09-16-2009, 03:39 PM
Seek to understand,^Value
Location: San Jose, California
Join Date: Mar 2008
Mein Auto: 02 BMW 525i M54 auto 130K
It always bothers me when people practically stop for speed bumps. Isn't that why they put a suspension on the vehicle in the first place? I barely slow down for 'em. Always have. Never before had this violent shudder.
Originally Posted by TheStig
25 mph is pretty fast for a speed bump
OK. Lemme take it step by step again ... (please doublecheck my math):
Originally Posted by MatWiz
Well, it's not mid air like you see in the cheesy 80's TV cop car chase in San Fransisco.
- BMW E39 approaches speed bump ...
- As the BMW "hits" the speed bump, the coil spring compresses (dampened by the shocks) ...
- On the ramp of the speed bump, the suspension is still compressed and the wheel travels some angle upward as it climbs the front ramp of the speed bump ...
- However, the speed bump is shaped such that there is a drop off on the back ramp ...
- So at the top of the speed bump, the wheel is still traveling on an upward force vector (essentially "lightening" the load on it) and the suspension is still compressed with potential energy ...
- KEY POINT (1): Just past the top of the speed bump, there is now barely any tire rubber touching the road (as there's that drop off on the other side of the speed bump yet the wheel force vector doesn't know that yet ... and ... worse yet, the coiled-spring suspension is now beginning to unload as the wheel sees nothing underneath it (sorta' kind') as it gets to the other side of the speed bump ...
- KEY POINT (2): Since one or more front shocks are "bad", the less-dampened suspension on one or both front wheels unloads the potential energy coiled up in the spring for that particular shock more than the BMW designers intended it to ...
- In effect, this additional springiness perhaps "pushes" one or both front wheels even further off the road contact surface than the engineers imagined ...
- At that point, the front wheel or wheels is, effectively, pushed up into the air by the three forces:
a) the force vector is upward at the speed-bump angle ...
b) there's only a thin stripe of road surface (traction) under the tire ...
c) the less-dampened springs are unwinding too rapidly just at the worst moment ...
- At this point, you purposely brake rather firmly ... (You see, they're right: it's all your fault!)
- Since there's very little to keep the front wheels moving, the front wheels slow down a LOT during this braking (relative to the rear wheels) ...
- If only one shock was bad, then that less-dampened front wheel is more unloaded than the other front wheel and now the ABS is REALLY confused since the two rear wheels are going at one rpm, the good front side is spinning at another rpm, while the bad front side (the one with the bad shock so the coil spring is unloading it even more) is spinning at yet a third (slower) rpm.
- This differential spinning makes the ABS dizzy and it reacts violently to the situation by shivering violently. Its equilibrium system had fully expected two of the three forces, but, the third force, i.e., that of the unduly undampened coil spring "pushing" one or both front wheels up in the air was unexpected, in effect causing the BMW ABS to puke violently.
- The solution is to give back to the ABS the three forces it expected (by removing the additional force of the undampened coil spring on one or more front wheels).
Is this the scenario?
I know you don't think it's that something called "bushings" ... but ... just to edify me ... (and others) ... can you show us on this RealOEM diagram for my 2002 525i where that "bushing" that some people think is also part of the problem lies?
Last edited by bluebee; 09-16-2009 at 03:52 PM.