Backspacing and spacers questions

I have american racing ATX series slots

I think they're 3.75 back spacing. my question is, what is the max spacer i could put on the front without rubbing, or minimum rubbing.

I know i want spidertrax 1.5 in the front, but i dont want to have rubbing.

ideal was 4.5 backspacing, but will the .75 more give me rubbing issues?
also to confirm the backspacing on the atx slots (black inside, chrome lip)

Im running 16x8 rims, with the Stock TRD OR tires (265/70/16 i believe)

 

If the wheels are 3.75" back spacing then adding 1.5 spacers in front will take the backspace to 2.25". Which depending on what size tires you have, lift or no lift? You may end up rubbing on the fenders. I would think the backspace they have now is ideal. If you wanted to add more backspacing, like up to 4.5" then I'm not really sure what your options are.

I just read the tire size... I don't think you'll have any rubbing issues but you may want you test beforehand.

 

backspacing is how far out they are from the mounts, so adding spacers gives more backspacing

 

Back spacing is the space from the hub mount on the rim to the edge of the rim. Turn the rim upside down with the face of the rim flush on the floor. Place a flat object across the back part of the rim. Take a measuring tool and measure from the wheel hub mount, to the flat object. That distance is the back spacing. So adding the spacer will take away from that distance. You would have to grind the hub mount down in order to add to the backspace.

Back spacing does determine how far out or in the wheel is, but because the wheel is pushed farther out doesnt necessarily mean there's more back spacing.

 

as you can see, the backspacing is on the inner side of the rim, right where the wheel spacers go, so if they already have 3.75 back spacing, adding 1.5 spacers to the rims would make it 5.25....

 

which brings me back to my original post, will having that much backspacing be in issue with the rims i have...

 

Spacers give you less backspacing and more offset.
Backspacing is the difference between the hub mount and the inner rim of the wheel.
Offset is the distance from the centerline of the rim to the hub face.

Why do you want more offset? If you are doing it for the stance, please understand that the trophy trucks get that look through a long travel suspension... they aren't running more offset, they are running longer control arms, and quite often are running LESS offset to reduce the loading on their bearings.

Increasing offset will increase wear on the bearings and ball joints.
Run as much offset as you need to clear the control arms and frame and no more.

 

the offset of the rims never change... spacers go into the hub, then the rims mount on the spacers, so they are adding distance from the hub... which is the backspacing. which creates more of a stance. and the reason I'm asking is because the suspension i want, which is a mid travel not long travel, requires a minimum of 4.75 backspacing, so the mounts and struts wont rub.

 

but i did make the mistake, you were right, back spacing is from the inner lip, the spacers wouldnt change the back spacing, just put the rims out further

which may be the opposite of what i want

i guess itll have to be trial and error, because the 4.75 backspacing is for a 9 inch wide rim... mine are 8

 

Backspacing is the distance from the edge of the rim to the hub mounting surface.
Offset is the distance from the center of the rim to the hub mounting surface.

Yes, the spacer bolts to the hub, but it does not move the hub mounting surface, it moves the wheel farther away from the hub mounting surface, which reduces backspacing and increases offset.... exactly the same as if the spacer were mounted to the wheel... or if the wheel were manufactured with the spacer built in.

Going to a mid-travel, you may need some trimming anyways.
Having the information about the MT would have been helpful in the original post... any answer to your question assuming stock suspension geometry would likely be incorrect for the MT.

 

9" wide rim with 4.75" backspacing = -0.25" offset (calculate adding one inch to the rim width).
An 8" rim with 4.75" backspacing will be +0.25" offset, so if they are specifying 4.75" for a 9" rim, then you'll be 1/2" "safer" for inside clearance on the arms and strut. On the fenders, you should still be okay since the outer edge of the wheel will still be about 1/2" inside of the design specs.

Okay... ^^^^ That part was correct.

Now... wheel is 8" with 3.75" backspacing, which is a 0.75" offset. Adding 1.5" spacers will push that offset out and reduce the backspacing to 2.25"

So the 9" rim with 4.75" backspacing carries 4.75" of the rim behind centerline and 5.25" of it outside of centerline.
An 8" rim with 2.25" backspacing (3.75+1.5 spacer) is going to carry 2.25" behind centerline and 6.75" outside of centerline.
An 8" rim with 3.75" backspacing is going to carry 3.75" behind centerline and 5.25" outside of centerline.

Honestly, I think you'll be okay without the spacers.
You'll have a full inch more clearance on the arms and struts, and the same exposure on the outside.

 

yeah i see that... like i said i made a mistake. My viewing of the dimensions of the picture were wrong, wrong reference points.

so with my rims, the width is 8, and backspacing 3.75... which means the offset is .25 from the center of the rim. by adding the spacer creates a 1.75 offset and pushes the tire out wider... correct?

me too... cause there are 1.25 spacers. which may be better?

 

See my edit... I think you're going to be perfect without the spacers.
Since your wheels have 1" less backspacing, there's more clearance on the inside, and since they are 1" narrower, they are the same on the outside.

Order the 1.25 spacers, but make sure they have a good return policy and don't open them unless you need them.

 

"So the 9" rim with 4.75" backspacing carries 4.75" of the rim behind centerline and 5.25" of it outside of centerline."
^ wouldn't it be 4.25?

"An 8" rim with 2.25" backspacing (3.75+1.5 spacer) is going to carry 2.25" behind centerline and 6.75" outside of centerline."
^ and 5.75 outside?

An 8" rim with 3.75" backspacing is going to carry 3.75" behind centerline and 5.25" outside of centerline."
^ and 4.25 outside?

 

but like you said.... doing the math, having no spacer actually gives me an inch more clearance with the struts/arms and then the same "fender" clearance

and all adding the spacers would do would be pushing my tires out wider, which would then result in higher possibility of rub and bending my fenders (if i run it hard enough to fully compress the shocks)

 

now reading and analyzing everything, allows me to possibly run a thicker/bigger tire and not have as many issues

So thanks for all the arguments and understanding haha

 

Rim width is measured on the tire mounting surface (inside of the bead seating area). Calculations are based on the actual rim width being 1" more than the stated width to account for the thickness of the rim, so the 9" rim with zero offset is 5" backspacing, rather than 4.5".

Ya, I think you'll be fine with the 8" wheel.
I'd try to have some spacers on hand just to be safe, but I'm pretty sure you'll be okay.

 

Gotcha! again thanks!

 

Exactly