Post from another forum by Tim Engel:
Lotus Europa and Elan bolt-on steel wheel
Size: ................ 13" x 4.5"
Bolt Pattern: ... 4 x 3.75" (4 x 95.25 mm)
Offset: ............. +1.09375" = +1 3/32" = +27.78 mm (+28)
Backspace: ...... 3.875" = 98.125 mm ("98 mm" backspace)
Frontspace: ..... 1.705"
Total Width: ... 5.580"
Center Bore: ... 2.262" ID
Triumph Spitfire bolt-on steel wheel (most/ earlier years)
(data on
http://auskellian.com)
Size: ................ 13" x 4.5"
Bolt Pattern: ... 4 x 3.75" (4 x 95.25 mm)
Offset: ............. +0.75" = +3/4" = +19.05 mm
Backspace: ...... 3.50" = 88.9 mm
Frontspace: ..... 2.00" = 50.8 mm
Total Width: ... 5.50" = 139.7 mm
Center Bore: ...
Weight: ........... ~13 Lbs
Triumph Spitfire bolt-on steel wheel (later with round holes)
Size: ................ 13" x 4.5"
Bolt Pattern: ... 4 x 3.75" (4 x 95.25 mm)
Offset: ............. +0.8125" = +13/16" = +20.6375 mm
Backspace: ......
Frontspace: .....
Total Width: ... 5.580" (5.625?? for an even fraction ??)
Center Bore: ... 2.250" ID = 57.15 mm
Panasport Alloy Minilite Clone
Size: ................ 13" x 5.0"
Bolt Pattern: ... 4 x 3.75" (4 x 95.25 mm)
Offset: ............. +0.79" = +20.07 mm
Backspace: ...... 3.750" = 95.25 mm
Frontspace: ..... 2.250" = 57.15 mm
Total Width: ... 6.000" = 152.40 mm
Weight: ........... ~ 13 Lbs
Panasport Alloy Minilite Clone
(I used on Type 65 Europa S2 with 185/70 all around)
Size: ................ 13" x 6.0"
Bolt Pattern: ... 4 x 3.75" (4 x 95.25 mm)
Offset: ............. +0.6299" = approx +5/8" = +16 mm
Backspace: ...... 4.052" = 102.93 mm
Frontspace: ..... 2.793" = 70.93 mm
Total Width: ... 6.845" = 173.86 mm
Rota RB (Minilite clone)
Size: ................ 15" x 6.0"
Bolt Pattern: ... 4 x 3.75" (4 x 95.25 mm)
Offset: ............. +0.984" = +63/64" = +24.994 mm (+25 mm)
Backspace: ......
Frontspace: .....
Total Width: ...
Center Bore: ... 2.248" ID = 57.1 mm
Weight: ........... 13.8 lbs
Cosmic Mk 2 Alloy (visually similar to Brand Lotus alloy)
Size: ................ 13" x 4.5"
Bolt Pattern: ... 4 x 3.75" (4 x 95.25 mm)
Offset: ............. +0.6299" = approx +5/8" = +16 mm
Cosmic Mk 2 Alloy (visually similar to Brand Lotus alloy)
Size: ................ 13" x 5.25"
Bolt Pattern: ... 4 x 3.75" (4 x 95.25 mm)
Offset: ............. +0.9375" = +1 15/16" = +23.81 mm
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Offset, ET, and Backspace/ Frontspace
ET = Einpresstiefe, or Einpress Tief (?? not sure which is correct).
It's German, and refers to a class of machined "fit", "Pushed in
Depth", or "Insertion Depth".
As it relates to automotive wheels, ET is similar to OFFSET, but
with a difference. ET and Offset both refer to the distance from
the wheel's mounting face/ flange to the centerline of the wheel,
but the difference between Offset and ET is in how the centerline is
calculated.
OFFSET is calculated from a wheel centerline half way between the
wheel's tire bead seats on the inside faces of the rim flanges.
Since the real "design" concern is the actual position of the TIRE
relative to the car/ suspension (scrub radius), it's more accurate
to measure from the bead seats. That's what Offset does.
