Machining wheel mating surfaces to change offset?

[Cloudsurfer]

So heres one for the smart, engineering types:

Is it possible for a good machine shop to re-machine the mating surfaces of a wheel to increase the offset? I have turbo twist hollow spokes on my car (18x10 ET40) that I would love to take to about ET45, so it possible for a shop to take off 5mm of the mating surface to achieve this?

I realize this would have to be done with extreme precision, but I'm inclined to think its possible since the machining of those surfaces are the last step performed in normal wheel manufacturing, and a shop that can precisely deck cylinder heads I would think could perform this properly.

Any input on this would be greatly appreciated.

Patrick

[MNBoxster]

Hi,

You do not want to remove any material from the wheels...

Happy Motoring!... Jim'99

[Rodger]

Normally a shop would use a vertical mill with an endmill bit to resurface a head. They run the bit back and forth across the head until it's level. It's crucial to ensure the head is properly leveled prior to starting the milling, to ensure the head is milled evenly.

With a wheel, I'm sure the preferred method would be to install the wheel on a lathe, then move in with a cutting bit at the center of the wheel, then bring the bit outward to remove the necessary amount of material. This would probably be done in several small cuts.

They're two completely different processes, but any machine shop (automotive o otherwise) with a large lathe can do the job.

[Cloudsurfer]

Jim-

I fully realize that this is probably not Kosher as far as Porsche is concerned, but I dont see why, if dont correctly, this would cause any concern for failure.

The real concern as far as I see it, is ensuring that the wheel's centerline is aligned perfectly parallel to the lathe so that, once milled, the mating surface is still parallel to the wheel centerline to avoid shimmies and vibration, etc.

Patrick

[MNBoxster]

Hi,

It can be done and on a Lathe, not a center pass machine. But, you're going to significantly weaken the wheel at it's main attachment point, not adviseable...

Happy Motoring!... Jim'99

[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