Machining wheels to change offset

[Bob_260]

I'm a newbie to the Ratsun community, and I hoping to tape into the tremendous amount of expertise.

 

I recently purchased a 1974 Datsun 260z and my first priority is to make sure the car goes where I point it and STOPS! So I'm upgrading to big disc brakes all around, adjustable Coilovers, bigger swaybars and polyurethane bushings. The big disc brakes have also meant that I need bigger rim. I don't want to add flares, but I have rolled the fender lips so I can put as much rubber as possible on the ground. Based on everything I've read and 8.5" wide rim was the biggest that would fit without flares. So I ordered a Rota RB's (17x8.5 with a +4mm offset). With all that said as background, here is my questions:

 

1) Obviously Rota has machined the rims down to a +4mm offset, how much more material can be machined off the 8.5" rims? Can I go to a Zero offset? -5mm -10mm? -15mm?

 

2) Other than than being mindful of the structural integrity of the rim are there any suspension or setup issues I need to be aware of?

[Z-train]

Don't do it.

[thisismatt]

You're already in the positive...you can't go negative.

[datzenmike]

Positive off set moves the rim inward away from the fender. Zero -5mm -10mm and -15mm moves it out into the fender. I assume you want positive off set so you don't need flares.

 

I would never feel safe milling the center of a mag rim to get the off set right. Order them in the correct off set.

[ggzilla]

Obligatory Datsun photo

 

[Bob_260]

You are right I meant that I want to move the wheels in towards the strut to get the wheels is the fender well without flares. So I'd like,to increase the offset in the "positive" direction. Thanks for sharing the "Obligatory Datsun Photo"!

[5t341tH]

usually there is about 5mm that can safely be machined away. I have not done it myself before, but I've had acquaintances do it before

so you would be +4 to +9

[Z-train]

And when you go into a tight turn,you have no idea if the rim will exit the turn in one piece. Mondo bad idea.

[Bob_260]

The basis of my question really comes down to the fundemantal question: If a wheel manufacturer offers multiple offsets for the same wheel, (+4mm, +10mm, +15mm....etc) where does the material come from to provide the different offsets?

 

As I look at it there are only a couple of options:

 

1) to optomize cost they cast one base casting, with excess material on the center section, and during the machining clean up phase they machine the various offsets that they plan to offer to the market. This would optomize their casting set up cost and would give them the greatest flexibility during the manufacturing process.

 

2) the second option is to determine through FEA analsis what the minimum center section would be to handle the stress any more importantly the cycle loads, then cast evey wheel to that dimension. But if they did that they would have relocate the wheel spokes on every wheel to position the backface to the proper offset location. In other words. If they offer a +4mm offset wheel (like Rota) and a +10mm offset wheel, the spokes of the +10mm wheel would have to be moved in by an additional +6mm to generate the additional offset.

 

When I look at the various wheels on the market, I don't get the impression that the spokes are moving. So this leads me to believe the wheel manufactures are utilizing option (1). So if I machine an additional +6mm, I'm not violation any of the original design considerations and therefore not risking a "wheel-off". I'm just taking the same material they would have taken during the original machining process.

 

So where is the material coming from?

[thisismatt]

I think you are correct that the center thickness is what changes, as you can definitely see the thickness difference between the +4 and +40 in the 15x8s. However, they also bore the lug nut recesses deeper for the thicker +4, and much less for the +40 for example, otherwise the thick centered +4 would need over an inch and a half longer studs. If you machine your wheels you will thin out yor lug nut seat, possibly getting into the taper to where your nuts bottom out on the hub or rotor hat, or just thin them out to a dangerous thickness.

[Jester]

I think you are correct that the center thickness is what changes, as you can definitely see the thickness difference between the +4 and +40 in the 15x8s. However, they also bore the lug nut recesses deeper for the thicker +4, and much less for the +40 for example, otherwise the thick centered +4 would need over an inch and a half longer studs. If you machine your wheels you will thin out yor lug nut seat, possibly getting into the taper to where your nuts bottom out on the hub or rotor hat, or just thin them out to a dangerous thickness.

