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Thread: Backspacing, offset and tire sizes explained

  1. #31
    Senior Member ir1d3d1rt's Avatar
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    i thought 35 and 40 was a 4x4 offset. ive heard you can lay a 2wd on the frame with a 8.5 with a 10mm offset and it is very close to hitting the fender. ive heard 14 or 15 is the best with a 8.5 wheel. thats with no bd.... this is all what ive read on this forum.
    it would be easier with more money.

    my project thread

  2. #32
    Registered User FabianSS357's Avatar
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    14mm and 30 mm or any size OFFSET for that matter is subjective to the width of the wheel

    there is NO set rule that a 14mm wheel is 2wd only as the offset is still subject to the wheel width properties.

    Refer to my first post


    I run a 35mm offset wheel on my first gen 2wd, but it is a 17X8 wheel and i did need a pretty thick spacer up front. had my truck been 4wd i wouldnt need the spacer.


    usually the higher offset wheels (35s for example) fit the 4X4s better because thier track is wider. In other words, it allows the wheel to tuck further into the wheel well since the mounting position of the wheel is further out.

    a 10-12mm offset wheel or less works well on 2wds (still subject to teh width rule) because two wheel drives have a narrow track. Meaning that the 2wds mounting point is further in than the 4wd allowing you to run a smaller offet (and in most cases a wider lip wheel)

    remember that all this is STILL subjective to the width of the wheel

    This is a little crude, but if you are unsure of what width and offset wheel to run this may help you find it

    Pull of one of your front wheels and remember to lift the truck so that it keeps the wheel pretty much in the normal driving position. (dont lift from teh frame as it will lower your wheel and give you inaccurete info.
    cut a sturdy peice of cardboard to the overall diameter of the wheel/tire size you plan on running.
    So if your considering getting 20 x whatever on 255 40s you should make a cardboard roughly 28inches in diameter since your tires will be 28 inches tall. also trace out 19.5 or so diameter of your wheel.

    now mount this cardboard to your rotor as your wheel would mount, and turn the wheel far to one side. get back underneath and measure the space you have between the cardboard and the closest area behind the cardboard (as this area will rub first)

    now turn the wheel the other way, and do the same.
    Dont forget tie rod ends and sway bars. (although with big diameter wheels the tie rods wont hit. as long as they fall between the area inside yoru wheel diameter markings.

    take all the measurements you took, and select the smallest one, as this will be the first place the tire/wheel rubs either turning left or right.

    for example, lets say you took a measurement of 6.5 inches to the nearest thing it would hit.

    i would allow pretty good space, an inch is comfortable clearance space. so that leaves us with a total of 5.5 inches of backspace that you can run.

    now if you want a 20X8.5 wheel, to equal a 5.5 backspace in OFFSET terms you would have to do the folllowing.

    20X8.5 is 9.5 wide (see first post)
    so you have an overall width of 9.5, 5.5 of which you want to be behind the mounting point.
    so you divide 9.5 in half, giving you 4.75

    that means the center point (0 offset) of the wheel is 4.75 inches from the rear and 4.75 inches from the front

    ok well you want 5.5" out back, so you have to figure the difference. you theoretically have 4.75 inches of backspace at your imaginary center point. so figure how much more you need.
    5.5" - 4.75'' = 0.75''

    So your wheel mounting point needs to be skewed .75 over towards the front (face) of the wheel in order to have a 5.5 spacing out back. now using conversion,
    .75 INCHES = 19mm

    so your new wheels should be 20X8.5 with a 19mm offset.
    The higher your offset goes on this size wheel, the more area of the rim you are putting behind the mounting point. in other words the closer your getting to hittting all the stuff thats back there


    now with that said if you wanna run a wider wheel say a 9.5 wheel, you still want that 5.5 backspacing, but now you have a wider area to cover, so simply do the math again, and you come out with a 6mm offset

    the backspacing is the same, your offset is different.
    REMEMBER THIS IS ONLY AN EXAMPLE, NOT ACTUAL MEASUREMENTS FOR YOUR SPECIFIC VEHCILE
    remember also that your TIRE SIZE has alot to do with rubbing.
    in the example i used a 255 wich is roughly 10inches wide, so that give us an additional .25 inch that would stick out from our RIM measurements.


