Quote:
Originally Posted by Purple Derple
So how does it cause wheel hop? Are the wheels receiving unsteady power because the pinion is climbing the ring gear initially and then falling when the wheels break traction? Is the twisting of the differential unevenly binding the half shaft u-joints? Does the differential pull on the half shafts changing the suspension geometry? Are the control arm bushings too soft allowing too much toe change on power? Why does the RR hop first?
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The process is as follows from the propeller shaft to the wheels;
Power applied to diff-causing pinion climb-this is correct. The oem rubber bushes allow for up and down movement,so as power is applied the nose of the differential climbs. This vertical movement is normal in any rwd arrangement, the problems start to arise because of the single mounting point in the rear-since the right side is unsupported,the combination of the nose of the diff rising coupled with the rotational energy applied allows for excessive movement there.this allows for movement, but the cause of hop itself only begins here, it's a reactive effect. It's reciprocation.
The energy is transferred through the differential to the axles. On the M3 the axle tubes are the floating style, the move side to side within the confines of their CV joints so binding isn't necessarily an issue,an suspension geometry is unaffected by any lateral movement of half shafts...it is however affected via weight transfer. This is where the reciprocation comes into play-even though the spring's ability to be,well,a spring- is suppressed via the shock it's extreme loading that allows it to fall slightly out of the shocks dampening control for a moment. Two very strong physical forces are at work here, forward movement and a downward thrust via weight transfer. As the tires fight for grip, an adverse reaction of spring energy is trying to push back up on the car, which every rwd car does by nature of design. This is where the return strain on the axles is sent back to the differential.
So now that the right side of the diff being unsupported and the rubber front bushes show their weakness. "Every action has an equal and opposite reaction",perfect classic phrase for the situation...this adverse reaction will thrust the diff in the other direction, yet the applied load via the engine is still pushing the other way. This push-pull effect amplifies itself rather quickly into the infamous "bang!bang!bang!" relatively quickly as more traction and power are applied. A system is only as good as it's weakest point.The RR being the only unsupported point of the diff, coupled with 3 other "soft" points ability to move make it the obvious place for the reaction to begin...
As far as the M suspension arm/link design goes, that doesn't really demand upgrading until you really start demanding more from the car in a road course situation,toe changes aren't really an issue here. I'm assuming the root of that question is based on why the cars pitch side to side aggressively as well, and i'll answer that as well. The unsupported RR causes a more aggressive adverse reaction on that side causing an imbalance in power applied to both tires. There's some other interesting factors involved here: the half-shafts are different lengths, the right side being the longest. Taking the M lsd apart unveils a host of other concerns, It's unlike any other clutch type lsd I've ever seen, I find myself asking "why the hell did they do this" rather often. It's a rather bizarre arrangement,both axle cup lengths are drastically different, the right side has all the clutches, the left side is essentially the same as an open diff, for it's driven via spider gears only and has very little support due to it's short spline shaft. I have seen a failure of bearings on a car that has seen track time. If you look at any other clutch style lsd like the OS Giken,etc,etc-the "proper" way to use a successful lsd in a performance application involves equal length input shafts,clutches on both sides of a center spider gear assembly. It's worth saying that side to side unpredictability has been tamed drastically by providing proper support to the oem diff unit, but I reccommend an OS lsd to anyone seeking the ultimate setup. They are pricey and require modification/machining for this platform, but are worth every dime if your serious about pushing your M on any track. Hopefully that covers your questions and then some
