I know you said you want to make them 18x8.5/10, but post up your current wheel specs with ETs and I'll help you get there .

From the picture you posted it looks like the pad height is about 25cm, which is roughly one inch(correct me if I'm wrong, the perspective of the picture is a little odd). With that, I should be able to come up with a recipe for you to get it spot.

 

I just got a set of BBS E28 as well
They're so beautiful, but they came off of a 993 Turbo (widebody). Mine is just a 964 C4 narrow. Obviously, I need to modify them to get them fit. I'm on the same boat as the other post. The current offsets on them are 11x18 ET38 and 10x18 ET50. The inner wheel halves are both 7.5" right now. My question is, what should be the width of the inner barrels in order to get them fit correctly on a narrow body C4?

thanks

minh

 

Hey Minh, will be glad to help!

Great looking wheels you have, and I'm curious how those center caps are in there! Mind taking pictures of them front and back?

Also, I'm a little confused by the measurements you gave me. The width of these wheels; is this measuring from bead seat to bead seat or edge to edge? There's a big difference when calculating offset and sizing. The reason I ask is because the 18x10 ET50 would mean with a 7.5 inch barrel, your outer shell should be 2.5 inches, but your picture indicates to me that it likely is not. Do you mind double checking all of those measurements for me with a tape measure?

I'm happy to explain how if you need guidance.

Thanks!

 

Thanks!!! Gonna send these out to Rotiform for refinishing and possible re-barrel. Don't know if I should go OG BBS color like silver or gold. Or should I go a different route. Maybe a different color. Not totally decided yet.

These are E28s so they are magnesium like what Thomas pointed out. The backspacing is 6.5/7.5. The exact spacing as BHCfarkas'. So I'm assuming the ET is close to his. I'm still trying to figure out how to measure it. Once I get the measurements, I'll let you know.

 

Hi Farkas
Thanks for replying. Yeah I can take a few more snaps for you to see the back later(at work right now). You're right about the rear outer barrels are 2.5".
I came up with that offset measuring the wheel backspace and center to the back method
I'm not sure if I did correctly either.

So can you help me figuring out what combination of barrel sizing to get in order to get the correct offset for the fronts and rears?

Thanks Farkas!!

 

Minh, let's double check your measurements after work so we can be sure!

This is how I would like you to measure your fronts and rear widths:


Also, let's get another unit of measure for your backspace as you did before.

Let me explain why we're doing this again:

The difference between the bead to bead measurement and edge to edge measurement is right around an entire inch difference! That means if you gave me 11 inches wide for the rear, I don't know if that's measuring from edge to edge or bead to bead. If it's bead to bead that means it's true width edge to edge is 12 inches! This difference matters when doing calculations.

The reason I'm asking specifically is because the dimensions you have given me don't totally add up. The rear wheel you picture was said to be 11 inches wide. You said all the inner barrels were 7.5 inches wide and you pictured the rear outer lip to be 2.5 inches. Add that up and you get 10 inches for your rear wheel instead of 11. Technically both numbers COULD be correct, but it depends from which measurement- bead to bead or edge to edge.

My guess is that the measurements you gave me were from edge to edge. If I go with each inner barrel being 7.5 inches, that would make your rear outer lip 2.5 inches and your front outer 1.5 inches. This could mean the offsets you calculated are off by 14+/- mm.


Now that you have the wheels in hand, it will be much easier for you to learn. The measuring is easy and you need only two measurements to figure out offset: Wheel Width(edge to edge) and Wheel Backspace.

The math is also very very simple:
1. Wheel Width / 2 = X
2. Backspace - X = Offset

I usually measure in inches so if that's the case you want to multiply your answer by 25.4 to get millimeters.

 

Just as info, my rims are bulit like this for my narrow body 964:

Front 7+1" ET50
Rear 8+2" ET65 (using a 21 mm spacer, gives ET44)

The centres to achive this is are front 0228041 and rear 0228040. 0228 means E28 and you'll find the number stamped on the front side of the centres. These centres are designed for the 964 RSR. You can use the 040 in the front to, but Porsche ordered the updated version 041 to get a different shape of the rear side of the spokes that would fit better over the big red calipers first used on the 964RSR.

So not E28 centres are the same. The ones used on the first 996 cup give different ET's, as an example.
Thomas

 

This is so educational you guys. Thanks for the detail info. I'll have to check the centers again for those numbers.

Thanks Farkas, I'll measure the wheels again and let you know.

much appreciated.

minh

 

Thomas
What if they're all 0228041?
Will that be a problem?

 

No, that will work. The ET is the same, it is just the inside radius of the spokes that are different. You wont see it mounted on the car. I'm not even sure it is needed but Porsche wanted it, and they got it

Thomas

 

That's good to know Thomas.
With the setup that you currently have, is there any rubbing issue?

 

Hi Farkas
So here are a few snap shots of the measurement. You're totally right about the measurement being way off. I measured them bead to bead using your method. Ended up with this:

rear 10x18 ET50.8
front 8.5x18 ET69.85

I also snap the outers measurement but I'm not exactly sure where to read...
It looks like 1" for front and 2.5" for rear. What do you think Farkas?

thanks for your help on this

minh

 

Good work, Minh! I agree with what you have measured!

Front: 1" outer, 7.5" inner = 8.5" bead to bead
Rear: 2.5" outer, 7.5" inner = 10" bead to bead

I see you took a stab at the offsets yourself. What was the measured backspace you got for the front and rear wheels? Remember, when you're calculating offset you use the end to end width! If you take the measurement from bead to bead and add an inch, it will also equal the end to end measurement. The importance of measuring bead to bead tells us the actual lip/barrel sizes. They don't measure lips/barrels by end to end, it's bead to mating surface.

Therefor:
8.5 bead to bead = 9.5" end to end
10 bead to beat = 11" end to end

So for example on the front wheel I'm going to use 6.5" as your backspace on these wheels(I realize that's not what it might be, but just for example):

9.5" / 2 = 4.75"
6.5" - 4.75" = 1.75"
1.75" x 25.4(convert to mm) = +44.45 offset

Again, that was only for example using 6.5" as the backspace... but what did you end up measuring, Minh?

I'm trying to explain as much as possible to everyone so they can do figures themselves, so hopefully I'm not losing anyone!

 

Awesome Farkas!
So it looks like my backspace is 7".

 

Great read. Very helpful.


I'll have some rs of my own to add to this thread shortly