    I hope i didnt confuse anyone with all this.
    and if it helps ill do a pictorial to ease the visualization.
    If your lazy or hate math, then figure out how much space you by doing the first part, and then refer to s10blaza's chart on the first page

    The backspacing you desire is on top, and simply go down the left side to different wheel sizes to see what offset you need to acheive that backspace

    actual self measurements > taking someone elses word
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  3. #33
    Registered User FabianSS357's Avatar
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    I also forgot to add that if your BD or bagged, you also have to take into consideration your FENDER edge and measure accordingly for that too
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  4. #34
    Double Bagged s10blaza's Avatar
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    Originally posted by FabianSS357
    I also forgot to add that if your BD or bagged, you also have to take into consideration your FENDER edge and measure accordingly for that too
    yup. I have +35mm on mine and its a 2wd, BDed and its dead on. I tried on a friends 20x9 +20mm and it hit horrible.

  5. #35
    Member #356 alowerlevel's Avatar
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    I found this on another forum and thought it might be useful here too


    Last edited by alowerlevel; 02-26-2007 at 12:02 PM.
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  6. #36
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    ok well, i got me some 24's for free from a buddy cause they wouldn't fit his truck, and i helped him change out a rear end. And i was wondering if i was to put them on my blazer would they fit/rub? I am thinking that they would rub at full lock, but i was just wondering???

  7. #37
    Registered User FabianSS357's Avatar
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    Quote Originally Posted by blazin_bushnell View Post
    ok well, i got me some 24's for free from a buddy cause they wouldn't fit his truck, and i helped him change out a rear end. And i was wondering if i was to put them on my blazer would they fit/rub? I am thinking that they would rub at full lock, but i was just wondering???
    You did not provide enough info for anyone to answer any of your questions.
    Did you actually read this informative thread or did you just post in hopes someone would reply?
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  8. #38
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    well, i didn't get any of the info, from my buddy who i got the rims from, so i true don't know anything except that they are 24's, i have to go get them after school, which is in like an hour. But i will get back with the information, But i didn't think it would be that big of an issue, if they would fit or rub or anything, sorry for the inconvience.
    Oh yea, i can tell you this though, that they will be going on a 00 2wd 4dr blazer, stock. hope it helps
    Last edited by blazin_bushnell; 04-17-2007 at 01:31 PM. Reason: forgot info

  9. #39
    Registered User FabianSS357's Avatar
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    Quote Originally Posted by blazin_bushnell View Post
    well, i didn't get any of the info, from my buddy who i got the rims from, so i true don't know anything except that they are 24's, i have to go get them after school, which is in like an hour. But i will get back with the information, But i didn't think it would be that big of an issue, if they would fit or rub or anything, sorry for the inconvience.
    Oh yea, i can tell you this though, that they will be going on a 00 2wd 4dr blazer, stock. hope it helps
    i wasnt trying to be too rude, its just with a 24" size you have to be specific with widths and offsets in order for anyone to be able to help you out. Since you dont have the info, it would be better if you just got a rim, mounted it up front and did a full turn left and right and checking it for clearance.

    LIke that you can be certain the wheel fits, and how much room you have to run the tire instead of having to depend on someone to reply who may not even be too certain if your wheel is gonna fit.

    From what i have heard most 24s rub a full lock due to their massive widths and fullsize truck/suv offsets.
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  10. #40
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    yea, thanks i didn't take it as rude or anything. I see wat ya mean. But thanks for the help anyways, i am gonna have to do wat ya sayed. Most people say that i am gonna have to put a lift on the blazer that way i would be able to turn at all, but forget raiseing it gonna bag it in july probly gonna just sell the rims and bag it sooner:D :D :D

  11. #41
    Registered User stateofidleness's Avatar
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    hey guys, im havin trouble understanding all this.. would these fit?

    http://cgi.ebay.com/ebaymotors/20-BM...4443QQtcZphoto


    how wide of a rim can i go with a 20" and whats a good offset for it?
    how wide of a rim for a 18" and a preferred offset?


