Dougs951S

sorry did I miss something here? Isn't 1018 ~120% stronger in tension than cast iron?

Dubai944

I have looked at this a few times and not done it. Anyone got any quantitative data on the improvement gained from moving the pick up points? I know it's better in theory, but how about in practise from those who have raced both with an optimised stock carrier and a Kokeln or other version modified carrier?

What is the noticeable difference in handling characteristic, or measurable difference in grip levels, lap times etc. I am skeptical there is a significant improvement over stock that translates to a better handling/quicker car particularly once you are running everything solid and a very stiff spring and the suspension isn't moving much anyway. Weight saving is good. Simpler adjustment is good but not completely necessary. Not sure it's worth it.

s14kev

This is material failure in bending. Compressive strength should not be so important. Compressive strength comes in to play if the tube/square section is under pure axial load which it is not in this case. The trailing arm mounts and torsion tube mounts apply a bending load to the torsion tube. Pound for pound with wall thickness being equal, a square tube is more efficient in resistance to bending than round tube. In a torsion bar delete setting, a square structure is stronger. In a non torsion bar delete setting, the round tube is stronger since the tube is also loaded in torsion by the torsion bars and a round structure is stronger in torsion than a square structure.

TonyG

The loads applied to the tube are primarily at the ends of the tube at the spring plate, not at the inner trailing arm mount point.

TonyG

s14kev

I would argue against this. The trailing arm, spring plate and torsion tube form a triangular structure. As we know triangular structures are one of the most rigid constructs we can make. This triangular structure acts as an incredibly strong lever arm focusing a large bending moment at the inner trailing arm mount point. The location that this torsion tube has bent is exactly where it should have since we have a relatively short and weak square tube attached to two rigid triangular constructs at each inner trailing arm mount point.

s14kev

The force applied by even a relatively large sway bar should be trivial in this situation given the location of the attachment points.

TonyG

Almost all of the acceleration and braking forces generated upon the tube are at the spring plate, which is the end of the tube.

The only real force applied to the middle of the tube is at the inner trailing arm mount. And those are primarily forces generated by cornering.

TonyG

s14kev

Hinges/rod ends/joints make no difference. In the end, it all forms the same rigid triangular construct regardless of whether the sides of the triangle are attached together by joints or solid welds

s14kev

That brace is to minimize twisting/flexing of the relatively flexible pressed steel control arm which causes toe/camber changes. It does not really play a significant part in the loads that the inner mount point sees which in the case of an E30 is really a rear axle carrier. If we are pedantic about it then yes it does take some load off the rear axle carrier between the two control arm pivots but does not have a great effect of the rear axle carrier inboard of the inner mounts.

Van

The factory part has a cast piece in the center - which has the torsion bar splines machined into it and the "collision ears" that interface with the torque tube. Then there are steel "pipe" pieces on each side that are welded to the casting, and then another cast piece at each end, which support the spring plate bushing and bolt onto the aluminum brackets.

eman930

Sounds to me like A lot more testing needs to be done, Some computer stress test ect. We can throw ideas around forever, The fact is this part was made in someone's garage/shop, and it seems that there is a lot more going on with loads and stress on that part then we truly know for sure. This just proves that A LOT of engineering goes into each part and the slightest change in angles, materials, and mounting points changes 10 fold on what those forces do.

Van

Looking at the pictures of the bent part again, it appears that the bend is in the vertical plane - e.g. the center of the square tube was pushed downward; as opposed to forward or rearward.

To make a bend in this direction, the inner portion of the control arms would have to be pushing down.

Because you're using a coil-over system (since obviously the torsion bars are gone), the weight of the car's chassis (the sprung weight) is pushing down on the shock/spring, which is then pushing down on the control arm about half way between the contact patch and the pivot point.

If North Coast Cab is using 1000 lb springs, and presumably stiff dampers to go with them, it's quite likely that a large bump in the road, while cornering right, caused the rear wheel to move up suddenly.

Because of the shock damping and heavy spring, the shock didn't compress much, and because of Newton's law about overcoming inertia, the car's sprung weight stayed pretty still. So, the upward motion of the wheel caused the control arm to pivot around the lower shock mount, which put a downward force on the inner control arm mount, thus bending the bar down.

The short answer is this: energy from bumps has to go somewhere... in this particular instance, the force needed to bend the bar was less than the force needed to compress the shock/spring.

Oddjob

Torsion bar carrier center piece is not cast iron. Absolutely not. Chance that its cast steel, but looks very much like a steel forging to me, as w/ the ends.

mikey_audiogeek

Agreed... and this force is applied via the cantilever beam that is the cast aluminium carrier arm. So does that suggest that the crossmember is not as strong as the hub carrier?

I can't tell just from the photo whether the crossmember has bent down or up - can someone confirm? (don't want to assume anything here)

mikey_audiogeek

The crossmember sits BELOW the torque tube, right?

And the redesigned crossmember sits higher than the factory one, right?

And the gearbox drops under acceleration, right? (torque reaction)

So...