Ed Scherer

Well, of course.

Then again, it's pretty unfortunate that we have to deal with inches and the rest of the mess of imperial and U.S. units anywhere.

Tires and wheels, though, are an almost unbelievable mess of units. Probably some interesting history there.

racer

No doubt, is the reason that most "americans" use the term "backspace" and mention it in inches.

Europeans, and lovers of the metric system, enjoy mm.. and ET is shorthand for the German word for offset.

You are in a forum for fans of a German (nee metric) car, hence the need to adapt!

Thank goodness you don't recall the Michelin TRX tires. They were a failed, in the US at least, METRIC measured Tire and Rim system. BMW used them in the mid '80s.. I think Ford tried them too on the Mustang.. Maybe Ferrari on the 308 as well. http://en.wikipedia.org/wiki/Michelin_TRX

On the otherhands.. since these questions about "what wheels will fit my car" etc have been asked since the begining of forums, there is a wealth of knowledge available to answer such questions.

Jerry Feather

The conversion factor of 25.4 will do it every time.

Jerry Feather

littleball_s4

Whenever I have to explain the complete tire "name", my students can't believe it. It is normally a funny class.

So 225/45 R16 92W

225 -> width in mm. Not very accurate, though.
45 -> aspect ratio. So how many "hundredths of widths" the tire side wall is. (not the most intuitive thing in the world) Not accurate at all. Rounded to the closest 5 multiple, to add fun.
16 -> rim diameter (of course, to be coherent with the width... in inches)
92 -> load rating. In no rational unit that I know of.
W -> speed code. In no rational unit that I know of.

It's quite high in my ranking of stupid engineer standards. Sure it has to have some logic (it's just it's beyond my limited mind). Something similar to interference fit standard or thread tolerances standard.

Mrmerlin

and since no wheel / tire discussion is complete without this , here you go have fun.

http://www.google.com/url?sa=t&sourc...honpNArOY9eXLw

Vilhuer

ET is shorthand for Einpresstiefe = insertion depth.

Not rational but easy to decipher.
http://www.tirerack.com/tires/tirete....jsp?techid=35

mark kibort

oh brother. Look, "not really" means that you will be hard pressed to find a drawing, like the one from Fiske that DOESNT show backspacing at the edge of the rim, AND that the mid line isnt the mounting surfaces distance edge, divided by 2, just as rim width uses those points as well.

So, prove me wrong and provide some evidence, before you "blast" me.

Neil Forn

Well I think I'm beginning to get it. The offset is the distance from the centerline of the wheel to the mounting face. So then the backspace less the offset should give us the centerline. I assume that the primary objective is to keep the center line of the wheel as close as possible to the original location in order to avoid changing the geometry of the front suspension. In a sense there are two offsets to consider, the offset of the wheel and the offset of the hub. Probably the factory designs the wheel to match the offset of the hub. When we use wheels with a different offset we move the centerline of the wheel from the designed location. Therefore when we use wheels with bigger offsets we move the centerline of the wheel inboard. Then we could restore the offset by inserting a spacer. I had always thought that you could reduce offset by machining the mounting face of the wheel. But, if you remove metal it would increase the offset. So if the factory hub had a 50mm offset, to use a round number, and the wheel had a 75mm offset we could restore the geometry by adding a 25mm spacer.

Sorry if this is old news to the old pros out there. I am finally starting to see the light. At least I hope so. Tell me if I’m wrong. I’ve never had clear understanding of the relationships between different offsets. In fact I thought it was the opposite.

Neil

Mrmerlin

FWIW dont start machining the rear side of the wheel .
this will reduce the grip length of the lug nut seating area and weaken the wheel it will then be easier for the nuts to be pulled through the seating surface and the wheel will come free

Vilhuer

ET number is one of those concepts which seem very hard to understand for most people. Plus sizing is another.



When wheel width and ET changes it is good idea to keep ET large enough that wheels centerline remains inside imaginary line drawn through upper and lower ball joint. In '78-86 suspension this is slightly larger at -16mm while S4 has -10.2mm as below picture shows. IIRR later 7.5" and 8" factory setups get closer to zero but they don't go over to positive side. If they did handling would suffer. Especially in braking.

Neil Forn

Don't worry I'm not planning any machining ... at least not yet. Did you ever notice that on the inside of the factory flats there are lines in the casting for design 90 wheels?

Neil

Jerry Feather

I think I noticed recently in a picturte posted somewhere where the lines are not in some of the flat wheel castings. Too, where they are I think it would make them more like the club sport wheels than the design 90's.

Jerry Feather

Neil Forn

You are right Club sport it is.

Mrmerlin

the casting lines are not on the seating surface so those can be cut out.
And there is someone here that has cut them out, its a lot of work but with the right machine it can be done