Performance Drive Shafts, Clutches, and Axles

Materials Used in High-Performance Driveshafts

Aluminum

  • Aluminum driveshafts are used in some racing applications.

  • Aluminum characteristics:

    • Strong

    • Reliable

    • Lightweight

  • Construction modifications by manufacturers may include:

    • Thicker and longer driveshaft tubes for added strength

    • Increased tube diameter without significantly increasing weight, to improve performance

Carbon Fiber

  • Carbon fiber driveshafts are used in some racing applications.

  • Characteristics of carbon fiber driveshafts:

    • Expensive, but highly efficient

    • Strong with high torsion strength, reducing twisting and shock to the rear wheels

    • High resistance to flexing at slower speeds, aiding vehicle launch in racing

Chrome Moly Steel

  • Steel is the most common material used in racecar driveshafts.

  • Chrome moly steel is the most commonly used steel type.

    • Relatively inexpensive

    • Strong, allowing for smaller diameter driveshafts compared to aluminum driveshafts

Driveshaft Dimensional Characteristics

Diameter

  • Driveshaft length and diameter directly affect racecar performance.

  • All driveshafts have a critical speed:

    • The speed at which the driveshaft becomes unstable and vibrates

    • Reaching critical speed may cause driveshaft failure

  • Manufacturers increase critical speed by:

    • Making the driveshaft lighter and stiffer

    • Increasing diameter without significantly increasing weight

  • Driveshaft diameter must match vehicle specifications to ensure:

    • Durability

    • Resistance to twisting

Balance

  • Driveshaft balancing is critical for high-performance vehicles.

  • Manufacturers use balance weights to meet vehicle requirements and specifications.

  • An improperly balanced driveshaft can:

    • Vibrate more at higher speeds

    • Cause driveshaft failure

    • Result in damage to expensive parts and components

Thickness

  • Driveshaft thickness depends on:

    • Material of the driveshaft

    • Type of driveshaft used in the vehicle

  • Diameter and thickness work together to affect performance.

  • Manufacturers must balance diameter and thickness to ensure the critical speed remains safe or unreachable.

High-Performance Driveshaft Modifications

Yoke Flange

  • Yoke flanges in high-performance driveshafts vary depending on application.

  • Most driveshaft yokes are made of forged steel or aluminum.

  • Manufacturers weld each yoke to the driveshaft tube to meet tight specifications and tolerances.

  • Improperly welded yokes can contribute to an out-of-balance driveshaft, potentially causing failure.

U-joints

  • Most common U-joints in high-performance vehicles are 1310 and 1330.

  • 1310 U-joint:

    • Used in many racing applications

    • Retained in the driveshaft using an external or internal snap ring, depending on application

  • 1330 U-joint:

    • Found near the pinion yoke going into the rear axle

    • Shares the same cap size as the 1310

    • Has a larger cross to handle higher torque

Materials in High Performance Clutches

Brass

  • Some high-performance clutches use brass in the friction material.

  • Brass benefits:

    • Better heat dissipation than stock clutch materials

    • Improved grip, reducing slippage between the clutch disc, pressure plate, and flywheel

Aluminum

  • High-performance clutches made of aluminum are lighter and offer a balance of performance and drivability.

  • Aluminum clutch assemblies are designed for:

    • Faster engine acceleration

    • Smoother shifting

  • To prevent corrosion, aluminum may be coated or anodized.

  • Aluminum flywheels often include iron inserts for the clutch friction material to engage with.

Metal Wire

  • High-performance clutches use metal wire woven through the friction material on the clutch disc.

  • The wire runs from the outer edge to the inner edge of the friction material.

  • Purpose of the wire:

    • Holds the friction material together

    • Prevents the friction material from breaking apart

Types of High Performance Clutches

Hydraulic Clutch - Overview

  • Hydraulic clutch is a self-adjusting clutch system.

  • Components include:

    • Master cylinder with an input piston connected to the clutch pedal

    • Slave cylinder (reaction or actuator cylinder) with an output piston attached to the clutch release lever

    • Hydraulic connection via plastic tubing or braided hose between master and slave cylinders

  • Advantages in high-performance vehicles:

    • Adjustable clutch pedal effort and travel to suit vehicle requirements

Applications

  • Hydraulic clutch systems in racing replace older pushrod and lever systems.

  • Provide smooth clutch engagement and minimum pedal effort, enabling fast shifting.

  • Offer optimal performance and control over clutch engagement and disengagement.

Mechanical Clutch - Overview

  • Most racing applications use a cable linkage mechanical clutch.

  • The cable allows the vehicle body and frame to flex while maintaining clutch control.

  • Cable connections:

    • Clutch pedal → Cable → Release fork

  • Operation: Pushing the clutch pedal pulls the cable and release fork, disengaging the pressure plate from the clutch.

Characteristics

  • Mechanical clutch cables in race cars are highly adjustable.

