Rear end width?

[redss86]

This one is for Norm and Marcus, or anybody else who wants to chime in.

I have an 86 Monte Carlo SS that I am building as a pt car. I am planning on going w/ the G5 front suspension, and have the truck arm setup for the rear.

I am going to build a custom 9in rear end. My question is this: Would be to any advantage, as far as handling goes, to either widen or narrow the actual width of the rear end?

I don't currently have any measurement of the factory width, or my actual suspension measurements yet.

Just wanting to get some ideas.

Thanks for the help,

Joe

[Travis B]

I don't see any real advantage or disadvantage......The narrower the rear the deeper dished wheel you can run?

[airrj1]

Joe,

I don't know you rear axle width for the G-body, but if you are going with a custom axle I would suggest that you determine the wheels and offset that you want to run an then size the axle appropriately. For example I have a '72 Chevelle and I am running Z06 rims with 2.125" adapters on each side. I have found that a 9" out of a Ford F-150 is exactly 4.250" wider than my stock axle. So when I build my 9" I will be able to eliminate my adapters. So if you are using rims that you can't custom order the backspacing you could use the axle to fit the tires in the wheelwells properly.

As far as running a wider axle with a rim with more offset vs. a narrower axle with a rim with less offset, I would guess that the narrower axle combonation would be less unsprung weight. But how much of a difference I don't know. Somewhere here on the board Katz talked about wheel offset and the loads placed on the wheels. He gave an example of a drag car that commonly has a narrow axle with the wheel sticking way out, and the loads on it as opposed to a wider axle and the rims offset towards the center of the car. He was suprised that more people didn't run a wider axle. (I hope I got that right, I'll search for the thread.)

Edit: This isn't the thread I was thinking of, but it answers some of your question I think.

[redss86]

I don't have the wheels yet, so I am kind of open in that area.

Zero offset on 12" rim (12" bead width) equals roughly 6.5" BS, depending on the lip width.

Typical semi-floaters (ie, Ford 9", Dana44, etc) and c-clip HSG has axle face to bearing centerline offset of 2.5~3". For best axle and bearing longevity, wheel centerline should be close to the neck of axle. This means slight positive offset, like 8.5" BS on 12" rims, for example.

On the other hand, shallow BS rims will save some unsprung mass (narrower axle). Think of current SCCA T/A cars. Flat-face rims up front to cut down scrub radius, and dished wheels rear for lighter unsprung mass. Keep in mind those cars use full floater axles with huge axle snouts, that can withstand the extra stress from negative offset rims.

Probably good street axles and bearings would tolerate neutral offset rims, but I wouldn't even consider using wheels with large negative offset on street cars that get driven hard.

Narrowing independent front and independent rear suspension to fit dished wheels just for the sake of aesthetics is dumb.

So, if I am going w/ a full floater rear end w/ SCP's GN axle snouts and hubs, what would you guys recommend for ideal backspacing. I am planning on going w/ 18x10's on the rear, and probably 18x8's up front. Any other recommendations on wheel size, bs, or axle width are welcome.

Thanks,

Joe

[Travis B]

Just based on looks I like a 4.5 bs on 10in rear wheels! gives it a 5in dish or so that looks very nice! However that is based on looks alone as far as performance goes Im not positive!

[airrj1]

Well, from what Katz said in that post he seemed mostly concerned about stock type axles and bearings. If you will have the good floater ends and bearings you should be able to do what you want. But I would follow what he said with a neutral or positive offset rim. For my personal taste I don't like much of a dish. I would look for a good size positive offset. But as Travis said I am speaking mostly for looks.

[USAZR1]

We built a 56" width Fab 9 rear for my 69 Elky. Stock width was approx. 62". I'm running 17x13's with 7" backspacing.

[Travis B]

We built a 56" width Fab 9 rear for my 69 Elky. Stock width was approx. 62". I'm running 17x13's with 7" backspacing.

And the dish on your wheels looks KILLER

[redss86]

So basically, it is a matter of asthetic opinion. There are some advantages/ dissadvantages, but not enough to be extremely particular about it.

