roadster front suspension

High performance Rambler American front suspension 1958 through 1963

11 apr 2022

new 10 feb 2018

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Introduction

This page is about advanced work on this unlikely suspension. The first level of concern was simple repair and reliability. That work is covered in the basic stock suspension page, which discusses design and construction shortcomings, parts availability, and moderate mods to make it streetable and safe. I'll refer to these as the 2010 mods, the first year that I started driving on them.

Work below is being done on my Rambler Roadster but could be applied to any of the 1958..1963 Americans, and probably the small Nash cars back to 1950.

The first serious mods I made to this suspension were to eliminate a ruined upper trunnion pivot system. The lower trunnions are differently terrible, but I found a set of 1960's aftermarket "trunnion repair kits" that removed most of the reliability problems, allowing me to defer lower-arm work. The upper arm/upper trunnion mods I'll call the 2018 mods, the first year I started driving on them. These mods include "Shelby drop", which raised the car's front roll center significantly, improving cornering.

Starting in 2010 I began driving this sports-car-hard, canyon and mountain roads, pounding the shit out of the chassis and suspension. This is not a Sunday/parade car, I drive with a crew of 60s/70s sport car enthusiasts on multi-day TT's. We drive hard and break things. See the SoCal TT website for some of the routes and cars. The 2018 mods have held up.

This pandemic year (2020) I have begun work on replacing the lower trunnion and arms with a ball joint system, and at the same time, further raise roll center and camber in turns. These will be the 202x mods, and are a work in progress.

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.

The goal

I'm not making a race car, nor claiming that early Ramblers are track kings. Incremental improvement is the goal, in performance and reliability. "Correct" parts are scarce and inadequate. I had to learn enough about roll center and camber in turns to make sense of the changes, and learned things along the way. Also I just like making things and driving on the result.

Suspension condition

It seems redundant to have to say this, but having suspension components in like-new condition is a requirement, and the first improvement to make for handling (and safety).

It is not easy to get the pre-1964 American suspensions in like-new condition, which is half of the impetus for the eventual ball joint switch. It took me years to find parts and work up replacements. 1964-up small cars are easy, all parts are available and the stampings are good by design. Not so these early cars. See the basic stock suspension page for this.

Wheels and tires

The factory wheels are 15" by 4" wide. The roadster got custom wheels from The Wheelsmith, about $110 each, perfectly round and true and pretty, chosen for the tire size I wanted: 205/50-16. The wheels are 16" by 7" wide. These clear the stock trunnion system without spacers, and large enough to clear the tie rod ends. No 14" wheel will fit these cars.

Offset is 3.5", centering the tire on the hub face, same as factory. I would have liked less offset (tire moved inboard) but then tires would rub on the upper trunnion. I am not rich; I buy one set and run them for as many years as I can (usually three). I'm currently using BF Goodrich Comp 2A in 205/50-16 and they are the stickiest tires I've ever had on this car.

The steel wheels were very heavy. I've since switched to Torque Thrust D's, 16 x 7". They weight 16 lbs each.

Springs

I made a number of changes to this car that were the right ones for the wrong reasons. (I will overlook my wrong decisions made for wrong reasons.) The foremost here was to redesign the rear suspension from scratch (for driveline reasons; this needs it's own page) which solved the rear understeer issue sort-of by accident. The other fortuitous change was to switch from wire springs to air springs. The adjustability alone might make this a worth performance-enhancing project. Air springs are inherently super-progressive; this compensates for the loss of travel, about 4".

Springs sag with time, if the springs are not new they will sag and push roll center deeper into the ground, which worsens lateral weight shift and body roll. The only cure is to replace the springs with new ones.

Suspension geometry

This suspension has a lot of flaws, but it is a clean double-wishbone design with good capabilities, even the spring-over-knuckle design, whose major drawback is one of styling (tall fenders).

A major design feature is modularity -- the suspension literally bolts onto the car. This modularity allows arbitrary changes to the location of control arm inner pivot heights. The major 2018 geometry mod moves the car's roll center from below ground to above ground.

Note on understeer -- it's in the rear of the car

It may surprise you to know that much of the understeer in this car is in the rear suspension, not the front.

The stock rear is a conventional longitudinal leaf spring system. This document is about front suspensions, but a quick review of the rear is warranted. Leaf springs are installed not exactly longitudinal, but cant inboard at the front, and the front spring eye (pivot bolt) is higher than the rear pivot (shackle).