 

This makes a lot sense. Never really thought about the lug recess as well as the base thickness. Good point! 

With that being pointed out I would just get the correct rims so you don't take a risk of busting a wheel and wrecking your car. 

 

Just my $.02       

[]2eDeYe]

The basis of my question really comes down to the fundemantal question: If a wheel manufacturer offers multiple offsets for the same wheel, (+4mm, +10mm, +15mm....etc) where does the material come from to provide the different offsets?

 

As I look at it there are only a couple of options:

 

1) to optomize cost they cast one base casting, with excess material on the center section, and during the machining clean up phase they machine the various offsets that they plan to offer to the market. This would optomize their casting set up cost and would give them the greatest flexibility during the manufacturing process.

 

2) the second option is to determine through FEA analsis what the minimum center section would be to handle the stress any more importantly the cycle loads, then cast evey wheel to that dimension. But if they did that they would have relocate the wheel spokes on every wheel to position the backface to the proper offset location. In other words. If they offer a +4mm offset wheel (like Rota) and a +10mm offset wheel, the spokes of the +10mm wheel would have to be moved in by an additional +6mm to generate the additional offset.

 

When I look at the various wheels on the market, I don't get the impression that the spokes are moving. So this leads me to believe the wheel manufactures are utilizing option (1). So if I machine an additional +6mm, I'm not violation any of the original design considerations and therefore not risking a "wheel-off". I'm just taking the same material they would have taken during the original machining process.

 

So where is the material coming from?

 

 

This is a massive assumption.

 

The barrel is usually shifted to increase or decrease offset not the casting thickness.

 

Also, casting is not that difficult a process for a company that is set up for it and multiple castings do not increase costs as much as you might think. 

[Bob_260]

As I mentioned in my previous response, I'm just trying to understand where the additional material is coming from to support the different offsets the manufactures offer. I think you are putting forward a 3rd option that I had not considered.

 

3) I assume when you say "the barrel is usually shifted to increase or decrease offset.." You are referring to the center section of the wheel. (Sorry I don't know the terms for the various parts of the wheel). If the Spokes remain in the same location relative to the tire OD, then wouldn't the arch of the spokes have to change to reach center section? Wheels with large positive offsets would need much flatter spokes and negative offsets would need deeper spoke arches to move the center section in. That would mean that looking at the wheels you would see a visual difference depending on the amount of offset. Looking at the pictures I don't get that impression, but I could be wrong, I have not seen the wheels in person. But my impression looking from the outside a17x8.5 +4 mm offset, looks identical to a 17x8.5 +25mm.

 

There are a number of YouTube clips showing offsets being machined down on a CNC milling machine, that's why I assume that they machined the backface to change the offsets

 

The response from "thisismatt" regarding the depth of the lug nut recess being different depending on the offset, to insure sufficient thread engagement on the lug nuts was a good insight that I had not considered. Has anyone come across a website that shows the technical dimensions of the Rota wheels?

 

I really appreciate the feedback and discussion on this! Thank you.

 

 

Sent from my iPad

[INDY510]

It's not going to be safe ... Every wheel I've ever tried has the exact same thickness between the lug nut seat and rotor. That's regardless of Rota, Watanabe, Panasport, SSR, Hayashi, Drag, OEM Datsun, Enkei, etc. ... they were all exactly the same thickness, and none required longer lug bolts. I have seen what you are talking about, when the lip size doesn't change, but the lug nut seats will be recessed deeper for a lower offset wheel.

 

Here is a 16x7 +0 Panasport, with the lug nuts fully flush with the wheel face:

 

 

 

 

And here is a 16x7 +22 Panasport, with the lug nuts sticking out 22mm farther because the rim disk is thinner:

 

 

Both Panasports were the same new type of Panasport Z, and the rim lip was almost the same size, but the lugnuts were recessed on the zero offset wheel to account for the thicker disk.