Joe

[Norm Peterson]

The inner limit at the rear is ultimately defined by the frame (or by however much you're willing to modify it - keeping in mind that you'll need lateral strength/stiffness to accommodate the panhard bar loads) plus an adequate amount of clearance for lateral tire deflection under cornering loads. The outer position and track are then defined from that point by your wheel width and tire size. I'm not as worried about a little negative offset here, because the moment loads applied to the axles/bearings from hard cornering tend to oppose the moment loads applied there by the corner weights on the more heavily loaded outside wheel. One might even argue that negative offset is preferable if there is much static negative camber, just to get the contact patch force centroid more directly under the bearing. A little negative camber (and a little toe-in) in that built axle is not only possible but desireable if you're actually going to run it hard (auto-x, open-tracking, open road events, etc.).

FWIW, USAZR1's 13" wide wheels are very close to zero offset, assuming ~1/2" thick flanges.

With the G5 suspension up front I'd seriously consider wider than 8" there. 8.5" wide will work with a 245 section tire with -12 or -13 mm offset and no spacer even with the OE G knuckle if you're only a tiny bit lucky with suspension component tolerances and are willing to trim the lips of the UCAs. Wider than 245's would probably involve the fenders. My understanding is that everything fits inside the barrel of the wheel with G5, so you can go closer to zero offset and add some width to the inside at the same time. Marcus should have information about how much wider is possible, but I'd expect that at least 9" is do-able.

Finally, I think you want to maintain the front and rear track dimensions (between wheel centers) fairly close to each other. Not necessarily identical, but not several inches different either.

Norm

[redss86]

The inner limit at the rear is ultimately defined by the frame (or by however much you're willing to modify it - keeping in mind that you'll need lateral strength/stiffness to accommodate the panhard bar loads) plus an adequate amount of clearance for lateral tire deflection under cornering loads. The outer position and track are then defined from that point by your wheel width and tire size. I'm not as worried about a little negative offset here, because the moment loads applied to the axles/bearings from hard cornering tend to oppose the moment loads applied there by the corner weights on the more heavily loaded outside wheel. One might even argue that negative offset is preferable if there is much static negative camber, just to get the contact patch force centroid more directly under the bearing. A little negative camber (and a little toe-in) in that built axle is not only possible but desireable if you're actually going to run it hard (auto-x, open-tracking, open road events, etc.).

FWIW, USAZR1's 13" wide wheels are very close to zero offset, assuming ~1/2" thick flanges.

With the G5 suspension up front I'd seriously consider wider than 8" there. 8.5" wide will work with a 245 section tire with -12 or -13 mm offset and no spacer even with the OE G knuckle if you're only a tiny bit lucky with suspension component tolerances and are willing to trim the lips of the UCAs. Wider than 245's would probably involve the fenders. My understanding is that everything fits inside the barrel of the wheel with G5, so you can go closer to zero offset and add some width to the inside at the same time. Marcus should have information about how much wider is possible, but I'd expect that at least 9" is do-able.

Finally, I think you want to maintain the front and rear track dimensions (between wheel centers) fairly close to each other. Not necessarily identical, but not several inches different either.

Norm

As far as the frame goes, I am custom fabricating the frame (from the front torque boxes back) out of 2x3x0.125 tubing. I plan on putting a brace between the frame rails above the panhard mount location for lateral support. I have added 1.5" to each rear wheel well on the body, and I am going to have the frame just inside the wells. I am figuring on running a 10" wheel w/ a tire from 295- 315 in width. The tire width kind of depends on how much lateral deflection you think there will be. I am also going to use rod ends for my panhard, and the truck arms have greasable monoballs in them.

As far as the front wheels go, the ones I found that I like only offer widths in 8, 10, and 12. The wheels I am wanting to get are the 18" Bonspeed Deltas. I don't think a 10" wheel will clear in the front w/o modifying the fender or inner fender. I haven't checked yet.

Any other ideas are appreciated.

Joe