Date: Mon, 17 Nov 1997 22:17:00 -0500
From: [email protected] (Dave Williams)
Subject: more than you ever wanted to know about wheel offset
To: [email protected]

I got a message from Michael Sen-Roy about wheel offset, pointing out
what I'd quoted from the SAE Handbook didn't jive with what I said in my
own text.  I got up, dragged the book back off the shelf, and checked
again.

He was right.  Duh.

SAE spec says a negative offset wheel sticks out, positive offset is
tucked in.

I *knew* otherwise, and even keying the text into a message didn't
register.  I hate it when that happens.

So where did I get the idea that things were the other way?

Puhn's "How To Make Your Car Handle", which I bought in 1981.  Page
126, Figure 34, has a nice drawing and a detailed caption showing
negative is in and positive is out.  The adjacent text also says
"Wheels built so the center of the rim is inboard of the wheel-mounting
surface, called *negative offset*, makes the track narrower."

Puhn was consistent, but backwards from the 1968 SAE spec.  And
Puhn is where I first learned about offsets, and it stuck.  And it
was reinforced by my earlier wheel purchases, from American Racing,
Center Line, and Saber Wheel, who *also* referred to offsets in the
same way Puhn did.

Somewhere along the way the wheel industry has changed to the SAE
terminology.  I hadn't noticed, even after recently being involved in
the purchase of some custom made 17x11 wheels for a friend's track car.
Boyd's and Fikse, among others, appeared to know what we were talking
about, and when we placed the order with Boyd's we got what we had
ordered.  Of course when it came down to writing the check we gave them
backspace figures instead of offset...

Here on Fordnatics, apparently quite a few other people used Puhn's
terminology; take a look at how many messages in the archives refer
to factory Mustang wheels as negative offset, even during the Offset
Wars a couple of years ago.  If anybody thought otherwise, nobody spoke
up.  Ditto for the autox, wheeltowheel, racefab, and homebuilt lists.

Time to check my library:


*Agrees with Puhn:

Don Alexander's "Performance Handling" from 1991. Oddly, it barely
mentions offset.  Pages 30 and 31 refer to "inset" and "large offset",
in context implying he's using Puhn's terminology.

"Peterson's Basic Chassis, Suspension, and Brakes", 1970, page 148,
refers to larger offsets as making the track wider, which would be the
way I learned it.  Positive numbers are larger than negative numbers,
right?

"How To Modify Your Mini" by David Vizard, 1977, page 179, also refers
to larger offsets as widening the track.

Paul Van Valkenburgh's "Race Car Engineering and Mechanics", 1992 ed.,
page 13, refers to larger offsets as wider track.

"Theory and Practice of Chassis Tuning" by Norbye, 1972, page 121,
refers to "larger" offsets as outboard.


*Agrees with SAE:

SAE Handbook, of course.

"Chassis Engineering" by Herb Adams, 1993, doesn't specifically say
positive or negative, but his dimensions come out SAE-style.

"Race Car Fabrication and Preparation" by Steve Smith, 1977, does it
SAE style.


*Doesn't say:

"New Directions in Suspension Design" by Colin Campbell, 1981,
doesn't mention offset at all car as as I could find.

"The Complete Handbook of Front Wheel Drive Cars" by Norbye, 1979,
mentions offset in passing but no details.

"Competition Car Suspension" by Allan Staniforth, 1991, doesn't
mention offset.

"The Anatomy & Development of the Sports Prototype Racing Car" by
Ian Bamsey, 1991, doesn't mention offset.

"The Car and Its Wheels - A Guide To Modern Suspension Systems" by
Norbye, 1980, doesn't even talk about wheels, just suspensions.

"Steering, Suspension, and Tyres" by JG Giles, 1968, doesn't talk
about wheels either.

"Advanced Race Car Suspension Development" by Steve Smith, 1974,
zip.

"Racing Car Design and Development" by Terry and Baker, 1973, zip.

"Racing and Sports Car Chassis Design" by Costin and Phipps, 1961,
zip.

"Fundamentals of Vehicle Dynamics" by Gillespie, 1992, zip.

"The Sports Car: Its Design and Performance" by Campbell, 1978, zip.


Finally, Tim Remus' "Boyd Coddington's How To Build Hot Rod Chassis"
from 1992 says,
---
"OFFSET:  This is where it gets confusing... Whether offset to the
outside of the car should be referred to as positive or negative
depends on whom you talk to."
---
No fake...


In conclusion:

Even though the Puhn terminology appears to be most common, the SAE
terminology is apparently catching on.  All SAE terminology is
theoretically "recommended", but since the FMVSS (Federal Motor Vehicle
Safety Standards) requires SAE terminology for Federal standards, most
OEMs are going along with all of it.  (like the change from "ECU" to
"PCM" for engine computers)

Considering the enormous hassle of the whole thing, I think I'll
just stick with backspace figures...


PS: it seems strange to me that someone whose business was selling
wheels wouldn't know all this stuff already...