ET is calculated from a wheel centerline that is 1/2 the wheel's
total outside width... ie, halfway between the wheel's inner and
outer faces. Since the inner and outer rim flanges can have
different shapes and widths, it's possible for the resulting overall
centerline location to differ from the tire/ bead seat centerline by
a significant amount. Therefore, ET can differ from Offset by a
similar amount.
With ET, it's much easier to measure the overall wheel width, and
the distance from the mounting flange to the inside edge, than it is
with Offset to pick up the working width on the tapered bead seat
between the flanges, or to measure from the mounting flange to the
inner bead seat when a metal wall separates them. The bead seat is
nowhere near as easy to measure as the rim face is. Not impossible,
just not as easy. However, I question the value of being able to
easily measure the wrong attribute.
So why use ET? Convenience, ET is the lazy man's Offset. ET is
for those who don't care about suspension geometry, and just want to
know if the wheel will fit in the wheel well without interference.
Back in the day, it was common for inner and outer wheel flanges to
be different in thickness and/or shape. Especially on stamped or
rolled steel wheels that often had the outer edge turned outward to
stiffen the flange. The turned edge mated with nothing and just
hung in the breeze, so there was little effort wasted on matching
the inside-outside edges, or on repeatability. For those wheels,
OFFSET based on bead seats was the dimension commonly spec'd, and the
value you're likely to find quoted in vintage archives.
The vast majority of modern wheels have symmetric inner/ outer
flanges which means the possibility of an ET error is minimal; so
"everybody" uses the ET edge-to-edge measurement instead of the
Offset bead-to-bead measurement. "Everybody" meaning new wheel
manufacturers and sellers. Vintage owners, on the other hand,
should tread there with caution.
If you are reading vintage Offset specs and then shopping for modern
ET wheels, be certain to account for any inner-outer differences in
the original rim flanges. Especially if the new wheels are designs
that have been around for a long time and come off of old tooling
(Minilite, Panasport, Superlite, Cosmic... etc
).
I've checked the OEM steel rims from my Europa S2 (same as Elan),
and the turned edges on the inner and outer flanges are different.
Similarly, the Speedline alloys on my Esprit S2 are different inner
to outer, but the Elite-Eclat alloy wheels are symmetric with nicely
machined faces (ie, zero ET error). In my stash of Lotus wheels,
only the Elite-Eclat rims could accurately use ET.
Offset has a significant effect on many elements of a car's
suspension, including suspension geometry, the scrub radius of the
steering system, clearance between the tire and suspension/ body
elements, clearance between the wheel and brakes, and the
relationship between the wheel faces and the bodywork (aesthetics).
To maintain a car's original handling characteristics and avoid undue
loads on bushings, ball joints, etc, the manufacturer's original
offset should be maintained when choosing new wheels unless there are
overriding clearance issues. Bling-Bling is not a valid reason for
taking liberties with Offset.
Zero Offset - The plane of the hub mounting surface is even with the
centerline of the wheel.
Positive Offset - The plane of the hub mounting surface is outboard
of the wheel's seat-to-seat centerline, toward the front or outside
face of the wheel. A more positive offset move the tire inward and
narrows the car's overall track width. Positive offset wheels are
generally found on front wheel drive cars in order to produce the
required negative scrub radius. It's also found in newer rear drive
cars simply since front wheel drive dominates in modern cars and
finding such FWD wheels is easier.
Negative Offset - The plane of the hub mounting surface is inboard of
the wheel's seat-to-seat centerline, toward the back or inside face
of the wheel. A more negative offset moves the tire outward and
widens the car's overall track width. In American vintage hotrod
terms, wheels with a deep negative offset were referred to as
"Reversed", or "Deep Dish".
Totally different...
Back Space is the distance from the wheel's mounting flange face to
the backside face of the inner flange.
Front Space is the similar distance to the frontside face of the
outer (front) flange.
Back Space + Front Space = Total Width.
BS/FS are handy for calculating interference issues with suspension
arms, fender lips, and such, but have nothing to do with Offset/ ET
or suspension/ steering geometry. Since BS/FS are measured from the
wheel's inner and outer faces, they relate better to ET numbers than
Offset numbers; but you need to keep in mind that BS/FS and ET are
measuring different things.
