The clutch acts as a mechanical link between the engine and transmission. When the clutch lever is pressed, this disconnects the engine from the transmission and the drive wheels, allowing the driver to smoothly change gears. As a result the clutch is an essential part of your car. Continue reading this competition clutches and flywheels guide for everything you need to know!
The clutch when engaged transmits the full power of the engine to the gearbox. The clutch also allows the car to perform starts from stationary that would stall the engine and be very damaging to the transmission.
A typical competition engine will in most cases have a lot more power and torque than the stock production car clutch was originally intended to transmit. If the torque of the engine exceeds the design limit of the clutch plate it will lose grip on the flywheel, commonly referred to as “clutch slip”. This clutch slip will mean the full power available will not make it to the driven wheels wasting the power advantage. Excessive slip will overheat the plates affecting reliability and will reduce clutch life. A production car clutch is designed to be pleasant for your grandmother to drive to the shops. Some of these design criteria work against the stock clutch when used in motorsport. The characteristics of a standard clutch can be improved for motorsport
Production car clutches
Stock clutches have an organic material that is gentle on bite, so it’s easy to use in traffic. This forgiving material often does not have enough grip for high torque applications. Organic material also only has limited heat capability. Motorsport organic or cerametallic compound plates have much more suitable material for competition use. Some race specific applications are sintered material, and carbon on the highest specification clutches.
– Better suited to road applications
– Can be used for light competition
– Offers the softest engagement
– Least prone to judder
– Available in both rigid and sprung hub formats
– Primarily used for race / rally applications
– Can be used for road use where engine torque requires it
– Greater resistance to high torque changes
– Smoother engagement than sintered material plates
– Less prone to judder than sintered material plates
– Available in both rigid and sprung centre formats
– Used primarily in race applications.
– Compact dimensions
– Lightweight – Low moment of inertia
– Used in race / rally / drift / drag applications
– Very high heat rating
– Ultra-lightweight, around 1/3 lighter than equivalent metal disc
– Heat expansion around 1/20th of metal discs
– Long Life compared with other motorsport materials
– High resistance to sticking
99.9% of road cars have single plate clutch. This has a torque limit due to the amount of surface area available combined with an acceptable to use cover spring rating. If the torque limit exceeds the capability of this combination, a change to twin or triple plate set up allows the surface area to increase but the cover spring rating can still be in a useable range. Motorsport clutch single plates are normally much lighter than stock plates, reducing inertia. Production cars have sprung centres to reduce shock loadings on the transmission, reduce noise, and make the car smoother. Competition clutch plates often have rigid centres to reduce weight and give quicker response and engagement.
The factory cover normally has a light action so anyone can operate regardless of physical strength. This means a lower clamp load that reduces the clutch’s ability to prevent the plate from slipping. Uprated clutches often have an increased clamp load to prevent slippage, some use clever design so the pedal pressure is unchanged. The standard cover is often heavy to give a long life and absorb vibrations and uses cheaper heavier materials to keep the costs low. Most motorsport cover combine lighter materials to reduce weight and aid cooling, this will mean less drivetrain inertia, giving better pick up.
Standard Type Flywheel
The stock flywheel will work with a motorsport clutch if the friction face is smooth and free from defects. If not, they can be machined, provided they stay within tolerances. Otherwise a replacement is the only option. If the stock flywheel is very heavy or needs replacing a lightened flywheel will reduce inertia and give much better engine response.
Dual mass flywheels
(DMFs) are fitted to many modern vehicles; they eliminate excessive transmission noise, protect the gearbox from damage, reduce gear change/shift effort. The DMF acts as a damper between the engine and the gearbox. DMFs are made of two parts connected to a central friction ring that allows slip between them. DMFs work by having a set of springs inserted between two rotating masses; the slip is cushioned by a set of torsional springs that smooth out irregular torque pulses from the engine. The springs are sized to absorb some of the resonant vibration from the engine under load conditions. All DMFs wear out over time, and they need to be replaced at equal intervals to the clutch. An engine with increased torque can mean the torsional springs are often overworked and can cause the flywheel to fail quicker, even on standard road use. The DMFs combined weight, due to its complex construction is quite heavy, this reduces the engine’s ability to pick up revs quickly.