  • Adjustability ensures correct clutch pedal travel and proper pedal height.

  • Adjuster nut is provided to make precise cable adjustments.

Centrifugal Clutch - Overview

  • Centrifugal clutches in racing vehicles adjust clamping force automatically.

  • As the engine speed increases, the pressure plate clamps the clutch disc tighter.

  • Manufacturers use a lighter spring on the pressure plate to reduce clutch pedal effort needed for disengagement.

Characteristics

  • The Borg and Beck style is the most popular centrifugal clutch in high-performance vehicles.

  • Release levers are placed in the clutch cover for:

    • Quicker pressure plate release

    • Positive clutch disengagement

    • Faster gear changes for the driver

Modifications of High Performance Clutches

Heat Treatment

  • Heavy-duty racing diaphragm springs are heat-treated to:

    • Increase spring tension

    • Provide more clamping force for a better hold on the clutch disc

  • High-performance manufacturers use this treatment to maintain high clamping force without compromising pressure plate integrity

Double Diaphragm

  • Double diaphragm design increases clamping force by stacking two diaphragm springs.

  • Special friction-reducing silicone material is placed between the diaphragms to:

    • Ensure smooth operation

    • Maintain effective clamping force

  • This design increases clutch pedal pressure significantly.

  • To offset higher pedal pressures, manufacturers modify the mechanical linkage to reduce effort needed to push the clutch pedal.

Pressure Plate Modifications

Diaphragm Pressure Plate

  • Diaphragm pressure plate features a heavy-duty cover to minimize flexibility and ensure:

    • Strength

    • Stability

    • Reliability

  • Includes a ductile iron pressure ring that:

    • Resists cracking

    • Dissipates heat quickly

  • Offers light pedal pressure and quick release for precise gear changes

  • Provides positive engagement capable of handling the horsepower of most high-performance vehicles.

Long Style Pressure Plate

  • Long-style pressure plate features forged and heat-treated heavy-duty levers for:

    • Quick release of the clutch disc

    • Positive gear changes

  • Assembled with a heavy-duty reinforced cover that includes:

    • Steel spring

    • Ductile iron pressure ring

Changes in the Clutch Hub

  • Clutch disc modifications may include additional torque-absorbing springs in the clutch hub to control driveline shock.

  • Center hubs are typically custom forged to accept the extra torque springs.

  • Aluminum is commonly used to lighten the clutch disc weight.

  • Depending on friction material type and engine power, the clutch hub may be a solid design to reduce dampening effects.

Types of Slave Cylinders

Aluminum Slave Cylinder

  • Aluminum slave cylinders are used in racing applications for their lightweight properties.

  • Bleeder screw and hydraulic line ports are located on top of the cylinder body for extra clearance in tight mounting spaces.

  • Provide ample hydraulic volume for a variety of custom race applications.

  • Slave cylinder piston can travel further than stock cylinders to ensure full clutch disengagement.

  • Adjustable pushrod allows fine-tuning of clutch engagement and disengagement.

Pull-type Cylinder

  • Pull-type slave cylinders are used in racing vehicles with limited mounting space.

  • Mounting position: behind the clutch release fork, instead of in front.

  • Operation: When the clutch pedal is pressed, the cylinder pulls the release fork rather than pushing it.

  • Adjustable to ensure proper clutch engagement and disengagement.

Bearing Modifications

Clutch Release Bearing

  • High-performance racing vehicles use high-strength clutch release bearings.

  • Release bearings are made from harder materials to withstand higher operating temperatures.

  • Manufacturers sometimes use a concentric slave cylinder assembly to replace the traditional release bearing.

    • In this setup, the slave cylinder is installed inside the transmission bellhousing.

    • This eliminates the need for a clutch release fork.

Pilot Bearing

  • Pilot bearing supports the transmission input shaft.

  • Heavy-duty roller pilot bearing installs at the rear of the crankshaft.

  • Function when clutch is disengaged: allows the transmission input shaft to rotate independently.

  • Function when clutch is fully engaged: pilot bearing is not needed, as the engine crankshaft and transmission input shaft rotate together.

Types of Drive Axles

Quick Change

  • High-performance vehicles sometimes use quick-change drive axle units.

  • Components include:

    • Forward end

    • Rear end

    • Transversely extending middle portion

  • Rear end contains a gear system that changes the gear ratio before transferring torque to the ring and pinion gears.

  • In quick-change axles, a jackshaft connects to the driveshaft:

    • One end connects to a quick-change gear

    • The other end connects to the pinion gear

  • Purpose: Provides a higher final drive ratio, suitable for high-performance applications with high ring and pinion gear ratios.

Rigid Axle

  • High-performance rigid axles are made from steel or aluminum.

  • Possible modifications include:

    • Axle shafts with more splines for increased strength

    • Improved bearings to withstand higher temperatures

    • Addition of a locking or limited-slip differential

  • Ring and pinion ratios may be changed depending on the racing vehicle requirements


Independent Rear Axle

  • Most open-wheel circle track vehicles use independent rear axles.

