20 mar 2020
new 10 feb 2018
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This document covers performance-oriented, substantial modifications to this suspension. There is a separate document for the repair and maintenance of the basic stock suspension, including non-factory modifications to re-use failed parts.
This document covers my modifications and redesign of the old nash-based front suspension in the 1958..1963 Rambler Americans. Lots of this information applies to stock rebuilds or problem diagnosis. Though the original impetus was to get around unavailable parts it's worked out to be a surprisingly easy to make it handle well.
My current path is to bring the suspension up to modern standards of performance and reliability, and have all wear parts be modern and obtainable.
(This page was moved from the Rambler Roadster page to it's own section here.)
No one was more surprised than I was at how easy it was to modify this old Nashcan (front) suspension, to reposition roll center to eliminate as much roll-based understeer as you might desire. This suspension has a lot of awful design flaws, mostly choices made around pivot construction and lousy materials. But underneath it all is a very clean double-wishbone with inherently good capabilities, mainly the spring-over-knuckle design.
The second major design feature is it's modularity -- the suspension literally bolts onto the car. This modularity allows arbitrary changes to the location of control arm inner pivot heights. The sole geometry mod here is what Ford folk call the "Shelby drop" -- lowering the upper control arm's pivot point -- to move the car's roll center from below ground to above ground, as desired.
It may surprise you to know that much, if not most, of the understeer in this car is in the rear suspension, not just the front. My roadster has a completely fabricated rear suspension.
The stock rear is a conventional longitudinal leaf spring system. This document is about front suspensions, but a quick review is called for I think. Leaf springs are not aligned exactly fore and aft, parallel to the direction of travel, but usually canted inward towards the front. Also the front spring eye (fixed height) is usually higher than the rear, on it's shackle.
Suspensions don't just go up and down; cars rotate, or roll. The alignment of the leaf springs doesn't matter (much) for up and down, but matters hugely in roll. Imagine a car in a left turn. Inertia "wants" the car to go in straight line as the steering wheel is turned, the right side springs are compressed, and the car leans (rolls). Each rear wheel swings in an arc (pivoting on the front eye), the outside wheel down, the inside wheel up (roll). The location of the spring front eye -- height and inboard/outboard -- determine the arc the wheel moves in. Rear wheel motion is not just up and down; the arc means it moves fore and aft simultaneously -- a very small but controlled amount.
Imagine a reference line drawn through the front and rear wheels (center of the tire). On the early Americans (1961 measured), the spring front eye is located (approximately) 9" inboard of this reference line. On a 1968 American, it is 6". The large inboard offset induces a lot of understeer in the rear: the outside wheel goes forward, the inside wheel rearward, in essence steering the rear axle assembly out of the turn -- understeer.
The short of it is, modifying the front without dealing with the rear will not make the car flat in a slalom. Doing both will come very close. The limitation I've come to now is lack of grip in the rear, due to terrible front/rear weight distribution (and my severely ligtened car). It now tends towards snap-oversteer.
vsusp.com has a lovely 2D suspension calculator. The calculators below have suspension component measurements derived by me. The A-arm and spindle measurements are fairly accurate I think.
This is the Rambler factory suspension component dimensions and recommended caster and camber settings. Though it's only 2D, it does a good job if showing how the car might behave in turns. These cars are burdened with notorious amounts of understeer.
|Factory (click graph for simulator)|
|roll center height||-1.142" (below ground)|
|roll center L/R motion per degree roll||-9.4" (out of turn)|
|camber change per degree roll||-1.17 degree|
This is the same suspension with the single modification of one inch of "Shelby drop" in the upper arm pivot bar.
|Modified (click graph for simulator)|
|roll center height||1.18" (above ground)|
|roll center L/R motion per degree roll||+7.8" (into turn)|
|camber change per degree roll||-0.90 degree|
Here is an excellent discussion of "race car" suspension details on Physics Forums/race car suspensions. I read all 60 pages, enjoyed every minute of it. one of the best behaved and respectful bunch of car folk i've ever read.
Some of the design and construction flaws are acceptable in light of it's ancient design, and some are not. the upper wishbone (A-arms) actually pivot on thin stamped sheet steel. This is just bad design. These have been notoriously short-lived throughout this car model's existence.