A dual mass is not recommended for sustained use above 6,500 RPM. A lightweight one-piece flywheel is usually the best for engine response and reliability in a motorsport environment. A one-piece flywheel will not need replacing when the clutch is replaced.
Choosing the correct clutch
Selecting the most suitable clutch for your application can seem daunting but the following will help choose the best one for you.
The criteria to decide on are pedal effort, engagement feel, shift response, acceleration response, and torque limit.
Power / Torque
The replacement clutch needs to have a higher torque figure than the engines rating. For example, if you have max 250 lb / ft of torque and you pick a clutch rated to 200 lb/ ft it is not going to be up to the job. If you know your torque figure it is always worth going a bit higher on the rating to help reliability and allow for future modifications. This especially applies on turbocharged and supercharged engines, where increases from further tuning can be quite large. If the engine is a new build and torque is unknown, you can often find dyno figures online for similar spec motors to give you an expected torque figure, if it is a complete one off you might have to perform some simulations or simply make an educated guess!
Type of Use
A car used mostly for road that does an occasional track day or navigation rally will have different clutch requirements from a pure race car. In the same vein a drag race car will need a different clutch compared with a trial’s car. An organic lined clutch will give similar feel to a standard clutch. This is good if fine clutch control is more important than a super quick change, if this is the case this is likely to be the best choice. If the opposite is true a superlight cerametallic clutch plate could be the answer. A basic guide is below:
Single Plate Organic Clutch: light pedal effort, excellent engagement feel, slower shift and acceleration response, lower torque level.
Use: Road, Track Day, Circuit Racing, Drifting.
Single Plate Cerametallic Clutch: medium pedal effort, reasonable engagement feel, slower shift and acceleration response, higher torque level.
Use: Extreme Road, Track Day, Circuit Racing, Time Attack, Drifting, Drag Racing, Rally.
Lightweight Single Plate Cerametallic Clutch: medium pedal effort, reasonable engagement feel, excellent shift and good acceleration response, higher torque level.
Use: Circuit Racing, Time Attack, Drifting.
Brands Exedy Stage 2 Racing
Flywheels: a lighter flywheel will reduce inertia meaning the revs pick up much faster, many of the these have vent holes to improve clutch cooling. Steel billet or Alloy construction means they are safer to higher rpm than stock flywheels. Some popular applications have a choice of light or ultra-light.
Dual Mass Replacement Flywheel Kits:
A smaller diameter clutch will have less inertia due to less weight. This will require a suitable flywheel to accept the smaller cover plate, these are available for many cars widely used in motorsport. A standard flywheel can also be modified but care must be taken to retain the unit’s integrity. Cast flywheels are not recommended above 8,000rpm.
Single Plate Cerametallic Clutch: medium pedal effort, reasonable engagement feel, excellent shift and acceleration response, higher torque level.
Use : Track Day, Circuit Racing, Time Attack, Drifting, Drag Racing.
Twin or Triple Plate Cerametallic Clutch: light pedal effort, reasonable engagement feel, reasonable shift response, excellent acceleration response, higher torque level.
Use: Road, Track Day, Circuit Racing, Time Attack, Drifting, Drag Racing.
Twin or Triple Plate Carbon Clutch: light pedal effort, excellent engagement feel, excellent shift and acceleration response, higher torque level.
Use: Track Day, Circuit Racing, Time Attack.
Brands Exedy Carbon D & R
Competition Race / Rally / Prototype Clutches
A low volume or bespoke race or rally vehicle often has a clutch that is a universal fit small diameter clutch unit. These are generally supplied by dimensions rather than by a specific fitment. An example might be a mid-engine car that runs a Hewland gearbox and a heavily modified engine with a bespoke bellhousing. These types of clutches come in a vast range of sizes, types, and ratings. Choosing a suitable clutch will depend on if the car has this type already fitted or if the car is being built for the first time.
Replacing an existing clutch
If this type of clutch is fitted already it will be necessary to determine the bolt pattern of the cover. This is normally measured by the diameter of an imaginary circle that would pass through the centre of each bolt (pitch circle diameter see below).
The outside diameter of the clutch plate will determine the clutch size, two popular sizes are 5.5” and 7.25” but many other sizes are available. The clutch disc size will be closely linked to the cover bolt pattern sizing. The centre of the drive plate will need checking if it is sprung or rigid.
