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Discussion Starter #1
I am about to order a 9 inch axle for my 67 with CBR multi-leaf springs. Factory tubs. I have my rear wheels already. 17x8 with 4.5" backspacing or 0 offset. I am going to run LS Fbody rear brakes. What axle length should I order? Here is what I know so far:

Factory backimg plate to backing plate is supposed to be: 50.75"
Factory wheel mounting to wheel mounting is supposed to be: 57.75"
I measured wheel mounting to wheel mounting to be 58.25" on my factory rear with LS brakes installed.
Outside of leaf to outside of leaf: 46"
Inside of tire to inside of tire (245/45R17): 48.75"

I'm thinking about trying to order the rear axle at 49.5" backing plate to backing plate. Thoughts?
 

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Put your wheels where you want them to sit under the car. Measure the distance between the two mounting surfaces. Subtract the thickness of the rotor hats. That's the flange to flange width you need.
 

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Starting from a “stock” setup on my 64 hardtop meaning, axle width 57.75”, drum brakes, suspension, and wheel houses an 8” wheel would need 5.25” of back space to clear all the points of contact inside and out It will be very close but clear.. If you are adding 0.50” with the brake kit then you’ll need to add that increase in width to the axle now you’re at 58.25” and will have compensate with more backspace. So, now you should be at 5.50” of BS. Your wheels only have 4.5” of backspacing you’re young to have to take 2” out of the axle housing so that the wheels should be in a right place..

I just had some 15x8 with 5.25 BS made for my 64.. I tried a wheel with 5.5” BS and it fit but rubbed the inner tub at the bottom edge articulating the axle going through a driveway ramp at an angle.. 5.25” makes a pretty equal amount or clearance inboard and out.. Approximately 0.50” of clearance..

I believe the 66-67s might have a little bit more space outboard then a 1st gen car but the inboard specs should be the same..
 

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The two previous responses pretty much covers it. The only thing I would add is reducing the total width by an additional 1/2 and using spacers will all but guarantee zero rubbing. :yes:
I’m suggesting a 2” trim job and you think adding an additional 0.50” more which comes to 2.50 total narrowing job for a margin that can be adjusted with spacers. This might not be a bad idea..
 

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Discussion Starter #7
All of those responses make a lot of sense. I'm a little surprised that the LS rotor hats added 0.25" over drum brakes each side but that is what my measurements are showing anyways.

I'm a bit concerned that if i go with 2" or 2.5" though I am going to have super tight clearance between the flange and the leaf spring perches. Might be hard to bolt the brakes on much less actuate the e brake lever.

Any thoughts on how to mitigate this?
 

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My experience is with 1st gen tire and wheel fitments. I use the reference points that I have found to be workable within the confines of the rear wheel house of my 1st gen bodies that I have on hand.

This is a 255/45/17 on a 17x8 offset 20 wheel which translates to 5.25 inches of BS. I feel this would be the maximum possible fitment that could live within the given space. It might rub if articulated but I haven’t had a chance to test a good driveway ramp to see.. What I can say is that there is more space with this at the outer wheel house then I have with my 215/60/15 on 15x7, 4.24” BS rally wheels. I do think a 245/45/17 would be an easier fit though..
 

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All of those responses make a lot of sense. I'm a little surprised that the LS rotor hats added 0.25" over drum brakes each side but that is what my measurements are showing anyways.

I'm a bit concerned that if i go with 2" or 2.5" though I am going to have super tight clearance between the flange and the leaf spring perches. Might be hard to bolt the brakes on much less actuate the e brake lever.

Any thoughts on how to mitigate this?
Your most accurate specs will be generated by sizing up the components you have to work with.. You might have a little more space outboard then my cars have. Placing your wheels in the position you want them and taking the dimension between the wheel hubs is about the best you can do.. Personally, I would rather play with wheel spacing then cut the axle but that is me and many people prefer to modify axles over the wheels.. There’s no right or wrong here. It’s just which parts you prefer to manipulate to get there.. If you have some really expensive wheels you are committed to then the axle is the expendable piece to make it all work.. What Pragmatist is suggesting is to take an additional 0.50”’ off of the axle and make up for it with wheel spacers to fine tune the wheel placement.. That is an option that you can either employ or forgo as you feel what best suits your needs and the confidence in your measurements..
 

