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Thread: Backspacing, offset and tire sizes explained

  1. #1
    Registered User FabianSS357's Avatar
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    Lightbulb Backspacing, offset and tire sizes explained

    Ok i have researched some info for for you guys with fitment questions that are too lazy to research yourself. ;)

    The bolt pattern on s10/sonoma/blazer/jimmy and sytys is
    5 X 4.75 and 5 X 120.65 mm

    a 5 x 120 wheel will bolt up fine as long as you have the small hubs (newer s series) since most of the 120 wheels are for bmws and are hubcentric.

    the best backspacing and offset for 2wds
    Originally posted by 98layinframe on s10forum

    The best backspace is:

    4" with a 7" wide rim

    4.5" with a 8" wide rim

    4.75" with a 8.5" wide rim

    best offset for 2wd

    7" --- 0 - 4mm

    8" --- 5 - 10mm

    8.5" --- 10 - 15mm

    now these are just approximations, the wheels you chose can vary from these measurements a little and still work.
    i wouldnt go more then 4.5 backspacing up front since you will probably have rubbing issues depending on tire sizes with anything bigger.


    backspacing refers to the measurement taken from the mounting pad on the wheel to the outside (back) lip in inches

    heres a chart to show you how to measure these two things thanks to summit


    offset is measured in mm and is the distance the mounting pad is skewed from the wheels centerline.

    for example, a +15mm offset wheel would mean that the mounting pad is moved 15mm from the wheels centerline towards the face
    heres a pic (right) that describes how offset is measured.


    dont be fooled by offsets though. the offset doesnt tell you anything if you dont know the width of the wheel.

    backspacing can be different for two wheels even if their offfset is the same. if you have a 15x7 wheel, with a 0 offset you will have 4" of backspacing, yet on a 15X9 wheel with a 0 offset you will have 5" of backspacing

    dont get confused thinking that since the wheel is 15x7 that its backspacing would be 3.5" with a zero offset since the mounting point is right in the middle.

    ZZ4 describes why there is this discrepancy
    Originally posted by ZZ4Blazer
    A wheel width is measured from the inside, like where the tire bead would set. but, If you measure the overall width of the rims, it will measure 1" wider. This is because of the outer lip of the rim.

    So, a rim thats 15x8", will measure 9" wide if you measure across it without the tire on.
    this is why you have a 4" backspace on a 7inch wide wheel because backspacing measures to the outside of the lip, which is .5" more.


    so an effective method of finding out backspace if you only know the offset and the width would be like this:
    18X8 wheel, with a +15mm offset.
    the wheel is actually 9" wide (see ZZ4 qoute above)
    divide the width by two (9 / 2 = 4.5)
    so your mounting center is at 4.5" (from the outside edge of wheel) but it has an offset of +15mm

    1" equals 25.4mm and 1mm = .04"

    15mm = .59inches so you add that to your 4.5 and you get a total of 5.09 inches of backspace:bigthumb:

    on to tire sizes
    245/40R17

    245 is the width of the tire in mm

    40 is the ratio of width to Height
    if im not mistaken it is the measurement of
    the side wall given in a percentage of width
    example, 40 in this case means that the sidewall height is 40% of the width (245mm) making the sidewall 98mm tall
    so the tire is 9.6457" wide and the sidewall is 3.8583" tall

    the last number is obviously the wheel size

    here is a link to a thread on s10forum where the author describes which tire sizes are the best to use on drops and bagged vehicles
    tire size info for drops and bags

    hopefully this thread helps people to understand measurements and how to find them and gives usufull info on tire sizes and the such

    i will try to find all the stuff for the 4x4s to post up soon.
    if im mistaken on anything feel free to correct me, also if you want to post more usefull stuff or info for the 4x4s please do so.
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  2. #2
    it fell? otter's Avatar
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    You go girl!












    :bigthumb:

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    Registered User ZZ4Blazer's Avatar
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    Damn..I love being quoted!! ;)
    Jim 94 Stepside
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    Quote Originally Posted by Harley
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    Dark Side Master LayinBodyBlazer's Avatar
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    Are these offsets and backspacing recommendations accurate for a wheel to tuck inside the fender lip? Reason being Im getting ready to order my rims and need to know what backspacing I need to get the 22"s to tuck in the rear...if they are, then thanks.

    If these numbers are more so suited to keep the edge of the wheel flush with the edge of the fender (or around that space as to keep it from tucking) what would u recommend for 22"s in the rear?