Suspensions don't just go up and down; cars rotate longitudinally, aka body roll. Leaf spring motion is complex; spring placement doesn't matter (much) for simple up and down, but matters hugely in roll.

Leaf springs are arched, upward at the ends. When leaf springs compress they lengthen; unloaded, they shorten. The front eye is fixed, but the rear is free to move on it's shackle; the result is that as the spring is compressed and released, the fore/aft position of the rear axle (and wheel) shift fore and aft a small amount. Front eye and rear shackle heights are different, causing up and down motion of each wheel relative to the body to be in an arc. All of these factors are important in understanding rear understeer.

Modifying the front without dealing with the rear will not make the car flat in a slalom. Doing both will come very close.

Roll center

I'm not going to regurgitate all the writing about sports car setup here, nor do I pretend to understand it all. My examinations below show that roll center first, camber in turns second, are the two characteristics that create the terrible handling of these cars.

Roll center on these cars is at, or below, ground; read on your own about roll center and it's effects. Stock, in a hard left turn the right front wheel has most of the car's weight on it, which feels like it wants to fold under. Raising roll center just two inches above ground causes the suspension components to shift the mass onto all four tires. The difference is immediately noticable.

Geometry simulation

vsusp.com is a lovely free 2D suspension calculator. The calculators below have suspension component measurements derived by me. I took better measurements recently (Oct 2020) but there could still be errors. The A-arm and spindle measurements are fairly accurate I think.

You may note the tire size is explicit, and not using the metric size (which is 205/50-16). The static tire size with 38 psi is slightly shorter than the specified diameter ("flat on the bottom") and this turned out to matter, a lot. This chassis is extremely sensitive to ride height (spring height)!

Notes on the simulator

The POV of the simulator is as if you were squatting down looking at front of the car, driving towards you. The tire on the left of the screen is the car's right tire. Positive "Roll angle" values are right turn, negative "Roll angle" is a left turn.

To change ride height (spring height) go to the Frame tab. Note that I used the lower arm's inner pivot as my measurement reference point, since there isn't a clean concept of "frame" in Rambler unibodies. The "Frame bottom to lower mount X" is accordingly zero.

Camber is adjusted exactly as it is on the car: by moving the lower arm's inner pivot inboard or outboard. On the car this is done with shims; in the simulator this is done by changing the "Frame center to lower mount X" variable in the Frame tab.

Stock, factory geometry, 2010 mod

This is the Rambler factory suspension component dimensions and recommended caster and camber settings (close enough). Though it's only 2D, it does a good job of showing how the car might behave in turns. The 2010 mods did not involve changes to geometry. (Measurements corrected 22 Oct 2020.)

Factory geometry

simulator
roll center height 0.276" above ground
camber change per degree roll -0.89 degree

The stock low RC isn't great, but it rapidly shifts about with any motion of the body on the suspension, at all.

Small changes in ride height/spring height make for large changes in roll center height. This is why switching to air springs, up front at least, might be the single best change to make for tuneability.

Here's a good rule of thumb for this car: The lower control arm's inner pivot must be equal to or higher than the outer pivot (trunnion here, ball joint on later cars):

Ensuring that the two lower pivots are at the same height is by itself a large performance gain. The car will be taller, but it will ride and handle better.

02 feb 2022: I've since determined that the inner pivot height is eleven (11) inches, but the geometry stated above remains correct. I run my roadster at 8 inches, and my 1960 American wagon at stock eleven inches, with good wire springs.

2018 modification, "Shelby drop"

This is the 2010 suspension with the single modification of one inch of "Shelby drop" in the upper arm pivot bar.

2018 modified (in use)

simulator
roll center height 2.41" (above ground)
camber change per degree roll -0.53 degree

While the raising of the roll center to just over two inches is great, it also hugely reduces the shifting about of RC with body roll. In stock form, the two control arms are nearly parallel, meaning that any change of angle changes the sign of the angle, flinging RC from one side to the other.

Increasing ride height (spring height) by half an inch raises RC nearly an inch; evidence to me that the single most cost effective performance improvement and tuneability might be air springs up front.

Camber increase in turns is slightly worse with this change, but raising the roll center brings so much improvement it hardly matters.