A permanent fix -- and substantial geometry improvement -- is described below. It's in place on my car and driving just fine, with 50,000 miles on it as of this writing (April 2020).
the lower wishbone/A-arm is worse. A multi-piece trunnion has caps that thread into each arm half, the trunnion itself threads into the caps, and the vertical steering knuckle threads into the trunnion. Unless assembled with excruciating precision binding occurs, which twists the caps out of the arm. (The caps are tool steel as a hint to the engineering fixups over the years). Therefore the pivots in both planes are threaded -- and so even properly lubricated -- which was historically rare -- metal grinds on metal. Lower arm trunnions have been known to disassemble themselves at speed. (The "big car" upper trunnion system is fine; this problem is peculiar to the Nash-inherited lower trunnion system.)
I was lucky enough to find aftermarket "trunnion repair kits" that effect an engineering change to make the lower trunnion vertical pivot be bronze bushed. these greatly improve things but the horizontal pivots still grind normally. the real solution here is to completely ditch the lower arm assembly; that is the next project (still pending April 2020).
After a year's research into suspension design and suitable build parts, I worked out that this little nash upper suspension is almost bolt-compatible with a mixture of Ford and Chevy parts (though it predates them a good quarter century).
The short of it is, the upper inner A-arm pivot bar is easily replaced with a "cross shaft bar" intended for a 1970 Ford Mustang, specifically one with "Shelby drop" built in, from RideTech. The mounting hole spacing was 0.125" too close together; I simply milled each (slotted) 0.0625" each towards the outside, and it just dropped into place. This part was the secret to success -- it allowed me to make simple tubular arms with standard parts from QA1.net (tube adapters and Endura series heim joints). The "Ford" cross shaft accepts 5/8" bolts, the same diameter as the threaded hole in the trunnion, allowing nicely standard parts usage.
The RideTech cross shaft part isn't in their online catalog, you have to order it on the phone. They were $75 each in 2018. For reference, here's the RideTech cross shaft installed in one of their StrongArm arm kits.
|cross shaft||Ride Tech 90000931||2||not online; phone order only|
And thanks to the very few aftermarket suspension parts providers that actually provide actual measurements in inches. Mainly QA1.net, RideTech, and SPC Performance. I guess the rest of the world just orders "ferd" or "chebby" and bolts together their prefab kit cars. The rest of us have to think and measure.
Here's an early mockup that shows the major relationships. It's missing some significant detail outlined below. but the bare mockup, installed, let me assemble the car and put weight on it so that I could work out the exact arm length to give me the desired adjustment range (+0.5 to -3 degrees camber or thereabouts). It took a couple of cuts to get it right and have it "land" in the middle of the adjustment range of the heims and the limitations imposed by the stiffening plate below.
On the bench I assembled the arms in the guesstimate length and made sure they were exactly the same, then welded the tabs on as shown. Without the stiffening plate the trunnion can shift back and forth (actually it swings in a funny shallow complex arc), the four heims making a parallelogram. The plate (not shown here) bolts across the top, onto the two two-hole tabs welded to the arms, and a bumper for the trunnion casting to hit when the suspension is at full drop (saving the air springs).
The tabs do require upper arm adjustment to be made only in full-turn increments, which works out to be about 3/8 of a degree of camber. Turning only one arm half halves that, and one turn of arm-length imbalance seems harmless. In fact I intentionally assembled it with the front half one turn longer to do some of the caster work of tilting the kingpin inclination.
The arm halves thread onto the cross shaft bar and heims previously attached to the chassis and set now to exactly the same length, then the front arm one turn longer.
The arms are then swung up to meet the trunnion casting and through-bolted. That's a chunk of low-carbon steel USA made threaded rod, 7.5" long, with QA1 1/4" (inner end heims) and 3/4" (trunnion end) high displacement spacers. There was no binding with short (0.33") spacers all around, but this makes the trapezoid a little more square, and there's plenty of room for it.
Here is the completed arm with stiffness plate installed. I originally imagined shifting the trunnion fore and aft to set caster but there's little movement there (which is good, actually) so the plate just gets bolted on. I left it loose and tighted them bolts after camber was set.
