Bill K

Does anyone know what the offsets were for the Ronal R9 Wheels 16X7 & 16X8 for a 911SC or Carrera? The seller doesn't have this info and since I'll be using them for SCCA Solo II events, the offset cannot be more than .25" different than stock.

TIA

Bill

ModusPonens

Bill, since it doesn't seem like anyone knows the answer to your question I would suggest you measure the offset yourself. Here's a link that will help you out if you've never done this before. How to measure Backspace and Offset

I hope the wheels are usable.

Danno

Be careful with that website. It uses a table to convert backspacing into offset. There is a WheelWidth measurement in there and they don't differentiate between OutsideWidth (aka OverallWidth) and BeadWidth (where the tire sits).

Those Ronal R9 16x7/8" rims most likely would measure 8/9" on the ouside. THESE are the widths you want to use, NOT the 7/8" widths stated on the wheel-size.

ModusPonens

Danno's right I wouldn't use that chart; I used that link because of the picture, their way of calculating the offset sucks. Here's my way.

(Overall Width/2)-Backspace = Offset

1. Sandwich the wheel between two pieces of thick plywood.
2. Measure the distance in millimeters between the inside edges of the pieces of plywood (this is the overall width).
3. Measure from where the wheel seats against the hub to the back piece of plywood (this is the Backspace).
4. Plug those two figures into the above equation.

Sorry about the crappy link, I know I have a better one some where around here in the virtual pile of paper in my computer.

ModusPonens

I browsed through my links and found a better one for helping you to understand offset.

http://www.usacomp.com/Offset.htm

Danno

Yeah, but that second site makes a slight error. Don't know if you got your equation from them. But the offset naming convention has switched in the last couple of decades.

It used to be that wheels with the mounting surface inboard of the centerline was said to have POSITIVE OFFSET. And wheels that had the mounting surface outboard of the centerline (like FWD cars and our Porsches) had NEGATIVE OFFSET. In which case, this equation works fine:

Let's try out a sample calculation with our Fuchs 16x8" wheel. Measuring the actual dimensions, we get:

BACKSPACE= 5-3/8" = 5.375"

OverallWidth= 9.0"



Plugging into the above equations, we get:

(Overall Width/2) - Backspace = Offset

(9.0/2) - 5.375 = 4.5-5.375 = -0.875" = -22.2mm



If you go ask the guy at the parts counter for a "Fuchs 16x8" wheel with -22.2mm offset", he'll just look at you and go, "huh?". That's because we all know that the 16x8" Fuchs wheels have 23.3mm offset (instead of -23.3mm), but why?

My assumption is that with the development of FWD cars and ABS systems, manufacturers needed more room behind the wheels and started to use wheels with the mounting surface more and more outboard of the wheel's centerline (at that time "negative offset). But it was too verbose to talk to everyone saying "wheel with X negative offset" so it was conveniently shorted to "X offset" with the negative assumed. This became common usage and everyone considers the outboard mounting surface as just "offset" (no negative).

So now the equation we really want to use is this:

Testing it out on our 16x8" Fuchs wheels, we get:

Backspace - (Overall Width/2) = Offset

5.375-(9/2) = 5.375-4.5 = 0.875" = 22.2mm offset



Which is a lot closer to the 23.3mm offset that's stamped on the wheel itself (no negative).

Again, I believe my 944 Wheels FAQ site sufficiently explains wheels measurements. And this diagram from it clearly shows the relationship between backspacing and offset:

ModusPonens

Whether or not the offset is positive or negative merely shows the relationship to the centerline. When using the way I calculate the offset if the result is negative it means the offset is from the centerline toward the outside of the car; the way you calculate the offset a negative result means the offset is from the centerline toward the inside of the car. The negative/positive doesn’t change the amount of offset, just it’s direction. Having the negative mean inside or outside the centerline is just a matter of taste.