Here's a thread I made for another forum years ago
A lot of people get confused by wheels, due to ET's, PCD's etc
So i thought i will try and explain them easily for you
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PCD
This is the "Pitch Circular Diameter"
Basically this is the number of holes, by the diameter
So if you have 4x100 wheels for example, then the 4 signifies the number of holes (in this case 4)
The 100 is the diameter (distance from one centre of the hole to another)
The way i think of it is, from the centre of the bolt hole to the centre of hub (this is radius) multiplied by 2 (which equals diameter)
This picture shows the PCD
Overcoming Close PCD Wheels
Close Size Wheels
Sometimes, if you get PCD's that are close to yours, you can use "wobble bolts" to help movements of 0.5-1.25mm per bolt, so 1.0-2.5mm overall
They sound dangerous, but they are not at all, they have special washers that when torqued up do not move about when driving, but they allow you to put close sized PCD wheels on
For example, mini wheels are 4x101.6 (or 4x4") as you can see there is a 1.6mm difference
So using wobble bolts, these wheels can be fitted to a 4x100 care
Adapters
Sometimes the PCD's are totally off, like porsche wheels have 5x130 PCD, but my car has 4x100
So using adapters (like spacers) you bolt the adapter to the hub with allen key head bolts
Then this then has a hubcentric centre, where you put the porsche wheel onto it, then bolt upto like usual
With the use of adapters, you can get desirable wheels you could not put on before onto cars they wouldn't normally fit, but there are always cases where you cannot put them on (is explained later on)
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Centrebore
Each make of car has different centrebores, which also need to be calculated, as this keeps the wheel "hubcentric" to the centre, so you don't get rotational vibrations (and doesn't put shearing stress on wheel bolts)
On the last pic it shows the centrebore too
Ways To Overcome different sizes
If you purchase new wheels, you can do 2 things depending on the size
Centrebore = too small?..... then you can get this increased from an engineering firm
Centrebore = too big?....well you can get "spigot rings", these are plastic/metal rings that fit on the inside of the wheels to make the Centrebore the correct size
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Offset (ET)
This is also very vital when buying new wheels for your car
ET = the distance from the rear mounting face to the centreline of the wheel
It can basically tell you how much your wheels will "poke out" or "tuck in"
Poke = literally that, how much it will poke out more that your wheels on at the moment (can cause rubbing on arches/illegal protuding wheels)
Tuck = how much it will go inwards more than normal wheels (causing possible scraping of the suspension struts)
Maths For ET
Things needed to work it out:
Width Of Wheel: from bead of wheel, to the other bead (as can be seen in the following pic)
Backspacing Of Wheel: This is the distance from the rear lip to the rear mounting face of the wheel (which is also shown on the above pic, again needs to be from bead.....or you can use the width above as lip to lip, so then when you do rear mounting face to rear lip it is constant)
If the width of the wheel is in inches, then multiply it by 25.4 to get total width
So here is an example of how to work it out
I have a wheel that is 8" wide, and has a backspacing of 153.9mm
ET = "Backspacing" - "Centreline"
Centreline = "wheel width" / 2 (divided by 2)
Centreline = 8 / 2 = 4"
Centreline = 4 x 25.4 = 101.6mm
ET = Backspacing - Centreline
ET = 153.9 - 101.6 = +52.3
*The positive sign, signifies the rear mounting face is closer to the outside of the wheel, meaning it will tuck more
A negative sign, signifies that the rear mounting face is closer to the inside of the wheel, meaning it will poke more
Helpful Sites To Work Out Differences
Wheel Offset Calculator
or
Online Wheel & Tyre Fitment calculator. Offset, Tyre stretch and Rolling Radius calculator
Typing in your old offset of wheels/width, then the same for the new, it will tell you how much poke/tuck you get on the wheels
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Common Problems
1) What happens to the offset if i get wheels that require adapters?
A) Well if you use adapters, the backspacing is the distance from the rear lip to the rear mounting face (where you would add on the adapter)
But the easier way would be to calculate the ET, and then just take away the width of the adapter
So for example if you have a wheel with ET52.3, and use a 20mm adapter, then the new offset will be ET52.3 - 20 = ET32.3
2) What width/ET wheels can i get away with without doing too much arch work?
A) Your best to use the site i posted above, and compare to the wheels you have, it will tell you how much extra poke/tuck you will achieve, so you can then get the measuring tape out, and see where the wheel will come upto on the car
Also the tyre you put on will also determine the scrubbing, that is why alot of people put slight stretch on tyres to overcome this
(stretch is putting a slightly smaller tyre, so the tyre wall becomes angled, eliminating/reducing scrubbing
3) Are there any PCDs that cannot be put together by using adapters?
A) Yes there are, there are a few that cause overlapping of bolt holes, so some companies will not make them, some have made a few ways to overcome them, but people think they are not safe, but that is always down to your own preference (I will not be held responsible if you buy them and they do fail, as I have ran them with no problems, but all cars are different)
4) How would i work out the ET of split rims, if the inside barrel stays the same, and I add wider lips?
A) This can be a little trickier, there is 2 methods to do this, you would work out the backspacing (as this will not change) then take away half the new width
So on some BBS RM's they are ET33, with a 6.5" width (0.5" lip), so Backspacing = ET + Centreline (rearranging the equation)
So backspacing = 33 + (6.5 / 2)
Backspacing = 33mm + 82.55 = 115.55mm
Then if you put on a 2" lip, this means the new width is 8" (as it had a 0.5" lip on it originally)
so new ET = "Backspacing" - "Centreline"
ET = 115.55 - 4"
ET = 115.55 - 101.6 = 13.95
ET14
Or you can do it another way i like is
If using a BBS RM again, ET33 on a 6.5" wide wheel (0.5" lip)
Then using a new 2" lip
It has a 1.5" wider lip
So divide by 2 (as it moves the centreline by half)
Increase in centreline = 0.75" (19.05mm)
New ET = old ET - "increase in centreline"
ET = 33 - 19.05 = 13.95mm
ET14
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Hope this helps, if you got any problems, give me a shout, and i will try and help if i can
always thought your Braids were 8x18!