Dec 2019: other fish to fry first; but still planned. the current system is stable so it remains for now. I now have a set of 'big car' knuckles and verified that the geometry is correct (inclination, mostly).
The basic idea: completely ditch the entire lower arm system; both arm halfs, the trunnion mess, and the "pivot bar". In it's place will go a "ball joint plate", probably from SPC or similar. Approximate design at the top of this page: SPC race parts.
The SPC parts on that page are "upper arm" parts (but an a-arm is an a-arm). an additional boon here is that extended-length ball joints simultaneously lower the car (raising the spindle) and induce "Shelby drop" upper arm geometry improvements without the use of the dropped upper arm pivot!
This requires replacing the steering knuckles with 1963..1967 "big car" knuckles. these already accept ball joints. I have verified that the upper trunnion pivot system is exactly the same and interchanges -- the 63 01 trunnion casting has the same bore and length as the 63 10 upper trunnion.
However the upper arm pivot on the big car is an inch or more outboard than the 01 chassis. The upper tubular arms must be replaced as well. This changes geometry in turns (less camber with body roll) so we'll see what that does to handling.
The lower ball joint tapers must be accommodated, through a combination of reaming and possibly reducers to accommodate GM ball joints. This work is TBD pending arrival of the 63 10 knuckles any day now.
The inner pivot bar in the 01 chassis also has bosses to attach the under-chassis K-brace. GM/Ford type bars will not have this. The K-brace will possibly be replaced with an adjustable fixed rod-end system and a fabricated L-bracket that bolts between the pivot bar and the chassis ears, the rod running under the engine.
I abandoned the wire springs, replaced them with Goodyear air springs to great effect. The wire springs were a bear to wrestle; 20" tall and only 5" diameter working with them was like arguing with a weapons-grade ball point pen spring. I made a tool to deal with them but no more; the Goodyear air springs are superior in every way and a fraction of the weight. And on-car adjustable.
The downside is that the air hose fittings seep and lose air, sometimes half an inch a month if height. Adding thousand bucks and a hundred pounds of complexity over comes that. I use a small double-action bicycle hand pump and a tape measure before each TT or road trip.
It's like Goodyear air springs were designed for this car. I made a stand out of an air spring mount plate from Ridetech and welded a small stand from hot rolled steel strip. The Rambler spring perch is a casting that holds the upper pivot, I made a clamp-on saddle for that.
A number of people have asked me about this air spring shoe, here are more pics of it. I fabricated it from steel stock.
Before the current work (feb 2018) I had done a thorough rebuild in 2010 when this suspension was still on my 1963 American 440. At disassembly here it had about 50,000 additional miles on it.
I decided that instead of repairing the crappy upper arms, to replace them with a new design that also addressed the designed-in chassis roll and understeer issues. Details follow.
20 feb 2020 note: The polyurethane bushings described below are still in service and haven't deformed in any way I can detect without tearing them out. I run toe very tight (1/32") which makes it sensitive to misalignment, and it's been no-change since set. Nor has this soft, low-durometer poly squeaked, ever.
This section produces new soft poly bushings that will fit into the existing stock stamped steel arms.
new-replacement rubber bushings were not available when I started on this "rebuild"-become-redesign. oh they're available for purchase, but nearly all of them were "new" in the 1980s, and old rubber is bad rubber. I just heard however that someone may be repopping these again. No matter, I found a way to fabricate new ones from the nice soft black polyurethane bushings from RideTech. And i'm way past "rebuild" now anyway.
The problem is that the Nash-lineage parts have very odd dimensions, 9/16" bore and uncommon length. Nothing is made in that size, every other OEM and all aftermarket seems to be sized in 1/8" increments. (I went through the dimensions of every bushing in Moog and other catalogs.)
Below is the inner end of the lower control arm (upper is identical here) and it's pivot bar. Though the rubber doesn't look too bad here, the center is offset somewhat. Nothing fatal, but i'm demanding.