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Discussion Starter #10
So my reasoning behind the axle notification is this. At the end of the day I want a 4.11 posi. I have 3.07 open with a worn cross pin in a 8.2" 10 bolt. Nothing swaps without cutting so for me it made more sense to replace the axle with one built to the correct length instead of replace everything in my original axle.
 

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So my reasoning behind the axle notification is this. At the end of the day I want a 4.11 posi. I have 3.07 open with a worn cross pin in a 8.2" 10 bolt. Nothing swaps without cutting so for me it made more sense to replace the axle with one built to the correct length instead of replace everything in my original axle.
This makes total sense to me.. If I were going to weigh the pros and cons, a stronger more easily serviced 9” would probably be strong argument to support over narrowing and rebuilding the 8.2.. I’m in the decision process right now on narrowing an 8.5 10 bolt for similar reasons.. I’ve got a couple of them on hand at this time, they are superior to the 8.2, I have disc and drum brakes on hand, and there more options available for them..
 

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Discussion Starter #12
I definitely searched for a 8.5" to build. Solid option. For me I wanted to narrow whatever if I was building. I wanted bolt in axles and an Eaton Truetrac regardless so really the housing was of minimal cost when considering axles and housing ends and labor to narrow a used housing but the 8.5" was a solid contender for me. The concept of a crate axle that just bolts in was worth a small premium for me.
 

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Discussion Starter #13
Thanks for all that replied. Even though I dreaded it I pulled the axle out today and got the wheels and tires mocked up in place. In case it might help anyone in the future here is what I found:

245/45R17
17"x8" 4.5" backspacing 0 offset
3/4" board between leaf spring and tire as a spacer
3/4" outer clearance on passenger side and 1" outer clearance on driver side
56-3/4" wheel mounting surface to wheel mounting surface

Closest spot is the bump stop bracket which has over 1/2" and I can get another 1/4" by clearancing it
 

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Thanks for all that replied. Even though I dreaded it I pulled the axle out today and got the wheels and tires mocked up in place. In case it might help anyone in the future here is what I found:

245/45R17
17"x8" 4.5" backspacing 0 offset
3/4" board between leaf spring and tire as a spacer
3/4" outer clearance on passenger side and 1" outer clearance on driver side
56-3/4" wheel mounting surface to wheel mounting surface

Closest spot is the bump stop bracket which has over 1/2" and I can get another 1/4" by clearancing it
Does this formula account for your 0.25” per side rotor hats?
 

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Discussion Starter #15
Does this formula account for your 0.25” per side rotor hats?
This is measuring from the back surface of the wheels. Any thickness for the brakes will need to be subtracted from this number. I have spoken to Quick Performance before and they were familiar with the fbody disc brake setup so they should be able to account for that thickness based on this number but I will look up the rotor thickness to provide then as well just to be safe.
 

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Discussion Starter #16
Rotor hats measure 1/4" thick (probably 6mm) so that is another 1/2" off of length which would make overall axle length 56-1/4".
 

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Discussion Starter #18
We all agree the stock axle width is 57.75" right? That's with drum brakes. When I switched to F body rear disc I removed 1/8" of drum brake material and added 1/4" of disc brake material. This put my total width at 58"
My total width after the new axle will be 56-3/4" which is 1-1/4" shorter so effectively the equivalent of adding 5/8" of backspacing to my 4-1/2". That would put me at an effective 5-1/8" compared to stock width. Typically I see 5-1/4" recommended so I feel pretty comfortable will all of the measurements the way they are working out.
 

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My total width after the new axle will be 56-3/4" which is 1-1/4" shorter so effectively the equivalent of adding 5/8" of backspacing to my 4-1/2". That would put me at an effective 5-1/8" compared to stock width. Typically I see 5-1/4" recommended so I feel pretty comfortable will all of the measurements the way they are working out.
Do yourself a favor and make it 56.5" or better yet 56":yes:
 

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Do yourself a favor and make it 56.5" or better yet 56":yes:
Maybe you could elaborate your thoughts on going narrower then what he has come up with per his dimensions.. I’m looking a doing an 8.5 in the future and will have to make a similar decision at some point.. I have worked out my wheel and tire sizes based off of the standard 8.2 axle with 57.75” and know what works with that setup.. Having wheels made to my spec is not a huge deal but most aftermarket wheels don’t offer adequate backspacing for 1st gen cars..
 
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