I'm no mathematician.....backspacing help

[cluxford]

OK guys, here goes. So I drop the car off today. we decide we will blast it then get all running gear in, mocked up and settled before starting serious planel work. Part of that includes 4 link rear, mini tubs and 9".

Now for the chicken and the egg. Chassis guys says what wheels do you want to run and when can we have em so we can build the diff / axles.

I say I can't get the wheels as they are custom Budnik's that require me to tell em backspace...can't work bacspace out with Diff / Axles decided.......

oh $hit....

So I need some advice. Plan is to run 11" rear rims, deep dished, at least 6" dish...

Can anyone give me some advice on steps I should take, do I do diff / axles first then wheels or wheels first and diff/axles second.

I'm thinking wheels first

Thanks in advance
Chris

[SirGeo]

How wide can you go with the tyres? and how wide a tyre do you want to run?

I'm no expert and i would disregard everything i say, but do your diff/axles and get your wheels to sute.. you'll need to take into account how deep you can go with the tubs etc and work from there...

Again ignore what i said as i know nothing about this :p

[EAT-30T]

CHris my Boyds are 4.5" BS if that helps and thats a nice fit.... so work off that I'd say.... diffs and sh*t is my job so just hit me up with any questions

For example

Stock
8" Rim 4.5 BS (in) 3.5" (out)
11" Rim with a 6" dish means a

5" back space and 6" out... now seeing how stock outer is 3.5" this means you have to shorten the diff 2.5" to compensate and obviously with the mini tubbing you'll have room for the 6" in....

I hope that makes sense????

Kurt

[Dillons69]

Chris,
I have been trying to do the math on my 69 camaro for a little while and have been doing the forums even once you have the measurements there can be millions of tyre combos ( there all have different heights and widths )to get that perfect look. I would suggest getting diff?axles and tubs done first to see what you have to play with. once this is done you can do the math easy. Guessing could be tricky.
jason

[OzRiv]

I spent 2 months in the US earlier this year, picked up my 72 Buick Riviera and looked for wheels.
Now a word of caution: What I found was that backspace and frontspace does not add up to the width of the wheel, but the width of the wheel + 1".
The wheel width is measured to the inside of where the tyre bead sits, whereas the back/front space is measured to the edge of the rim so the stock 8" rim mentioned will actually have a 5" BS and a 4" FS......confused the **** out of me since I could never get the width, offset and backspace to add up.

CHris my Boyds are 4.5" BS if that helps and thats a nice fit.... so work off that I'd say.... diffs and sh*t is my job so just hit me up with any questions

For example

Stock
8" Rim 4.5 BS (in) 3.5" (out)
11" Rim with a 6" dish means a

5" back space and 6" out... now seeing how stock outer is 3.5" this means you have to shorten the diff 2.5" to compensate and obviously with the mini tubbing you'll have room for the 6" in....

I hope that makes sense????

Kurt

[SirGeo]

Just reading what's been written here confuses the hell out of me...

[OzRiv]

Ok, I'll try to be clear,
wheel width is measured where the tyre bead sits in the wheel
http://www.americanwheelandtire.com/images/tech/offset.jpg
as you can see, there is an extra bit of rim sticking out either side of where the width is measured, this means that what is called an 8" rim will actually measure 9" from edge to edge.
Backspace/dish is measured off the edge of the rim
http://www.americanwheelandtire.com/images/tech/terminology.jpg
so... an 8" rim with a 0 offset will have a backspace of 4.5"

If you are not aware of the difference in how the measurements are made, you could get a 1" supprise when you go to fit the wheels.

OzRiv

[cluxford]

How about a 1.35" surprise !!!

I was told I needed a raised mounting hub to clear the Corvette C5 calipers (of 1.35"). I did this on front....all good.

Did the same on the back (with 5" backspace)....now got the discs and calipers....and guess what don't need the 1.35" pad on the rear....so I lost 1.35" "dish" on the rear....may need to buy some new rear rims (can't re do em as it's solid aluminium) and I want / need 5" backspace.

Expensive lessons...but i love learning....I now know a lot more about backspace measurements.

OzRiv, I understand your post perfectly...well explained

[J.J.]

I spent 2 months in the US earlier this year, picked up my 72 Buick Riviera and looked for wheels.
Now a word of caution: What I found was that backspace and frontspace does not add up to the width of the wheel, but the width of the wheel + 1".
The wheel width is measured to the inside of where the tyre bead sits, whereas the back/front space is measured to the edge of the rim so the stock 8" rim mentioned will actually have a 5" BS and a 4" FS......confused the **** out of me since I could never get the width, offset and backspace to add up.

sorry to dig up an old thread, as i'm trying to work out what size wheels will fit my 69 camaro.

what your saying above is that an 8" rim is actually 9"

[cluxford]

on Pro-touring.com the most common response is you can go a 9" or 10" inch rim without minitubs depending on diff length and backspace.

Standard diff you can fit a 285 easy...

[J.J.]

on Pro-touring.com the most common response is you can go a 9" or 10" inch rim without minitubs depending on diff length and backspace.

Standard diff you can fit a 285 easy...

i wanna run a 275/60/15 on either a 15x10 with a 5.5" back space or a 15x9 with a 4.5" back space

i want the car very low also

[cluxford]

your only issue will be clearance on the inner guard

I am running 17's admitedly with a 50 series profile tyre and have the car 100mm off the ground and had less than half an inch clearance to the inner guard

with 15's and a reasonable profile tyre I think your choice will be fine....

Go do a search on Pro-touring.com for rim width and backspace....there is heaps of info and lots of examples of what guys have already done.

That's where I did a lot of research when I was looking at backspacing

[cluxford]

Here is a good example

http://www.pro-touring.com/forum/showthread.php?t=28454&highlight=biggest+rim

I trust Frank at Prodigy he has helped me a lot along the way and has built a lot of cars including many first gens....

You'll see his post where he says you will be fine with a 275 on a 9" rim with a 5.5" bacspace

[RSZ28]

I am running 17's admitedly with a 50 series profile tyre and have the car 100mm off the ground and had less than half an inch clearance to the inner guard



Where are you measuring the 100mm? Front cross member/sub frame? Rockers?

That's nicely low! :cool:

[TUFF68]

i bought the wheels and tyres i wanted first and then had the diff built to suit. Convo Pro's only come in 2 offsets, 5" and the deeper dish 3-1/2" backspacing. I wanted the deep dish ones so the diff had to be roughly 2" shorter each side than the stock diff. Once the tyres were mounted on the rims i sat the wheels under the guards exactly where i wanted them in terms of clearance on both inner and outer guard and then measured the distance between the rims where they bolt to the hub. Gave the measurment to the diff builder and he built the diff spot on. No clearance problems once everything was assembled

Only problem i have now with a shortened diff is that none of the stock rims i have lying around fit to use for a bit of fun. I have one pair of 8" cragars that fit using a spacer and will have to get another pair of 6" stockies modified before powercruise 15

[OzRiv]

An 8" rim is 9" wide edge to edge, I fixed the picture links in my original post, they show it pretty clearly.

[cluxford]

Where are you measuring the 100mm? Front cross member/sub frame? Rockers?

That's nicely low! :cool:

subframe...and yeah its low...roof line is chest high...

[J.J.]

pics