The pivot bushings, eight per side, though of typical construction (press-fit assembly composed of concentric steel sleeves with rubber between) the sizes are peculiar to Nash/AMC, which uses 11/16" pivots. Availability of these bushings come and go; when I first wrote this page they were not available. Now (2020) they are:
Here is the shopping list for soft-poly bushing construction shown below.
|poly bush||Ride Tech 90001596||16||poly bush 3/4" ID (per half)|
|spacer||fabbed||16||3/32" or so fiber, alum, steel|
|shells||used or junk AMC||8||salvage from bad bushings|
The trick to fabricating poly replacements is to reuse the outer and inner steel parts of the OEM bushings and replace the inner "rubber" with two poly halves. I drilled out enough rubber to squirt some Kroil in, and pressed out the inner sleeve in a vise. To make things easier the outer bush sleeve conveniently remains in the arm, as it's a huge PITA to press in and out.
The poly bushings from RideTech are two bucks each, 16 required if you're doing top and bottom bushings. I made a jig to flycut them in my mill-drill. Another complication is that the outer end of the OEM bushings are domed to meet the cup washers, where the RideTech bushings are of course flat. For that I had to fabricate thin phenolic rings. I cut the rings using a pair of orginary hole saws, the ODs first, leaving slugs with the holesaw pilot hole. Then with careful edge-clamping each slug hole-sawed out the ID. any non-compressible material would do, I happen to have a lot of scrap Micarta around.
The pictures below are in assembly-sequence order. it's not as difficult as it looks. I coated everything including my hands tools parts clothing the dog out in the yard etc with silicone, though I applied it originally to the poly only. nasty stuff gets everywhere. Dr. Seuss's ooblick.
Given that the ID of the poly is .0625" smaller than the OD of the bushing sleeve, it's under compression, which requires a particular sequence to assemble them. I left one bolted tight for a month then disassembled to look for deformity and creeping. looks fine.
If you look carefully at the old shells, the end with the flare is radiused, the other end sharp. you want to insert the bushing-with-sleeve intially into the sharp end. The reason becomes more obvious when you insert the second poly half, below.
It's all goopy, it likes to pop out.
Press the sleeve and poly half into the sharp end of the housing until the sleeve is flush, eg. all the way as shown.
With a socket, press it in "a little more", until 1/16" protrudes still out of the first poly half.
Out of the vise, looking into the open, flared end, the inner sleeve protrudes just past the end of the housing. Onto this intially press the second poly half. Aha! Now you can see why you want that flared end here; it helps guide the expanding poly into the hole (because the inner sleeve is 11/16" OD, into the 5/8" ID of the poly).
At this step I use the Nash pivot bar as a tool to complete assembly. Note the inner cup washer (large ID) and the thin spacer, and the outer ring and then small hole cup washer. That's just an old lugnut, 1/2-20 thread, to assemble with (not the aircraft lock nut).
Here you can see the relationship of the spacer ring to the inner sleeve and poly. The sleeve extends somewhat, the ring fills the gap and retains the poly in place without compressing it.
For years I ran this car and the previous 1963 American with shocks listed in parts catalogs online, and the ride was harsh. They fit in the hole, but they are the wrong rate for this earlier, much lighter chassis. They are only "suggested" due to the ongoing parts-catalog collapse. Having seriously lightened this car, approximately 800 lbs lighter, bumps and potholes were like a kick in the ass.
The original front shock on this car has a lower mount specific to this one car, only. I made a bracket to accept a common lower stud mount. It involves welding but it's simple.
I made a spreadsheet of every shock I could find that had the right physical dimensions (thank you Gabriel for putting your full catalog online as a PDF). Then for each shock researched the curb weight (not the manufacturers claimed weight, which is for ICC shipping tariff determination) and chose shocks by weight. That solved my harshness problem. There is lore that says stiff shocks improve handling but that doesn't make sense to me. Spring rate determines response to impulse. Shocks dampen spring rebound. Stiff shocks transfer too much of that impulse directly to the chassis. You want correct shocks, not automatically stiffer shocks. The car now recovers in a half cycle of bounce.
