Verify backspace calculation

[Jimmy Sean]

I’m getting ready to order my wheels and would like someone to verify that my math is correct.

The wheels are 18x9.5 ET20. Do you come up with a backspace of 6.04 inches?

Thanks

[ProdigyCustoms]

If it is a Camaro it should be 5.5" backspace?

[Jimmy Sean]

If it is a Camaro it should be 5.5" backspace?

Thanks for the reply,

Agreed, if not customizing/modifying the car.

I would still like to know if my calculation for backspace of the wheel is correct. FWIW the rear end in my firebird is out of a Chevelle.

[MarkM66]

I’m getting ready to order my wheels and would like someone to verify that my math is correct.

The wheels are 18x9.5 ET20. Do you come up with a backspace of 6.04 inches?

Thanks

How can we calculate with no measurements?

[Jimmy Sean]

How can we calculate with no measurements?


Ok let me rephrase the question. I'm trying to figure out what the distance is from the mounting surface to the back edge of the wheel. I calulated that from the mounting surface to the back edge of the wheel would be 6.04 inches. The wheel is 9.5 inches wide and has an ET of 20. Is that correct?

9.5*25.4 = 241.3 mm
241.3mm/2 = 120.65mm
120.65mm + 20mm(ET) + 12.7mm (1/2 inch for the lip converted into mm) = 153.35mm
153.35mm/25.4 = 6.0374 inches

Thoughts?

Thanks

[wiedemab]

ET 20

Is that the offset of 20mm? If so is it positive or negative?

You need to know the width of the lip of the wheel. Offset is measured from the centerline of the wheel as determined by the inner and outer tire mounting surface. The backspace is the measurement from the mount surface to the inner most part of the wheel lip. Just to make sure I'm making sense, by the width of the wheel lip, I mean the distance from the tire mounting surface to the outside of the wheel's lip. I usually assume this at .5", but depending on the wheel it could vary.

Using your numbers and assuming that the offset is positive (more dished inward) here is what I come up with.

9.5"/2 + (20mm/25.4"/mm) + .5" = 6.03" Backspace

Here is a good diagram I found on TireRack

http://www.tirerack.com/images/wheels/tech/offset.gif

[Jimmy Sean]

ET 20

Is that the offset of 20mm? If so is it positive or negative?


Yes as I understand it ET is equal to offset and for this wheel it is positive 20.

http://www.matey-matey.com/offset_diagram.gif

"Offset is usually stamped or engraved into the wheel and is measured in millimetres of 'ET' [ET is the short form of the German word 'Einpresstiefe' which literally translates as 'insertion depth']"


Thanks

[wiedemab]

[ET is the short form of the German word 'Einpresstiefe' which literally translates as 'insertion depth']"

Thanks

Sounds Kinky????:lol:

[Jmartorana]

to be exact 6.023622"

Plug your numbers into this calculator..... http://www.rims-n-tires.com/rt_specs.jsp?item=Wheel-Tire%20size%20comparer&showRim1=1&showTire1=1&sw1=235&ar1=60&bd1=15&rd1=15&rw1=7.0&et1=1&showRim2=1&showTire2=1&sw2=275&ar2=40&bd2=18&rd2=18&rw2=8.0&et2=13&text1=&text2=

[ProdigyCustoms]

If the rear is out of a Chevelle, you should more then 6" backspace. Would need to know exactly what year Chevelle, but it COULD be up to one inch wider per side then a stcok Camaro rear.