Any stamped or engraved part numbers that are visible are worth making a note of. The outside diameter of the splines and the number of splines will also require checking. A typical example would be 25.4mm x 23T spline, this will measure 25.4mm on the outer edge of the gearbox spline and have 23 teeth. If the transmission is a commonly used type, this may be easy to look up. If the clutch is twin plate the plates may be the same, but with triple or quadruple plate clutches the inner plates will have flatter centre spigots or plates. The pressure and floater plates that fit between the plates on multi plate clutches should be checked for wear and warping.
Choosing a clutch for a new application.
If the clutch is being used for a bespoke installation, the flywheel will be the starting point to determine what clutch type is used. If the engine is using an aftermarket flywheel the clutch cover and corresponding plates will need matching to the flywheel and its predetermined bolt pattern (see the flywheel links for the range available for popular engines). Most clutch covers fit a stepped flywheel where the cover is radially located by sitting over a stepdown at the edge of the friction face. The other possible type of flywheel is classed as flat face or pot type where the cover is located by its outer edge, the flywheel has a lip around the outer to locate the cover (shaped like a pot hence the name). If there is no flywheel available, the cover plate bolt pattern will need machining to suit the clutch of your choice. Drawing with these dimensions are usually viewable on the manufacturer’s website. The following information will need determining to pick a suitable clutch unit.
Clutch Cover Bolt pattern (Pitch Circle Diameter) see above diagram
Clutch plate diameter (most likely linked with the above)
Gearbox Input Shaft Spline Size (outer diameter and number of teeth)
Torque Rating Required
Cover Bearing Operation : Push or Pull (push is much more popular)
Flywheel Cover Location : Stepped or Flat / Pot Type
Bell Housing Clearance (compact housings might not accept 3-4 plate types)
When fitting a non-standard clutch & flywheel as a replacement for the original certain parameters must be measured. As shown below A, B & C are taken to ensure the replacement kit operates correctly.
A = The distance from the flywheel mounting face of the crank shaft to the contact face of the release bearing at maximum travel.
B = The distance from the flywheel mounting face of the crank shaft to the friction face of the flywheel.
C = Kit Set Up Height, the distance from the flywheel mounting face on the crank shaft to the contact point of the release bearing on the diaphragm spring.
On fitting the release bearing it should be arranged so that the bearing is not in contact with the diaphragm spring fingers when the clutch is fully engaged. Constant contact will result in excessive wear on both the diaphragm spring fingers and the release bearing. It is also important that the travel of the bearing when operated is to a controlled distance, otherwise damage to the diaphragm spring can occur, this travel can be limited by means of an external stop. The pedal should not travel typically more than 6mm / ¼” once the drive is disconnected.
If a small diameter clutch is being used, the bellhousing must be checked to see if it is both concentric and parallel while bolted to the engine. This is easily checked with a dial indicator mounted to the bare crankshaft. If either of these are outside of recommended tolerances for the clutch selected, it can cause the following issues: hub spline wear, cracking of friction discs, premature bearing wear and/or failed disc rivets. This should be corrected by careful machining if either is outside of manufacturers stated tolerances.
Hydraulic release bearings
If the stock release bearing does not suit the new clutch diameter or does not have a suitable operating depth, there are many after-market hydraulic release bearings available in different diameter and mounting depths. If the release bearing suits the setup height but does not have a correct diameter face bearing to suit the smaller diameter clutch fingers, a wide selection of bearings are available. These are sold by dimensions rather than by fitment. The below guide shows typical diameters for most common size clutches:
48 → 52mm for a clutch of Ø 140mm
48 → 54mm for a clutch of Ø 184mm
52 → 54mm for a clutch of Ø 200mm & 215mm
Most release bearings are radius face for flat diaphragm spring fingers, but some are flat faced to suit round nose diaphragm spring fingers.
Hydraulic release bearings can be found by clicking here.
Competition Clutches And Flywheels Useful Links :
AP Racing : Cerametallic & Sintered Clutches
Helix : Cerametallic & Sintered Clutches
Tilton : Cerametallic & Sintered Clutches
Links to Brands we supply :
We hope the guide is useful to select a suitable motorsport clutch or flywheel for your intended competition. We understand the choice can be quite complex and some information may be hard to determine without the clutch apart. If you have clutch questions that you would like help with, please do not hesitate to contact us by phone or e-mail, 01978 664466 or firstname.lastname@example.org
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