In the Clutch
This transaxle was removed allowing access to the clutch of a Ferrari F355 Challenge.
An often overlooked automotive system for those of us who like to row our own gears is the clutch and associated parts. The proper care and operation of the clutch and transmission for maximum performance requires expert attention. As one would wear out brakes more quickly living amongst hills, clutch problems can be greatly exacerbated with a spirited driver.
With experience servicing everything from dedicated racing machines to grocery getters, Cantech is very familiar with clutch failure of all kinds. We have a great deal of experience with Mini Coopers here at Cantech and at times it becomes obvious their owners thoroughly enjoy what are in honesty very focused sports coupes. A hands down favorite for winding roads, its hard to go wrong with a very short wheelbase mated to a strong engine with BMW inspired handling. In this author’s experience it is one of the most enjoyable sports coupes to nail shifts through a series of turns.
Shown is the heat damaged flywheel of a Volkswagen CC 2.0T.
A clutch is essentially a friction plate, or a disc of friction pads not unlike brake pads, that use that frictional bonding force to connect the engine to the drivetrain/ powered wheels. The clutch plate, with friction pads on both sides, is sandwiched between the flywheel- connected to the engine, and pressure plate-connected to the drivetrain. A concave spring diaphragm on the inside of this pressure plate pushes it towards or away from the clutch plate to engage or disengage the engine. When the gear is engaged the clutch plate is gripping both the flywheel and the pressure plate. When it is disengaged the clutch plate is independent of both the flywheel and the pressure plate.
COMMON CLUTCH PROBLEMS
The most common sensation of a worn out clutch would be slippage. The engine revs increase without much forward progress but eventually settle down once the clutch has sufficient grip. This is usually experienced while driving up a steep hill or under high throttle/high torque situations.
On a docile high mileage car, residual clutch material would be deposited over time and glued to the inside of the bell housing. This Mini Cooper’s clutch had a violent end, as seen by the downy material and accompanying shrapnel.
Neglecting to have a slipping clutch repaired, misalignment, or a bad component can cause the pressure plate, clutch plate or flywheel to warp, crack and eventually disintegrate throwing metal shrapnel. A common problem with spirited drivers or cars with extra horsepower is warping of the flywheel due to built up heat from constant, sometimes jolting frictional force of the clutch pack. The metal can create soft spots, twist around its axis and become uneven or crack, causing vibrations or catastrophic failure.
To the right is what is left of the clutch disc from a Mini Cooper. On the left is a cracked pressure plate from the same car.
Another issue we regularly see among hard driven cars is when the transmission refuses to go into gear. This can be due to a high wearing component- the throw-out bearing. This thick metal puck shaped bearing physically pushes the pressure plate center towards or away from the spinning clutch and can wear out after heavy use. An unresponsive shifter gate coupled with a clutch pedal with no resistance usually means something has failed in the hydraulic system. If the clutch requires constant pumping to allow the shifter into gear it could point to a bad seal in the master cylinder or air bubbles somewhere in the clutch fluid line.
Shown here is BMW’s dual clutch Seven Speed.
An inherent downside to a manual gearbox is how quickly and smoothly the driver can get to the next gear while maintaining linear power delivery. Double clutch transmissions available from Audi, BMW, Mercedes, Ferrari and others have solved much of this issue when coupled with modern engine management. The two clutches are situated on the same axis with an inner and an outer shaft of gears.
Each odd/even gear is on a separate driveshaft. When a gearshift happens, the inner clutch engages while the outer clutch disengages. This allows the next gear to be selected before the last gear is disengaged, with mere fractions of time and rpm lost in the transaction and little torque interruption. The Supercharged M3 from the previous post has one of the better examples in production today. The shift takes about eight milliseconds (.008) to complete while the engine cuts spark and fuel at that precise moment. Not only is the service life of the clutch packs increased, but the choice of the seven speed over the six speed manual dropped its 0-60 time by two tenths of a second and its quarter mile time by three tenths.
There are obviously plenty of ways a clutch system can ruin your day if it is not running optimally, and a lot of the signs can be too subtle to notice, which is why they should be handled by professionals. We can always appreciate a challenge; especially when there is a performance solution.