Custom Offsets & PCD - a curiosity, and a question

[Calps]

Hi all,

I'm doing the groundwork for a new set of wheels at the moment, and noticed that a few sellers have wheels that are ordered to customer specification, in that the PCD and the offset are made to whatever I request (within a given range).

In my (very limited) understanding of offset, the number indicates how far in/out the wheels sits in relation to the hub. I had long associated this with the amount of concave a wheel had, assuming that the spokes/arms of the same wheel model would differ in angle from wheel to wheel when comparing a high offset vs low offset version of the same wheel - similar to how CSLs on the front of a car usually have a flatter curve than those on the rear for example. I'm guessing this is not the case for these custom specced wheels, as they presumably all start off from a common base? Do they just machine away the back of the centre bore until they achieve the desired offset?

For context - I'm interested in a set of wheels with a nice degree of concave to them, and want to know if the variable offset on these types of wheels is a good indicator of that or not.

As a follow on Q - what is the max offset that can be had on a set of 8.5j front and rear on a Z4C? (I had been working with front 19×8.5 ET38 & rear 19×9.5 ET43, but may switch to a square 8.5j setup and interested if any change needed for the rear)

 

[True-Blue]

You can definitely use a wheel with more poke than et 43 on the rear, giving the potential for greater concave.

I realise it’s a 9J and 18’’, but have a look at the way a genuine style 224 fills the arch on an E85/E86

 

[Calps]

You can definitely use a wheel with more poke than et 43 on the rear, giving the potential for greater concave.

I realise it’s a 9J and 18’’, but have a look at the way a genuine style 224 fills the arch on an E85/E86

Thanks True-Blue. Are there known ranges for min/max offsets for front/rear?

I found the following from offset guru Mr Wilks, which has been my point of reference so far, but interested if there are changes needed if the wheel width on the rear is reduced/increased (reduced to 8.5j in my case):

"For Si Z4 as a rule of thumb aim for front 8.5j et 38/42, Rear 9.5j et 35/42"

 

[True-Blue]

You can definitely use a wheel with more poke than et 43 on the rear, giving the potential for greater concave.

I realise it’s a 9J and 18’’, but have a look at the way a genuine style 224 fills the arch on an E85/E86

Thanks True-Blue. Are there known ranges for min/max offsets for front/rear?

I found the following from offset guru Mr Wilks, which has been my point of reference so far, but interested if there are changes needed if the wheel width on the rear is reduced/increased (reduced to 8.5j in my case):

"For Si Z4 as a rule of thumb aim for front 8.5j et 38/42, Rear 9.5j et 35/42"

There are some that definitely won’t work, and Mr Wilks ‘rule of thumb’ is good starting point. I’d look at the specs for the 224’s for the rear and compare to this on ‘will they fit’ with the options you are considering. I can’t remember too much about the front options, but 224’s fit almost perfectly on the rear of an E85/E86 - I think front 224’s work with a 5 or 10mm spacer.

The other thing to consider is that tire sizes also have an impact on rub/no rub -255/35/19 is a good compromise on 9.5J wheel and a lot cheaper than 265/30 or 275/30 options.

 

[Mister T]

This is probably one of the best guides for fitment:

https://support.apexraceparts.com/hc/en-us/articles/360048839813-E85-86-Z4-Non-M-Wheel-Tire-Fitment-Guide

To answer your other question, most custom one piece wheels come in 3 different 'disk' options, this determines how concave the face is and also how much brake caliper clearance there is. You can then select a range of offsets specific to each 'disk'.

 

[Calps]

This is probably one of the best guides for fitment:

https://support.apexraceparts.com/hc/en-us/articles/360048839813-E85-86-Z4-Non-M-Wheel-Tire-Fitment-Guide

To answer your other question, most custom one piece wheels come in 3 different 'disk' options, this determines how concave the face is and also how much brake caliper clearance there is. You can then select a range of offsets specific to each 'disk'.

Thanks Mister T, that makes a lot of sense.

The link is is a good read, however stops short of square setups for 19" wheels. It only gives guidelines for 19's in a staggered setup, and the smallest rear measurement is for 9.5J, but I could probably work back from that.

It looks like somewhere between ET35 or ET38 would be do-able for 19" 8.5J rears. What do you think?