[INDY510]

If you look at new Work or Volk wheels, they specify the thickness of each disk size and how it changes the lip size.. you can see on this chart how O-disk wheels have a 10mm higher offset than A-disk wheels with the same lip size:

 

[bonvo]

why dont you wait and get the rims in and see what kind of clearence you have? might have to slightly pull the fenders and run a stretch tire but i dont see why it wouldnt clear without endangering yourself and everyone on the road 

[]2eDeYe]

As I mentioned in my previous response, I'm just trying to understand where the additional material is coming from to support the different offsets the manufactures offer. I think you are putting forward a 3rd option that I had not considered.

 

3) I assume when you say "the barrel is usually shifted to increase or decrease offset.." You are referring to the center section of the wheel. (Sorry I don't know the terms for the various parts of the wheel). If the Spokes remain in the same location relative to the tire OD, then wouldn't the arch of the spokes have to change to reach center section? Wheels with large positive offsets would need much flatter spokes and negative offsets would need deeper spoke arches to move the center section in. That would mean that looking at the wheels you would see a visual difference depending on the amount of offset. Looking at the pictures I don't get that impression, but I could be wrong, I have not seen the wheels in person. But my impression looking from the outside a17x8.5 +4 mm offset, looks identical to a 17x8.5 +25mm.

 

There are a number of YouTube clips showing offsets being machined down on a CNC milling machine, that's why I assume that they machined the backface to change the offsets

 

The response from "thisismatt" regarding the depth of the lug nut recess being different depending on the offset, to insure sufficient thread engagement on the lug nuts was a good insight that I had not considered. Has anyone come across a website that shows the technical dimensions of the Rota wheels?

 

I really appreciate the feedback and discussion on this! Thank you.

 

 

Sent from my iPad

 

 

The barrel is the outer portion of the wheel. The center section is the portion with your lug pattern. 

 

This is a three piece wheel but it shows how you can change offsets and wheel widths by using different components. 

 

 

Most old hot rodders would flip the barrel on their steel wheels to get deeper offsets, known as reverse dish. Old school lip enhancment, you can even widen them 

 

Steel barrels

 

 

http://www.fordmuscle.com/archives/2006/02/Steelies/

 

 

Notice that all these examples use the same wheel center. 

 

 

 

 

Machining an entire wheel is not much different, other than the programming setting everything. You can choose whatever offset you want to machine to, but it still adjusts the offset the same way. 

[Dime Dave]

VTO sells wheels where the offset can be custom ordered, but you must order a custom drilled wheel to do it.

 

A VTO wheel of any given diameter/width, regardless of offset is made using the same mold. If you buy an un-drilled VTO you can have it machined to a custom offset before the bolt circle is drilled into it. The lug nut seat to wheel mounting surface is the same thickness on all VTO wheel, so the lower the wheel offset is, the more recessed the lug nuts are from the face of the wheel (like the Panasport Z's shown above)

 

As an example of how VTO wheels can be ordered in custom offsets, VTO offers a 16x7 +12 that is manufactured as un-drilled for custom patterns. This wheel can be machined to a custom offset from +12 to as much as +35 before the bolt pattern is drilled. 

[Bob_260]

Thanks to everyone how responded back to my post. I'm convinced that this is not the way to go. I come away a little more educated on "Offsets" and "Wheel construction" . I really appreciate being able to tap into the knowledge and experience of the community! As a Newbie to the Z community the whole issue of what wheel/tire/suspension/car model combinations work with or,without Flare is more than a little confusing. I greatly appreciate your help

 

I'll go ahead and get my tires mounted. (In response to one of the post: I didn't want to mount the tires if I was going to mill the wheels) As I mentioned originally I've already rolled the fender lip. If I a clearance issue, I may run a little camber in the rear or as suggested pull fenders to create the necessary clearance.

 

Once I get the suspension back in the car I'll, try and post some follow-up pictures.

 

Thanks everybody

Bob_260