I don't think things will improve much past this point, beyond minor tweaks to ride height or better tires. But see the Ambassador adaptation section for advanced work.


For comparison/amusement purposes, here is what a real sports car suspension can be like. This is a "Locost", a US made Lotus 7 replica. The roll center is very low, and does not move when the body rolls (it shifts somewhat with one wheel in bump). This isn't a reasonable comparison for any production street car!

Locost (Lotus 7) simulator
roll center height 1" (above ground)
camber change per degree roll nope



2010 modifications

The 2010 modifications are basically the repair work done in the other suspension page, and adapting a modern shock absorber. Some of the OEM design and construction flaws are acceptable in light of it's ancient design, and some are not.

I cover most of the flaws, and some fixes in the stock suspension page.


2018 modifications

upper A-arm redesign

My current solution replaces the upper arm system with standard parts, retaining the central trunnion casting. The upper inner A-arm pivot bar is replaced with a 1970 Ford Mustang "cross shaft bar"that includes built-in "Shelby drop", from RideTech. The mounting hole spacing was 0.125" too close together; I milled (slotted) 0.0625" each towards the outside. This allowed me to make simple tubular arms with tube adapters and Endura series heim joints from QA1.net.

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.

part   quan notes
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 prefab kits. 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. 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 is negligible. In fact I intentionally assembled it with the front leg 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.

upper wishbone/A-arm parts

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.

part   quan notes
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

air springs

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.

part   quan notes
air spring Firestone 267c Double Convoluted 2 RideTech 90006781

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 of height. 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.


fabricating poly bushings

Oct 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. 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. The inner sleeve and outer shell of the worm OEM bushing are reused. Here is the shopping list for soft-poly bushing construction shown below.

part   quan notes
poly bush Ride Tech 90001596 16 poly bush 3/4" ID (per half)
spacer Precision Brand 25199 16 7/8"ID x 1-3/8"OD x .125" thick
shells used or junk AMC 8 salvage from bad bushings

(As of October 2020, the 90001596 part does not appear on their online catalog, or at Summit, but RideTech sales says the part is available for order.)

Preparation

Extract bushing shell and sleeve

Leave the old OEM type bushing installed in the arm. There is no benefit to removing the shell. You need to get the inner sleeve out and all the rubber between removed. I use a hand drill and 1/8" bit to remove enough rubber to loosen it's grip on the sleeve. This also allows oil etc in to assist.

The inner sleeve is longer than the shell, making it easier to grip in a vise to rotate it loose.

Though painted here and so not easy to see, the shells are ready for assembly.

Cut poly to length

The RideTech poly halfs are .75" and accommodate "most" car pivot bushings which are typically 1.5" inside. Our bushings are only 1" in length, so the poly needs to be shortened. I made a jig to flycut them in my mill-drill.




Poly bushing assembly

The lower arm will pivot on this part in the poly; get it smooth. I roll it around on a small 2" belt sander.

Lubricate the first poly half with silicone suspension snot.

Insert the inner sleeve half way into the bushing half. The sleeve is larger than the ID of the poly; this bulges the poly. The inside end of the old press-fit bushing sleeve has a square (sharp) edge, and the other, outer, side has a radiused edge. We use this to advantage in subsequent steps.

Insert this assembly into the arm, from the inside out as shown. The un-bulged poly fits in snugly, then stops at the bulge.

Use a vise and short tube or socket to press this half assembly into the arm. It may spring back a bit. This is OK.

After pressing the other side will look like this.

To the inner sleeve stickout add the other poly half, lubricated.

Put this end of the arm in the vise and squeeze it together. Note that this will leave a small gap under the flange. If you got silicone on the outer diameter of the poly the bushing(s) may pop out. Just reassemble as best you can.

The assembled bushing ideally looks like this.

Cleanup the pivot bar. I run a die (1/2-20) over the threads, wire brush, sand off rust.

Though this is not a moving part, a thin film of oil will prevent it from rusting together later.

Add large ID cup washer and large-ID shims that total .100" to .125".

Slide the bushed arm assembly onto the pivot bar, and add shim(s) and the small ID cup washer. Loosely start the nut.

Note that the shims will stick out since their ID is necessarily large. Here I finger tighten the nut, not bending the thin shims, then just poke them in with the end of a screwdriver, taking up slack with the nut. Be sure to look all around the circumference. The shims want to sit within the cup and apply pressure to the poly evenly.

