ALFAGTV6.COM

[Greg Gordon]

That's a good point about the see-saw action. Still, if you do this and watch, you can clearly see when the back end stops moving and see how much the chassis flexes, and it's quite a lot. I should probably re-phrase that next time I make changes to that page.

Greg

[Mats]

If you drive in a parking garage and listen when you go from a flat area and turn down a ramp you can actually hear the doors move around in the seals when the first front wheel unloads...

[MALDI]

The torsion bars transfer the car's (front end) load to the center of the chassis. This is one of the more clever aspects of the car's suspension. This makes the front-end jacking test a bit of a red herring. I won't argue how stiff the GTV6 is relative to other vehicles (because we don't have actual numbers!) but I would say it is stiff as it needs to be for a road going car. Of course, high power modifications change everything and do not fit the original designer's intensions.

Also, note that the torsion bars on 911s (older ones that have them) face the opposite way from the GTV6 (toward the front) thus transferring load even further toward the edges of the chassis. Torsion bars were used instead of coil springs to give more trunk space, why the torsion bars were not turned around I am not sure, probably because they would interfere with the foot wells.

[Mats]

Modification of the truth that one, the load does indeed go into the chassis at the LCA fixing points, the torque from the bar goes into the chassis bar at the middle of the car though.

[MALDI]

the load does indeed go into the chassis at the LCA fixing points

Mats, wouldn't you say these loads are largely lateral (in the plane of the road surface) and not vertical? It would seem so since both control arms are hinged at the chassis, therefore, vertical loads should be at a minimum (until you hit the bump stops). I have tried to find data on this topic (for transaxle Alfas or any other car) and have always come up short. Have you ever come upon this information?

[Mats]

Simple mechanics, if you have a vertical force at the contact patch between the road and tire you will have a vertical force in the ball-joint and one in the hinge at the chassis. They all cancel each other out or something will accelerate (i.e. equal size on the one from road surface to tire, spindle to LCA and LCA to chassis).
The torque is a different question.

[MALDI]

But the torque in the torsion bar is resisting the normal force applied at the contact patch. If that were not the case the car would simply settle onto its bump stops. Also, the mounting points of the control arms at the chassis are rubber. Even in compression rubber will not withstand the weight of the car. In a suspension with normal coil springs the springs are mounted metal-on-metal at both the control arm and chassis ends.

My (unsubstantiated!) guess is the >90% of the front normal (normal to the road surface that is) load of the car is handled by the torsion bars and less than 10% is loaded onto the control arm hard points. The cross member under the front seats is what is primarily holding up the (front) end of the car, it is under a twist load as the two torsion bars torque it in opposite directions. In this way chassis flex is limted and the chassis can be lighter (for which we are all thankful).

The (much lower) lateral forces due to cornering and vehicle accelertion/deceleration (these could be called longitudinal) must be taken up by the control arm mounting points.

[Mats]

Take a broom-stick, add some kind of lever at one end and hang 10Kg at the end of the lever, now grab the other end and try to keep the broom-stick straight out and the lever level.
Will the broom-stick somehow keep itself level by magic or will you have to keep it level by adding force with your hands?
Would it be a simpler task if you add some kind of support that keeps the broom-stick level so you only have to add torque to keep the lever level?

Which one would best represent the torsion bar and LCA fixings?

[MALDI]

Ahh, I see what you are saying now!

Care to hazard a guess as to what percentage of the static load on the front end of the vehicle is supported by each? Is it opposite to what I said: 90% on the control arms and 10% on the torsion bars? 50/50 perhaps?

[Mats]

100% on the control arm and add the torsion on that.
Torsion per definition cannot take any vertical load in a pivot point.

[MALDI]

Thanks, Mats.

[Mats]

No Problem

[grant]

OK, new issue:

I want to switch from Bilsteins to Koni Yellows. Will I blow Koni Yellows if I run the rears only with a #350 lb/inch spring? (about 7kg/mm).

Reasoning, the RSR coilovers have like zero rebound (for comfort? Or V6 cars are just to damned heavy!) and the bilsteins are working too well in the back - bobbing. Any thoughts?

[Micke]

I use yellow Koni's in the back together with 520 lb/in springs. No problems.

[grant]

Grazie!!