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  5. #5
    Registered User FabianSS357's Avatar
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    Soloud, i believe the offset is for the wheel to sit as close to factory as possible, which means on the inside of the lip.

    but to make things easier you should probably just measure on your blaze to be exact.

    you will have to measure the distance from the lip to your mounting pad on the axle. if your rear is bagged already, just let the axle go up as far as possible and measure the distance, and subtract an inch or two for the wheel to actually be able to ride inside the alloted space.

    from that measurement you can then figure out how much backspacing to use with the size of the wheel your running.

    not sure what the specs are on 22s but arent they like 9 or 10 inches wide?

    for example, say you are running a 22 by 9 your gonna have 10 inches of overall rim/wheel width.
    if you have say 6 inches of clearance between the lip and the mounting pad. that gives you 4 inches of backspacing if you want to run the wheel all the way to the lip and have a minimal clearance to be able to tuck

    so you just reverse the process above... you have a four inch backspacing on a 10" (20X9) wide wheel. your 0 offest will be at dead center, being at 5" (from outer edge of the wheel) since you need 4 of backspacing to fit your needs you will have a -offset of 1 inch
    so you need to look for a wheel that has an offest of -25mm this would typically be a big deep dish type wheel.

    a positive offset wheel moves teh mounting pad forward like on a FWD wheel, a negative offest moves the wheel face inward aka a deep dish wheel
    remember though, all these numbers are dependant of the numbers you measure.

    also if you have adaquate space to move your wheels inward the more you move them in the more backspacing will increase. (since you have a negative offset your negative offset will dimish and go towards a zero offset, and if you move the wheel more in you will eventually gain offset making it positve at some point)

    hope you understand my mumbling mess. if not i can sketch up a pic with some sample demensions to clear any confusion up
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    Dark Side Master LayinBodyBlazer's Avatar
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    gah! forgot I posted in this lol


    ok, now that Im kinda confuzzled...what would you suggest the spacing be for the rear wheels to have a nice big lip on them?

    I will most likely be using a Ford 9" rear that Kyle (GBD) is going to narrow for me so keep that in mind.


    Also, will most custom wheel manufacturers have sizes available with deep lip spacing or am I looking at getting someone like BAD or Boyd to do my wheels?

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  7. #7
    trueblood
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    what is the width of the center bore(hubcentric) of 2nd gens?

  8. #8
    Registered User ZZ4Blazer's Avatar
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    Originally posted by SoLoudItHtz
    gah! forgot I posted in this lol


    ok, now that Im kinda confuzzled...what would you suggest the spacing be for the rear wheels to have a nice big lip on them?

    I will most likely be using a Ford 9" rear that Kyle (GBD) is going to narrow for me so keep that in mind.


    Also, will most custom wheel manufacturers have sizes available with deep lip spacing or am I looking at getting someone like BAD or Boyd to do my wheels?
    A deep wheel lip is more likely gonna depend on the wheel styling. The wider the rim, and less the backspacing, yeah, its gonna appear to have a wider lip on it. But, for an s-series, your most likely to need more of a zero offset, so your probably not gonna get it unless you've got that style of rim in mind.

    22's are really gonna depend on how wide it is.
    Jim 94 Stepside
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    Quote Originally Posted by Harley
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  9. #9
    Dark Side Master LayinBodyBlazer's Avatar
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    Im getting a Ford 9" for the rear that Kyle (GBD) is narrowing for me so I can get the deep look Im after ;)

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  10. #10
    Registered User FabianSS357's Avatar
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    Small update.

    Here is a tire size calculator.

    I know a lot of memebers have questions about what size tires to use when they upgrade to larger wheels.

    This calculator provides valuable info for people that are looking to change wheel size in relation to your orginal wheel/tire combo

    It gives diameters, radius' overall width and other info.

    It also gives percentages of how big/small another size will be and a percentage of speed difference. it tells you exactly how much your speedometer will be off.

    Just select your OE tire size, and the sizes of tires you are looking at.

    Tire Size Calculator
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  11. #11
    Double Bagged s10blaza's Avatar
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    I found this chart online and found it extremely useful


  12. #12
    Senior Member ir1d3d1rt's Avatar
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    what kind of backspacing are you guys running laid out on 20s? it is 4.5 all the way around for a 20x8?
    it would be easier with more money.

    my project thread

  13. #13
    Double Bagged s10blaza's Avatar
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    Bagged or bodydropped? I would say 5" if bodydropped since a 4.5" would most likely hit the quarter panel

  14. #14
    Senior Member ir1d3d1rt's Avatar
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    yea its gonna be body dropped to the rocker...
    could i run a 20x8.5 with 5.264 backspacing... its 14mm offset. thanks
    Last edited by ir1d3d1rt; 06-15-2006 at 02:08 PM.
    it would be easier with more money.

    my project thread

  15. #15
    Double Bagged s10blaza's Avatar
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    Originally posted by ir1d3d1rt
    yea its gonna be body dropped to the rocker...
    could i run a 20x8.5 with 5.264 backspacing... its 14mm offset. thanks
    That's a no. I borrowed a rim in those exact dimensions and mocked it up when laid out. It rubbed the quarter panel. You definitely need a +20 offset or 5.5" of backspacing to clear.

    There's always the narrow the rearend alternative :D

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