Wheel Guide
Results 1 to 10 of 10

Thread: Wheel Guide

  1. #1
    BFI Stalwart
    suj's Avatar
    Join Date
    Aug 2012
    Location
    Birmingham
    Posts
    674
    Car Specs:
    Year
    1997 / 1999
    Model
    E36 / E38
    Engine
    328i / 740iL
    Fuel
    Petrol
    Thanks
    685
    Thanked 513 Times in 368 Posts


    Donate Please click the
    image to support
    BMWForums.info
    Click Here To Contribute To Our Site

    Wheel Guide

    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

    ----------------------------------------------------------------------------------------------------------------------------------------------

    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)

    ----------------------------------------------------------------------------------------------------------------------------------------------

    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

    ----------------------------------------------------------------------------------------------------------------------------------------------

    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

    ----------------------------------------------------------------------------------------------------------------------------------------------

    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

    ----------------------------------------------------------------------------------------------------------------------------------------------

    Hope this helps, if you got any problems, give me a shout, and i will try and help if i can

    Suj
    Last edited by suj; 01-10-2012 at 09:28. Reason: My OCD'ness to layout.

  2. The Following 8 Users Say Thank You to suj For This Useful Post:

    ABM (01-10-2012),Chuckb (02-11-2012),Devil (21-10-2012),Ferret (01-10-2012),Mark@Vorny.co.uk (01-10-2012),NealP (01-10-2012),Spirit (01-10-2012),wanda (01-10-2012)

  3. #2
    ...is not Kevin Finnerty
    Moderator
    ABM's Avatar
    Join Date
    May 2012
    Location
    Somerset
    Posts
    4,680
    Car Specs:
    Year
    -
    Model
    -
    Engine
    -
    Fuel
    -
    Thanks
    2,943
    Thanked 4,561 Times in 2,731 Posts


    This site's also very useful for working out what will work and what won't Online Wheel & Tyre Fitment calculator. Offset, Tyre stretch and Rolling Radius calculator

    Good guide

  4. The Following User Says Thank You to ABM For This Useful Post:

    suj (01-10-2012)

  5. #3
    BFI Stalwart
    suj's Avatar
    Join Date
    Aug 2012
    Location
    Birmingham
    Posts
    674
    Car Specs:
    Year
    1997 / 1999
    Model
    E36 / E38
    Engine
    328i / 740iL
    Fuel
    Petrol
    Thanks
    685
    Thanked 513 Times in 368 Posts


    Quote Originally Posted by ABM View Post
    This site's also very useful for working out what will work and what won't Online Wheel & Tyre Fitment calculator. Offset, Tyre stretch and Rolling Radius calculator

    Good guide
    Thanks dude, yeah I got that link up there lol

    I made this a while back for another forum, so thought i'd share it.

    Helped a few dub boys when I was into them

  6. #4
    ...is not Kevin Finnerty
    Moderator
    ABM's Avatar
    Join Date
    May 2012
    Location
    Somerset
    Posts
    4,680
    Car Specs:
    Year
    -
    Model
    -
    Engine
    -
    Fuel
    -
    Thanks
    2,943
    Thanked 4,561 Times in 2,731 Posts


    So you have, I missed that

  7. The Following User Says Thank You to ABM For This Useful Post:

    suj (01-10-2012)

  8. #5
    Work, Work, Work & No Play
    BFI Supreme
    Devil's Avatar
    Join Date
    Apr 2012
    Location
    Norfolk
    Posts
    3,240
    Car Specs:
    Year
    2013
    Model
    E82
    Engine
    120d (N47D20)
    Fuel
    V-Power Nitro+ Diesel
    Thanks
    4,854
    Thanked 1,737 Times in 1,303 Posts


    Legend! Really good write up here!

  9. The Following User Says Thank You to Devil For This Useful Post:

    suj (23-10-2012)

  10. #6
    BFI Stalwart
    suj's Avatar
    Join Date
    Aug 2012
    Location
    Birmingham
    Posts
    674
    Car Specs:
    Year
    1997 / 1999
    Model
    E36 / E38
    Engine
    328i / 740iL
    Fuel
    Petrol
    Thanks
    685
    Thanked 513 Times in 368 Posts


    No problem dude, always here to help

  11. #7
    BFI Noob
    Mapo's Avatar
    Join Date
    Nov 2012
    Location
    Bulgaria Varna city
    Posts
    10
    Car Specs:
    Year
    1991
    Model
    850 K55
    Engine
    m70
    Fuel
    petrol
    Thanks
    0
    Thanked 4 Times in 3 Posts


    Hello ,and thanks!
    I am working on a project on which I use BMW components and hava a real huss around these matters . Working on a Polish car-Upgrading{modyf.}.Front axle is ready-using orig.BMW wheels with Is-47.I would like to use a little bigger Is-#`cause even these a wider more than I would like...`wondering if does greater "Is -#"exists...

  12. #8
    BFI Stalwart
    suj's Avatar
    Join Date
    Aug 2012
    Location
    Birmingham
    Posts
    674
    Car Specs:
    Year
    1997 / 1999
    Model
    E36 / E38
    Engine
    328i / 740iL
    Fuel
    Petrol
    Thanks
    685
    Thanked 513 Times in 368 Posts


    Ermmm...sorry I know you are from Bulgaria, and your english is better than my Bulgarian, but I don't understand what you have written?

  13. #9
    My car,my rules and anti oem resident
    BFI God you need to get out more!
    Spirit's Avatar
    Join Date
    Apr 2012
    Location
    kings lynn-norfolk
    Posts
    6,190
    Car Specs:
    Year
    2002
    Model
    e39 m5
    Engine
    5.0 v8
    Fuel
    petrol
    Thanks
    6,219
    Thanked 4,960 Times in 3,195 Posts


    Re: Wheel Guide

    his et alloy is 47

    what alloy he has god knows though:o:o:o:o:o:o

  14. #10
    BMW Enthusiast
    Unverified Member
    TDillon22's Avatar
    Join Date
    Mar 2017
    Location
    Cardiff
    Posts
    3
    Car Specs:
    Year
    2005
    Model
    M3
    Engine
    Straight 6
    Fuel
    Petrol
    Thanks
    0
    Thanked 0 Times in 0 Posts


    Awesome post, thank you!

User Tag List

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •