We all have heard the infamous M3 clunk, but there are a few reasons why the rear end clunk: loose diff mounting bolts, excessive drive shaft plays, torn subframe mounting floor, or just the LSD itself.
This topic is about the LSD clunk. This happened when the clutch stack suddenly slipped causing the spiders to hit the side-gears very hard when we start moving the car from zero.
There are backlash between side-gears and spider gears, as all diff have and this is normal. In the open-diff, the spider gears drive the side-gears which turn the output flanges. As the tranny clutch is releasing, the drive shaft starts turning and pinion turns the ring gear, and the spiders turn the side-gears without a big clunk noise even though there is a fair amount of backlash between spiders and side-gears. Why? Because the backlash is taken up smoothly by the spiders free movement (no clutch stack) and not a sudden jerk.
In the LSD diff, things happen differently. Ideally, the clutch stack should not have preload -- slipping freely at the beginning -- and it should be only loaded up whenever the two wheels are at different speed. But the fact is that all M3 LSD have the clutch stack preloaded (jack up the wheels and turning one wheel will turn the other one in the same direction which means clutch is preloaded). What is the negative effect of preloaded clutch? Clunk noise.
So when the pinion started turning the ring and the carrier housing, which turning the clutch plates (by the outer diameter teeth). The clutch stack inner diameter teeth drive the left output flange splines. So when you started moving the car, the carrier turned the clutch stack which turned the left spline shaft and the left wheel (the right wheel has no power at this time), and the spiders/side-gears do nothing because there is more backlash between spiders and side-gears and so they are not meshed and loaded up. This all depends on how much the clutch stack is preloaded. If it was preloaded enough for the clutch stack to turn the left wheel, then the left wheel is turning by the clutch stack and not by the spiders/side-gear. But when you gave more gas, then the preloaded clutch disks slipped (as the oil pump did not press more on the clutch since no differential in wheel speed). When the clutch disks suddenly slipped, causing the carrier accelerating fast, causing the spiders to hit the side-gears very hard, and the famous clunk noise -- the Big Bang created.
To fix this factory made clunk noise, the clutch stack has to be shimmed so it just has enough preload but not enough to move the car. When moving from zero, the correctly shimmed clutch stack should slip smoothly to allow the spiders mesh with the side-gears and drive them from the get-go.
Edited: This discussion is only for most of the M3 with stock GKN version V1. Some 2006 cars have the V2 version LSD which is quite different than V1.
This topic is about the LSD clunk. This happened when the clutch stack suddenly slipped causing the spiders to hit the side-gears very hard when we start moving the car from zero.
There are backlash between side-gears and spider gears, as all diff have and this is normal. In the open-diff, the spider gears drive the side-gears which turn the output flanges. As the tranny clutch is releasing, the drive shaft starts turning and pinion turns the ring gear, and the spiders turn the side-gears without a big clunk noise even though there is a fair amount of backlash between spiders and side-gears. Why? Because the backlash is taken up smoothly by the spiders free movement (no clutch stack) and not a sudden jerk.
In the LSD diff, things happen differently. Ideally, the clutch stack should not have preload -- slipping freely at the beginning -- and it should be only loaded up whenever the two wheels are at different speed. But the fact is that all M3 LSD have the clutch stack preloaded (jack up the wheels and turning one wheel will turn the other one in the same direction which means clutch is preloaded). What is the negative effect of preloaded clutch? Clunk noise.
So when the pinion started turning the ring and the carrier housing, which turning the clutch plates (by the outer diameter teeth). The clutch stack inner diameter teeth drive the left output flange splines. So when you started moving the car, the carrier turned the clutch stack which turned the left spline shaft and the left wheel (the right wheel has no power at this time), and the spiders/side-gears do nothing because there is more backlash between spiders and side-gears and so they are not meshed and loaded up. This all depends on how much the clutch stack is preloaded. If it was preloaded enough for the clutch stack to turn the left wheel, then the left wheel is turning by the clutch stack and not by the spiders/side-gear. But when you gave more gas, then the preloaded clutch disks slipped (as the oil pump did not press more on the clutch since no differential in wheel speed). When the clutch disks suddenly slipped, causing the carrier accelerating fast, causing the spiders to hit the side-gears very hard, and the famous clunk noise -- the Big Bang created.
To fix this factory made clunk noise, the clutch stack has to be shimmed so it just has enough preload but not enough to move the car. When moving from zero, the correctly shimmed clutch stack should slip smoothly to allow the spiders mesh with the side-gears and drive them from the get-go.
Edited: This discussion is only for most of the M3 with stock GKN version V1. Some 2006 cars have the V2 version LSD which is quite different than V1.
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