Here's a closer shot of the joint. Note the large round joint on the left, with the remnant of the threaded portion - badly bent - coming out the bottom (I'm assuming it just freely rotated to that position).
Unusual rear inner toe link failure
First of all, this wasn't my car!
An Elise had an unfortunate 'off' at a track day today; the car suffered a very heavy hit to the front 3/4, though the rear left wheel had enough of a hit to destroy the clamshell and fracture the brake disc where it connects to the disc's hat section. Anyway, the odd thing was the rear inner toe link on that impacted corner. With all the talk about this joint, checking its torque and all, I'd come to the belief that if it fails, it would be due to shear of the bolt itself. Hence the nature of the rear toe link braces, that put that bolt in double shear to increase its strength. Now, this Elise, we don't know exactly what happened to it, though the images show enough to judge how the impacts were. We DO know that the car dug in the dirt quite extensively, which could explain the failure here. You see, the rear inner toe link broke, but it didn't shear, the threaded portion of the link itself bent and finally snapped. See the photo below. It's a strange failure, any guesses as to why it broke like this rather than shearing the attaching bolt?
An Elise had an unfortunate 'off' at a track day today; the car suffered a very heavy hit to the front 3/4, though the rear left wheel had enough of a hit to destroy the clamshell and fracture the brake disc where it connects to the disc's hat section. Anyway, the odd thing was the rear inner toe link on that impacted corner. With all the talk about this joint, checking its torque and all, I'd come to the belief that if it fails, it would be due to shear of the bolt itself. Hence the nature of the rear toe link braces, that put that bolt in double shear to increase its strength. Now, this Elise, we don't know exactly what happened to it, though the images show enough to judge how the impacts were. We DO know that the car dug in the dirt quite extensively, which could explain the failure here. You see, the rear inner toe link broke, but it didn't shear, the threaded portion of the link itself bent and finally snapped. See the photo below. It's a strange failure, any guesses as to why it broke like this rather than shearing the attaching bolt?
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It should "fail" exactly the way that it did.MattG said:
In the first diagram below, the force (in red) is exerted along the toe-link to the ball joint. The ball joint resists this force by holding laterally in single shear with the mounting bolt (the blue lines). But part of the force exerted on the ball joint acts as a rotational moment around the ball joint. That is resisted as a torque (bending) of the ball joint mounting stud. The premature failure of the inner ball joint happens when the ball joint mounting stud is not tight - this allows the mall joint to move around and causes the mounting bolt to dynamically bend back and forth causing the bolt to fatigue and fail.
But the designed in "failure mode" is shown in the second diagram below. In this case, the force on the toe-link is greater than it should be (due to impact). The bending moment about the ball joint is greater than the strength of the threaded connection between the ball joint and the toe-link rod, causing that threaded section to yield (bend). When the force exceeds the ultimate strength of that section, it breaks, stopping the force from being transfered to the chassis by the ball joint. The ball joint fails, and the chassis is undamaged.
In the third diagram, the double shear joint is shown. Again, the force is in red, exerted along the toe-link rod. But there is not bending moment about the Heim joint at the chassis mount. The force is exerted laterally by the rod and resisted latterly by mounting bolts and brackets. There is no bending moment. The result is that if the force along the toe-link rod is too great (as from an impact), that force will be completely transfered to the chassis. There is no part that will bend and fail before transferring that load to the chassis. There is no sacrificial part. The joint is stronger, but the failure mode transfers the damage to the chassis.
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I stand corrected.MattG said:
Signed, Jon -- NOT an engineer.
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