Machining wheel mating surfaces to change offset?

[Cloudsurfer]

Jim-

I thought about the potential weakening to the wheel center, however (and please do correct me if I am wrong), the center hub area is solid alloy, as its where the lug bolts go through, so taking off a few mm of material shouldnt have a profound effect on the overall integrity of the wheel as a unit, or am I completely off base with my thinking?

Patrick

[eslai]

Hard to say how much that would weaken the wheel. It would depend on the wheel design. I say it's risky, especially for such a wide wheel.

[MNBoxster]

Originally Posted by bmwm750

Jim-

I thought about the potential weakening to the wheel center, however (and please do correct me if I am wrong), the center hub area is solid alloy, as its where the lug bolts go through, so taking off a few mm of material shouldnt have a profound effect on the overall integrity of the wheel as a unit, or am I completely off base with my thinking?

Patrick

Hi,

Well, there are several things to consider. Tensile Strength will be the result of the Alloy used, it's thickness and temperature.

Since lightness/strength is the goal of the Wheel Manufacturer, I doubt that there is a lot of excess material. Remove some, and you can quite easily end up with an Ultimate Tensile Strength (the point at which the metal fails) which is much closer to the forces which the Wheel can experience, even the removal of just a few mm.

Additionally, Aluminum is subject to Fatigue failure (referred to as its Endurance Limit). The design solution is to increase the thickness so that deflection is kept within the allowable range. Again, machining away material can easily compromise the design.

Then, the Machining itself must be considered. If not done properly, the process can create lots of micro-cracks, which could spread when under stress leading the Wheel failure.

The forces to consider are not those normally encountered, but rather those brief sudden high impacts such as potholes and such where the forces against the Wheel are multiplied many hundreds of times. I would be more concerned with the cumulative, not the instant effects of these impacts. This cumulative effect is a concern for non-altered wheels, more so if you start machining them.

Too much information is lacking to state unequivocally that a few mm wouldn't make a difference. They could, in fact, make ALL the difference. The only way to be certain would be some destructive testing, which you cannot do. Again, I think it's ill-advised...

Happy Motoring!... Jim'99

[Cloudsurfer]

Well, factoring all that into account, I think I agree that I'd rather not be the guinea pig on this experiment. As you pointed out with regard to the wheel's design, these are factory hollow spokes, which as we all know were designed to be as light as possible. I might feel better doing this to a heavier (and less expensive) wheel.

All said and done, we're not going to try this. Judgeing by the pics that I posted on the "wheel offset question" thread, am I going to run into outer clearance issues with my 18x10 ET40 rears once I get the proper (265/35) tire on and lower the car?

Patrick