this car came equipped with typical longitudinal leaf springs, soft, squishy, compliant in all directions, and of course worn out. these cars were not known for their precise handling, let's leave it at that. i'm fairly sure Nash invented understeer and installed a lot of it just to be safe. the rear axle was a typical AMC15, the small Dana axle. reliable except worn out, 9" drum brakes of course, and the Nash-specific sliding pinion yoke with novelty oversized locknut that uses an unobtanium pinion seal.
rather than attempt to upgrade or repair anything it was all extracted and replaced with a narrowed and re-geared 1998 Mustang 7.5" rear axle assembly. it of course has disc brakes which makes the car discs all around now. i chose the 7.5" axle because it is 50 lbs lighter than the larger more popular axle and i'll never install enough power to break it, plus the car is very light, 2250 lbs on the truck scale.
wishbones are an old design, but not a bad one. wishbones are simply truck arms extended forward until the arms meet, and share a single front joint. it solves a lot of problems that more-popular rear systems like truck arms and four-bar links have. it's easy to make it have zero bind due to body roll. pinion angle change is very low. anti-dive is fairly good (the front pivot same height as the rear axle). it requires some care to make it stiff, but no worse here than truck arms.
a wishbone has dynamic geometry similar to a torque arm setup, but the wishbone inherently prevents lateral axle rotation and doesn't need the two lower arms shown in this how-to-build-hotrods.com page. it of course requires lateral location, here via panhard.
this wishbone has pinion angle adjustment built in. axle "rotation" is adjustable also, eg. if viewed from abovethe rear axle made parallel to the front "axle". this is actually the third wishbone i've constructed; the first was non-adjustable, fairly weak, and clamped to the old AMC axle spring mount pads. i ditched that axle and made a second version that was fully adjustable. the most recent version, fall of 2016, was doubled in strength to accommodate the built 195.6 OHV engine, T5 transmission, and re-gearing of the axle to match. this one solves all problems of the past and has no noticable deflection.
the axle was narrowed by Cook's Machine Works here in Los Angeles. they cut from the middle of the tube and butt weld, the whole assembly held in a large lathe to keep it true. i had the pinion centered. i retained the OEM four-link brackets to mount the wishbone, and extended them upward a bit to add the anti-wrap braces, which also serve as geometry and pinion angle adjustments. though these joints are rigid in operation i used heims to accommodate the angles involved; also rotating the assembly about the axle axis changes the triangle angles, so some way to accommodate the geometry change is required.
in fall of 2016 i had the axle re-geared to match the new engine and T5 transmission by J & S Gear Co., Huntington Beach, who also installed new axles. Cook Machinery and J & S, both top-notch shops generally work together to do these two tasks. i highly recommend both.
the factory four-link brackets welded onto the axle tube were used to mount the wishbone via heim joints. metal was added to the tube at the top so that all four heims would be in double shear. the air spring axle perch is welded across the tops of that. though once in place these four do not move, heim joints provided a simple and rliable means of attaching the wishbone to the axle. and when the wishbone legs are adjusted to dial in squareness and pinion angle those triangles do change and the heims accommodate that.
on the back side of the axle on the drivers side is a nut on brackets to accommodate half of the panhard axle mount, visible below, barely.
at this time i added the air spring perches (simple), added metal so that all four heims, mounted on the Mustang four-link brackets are in double shear, and the axle end of the panhard rod mount. this had to clear all the standard junk attached to the axle (shocks, the protruding differential cover, brakes, etc) so this meant the axle panhard mount had to be dangling in the air. this adds a fore/aft force vector that necessitated careful bracing and triangulation. it just barely allows the shock to be installed/removed.
the outboard end of the panhard axle mount attaches to the anti-vibration thingie. all of the force here is lateral, into/out of the axle flange. the stiffly boxed and braced bracket reaches over to the nut welded into a stantion on the axle tube near the differential housing.
the leaf spring system was replaced with small Firestone air springs.(i'm using one old leaf pack as edging in a garden; works great!) these were short enough to fit between the axle and the chassis kickup, putting them far out near each wheel and eliminating body roll spring geometry changes that occurs when springs are inboard. the apri spring axle tube perches are visible on many of the photos above and below. a single 3/8" bolt up through the horizontal plate attaches each air spring to the axle, and precut disc of steel braced into the chassis above holds the top of the spring, from which the air line extends to the rear fender Schrader valve.
the wishbone and panhard and panhard mount were constructed from 1.25" and some 1" DOM tubing of moderate wall thickness, and heim joints and tube adapters from QA1.net. everything was MIG welded.
the wishbone was aligned using V blocks and a cheapo laser. (the image shows the second wishbone, not the current version, and is for illustration only). the V blocks align to the centerline with a mechanics square. the triangle was then tweaked to put the dot on the pinion nut horizontally. vertical alignment in this picture is done by the jackstands, which hold the wishbone pointy end at more or less install height, and the pinion flange more or less vertical. it only has to be "close" as the rear heims provide adjustment for squareness on the car.
in this photo you can see the final strut assembly and the air spring perches welded to the axle tube. the upper tubes with tapered ends are swaged tubes from QA1.net with a left hand and right hand heim joints so that loosening the nut and turning the tube adjusts pinion angle.