    98 4door 2wd blazer
    thanks

  12. #42
    Registered User ZZ4Blazer's Avatar
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    Its simple math man, once you get it, you'll see its way easy.

    Trust me, math was my absolute worst subject, lol.

    On those rims, they're 8.5" wide. Every single rim made, is measured on the inside lip where the tire mounts. Theres actually 1/2 more rim on each side of the rim. So essentually, if you got those wheels, and measured them with a tape measure across them, they will measure 9.5" wide.

    Knowing that, its listed that they have a 15 offset. Offset it always measured in millimeters. (mm) Offset also always is measured from the rim centerline. So, finding the rim centerline, just divide the rim width in half. Then, add the offset number to that.

    On those rims, with 15mm offset, they will have 5.34" of backspacing
    (9.5" rim in half is 4.75") 4.75" centerline meausurement + 15MM(15/25.4= .59") {converts mm to inches, 25.4MM=1 inch}

    So 4.75"centerline measurement + .59" offset= 5.34" of backspacing. Backspacing is measured from the wheel flange, to the very wheel lip.

    That would work in the back, but the rims would sit inside the fender a bit. Me personally, I like them as close to the fenderlip as possible.

    Thats too much bs for the front.

    Generally, stick to no wider than 8.5" on the front of a 2nd gen truck, and no more than 4.75" backspacing. More than that, your gonna get bad rubbing on the swaybar, and frame/inner fenderwells.

    On a side note, I know those say s10 and lists lots of chevys, but those arent exactly the chevy bolt pattern. those are 5x120(bmw pattern), the small chevy/s-10 bolt pattern is 5x 120.65" . I know its only .65" difference, but its my peference not to push it. Technically they will bolt up. but I know guys have run them and have had them loosen up and wollow out the bolt holes. Some guys run them with no problems. But Im kind anal but that kinda stuff.
    Last edited by ZZ4Blazer; 05-02-2007 at 12:09 AM.
    Jim 94 Stepside
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  13. #43
    Registered User stateofidleness's Avatar
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    hey man, first i wanna say thanks for actually responding, some other s10 forums dont lol.. 'ppreciate it.

    so a 8.5" rim will fit all the way around (front and rear), but the offsets need to be different?

    also, i agree with you about being as close to the lip as possible. that is the look im goin for. what would be a good rim size for this.. im probably going to have a 2/3 drop depending on how well i can fill the fender well with new rims/tires.

    id like a 20" with a 40 series tire on as wide a rim as possible all the way around

    i dont mind going with an 18" rim (its my daily driver), but want to keep the low profile look.

    again, really appreciate the replies man..
    -steve

  14. #44
    Registered User ZZ4Blazer's Avatar
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    Its cool man, not working tonight and Im bored, lol.

    No you dont need 2 different offsets, less your running different sized widths. Those will work on the rear because it doenst turn, they will bolt up to the front no problem, but they will rub like hell when you turn.

    18's or 20's really doenst matter. They'll fit the same if the tire size is roughly the same overall diameter.

    Most chrome 18" or 20's will come in an 8.5" wide rim. I would try to find one with zero offset. That will give you 4.75" of backspacing.

    an 8.5" rim isnt gonna fill the rear wheel well. If you want that your gonna need to go to a billet wheel, because theres almost no companies that make a chrome rim in different widths. You can easily fit a 10" wide rim in the back of your truck, even an 11" with the right backspacing.
    Jim 94 Stepside
    ZZ430 clone V8
    NX kit in the works
    Tremec T56 6 speed
    8.5" rear
    Quote Originally Posted by Harley
    Go before show any day of the week.

  15. #45
    Registered User stateofidleness's Avatar
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    thanks man...

    so what do you recommend if im wanting to buy a whole set of 4 (most likely ebay)

    18x8? 20x8? (does that exist)

    lol hey man, just wanted to add that i got 7" wide rims on right now, so a 8.5" would look a lot better than 7" lol
    so an 8.5 will rub on the front no matter what?
    Last edited by stateofidleness; 05-02-2007 at 12:58 AM.

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