Found this when I couldnt work out why mine slips in 3 and 4 and not in 1 or 2. Put it down to the extra torque output from the PSI box fitted. Great example of how PSI and autoboxes arent a happy marriage.
WHY DO CLUTCHES SLIP IN 4th GEAR AND NOT AS OFTEN IN 1st, 2nd or 3rd GEAR?
In a car often things such as engine RPM, engine torque and engine power output are varying all the time. One initial concept that helps a little in understanding the forces on clutches and gears in the drive chain is to fix one variable and consider a concept of a constant engine power output and call it CEP.
We can define CEP as an engine power output that is some continuous stable value and such that the car has acquired a stable road speed. Under these circumstances the drive power is continuously supplied via a torque (turning force) through the drive train to the wheels. This CEP is exactly equal to all of the losses due to friction of every kind in the car and in the cars interaction with its environment, including wind & tire resistance etc. The car has no unbalanced force acting on it under this condition and therefore it is not accelerating or decelerating and has a stable road speed. Engine power and torque do in practice vary with RPM but it can be useful to think of it as stable over a range of RPMs as will be explained.
Torque itself is a measure of force (Newtons) applied perpendicular to a fulcrum arm of some radius in meters and we generally are familiar with it as a turning force to tighten nuts using a Torque wrench. Torque is a product of force and perpendicular distance (┘distance) and therefore is a measure of work in joules and the work is done in pushing mass around a circumferential pathway. The relationship between torque and power (joules per second or Watts) supplied by the engine is:
Power (engine) = Torque x angular velocity ( of flywheel or crankshaft).
The angular velocity has units of radians per second. There are 2π radians in one rotation so to convert this into RPM then the angular velocity of say your engines flywheel is:
Angular velocity (flywheel) = (2 x π x RPM)/ 60 = approximately 0.1 x RPM.
So using the two relations above and breaking the torque into its two components of force and ┘distance and making the engine power a CEP we have:
CEP = 0.1 x Force x ┘Distance x RPM
All of the ┘distances or fulcrums if you like, at any place in the drive train or gearbox, overdrive, clutches and gears, differential and tires size etc are fixed by their geometry. So we can simplify the above scenario further and say for this approximation:
Force x RPM = Constant.
Or equally: Torque x RPM = another Constant.
Any clutch surfaces in the drive train, whether they be in the O/D unit, or the G/boxes main clutch have to transmit the torque or force without slipping. If we assume that the engine power is approximately constant (CEP) over a range of RPM say between 2000 and 3000 rpm then we can see from the relation above what an imposed change of RPM on the engine will do to the torque and therefore the forces on the clutch surfaces. Forced changes in engine rpm at some car road speed occur when you change gears or when the overdrive unit is switched on or switched off which changes the gearing.
Therefore at some CEP if the RPM is forced up by gearing then the clutch surfaces experience a decrease in force and if the RPM is forced down by gearing the clutch surface experience an increase in force.
This is the reason why a failing gearbox clutch will slip more easily in 4th gear than lower gears, because the forces on its surfaces are higher in 4th gear than lower gears for any CEP. Usually with a slipping clutch it seems fine in low gears, then you change from 3rd to 4th gear and attempt to accelerate in 4th gear and it begins to slip as the forces exceed the holding frictional forces of the clutch surface.
have you got it sorted out just by changing the clutch plate or the entire kit with pressure plate and/or release bearing?