  • Components include:

    • Upper and lower control arms

    • Coil springs and shocks

  • Main advantage: Each wheel can react to uneven track surfaces independently, without affecting the other wheel.

Short Axle Housing

  • Short axle housings have axle tubes cut down to reduce the overall axle length.

  • Applications: Most hot rods and drag racing vehicles use short axle housings.

  • Reason: Allows larger and wider tires to fit under the rear fenders of the vehicle.

Custom-built Housing

  • Axle housings can be custom-built from scratch based on customer requirements.

  • Construction methods include:

    • Welding axle tubes directly to the axle housing

    • Manufacturing the center section first, then welding the sides to fit the axle tubes

Modifications to Differentials

Center Section - Overview

  • Removable center section rear axles are popular in racing vehicles.

  • Center section is usually made from aluminum for lightweight construction.

  • Reinforcement:

    • Cross web design with horizontal and lateral supports

    • Reduces twisting in racing axles

  • Front pinion bearing pockets may also be made from aluminum and can be customized for different bearing sizes.

  • Differential options: spool, locker, or limited-slip, depending on the ring and pinion gearset application.

Applications

  • Removable center section axles are common in many racing car applications.

  • Advantages:

    • Easy to disassemble for part replacement

    • Strong and reliable

  • Commonly used in a Ford 9-inch axle.

  • Versatility: Can be modified to fit almost any vehicle, suitable for different competition classes.

Differential Housing Cover - Overview

  • Rear differential cover is used in racing axles with an integral housing.

  • Strength enhancements:

    • Supports added to the cover, which can be lateral, horizontal, or both

    • Supports increase rigidity and reduce axle flex

  • Cooling benefits: Supports act as cooling fins, lowering differential fluid temperature.

  • Additional features:

    • Fluid fill hole to aid in filling the differential

    • Some covers have bearing cap studs and fasteners to further stiffen the axle assembly

Applications

  • Differential housing covers are used in many race car applications.

  • Removable covers are commonly found in hot rod vehicles.

  • Can be either:

    • Integral part of the axle

    • Added for appearance

Differential Cooler - Overview

  • Differential cooler is a small radiator that cools differential fluid.

  • Pump types:

    • Electric pump:

      • Includes an electric motor and electric cooling fan

      • Motor drives the impeller to pump differential fluid through the cooler

      • Electric fan cools the motor

    • Mechanical pump:

      • Driven off the driveshaft or transmission output shaft

  • Additional features:

    • Filter screen to prevent large metal particles from entering the pump

    • Fluid lines route fluid from the differential housing → cooler and pump → back to the differential housing

Applications

  • Differential coolers are commonly used in circle track racing.

  • Purpose: Prevent high differential temperatures during long race times.

Ring and Pinion Gears - Overview

  • Ring and pinion gearsets in high-performance vehicles are often modified by manufacturers.

  • Material: Mostly hardened steel.

  • Purpose: Modified to meet specific race track requirements.

Applications

  • Racing vehicles with solid or independent drive axles use ring and pinion gears.

  • Manufacturer modifications include:

    • Gear ratio adjustments

    • Gear teeth contact pattern modifications

  • Purpose: Ensure performance meets racing car requirements.

Yokes

Transmission Yoke

  • Transmission yokes in high-performance vehicles are made from steel or aluminum.

  • Many manufacturers upgrade the yokes to accommodate larger U-joint series.

  • U-joint yoke conversion kits are available to convert transmission yokes for use with larger U-joints.

Driveshaft Yoke

  • High-performance driveshaft yokes are made of steel or aluminum.

  • Yokes attach to the driveshaft during assembly.

  • Proper phasing of the driveshaft is required before welding to ensure:

    • U-joints operate properly

    • No vibration or binding occurs during launch or high-speed travel

Modifications to Axle Shafts

Materials

  • Racing axles are made from high-strength steel, with the steel grade varying by manufacturer.

  • Key features:

    • Hardened steel construction

    • Large number of splines at the shaft ends

  • Purpose of more splines:

    • Handle higher power without breaking

    • Increase overall axle strength

Wheel Studs and Bolts

  • Wheel studs and bolts in most racing vehicles are made of chrome moly steel.

  • Key properties:

    • High strength

    • Can handle heavy loads and high lateral forces common in racing

C-clip Eliminator

  • C-clip function: Holds the axle shaft in the differential side gear.

  • C-clip eliminator kit:

    • Replaces the C-clips in the differential

    • Installs on the axle tubes and accommodates axle bearings (press-fit on the axle)

    • Includes a bushing to reduce bearing diameter to fit the shaft

  • Purpose in racing vehicles:

    • Keeps the axle inside the axle tube if it breaks

    • Prevents axle separation from the housing