Should look like this. Shims are not visible. Tighten the nuts moderately for working convenience as desired now.

After tightening it begins to look like a control arm. The distance between the two stiffener mounting flats should be 1.5" plus/minus 1/8" or less.

Compressed bushing detail.

SHOCK ABSORBERS

The original front shock on this car has a lower mount specific to this one car, only. It is available only from AMC/Rambler/Nash suppliers, there is not one single replacement part available from any conventional parts source. If they list one it is wrong.

The correct Nash/Rambler shock absorber is available from AMC parts suppliers such as Galvin's AMC Rambler Parts.

I made an adapter that bolts into the lower control arm and accepts a more common shock absorber, but it the stiffener mention above is required. You can read about this front shock adapter that allows the use of conventional shocks. This increases shock absorber overall length by one inch, since the bottom mount is lower.

FRONT BRAKES

Originally these cars had 9" x 2" drum brakes on all four corners, variably Wagner or Bendix. These were barely adequate when new, but one quick stop from 60 MPH and they've faded.

I originally ran a modified 9" x 2.5" front brake system (Gremlin donor) which work surprisingly well, but when I switched to a 1998 Mustang rear axle that had discs I installed the current Scarebird setup for four-wheel discs. No regrets, Scarebird is definitely the way to go. (Here is a page on my 2015 Scarebird disc brake conversion on a 1963 American. This older page to refers to a previous Scarebird bracket; the current one clears all AMC cars now, and uses different cailiper and rotor.)

Here is the hot-rodded drum brake setup. Many of the photos on this page were taken that drump setup.

alignment procedure

I use a Longacre Caster/Camber tool that mounts magnetically 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".

For setting toe I use my own toe tool.


Big car steering knuckle and ball joint (2020 mods)

The 2020/2021 mods are a work in progress, documented below. They basically involve ditching the lower trunnion for a ball joint and knuckle and upper trunnion from a 1963-1964 "big car" (Classic/Ambassador).

Assumptions/notes: Use short spindles, assumes Scarebird plate. Sensitive to 'hub to lower ball joint Y', 1/2" taller K719 is a large improvement QA1.net 1210-207S. Sensitive to ride height. Below is std ball joint.

2021 modified (in progress)

simulator
roll center height 2.2" to ~5" (above ground)
camber change per degree roll -0.54 degree

This is a work in progress and could change at any time. So far, the 1964 Ambassador steering knuckle seems likely to work out, pending steering arm issues. The 64 Ambo knuckle is heftier and lowers the car by raising the spindle, probably for styling reasons. I can position the ball joint fitment business anywhere within an inch of height, allowing me to raise both upper and lower outer pivots, which then permits full non-parallelism.

In 2020 I was assuming I would build a lower arm from "race car" parts, but that affected everything down to the chassis and K-brace. My current plan is to take advantage of the pile of bent control arms, modifying a set to bolt on a thread-in ball joint adapter (itself TBD, but it will be a K719 ball joint part). This is the path of least change.

The resulting geometry will be more or less the same as 2018, but stronger and more reliable, with the added advantage of lowering the car by raising the spindle (probably a design goal for the newer suspension).

3 nov 2020 note to self: RC rises rapidly with "Hub to lower ball joint Y" (BJH) increase; with BJ set on top of arm it's 4.8", pushing RC way up (5" - 6"). BJ height of 4" puts RC at 2.6", close to current (2018) 2.4", and dropping frame height 0.1" even puts RC back to 2.4". (Note that shelby drop is removed here.) Though RC is very sensitive to ride height with this config lateral RC shift is reduced. It may be the case that the current setup is fine, the BJ solution not worth the effort, as fabbing a lower arm critically safe enough not worth the design instability.

Photos of the ball joint suspension work so far. The remaining problem is the steering arms; the 63, 64 big car arms have the tie rod end much further inboard. This will mess up Ackermann angle and for the roadster performance is a big deal, I have the 2018 mod working surprisingly well, so this isn't usable. But for a non-performance car it might be OK.

The spring perch will need to be substantially shortened.

Here's how the lower arms were cut. These were stripped out and unusable to beghin with.




Benchtop metal "simulator"

I made this test jig that holds one side's components in the correct alignment. I'll use this to fabricate the new lower arms. Measurements here match measurements from the car and is a lot easier to pick at.