the wishbone legs are each triangulated with the rear section adjustable by rotating the knurled-tube section. it wasn't possible to make the triangle perfectly straight due to clearance reasons. this lowers it's strength somewhat but it's already overkill. the lower tube is thickwall 1.25" to guard against road damage.
front joinery appears excessive, and i hope it is. this was where the previous wishbone deflected. the force peaks just beyond the five-way joint and before the tube adapter insert. it's possible i will get paranoid this winter and weld a rib top and bottom along this path, though i have no reason to think it will fail.
since this is a unibody, and there is no transmission crossmember handy to bolt things to, i made a spider that positions the wishbone's front pivot below and forward of the front U-joint. unfortunately i didn't take any photos of it before i installed it. the first photo below shows it under construction. the two vertical tabs with 5/8" holes will hold the big heim joint below the transmission tail and centered in the chassis. (the driveline is offset an inch to the passenger side). you can see the nose of the wishbone more or less in place on a jackstand to the left. this partial assembly bolts upwards onto the reinforced floor pan with captive nuts in the pan. these two short uprights handle axle upward wrap; 1000 ft/lbs of axle torque becomes 200 lbs of upward force on these two uprights, right onto the side walls of the transmission hump.
hiding inside the spider is a stock 1998 Mustang transmission mount, the "5th motor mount" necessary to prevent torque rotation of the transmission tail. engine torque twisting against the chassis and rubber isolated compliance makes the output yoke lift up and rotate towards the drivers side. if held centered, the force is quite small and easily restrained. as it moves out of it's centered position its "leverage" increases, which becomes positive feedback.
this post-assembly view shows the two trailing struts of the spider that transmit fore/aft force from the rear axle to the chassis, at the kickup to the rear foot well, which is close to the pan anchors of the B pillar to B pillar lateral bracing inside the car. the photo angle conveniently exaggerates the driveline offset.
when i first considered this scheme i assumed the most difficult part would be clearance issues around the rear U-joint, yoke, transmission, and front joint. there's no issue at all. though not obvious here there's an inch of clearance between driveshaft yoke and wishbone. i tested everything for clearance with all air out of all four springs. given the length of the lever, the distance remains essentially the same.
all of this stuff neatly tucks up into the chassis; nothing protrudes or hangs lower than the rear engine cross member or side sills.
the panhard rod itself is uninteresting; a 1" DOM tube with tube adapters and left and right handed heim joints. (note: the unpainted rod in this image is made from crappy thinwall tubing while i awaited the right parts to arrive from QA1.net.) all the work is in the mounts and geometry. the panhard rod runs 6 inches behind the axle to clear the differential cover and shocks and brakes. on the drivers side it attaches to the axle, and passenger side to the fabricated panhard mount.
the chassis mount was more complex due to the monocoque chassis. there is no single point to attach the chassis end of the panhard to in a sheet metal car. i fabricated a tubular mount that ties both sides of the chassis together, places the rigid mount in the right place, and braces rearward for good measure.
at the right side chassis end the force on the chassis is pretty much lateral. from the chassis-end panhard rod pivot, a short stiff stub runs more or less straight up and locates it. a long DOM tube runs to the left side of the car, transferring tire side force almost-not-quite laterally to the chassis; the tube is long enough and the triangle short enough i'm not worried about the offset vector. but in any case, on the right side near the vertical stub is a rearward brace, about 45 degrees, which stablilizes the system from non-lateral force. (in these photos you can just make out the real DOM tubed panhard rod.)
i suspect that few cars of this vintage were delivered new with actually-square chassis, and they sure don't get squarer as they are driven. i had an old station wagon long ago that had the rear offset a full inch to one side. you could see it from behind. it tracked perfectly, tires and alignments were fine and i drove it that way for 21 years.
to an already low-precision system i am adding my own construction makes real any measurement errors i might have made. so i made it all adjustable.
i worked out a trivially simple way to get accurate measurements on all four corners. up on the front of the car, the lower trunnions have a Zerk grease fitting. (a ball joint car is exactly the same). i chopped the greasgun fitting from a pair of zerks, inserted a medium cotter pin, loop end out, slightly bowed the legs so it would stay together, and screwed them into the trunnions. now a tape measure tip will snap into the gap in the cotter pin and stay there, allowing precise measurements to the car's opposite corner. i found a symmetrical spot on each rear axle tube to use as reference and with a pair of 12 foot tape measures looped around the axle, i was able to dial in chassis squareness to within 1/16". done and done.
the parking brake was surprisingly straightforward. the 1961 Rambler ratcheted pull lever, it's jacklever and pull rod under the car mated up to new 1998 parking brake cables. i just had to make a funny little adapter to mate the pull rod to the cables, and a mount for the cable housings. the mount welded onto the front drivers side corner of the wishbone. the only image i have is from the second version when i still had the T14 installed, and a spider that braces forward to the crossmember.
the driveshaft tunnel on this car is comically small, and would not accommodate the Mustang driveshaft. i needed a custom length anyway so i used an early 70's AMC driveshaft and yokes. i'm using dual pattern 1310/1350 U-joints. driveshaft was shortened and balanced by Cook Machine Works here in Los Angeles.
here are all three versions of wishbone, left to right.