Author Topic: Backspacing help  (Read 3534 times)

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Offline Wreck

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Backspacing help
« on: August 28, 2007, 07:21:50 AM »
I know the stock backspacing for a 78 TransAm is 4".   That's for a 17x8 wheel.  Can anyone tell me the backspacing for a 17x9.5 wheel?  I've searched the internet high and low, and even ordered the wrong wheels based on some bad advice from Ecklers Camaro.  They told me 6.5 inch backspacing for the rear of my car and they custom made the wheels.  They're no where near a fit for the car and they won't return the wheels. 


78 Trans Am - 472 ci 7.7 liter. 
JH Longtubes, mid dumps
3.36 gears

95 Z28 - 350 ci
JH Longtubes, Dynamax 3", cutout
3.73 Gears
Hotchkis F\R Swaybars \ 2" Drop
KB Sub Frame
4 Point Cage

Offline Rick

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Re: Backspacing help
« Reply #1 on: September 27, 2007, 11:40:28 PM »
Did yoiu ever get an answer to your question, Wreck?  Did you get any joy on the incorrect wheels? ??? ???

I always thought that in order to maintain the proper handling you needed to basically keep the baskspacing the same as stock, and modify the wheel wells to accomodate wider rims.  While that might seem obvious with the front wheels, the rear wheels also have stuff like wheel bearing loading and axle stress to consider.  I've seen several cars with funky offsets break rear axles and actually have the wheel/axle separate from the car on the road. :o :o

Offline Joker (告r茺agon)

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Re: Backspacing help
« Reply #2 on: September 28, 2007, 02:05:07 AM »
I wish I had seen this before.
The backspacing can be different, but wider wheels should be evenly extended in and out. If the 8" wheel's backspacing is 4" (basicly centered) then the 9.5" wheel should also be pretty close to centered to maintain the same load on the axle and bearings. Altering it a little one way or the other shouldn't make that much difference on a street car. But that figure they gave you moves the weight load way to the inside by a factor of 2:1 (6.5" on the inside and only 3" on the outside). And there's no way you have enough room for 6.5" of weel plus more for tire buldge between the mounting plane of the wheel and the inside of the wheel well. That's the width of some stock wheels.

The best way to find how much space you have is to take a flat edge laid on the drum/rotor where the wheel would mount and measure in from that plane to the inside of the wheel well. Then subtract the amount of tire buldge for your tires when mounted on that width rim. Should be easy if you still have them so that you can just measure from the outer width of the tire using the straight edge again down to the bead of the tire/lip of the rim. (remember to add a little for the thickness of the rim's lip since they are measured on the inner surface.) I'm sure you also know to leave a little extra room for tire flex. And that you already measured between the inner well and the outer lip of the fender/quarter panel, also leaving room for tire flex.
Larry


Offline Rick

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Re: Backspacing help
« Reply #3 on: September 29, 2007, 11:43:36 PM »
That's what I was trying to say, Larry.  I've always been told the critical thing is to keep the relative balance (in/out) the same when widening the wheels to avoid loading the spindles/axles in ways they weren't designed to handle. ;)

Offline Joker (告r茺agon)

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Re: Backspacing help
« Reply #4 on: September 29, 2007, 11:46:10 PM »
Right. I've seen failures like you have. When I was working in my own shop for a living I wouldn't mount wheels that stuck out a lot on the lowriders to keep from being liable for something like that.
Larry


Offline Rick

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Re: Backspacing help
« Reply #5 on: September 29, 2007, 11:48:25 PM »
You gotta admit, it's pretty funny to see a jacked-up 4X4 lose an axle at 75 MPH! :D

I did feel sorry for his wife and baby, though -- scared the crap out of them. :(

Offline Prettybird81

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Re: Backspacing help
« Reply #6 on: October 25, 2007, 02:15:41 PM »
Dude... maybe funny at 17.5 mph but not 75 mph!!!!!  Scary as heck!

OK, now that i've read this... i have to go learn what backspacing is...  ::)
Jeri

"Prettybird" ~ 81 T/A 301T/Auto, 23K.... oh... wait... make that 26K... nope... 27K Orig Miles (survivor)

"Arlington Cemetery has no section for Republicans, Democrats or Independents so get over yourself & suck it up"

Offline Joker (告r茺agon)

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Re: Backspacing help
« Reply #7 on: October 25, 2007, 02:41:20 PM »
Backspacing is the distance from the mounting surface (the surface of the rotor/drum) to the back edge of the wheel.

http://www.78ta.com/smf/index.php?topic=5386.new#new

Larry


Offline Rick

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Re: Backspacing help
« Reply #8 on: October 25, 2007, 04:00:07 PM »
Dude... maybe funny at 17.5 mph but not 75 mph!!!!!  Scary as heck!

Yeah, it looks REALLY odd when it happens...first the wheel starts to pull out of the wheel well...and it just keeps going...and going...and going.  When it gets so far out that you can see the axle sticking out of the wheel, the whole assembly goes astable and starts wobbling all around -- then it pulls out completely and goes spinning and skipping down the road...meanwhile the vehicle sags in that corner and the brake assembly hits the pavement, making a shower of sparks.  Meanwhile the occupants are staring at the wheel whizzing past them down the side of the car...and the looks on their faces is PRICELESS!  OK, so maybe it's not too funny to THEM, but I was grinning as I pulled in behind them and used my fire extinguisher on their flaming brake assembly.  :D :D ;)

The lesson here is to get the backspacing right so you're not unduly stressing the axle, causing premature catastropnic failure... 8) 8) 8)

Offline eroc022

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Re: Backspacing help
« Reply #9 on: October 26, 2007, 02:21:41 PM »
did you try a wheel spacer of any sort...i know dave hall is running a 10 in wide on the rear and thats what im gonna run....
Eroc
Fixing the car that Restore A Muscle Car jacked up....