: 1968 Deville convertible tire rear axle and tire upgrade question



hermanpj
04-11-11, 10:56 AM
I searched the forum and didnt find this answer so if you know of a thread to point me to, please do, and apologies for the repeat here.
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I decided to put in an Art Morrison front bikini clip and replace rear axle with a ford 9 inch. Will have discs, tough axle shafts, locker, air bag, 4 link and a panhard bar. At same time am replacing tires. Here is the rub, literally.

Stock tire and wheel combination is:
-9.00 x 15" tire 4ply rated 2 ply on a 15" i diameter wheel 6" width between inner and outer mounting flanges, 0.54" positive offset.
-Closest modern tire per this forum is 235/75R15. This calculates to a 28.88 diameter tire with a maximum section of 8.5 inches and a tread width of 7"
-Assuming this tire is mounted on same wheel, mounted on drum, there is 1.5" clearance between tire and frame, and only 1" clearance between tire and lower
control arm mount.
-total backspace is 3 inches (1/2 wheel width) + 0.54" pos offset = 3.54"
-Outer wall clearance to the fender and fender skirt is not great, definitely would not want to push the outer wall of the tire much further out.

New tire and wheel
-I'd like to stay with roughly same rolling diameter. Want a larger wheel without the low profile look. Looking at a 225/65R17 H rated tire on a 17 x 8 wheel.
-Tire works out to 28.52" diameter, max section of 9", tread width 7"
-AME Ford 9" axle will relocate control arms inboard of frame so no concern with clearance to control arm mount
-However, the wheel I like (which retains some of the look of the original sombrero wheel cover) is a Centerline California Smoothie.
-It retains 5x5.0" BP, is 17 inch diameter. Problem is its 8" flange to flange and
has a 5" backspace includiung the offset.
-this means backspace is 1.46" greater than stock, and with max tire section at 9" vs stock 8.5, the tire itself will extend 0.25" further inboard on any rim.
-adding those, the increase inboard from the mounting surface (assuming same axle width disc to disc as existing hub to hub) would be 1.71" which means the wheel won't mount - it will hit the frame first.

So the questions
a) What is the minimum recommended distance between frame and tire
b) How much clearance is there outer face of stock tire to fender skirt? Maybe I could push the axle width out to move tire out to mimumum recommended distance
c) What is minumum recommended distance between fender skirt and wheel arch and the outer face of rear tire?
D) Does anyone have a performance tire and wheel combination that does not
have a frame interference problem, preferably on 17 inch rim?

70sdv
04-12-11, 12:41 AM
Did you look at the Centerline Billet Series Stealth? It's got a smooth look and you can get them in a 5X5 pattern, 17" by 7" width in a custom backspace. You can obviously specify a backspace of 4.5" or less and gain the clearance you're looking for.
Try searching for Chevy/GMC Truck & SUV 2WD (5-lug, pre-99) wheels which will fit our Caddies for some more options.
BTW a 235/65-17 is a popular tire size with a 29.1" diameter and section width's in the 9.5 range and will fit a 7" wide rim. As far as I know as long as you're not rubbing in the rear there's no problem, though I'd be shooting for a minimum of a 1/4' to 1/2' clearance on each side to avoid any problems if your tire ever gets low on pressure and expands a bit. If my math is right a backspace of about 4.25" should squeeze it in without rubbing.
This is the combo I'm eyeing for my 70 which has the same constraints as your 68. HTH

hermanpj
04-13-11, 08:54 PM
THANKS - we are definitely thinking alike. I like the billet stealth, -the california smoothie is very similar, main difference is the lug nuts show on the california. Is the Billet Series more costly?

I went out and put the fender skirt back on and measured minium clearance within the tire's diameter. The stock tire is 28.88 inches which is close enough to what I'll be running. I found the minimum distance from the inside edge of the fender skirt at its narrowest to the frame is 11.25 inches.

So I'm planning to go with a 235/60R18 Goodyear Eagle GT. Thats a 29.1 inch rolling diameter tire with a 9.5 inch max section width so there's 1.75 inches of clearance to be divided between inside and outside, so I thikn I have that solved now.

It accepts up to 8.5 inch nominal width wheels. so I plan on going with 8x18 wheel with 5 inch backspace, including 0.5 inch for the outer lip to mounting flange, 4 inches to wheel centerline, and 0.5 inch positive offset.

I made a backspace calculator to verify all my numbers, then I fed all this info plus the factory frame specs into Art Morrison and they are custom building the Ford 9 inch housing to width to the exact dimensions.

I put half down today so I should have engineering specs in a week or so... now i just have to improve my welding skills so I can weld the in the new cross member, and brackets for 4 bar, panhard bar, upper perches for th air springs, and new shock mounts.... not to mention cutting the front of the frame off and grafting in their bikini clip. :-)