From Tim Engel on the Yahoo/IO list:
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.
