roadster front suspension

quickie jump list:

10 feb 2018

this page documents my redesign of the old nash-based front suspension as used in the 1958 through 1963 Rambler American cars. though the overall design and geometry isn't bad, and turns out to be quite amenable to improvement, design detail and contruction is apallingly bad. needless to say no one else has paid any attention to this system but me.

foolish or not i've decided to make this car handle well, at least by 1960's sports car standards. at least half of the notorious Rambler understeer (push) is in the rear suspension, via the rather extreme inboard location of the front spring eyes in addition to the usual vertical location of the front eye and rear shackle. i eliminated all of that when i installed the narrowed 1998 Mustang rear axle on a rigid wishbone/air spring system.

i am usually happy to work with long-obsolete designs and construction, procedures and materials. i'm one of the few people to defend the trunnion suspension in the '63-up "big car" chassis, having personally put 350,000 highway miles on one set of upper trunnions. but this ancient and ailing "small car" suspension is terrible in many respects, even accounting for pre-interstate use patterns. AMC agreed with me -- it was one of the first things replaced by AMC in their then-new models, eg. the aforementioned 350K mile car. the system in these pre-1964 Americans is crap.

there are two design flaws, either of which would be enough reason to ditch it: threaded pivoting parts (instead of bearings or bushings) and unbushed stamped sheet steel arms. it's just stupid, even for 1948.

prior modifications

i'd already abandoned the wire springs, replaced them with Goodyear air springs to great effect. the wire springs were a bear to wrestle; 18" 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.

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.

geometry modifications

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 pretty good i think.

stock (vsusp.com link)
roll center height above ground -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
 
modified (vsusp.com link) mods: 1" so-called Shelby drop; upper arm 0.5" shorter, spaced out from chassis 0.5"
roll center height above ground 1.18"
roll center L/R motion per degree roll +7.8" (into turn)
camber change per degree roll -0.90 degree

excellent discussion of "race car" suspension details here.

as always i packrat key components for times like this. below is the original 1961 suspension i pulled from the roadster's donor ('61 American). the amount of accumulated grease and dirt is amazing. note the ovalled hole of the top control arm. i was luck that the bolt actually came out of this one.

below are photos of the suspension extracted in winter of 2017/2018. i'd built and installed it in 2010 (in my then 1963 American) and replacing the flawed upper trunnion pivot system with my own flawed (but slightly better) design. the wear on this is shown here.

wear on installed parts

below is the threaded rod upper pivot with the visible wear from seven years of moderately hard driving. the OEM part was a wierd tapered bolt made from extremely hard tool steel. to make a long story short, the design did not allow for lubrication inside the casting; they froze in the casting instead of rotating, and this rotation turned the bolt/rod in the stamped arm ends, causing them to oval badly and rattle loudly. this design ruins hard parts in under a decade.

the same screwball pivoting-thread idea is also used on the lower arms, shown further below, to a similar effect. the lower arm assembly procedure is bizarre, and completely undocumented in the factory manuals, but mentioned somewhat in aftermarket "trunnion repair kit" install sheets. more on that below.

justforthehellofit here is a photo of the OEM upper control arm minus trunnion.

after cleaning of the donor parts and disassembly and cleaning of the previously-installed parts, i had this motley assortment of worn out stamped arms. the holes in each end of both upper and lower arms have threads that are cut by the part that threads into them. seriously, like self-tapping sheet metal screws. Oh grandpa Nash, you can and did do better work than this. (on one of the rare moments this area was addressed in the TSM it warns about the big tapered bolt cutting the threads at different sizes (!) so be sure to reinstall the bolt same way it came out.)

as you can see above the small holes (upper arms) get ovalled and the large hole (bottom arm) has self-tapped threads and metal displacement. the tapped holes loosen even with new trunnion cups. terrible system.

fabricating poly bushings

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 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-comprssible 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. "ship it". good enough to test on the road.

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 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.

upper A-arm redesign

unfortunately i did not photograph the wear and inherent slop in the upper arm's outer end where it meets the trunnion casting. it's somewhat visible in the photos at the top. it's a major weak link in this suspension, which obscures the basic decent geometry.

after two year's worth of research into not just suspension design but suitable build parts, i worked out that this oddball little nash suspension is almost bolt-compatible with a mixture of Ford and Chevy parts (and predates them a good quarter century). after all, it is just a classic double-wishbone...

the short of it is, the upper inner A-arm pivot bar was easily replaced with a "cross shaft bar" to fit a 1970 Ford Mustang. and 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.

for reference, here's the RideTech cross shaft installed in one of their StrongArm arm kits.

and thanks to the very few aftermarket suspension parts providers that actually provide on their page or via email, 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.

lower control arm

things are actually worse at the bottom end. far worse. it's nearly impossible to assemble correctly. i need more documentation here but basically the trunnion caps are threaded inside (3/4" coarse thread but i dont think UNC) and cap outer has a very strange shallow roundy thread that gouges into the stamped arm. if i didn't have photos for proof no one would believe how stupid this is. you need to assemble them both together at once, threading two double-threaded nuts into to arms and onto the central trunnion casting,have the nuts bottom out (and gouge) the lower arm, and simultaneously have the anonymous looking (but absolutely mission-critical) spacer fit tightly -- within 0.01" of correct. and, simultaneously, the inner bushings in place over the pivot shaft. which of course requires that the bushings be press fit to the precise correct depth at their end. i am a lifetime AMC rambler geek, don't readily bad-mouth the occasional less-than-ideal design, but WOW WHAT CRAP THIS IS.

as further evidence i'm not just making this up, by the middle 1960's there were aftermarket trunnion repair kits that replaced the lower trunnion with an improved design. yes, the steering spindle also threads into the trunnion, so yes, the car goes up in the air (not much, 1/8") turned to the right, and down when turned to the left. and of course grinds the metal into dust.

note the suggestion to use the "other" kit if the parts were not severely worn. the assumption was that they wereseverely worn... as is generally the case.

alignment procedure

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.

SHOCK ABSORBERS

for years i ran this car and the previous 1963 American with the wrong shocks, and the ride was harsh. by "wrong shocks" i mean the ones that all that parts catalogs and suppliers suggest -- 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. absolutely unavailable new, there are some NOS replacements available from the AMC suppliers, but i don't think those are practical (or affordable) for driver use. 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 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 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.

FRONT BRAKES

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.)

parts list

these are the major components used for this work. 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.
part supplier/number quantity notes
cross shaft RideTech 90000931 2 not online; phone order only
poly bushing RideTech 90001596 16 poly bushing 3/4" ID (per half)
arm tube stock DOM tube 7/8" OD .065" wall 2 dimensions 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
front shock Gabriel 81270 2 Volvo 122 front, 2400 lbs, requires fab mount
rear shock Gabriel 8144 2 81..84 Toyota Starlet, 1600 lb