can be attained for the pressure plate.  Borg & Beck clutches rely completely on static pressure which is unaffected by engine RPM.

Borg & Beck pressure plates are found in GM, Chrysler, and AMC early model applications with mechanical linkage.  Borg & Beck clutches and can be identified by looking at the width of the clutch fingers, which is about one inch. The coil spring design by nature will require more pedal effort to engage and disengage the clutch.

Borg and Beck clutches are best suited to street and heavy duty street applications for older muscle cars and trucks.

Tech Tip

CENTRIFUGAL OR COUNTERWEIGHT LOADING

Centrifugal pressure is additional clamping pressure on the clutch disc that occurs as a function of engine RPM.  As engine RPM increases, centrifugal pressure forces the clutch fingers of the Long Style pressure plate outward.  As this occurs, the clutch fingers pivot against the cover and apply additional clamp loading to the clutch disc. 

Long Style Clutches

Exploded view

Counterweighted lever (top) and standard lever (bottom)

Typical Long Style pressure plate

Weights are added to the Clutch levers to increase the plate pressure as RPM increases.

Long style pressure plates are the Ford version of a three lever, coil spring pressure plate.  The Long Style is the most popular type of pressure plate for drag racing applications.  It’s design is the basis for today’s professional drag racing clutches.

The inherent advantage of Long Style pressure plates is their ability to apply centrifugal clamping pressure. As engine RPM increases, the levers in the plate pivot against the cover and apply additional clamp load to the clutch disc.  This is true in both counterweighted lever Long Style pressure plates and also non-counterweighted designs.  (Counterweighted levers have provision for installation of weights to the backside of the levers to further increase the centrifugal clamp effect.)

The The Long Style pressure plate is best utilized with mechanical release linkages.  Long Style clutches were prevalent in early Ford muscle cars and trucks.

Some Long Style clutches feature adjustable static pressure.  By turning an allen screw located on top of the spring, the pressure can be increased.

Tech Tip

ADJUSTING STATIC PRESSURE ON RAM LONG STYLE CLUTCHES

Increasing the static pressure of a RAM Long Style pressure plate is achieved by turning the allen adjuster screw COUNTERCLOCKWISE to increase the static load.  When the screw is turned, it pushed against the cover and the adjuster base compresses the spring, increasing the spring rate.  Most pressure plates have a maximum of 7 turns adjustment.  Turning the adjuster screw further will cause it to come out of the adjuster base.  When this occurs, the entire plate must be disassembled for repair.  Some racers mark the turns they have in the clutch on the side of the bellhousing in order to keep track of where they are set. 

As the adjuster screw is turned counterclockwise, the spring is compressed to increase clamp pressure.

What is a clutch?

A clutch is the device that couples the engine to the drivetrain.  It is designed so that it can be positively disengaged by depressing the clutch pedal.  This disconnects the engine and drivetrain.  Releasing the clutch pedal engages the clutch, connecting the engine and drivetrain.

There are several different types of clutches depending on the application or use of the vehicle.