What's offset, PCD and Backspace?

[Bananaman]

http://www.rsracing.com/tech-wheel.html

[Captain Colonial]

Another helpful link for those changing wheel size:

 

https://www.1010tires.com/Tools/Wheel-Offset-Calculator

 

Note that earlier alloys often had their offset stamped as "E" instead of "ET" for some reason - if yours has something like E20, that's likely to be its offset, but regardless of "E" or "ET", always measure, do not assume!

 

Fun fact: ET is the short form of the German word 'Einpresstiefe' which literally translates as 'insertion depth'.

[Mark (smokey mow)]

I posted this a few years back as a basic method of measuring the offset.

 

http://forum.wscc.co.uk/forum/index.php/topic/80975-wheel-offset/?p=809176

 

(Actual excellent post from link above is shown here - Mod)

 

I'll try to simplify this by using pictures in this example using a Mazda MX5 wheel.

Begin by taking the wheel off and laying it face down on the garage floor.

Lay a straight edge across the top and measure the distance from this to the floor (in this example it's about 190mm), divide this by 2 and it will give you a measurement for the centre of the wheel (190/2 = 95mm).



Now repeat the process but this time measure the distance from the straight edge to the mounting face on the inside of the wheel (here that's about 140mm)



Now take the result of the first calculation (95mm) and subtract this from the second measurement (140mm); so 140-95 = 45mm.

So the offset of this MX5 wheel is 45mm.

The PCD of the wheels (or Bolt Circle for our American cousin) is measured from centre to centre of opposite holes so here you can see that this mazda wheel is a 4-stud 100mm PCD.



 

[funky munky]

Looking to get new wheels this year and using the excellent info on hear sussed that currently the spec is:

 

15"

7j

pcd 108 (ford) x 4

Offset 15

 

is this quite common ? car is fixed arch 1997 widebody, hoping to get some Team Dynamics jobbies

[stu999]

Your wheels are almost definitely 108PCD (unless Mazda SDV...)

However, the width and offset are best confirmed using your original wheels before purchasing - and ideally measuring front and rear too :-)

[stu999]

Sorry, perhaps misunderstood, are they the measurements you have made?

If so, in answer to your question (and my revised understanding) is yes! :-D

[funky munky]

Hi Stu

 

think I have followed correctly, first measure floor to top of wheel (using straight edge) was 210mm then highest part of inside wheel to top was 120mm.So working out 120mm - 105 mm = 15, wheels are the same all round ( no pun intended)

 

Thanks

Pete

[stu999]

Yup, sounds right. And yes, quite a popular size, shouldn't have too much trouble sourcing the wheels you want :-)

[Flying Tango]

Quick question..... The distance from the inside rim to the floor for the first measurement will be different with a tyre on. Are people taking this into account?

I've got miniltes on my narrow bodied westy and have worked out an offset of 12.5 with the tire on. Suspect it'll be 15 without the tyre!

[Dommo]

If the wheel/tyre combo is laid flat you should be able to measure how high the rim is off the floor to take that into account

[Martyn Vann - Warwickshire AO]

14 minutes ago, Flying Tango said:

Quick question..... The distance from the inside rim to the floor for the first measurement will be different with a tyre on. Are people taking this into account?

I've got miniltes on my narrow bodied westy and have worked out an offset of 12.5 with the tire on. Suspect it'll be 15 without the tyre!

Yes, it should be without the tyre. Try deflating the tyre and/or applying weight to the rim to get it to touch the floor if you want to be more accurate.

[Matta]

On ‎30‎/‎03‎/‎2017 at 13:26, Martyn Vann - Joint Warwickshire AO said:

Yes, it should be without the tyre. Try deflating the tyre and/or applying weight to the rim to get it to touch the floor if you want to be more accurate.

It shouldn't make a difference having the tyre on? Assuming the inflated tyre extends an equal distance both sides of the rim and you measure across the 'top' of the tyre to the floor in this example, the offset calculation would be exactly the same as without the tyre. IMO obviously!

Matt

[Martyn Vann - Warwickshire AO]

1 hour ago, Matta said:

It shouldn't make a difference having the tyre on? Assuming the inflated tyre extends an equal distance both sides of the rim and you measure across the 'top' of the tyre to the floor in this example, the offset calculation would be exactly the same as without the tyre. IMO obviously!

Matt

Yes, you are right but it would be more accurate to measure from rim to rim as there can be some variation in sidewalls!

[SootySport]

ET or offset has nothing to do with the tyre, it's a rim measurement.  It's the distance from the inner hub face to half the distance of the wheel rim width.  
SEIW Kit bodywork usually requires ET 23 for 7" wide rims and thats what the factory supplied TD 1.2 wheels come as.   The front arches are often changed and repositioned post build so affect the required ET value on front wheels.