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Thread: The Z8 frame
01-20-2006 08:10 AM #1
The Z8 frame
Here is one idea. If the Z8 subframe is well designed it should be able to deal with road shock and not distort. The theory of a frame being out of alignment due to an average pothole is unlikely, very unlikely. But another issue may be involved here, and I hope BMW has checked it out--torque twist. Under hard acceleration, or when shifting gears, the front end and rear end of a rear wheel drive car tend to "twist" counter to each other. This is especially evident in high powered cars like the BMW. Steel frames have no problem with this issue due to their tendency to to be able to take a bit of flexing without distortion. Aluminium is different. It is more "stiff"than steel and will tend to crack and give way under stress. This is why torque values are so important with aluminium components. Over torque wheel lugs on an aluminium wheel and a steel wheel and you will see the first crack and the second give. If the BMW frame is stressed by engine torque it will fail if it has not been designed to deal with it. And this may be the issue as noted in a recent article about the Z8 body panels getting strangely out of alignment.A torque tube could be installed between the engine and the differential to prevent this twisting. I would not want to have any bracing welded onto my car. Welding is very tricky with aluminium and and create a weak point that later fails. Hope this will not be an issue for you guys. You've still got a great car as far as I'm concerned.Does 10% ethanol gas hurt the BMW fuel system?
01-20-2006 04:56 PM #2
Re: The Z8 frame
Thanks for your thoughts on this issue but I believe your conjecture is way off base. To begin with, aluminum is an alloy and there are a great variety of aluminum alloys, each having very different properties. The aluminum used in the Z8's frame (6063) is specifically designed to deal with twisting forces and the structure itself is one of the stiffest automotive frames ever designed, capable of withstanding 10,500 Nm of force per degree of deflection and boasts 21 Hz of bending rigidity and 23 Hz of torsional rigidity. Moreover, there are supercharged Z8's running around with almost double the stock torque output with no reports of frame damage.
As far as welding and the Z8's chassis is concerned, you should know that 187 feet of welding is used to assemble the frame, and damaged components can be replaced with new ones that are welded in by BMW's Aluminum Repair Centers. If done properly, no weaknesses are created as a result of these repairs.
Furthermore, the reported damage is not consistent with the kind of "torque twisting" you described: there have been no reported cracks in the frame. What is happening is the aluminum used to make the shock towers is deforming coupled with the possibility that one or more support braces may be bent. The problems reported are very much a localized event, not systemic.
As far as a torque tube fix is concerned, I must point out that torque tubes are normally used to prevent the rotational forces generated within the rear mounted differential from negatively affecting the performance of the rear suspension of the car, not to prevent frame damage. In the Z8, those forces are effectively dealt with by the car's multi-link independent rear suspension design.
Given the nature of the reported damage, the frame's extraordinary stiffness, and the compliant coupling of the suspension, I do not believe a torque tube would serve any beneficial purpose on a Z8 and I can see no way the Z8's chassis is suffering from the kind of "torque twist" you described.Grease Monkey
01-20-2006 06:41 PM #3
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01-20-2006 08:41 PM #4
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