I previously had Gabriel 82069/NAPA 5819 up front, KYB 343135 in rear. I used these in both the stock, heavier (3200lbs), 1963 American, with stock steel wire springs in front and "heavy duty" leafs in rear, and on the roadster with Goodyear air springs (2500 lbs with me in it). Both cars rode OK but harsh, and the roadster was actually painful. The air springs have a much higher and progressive rate and shorter travel, so you'd think the stiffer shocks would be a better match. But the softer (sic) shocks made huge and immediate improvement. YMMV, but it's a cheap bolt-on experiment to do.
The modified drum brakes (see below) have since been replaced with a recent Scarebird setup, which uses Previa rotors and some middling Chevy caliper. Though the hacked drums were quite fine the '98 Mustang axle had discs on it, so discs up front headed off any weird balance issues.
of course this car came with drum brakes, terrifyingly small ones. picture below shows the upgraded 9 x 2.5" drums setup from a Gremlin, that were also modded for high performance. in 2015 I stuck Scarebird disc brake conversion on it, end of story. (the page linked to refers to the previous Scarebird bracket; the current one clears all AMC cars now, and uses different cailiper and rotor.)
This replaces the upper stamped arm with tubular arms and heim joints, and renders the upper arms adjustable for camber and caster. only quality DOM tubing is suitable for suspension components! mild steel is fine, but it must be DOM. dimensions of the tube (ID, mainly) and the tube adapter (OD, QA1.net's "D" dimension) is critical. The combination below gives a correct slip-fit. 1" OD tube, .120" wall works but is overkill and heavier.
|cross shaft||Ride Tech 90000931||2||not online; phone order only|
|arm tube stock||DOM tube 7/8" OD .065" wall||2||size critical; fits tube adapters|
|heim joint||QA1.net EXMR10||4||Endura series, carbon steel, 5/8" RIGHT HAND THREAD|
|heim joint||QA1.net EXML10||4||Endura series, carbon steel, 5/8" LEFT HAND THREAD|
|tube adapter||QA1.net 1844-121||4||1" OD, 5/8-18 thread, RIGHT HAND THREAD|
|tube adapter||QA1.net 1844-122||4||1" OD, 5/8-18 thread, LEFT HAND THREAD|
|jam nut||QA1.net JNR10S-1||4||5/8-18 jam nut, RIGHT HAND THREAD|
|jam nut||QA1.net JNL10S-1||4||5/8-18 jam nut, LEFT HAND THREAD|
|spacer||QA1.net SG1012||4||5/8" bore spacer .75" long|
|spacer||QA1.net SG104||4||5/8" bore spacer .25" long|
The front coil springs were replaced with these small Firestone air springs, a perch/stand tack welded to the upper spring socket, and a saddle that clamps onto the trunnion casting. see the text for details.
|air spring||Firestone 267c Double Convoluted||2||RideTech 90006781|
|steering knuckle||AMC 316 2825||2||steering knuckle ("pin") fro 1963-up classic or ambassador|
These shock absorbers are a better match for the 1958..1963 Americans. The parts-store recommended shocks are far too stiff, and are the result of "catalog collapse".
|front shock||Gabriel 81270||2||Volvo 122 front, 2400 lbs, requires fab mount|
|rear shock||Gabriel 8144||2||81..84 Toyota Starlet, 1600 lb|
Rotating each upper arm half (each the same amount) sets camber, and traditional shims between pivot bar and chassis in the lower arm sets caster. The two interact remarkably little.
I use a Longacre Caster/Camber tool that mag-mounts onto the end of the spindle (taking off the grease cap).
It is of critical importance that ride height be correct, and set before doing any other adjustments. This is easily done with air springs. Using a tape measure, adjust spring each side so that the lower control arm pivot points are exactly the same height off the ground. This is true for stock or modified cars. Dynamic camber and bump steer requires this basic geometry precondition.
The wheel to adjust must be straight ahead. The stiffening plate must be removed to adjust (just don't turn the wheels lock to lock without it, the trunnion shifts fore and aft). Turn the arms in or out to set camber. That's it. Install the stiffening plate.
caster is set via shims behind the lower pivot bar. The pivot bar is
assembled with one fat factory washer only. shims go only in the front. Mine
needed one shim left and two right, about 1/8" and 3/16". This reduced camber
by about 0.3 degrees. I didn't bother to reset camber.