Help with backspacing on 71 Firebird...

[cpd004]

Looking to run 18x10 rear 18x9 front. The rear is mini tubbed and running Ridetech level 2 all around. I need to figure out back spacing and offset. The tires look like they'll be 285/40/18 and 245/40/18. Any help would be appreciated.

[Vince@Meanstreets]

Looking to run 18x10 rear 18x9 front. The rear is mini tubbed and running Ridetech level 2 all around. I need to figure out back spacing and offset. The tires look like they'll be 285/40/18 and 245/40/18. Any help would be appreciated.

don't wanna run 11's in the rear?

[cpd004]

don't wanna run 11's in the rear?

I'd like to, but it doesn't appear they'd fit.

Here's a 275/40/17 on the car now:

http://imagizer.imageshack.us/v2/640x480q90/661/ipS0gX.jpg

Also having a hard time finding a tire size for an 11" rim that doesn't have too little sidewall.

[Vince@Meanstreets]

What size wheel size is on there now?
How much room do you have between the tire and the outer wheel house?
How much room do you have between the tire and the inner well and suspension components?

On link suspended cars I like to shoot for 1/2" minimum to the outer, 1" minimum to the inner wheel house and 1/2" to axle mounted suspension components.

[cpd004]

The wheel that's on there now is a 17x10 if I remember correctly. It has 5.5" backspacing and the tire is a 275/40/17. From the surface of the disc to the wheel lip looks to be 4 5/16". I can roll the lip and get a little bit there. From the front surface of the disc to that lower mount for the coilover appears to be 7". Thanks for the help.

[Vince@Meanstreets]

The wheel that's on there now is a 17x10 if I remember correctly. It has 5.5" backspacing and the tire is a 275/40/17. From the surface of the disc to the wheel lip looks to be 4 5/16". I can roll the lip and get a little bit there. From the front surface of the disc to that lower mount for the coilover appears to be 7". Thanks for the help.

No problem.

On the 2nd gens the wheel lip isn't the problem. It's the outer wheel house above it that gets on the way. If your outer is fine you can go in more.

Since you have a wheel and tire on there that you know the specs on, use them to figure your backspace from there. Find your distances from the tire / wheel to the parts mentioned. One important dim is the distance between the outer tire to the outer wheel house.

Going from 10 to 11 you will just have to increase the back space to 6.5". Since the axle mounted parts move with the tire you can get away with it being close. 1/2" should be fine.

[cpd004]

No problem.

On the 2nd gens the wheel lip isn't the problem. It's the outer wheel house above it that gets on the way. If your outer is fine you can go in more.

Since you have a wheel and tire on there that you know the specs on, use them to figure your backspace from there. Find your distances from the tire / wheel to the parts mentioned. One important dim is the distance between the outer tire to the outer wheel house.

Going from 10 to 11 you will just have to increase the back space to 6.5". Since the axle mounted parts move with the tire you can get away with it being close. 1/2" should be fine.


Thanks for your help! The tire just barely touches the lip only on the drivers side on bigger dips in the road. It appears as though just tapping it up a bit or slightly rolling it would take care of that.

I might be mistaken, but I thought I read somewhere that on an 11" rim, the tire would be about 12" wide. I'm assuming this means sticking out about 1/2" on each side of the rim. What's got me worried is that adding the 2 measurements I got from the surface of the disc, I get 11 5/16". It just doesn't appear that a 12" wide tire will fit in there.

Others running my combo seem to be sticking 335/30's back there and even Ridetech said it was possible, I just don't see how.

[OLDFLM]

FWIW I'm running 255/45/18s on 18x8 front and 295/40/18s on 18x10 rear with stock tubs. With minitubs I'd think you'd be able to put a lot more tire in the rear.

[cpd004]

FWIW I'm running 255/45/18s on 18x8 front and 295/40/18s on 18x10 rear with stock tubs. With minitubs I'd think you'd be able to put a lot more tire in the rear.

The tub isn't the problem any more....it seems that lower coil over bracket as pictured above is.

[tazzz2_ca]

I'd like to, but it doesn't appear they'd fit.

Here's a 275/40/17 on the car now:

http://imagizer.imageshack.us/v2/640x480q90/661/ipS0gX.jpg

Also having a hard time finding a tire size for an 11" rim that doesn't have too little sidewall.

I would call ride tech and ask about your spring perches,,, the shock looks to be laid way over to the outboard side in the photo... Maybe just the angle but my 70 Formula looks like this....
http://i997.photobucket.com/albums/af93/projectwarpath/Project%20Cochise/albertassem014_zpsf2858861.jpg (http://s997.photobucket.com/user/projectwarpath/media/Project%20Cochise/albertassem014_zpsf2858861.jpg.html)

[Vince@Meanstreets]

The tub isn't the problem any more....it seems that lower coil over bracket as pictured above is.

I would call ride tech and ask about your spring perches,,, the shock looks to be laid way over to the outboard side in the photo... Maybe just the angle but my 70 Formula looks like this....
http://i997.photobucket.com/albums/af93/projectwarpath/Project%20Cochise/albertassem014_zpsf2858861.jpg (http://s997.photobucket.com/user/projectwarpath/media/Project%20Cochise/albertassem014_zpsf2858861.jpg.html)

CPD004,

Are your lower links off set at the front like the 1st gen Camaro kit is?

If so you can switch to weld on brackets and flip your links so they offset to the inside which will move your lower mount inwards.

[cpd004]

Albert, it definitely looks very different on my car than on yours. The rear was installed by a Ridetech recommended installer. I'll have to call them to see what they say.

Vince, I'm not sure what the first gen kit looks like. I'll take a pic and post what it looks like from the other side if that helps.

I've been dealing with the engine swap right now as I have no idea which direction to go with the rim selection. I'll have to eventually make some decisions though.

[Nativefx]

CPD004,

Are your lower links off set at the front like the 1st gen Camaro kit is?

If so you can switch to weld on brackets and flip your links so they offset to the inside which will move your lower mount inwards.

Hey Vince, I'm assuming your talking about the front mounting points of the lower arm? If you switch these how far inward does this place your mounting points? I'm looking to get a new shortened rear end built, but when I placed my wheels inside the wheel wells, one side was really close to the inner rim. I know there are slight adjustments that can be made to the front mount in order to align the axle, but I'm worried about how close it will be once final adjustments are made. If I decide to go with the weld-on brackets, how would I locate the point they needed to be welded given the wide tolerances built into older cars? This is going into a 69 Firebird.

I'm also trying to figure out the best way to locate my rims and determine the spacing to both the inner and out fenders. I'm running 325/30/19 Pilot Super Sports on Forgeline wheels 19 x 11.5 with a 5" BS. It looks like I have plenty of room with the 325's as I have a '69 I'm trying to move them out as close as I can to the outside fender to get the look I want, but I'm worried about rubbing during driving and also when I lay it out with AirRide setup. I don't want the top of the tires to rest on the inside fenders.

Last night we tried placing the wheels under the car but trying to get them to ride height was difficult, and then trying to measure WMS to WMS without moving them was FRUSTRATING!! So I downloaded this https://www.cachassisworks.com/Attachments/worksheets/fab9_instructions.pdf and thought I would see if this would make things easier? I noticed that above the inside fender lip, the inner fender also comes in a bit which is what started to concern me.