GUIDE ROD
Attachment 1143090
The Guide Rod and Trailing Arm react thrust and braking loads. A simple analysis of the in-service load levels is impractical due to the sophisticated geometry of the multi-link suspension.
The OE Guide Rod is a pair of pressed steel halves welded together with inboard and outboard rubber bushings. LH and RH parts are identical as the welded assembly is straight. The two halves of the weldment form two parallel columns that fail by buckling away from each other at a load in excess of 14,000 lbs, based on testing. The unit would be stronger if it were fully welded, but the partially welded design provides for controlled failure.
Attachment 1143091
The M3 Guide Rod is forged aluminum with inboard ball joint and outboard rubber bushing. The forged bodies of the LH and RH arms are identical but installation of the bushings makes them handed. The bodies are curved (downward), not for installation clearance issues, but to ensure that the compressive failure of this member will be by progressive bending. The ball joint used at the inboard location effectively doubles the stiffness and halves the deflection under axial loading versus the OE Guide Rod, which will alter (and presumably decrease) compliance steer effects.
Attachment 1143092
Hello,
Thank you for all this info.
Did you toke note of the compression spring rate of the guide rod before it colapsed?
By the way, I believe the reason for the M3 aluminium arms being curved and flexible is to be compatible the over-restrained geometry of the 5 link suspension.
This means that if you make all suspension arms and the subframe solid enough, the suspension would be locked.
In real world terms, if you make all arms solid, the weakest itens in the suspension arrangement would be forced to flex or bend, creating a failure condition after a number of cycles.
There is a reason for most race cars suspension hub being controled by a 3 ball joint arrangement. Because 3 points define a plane with no need for arm flexibility.
So why BMW spend money designing a 5 link suspension? To acomodate toe-in increase only during cornering and braking, and not during bump/rebound suspension movement, to add stability to the car without adding tyre wear and fuel consumption typical of lots of toe-in setup.
Hope this helps
Joao