I suspect that this is probably the chapter that people first flip to while perusing this book in a bookstore. Another racing invention of the early 1920s, tubular A-arms and trailing arms have taken the automotive aftermarket by storm. Every hot rodding magazine has articles and ads about them, so they must be pretty important, right? Yes, of course they play a very important role in your suspension, but you may be surprised to learn that it is only a supporting role.
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Earlier, I addressed suspension pickup points and how they define the suspension geometry. Look at any serious engineering book on suspension design and you see endless discussion about where these points should be located and why. Curiously, you find very little about the design of the A-arms. References are usually along the lines of, “…moving the lower ball-joint pickup point outboard may improve the scrub radius, but will necessitate a longer lower A-arm.” That’s because the arms connect some of the pickup points together but they don’t define them.
For example, let’s say a stock upper A-arm is 9 inches long from the center-line of the cross shaft to the pickup point of the upper ball joint. Then shorten it 1/2 inch, which gives you more static negative camber in your alignment (assuming you don’t change the number of alignment shims). Then take the same car, leave the upper A-arm at the original 9-inch dimension, and add 1/2 inch of alignment shims. You still get the same static camber change. Doesn’t the shortened arm yield a more aggressive camber curve because the radius that the upper ball-joint pickup swings in is more abrupt? Sure, technically, but by such a small degree you can barely measure it. Any benefits are clearly coming from the substantial alignment change, not the tiny incidental geometry change.
However, most tubular A-arms also advertise improved caster. Sure, many of them have the ball-joint mounting plate offset toward the firewall more than the original factory arms, which gives you a head start on achieving more positive caster in your front end alignment. That’s a good thing but, again, it’s an alignment change more than a geometry change. The camber curves, roll center height, etc., remain almost unchanged.
I’m not claiming tubular A-arms are a waste of money. I’m just saying that when you make the kinds of major suspension changes required on most muscle cars to bring them up to par with modern performance cars, you usually find that the stock A-arms (especially the upper ones) simply don’t fit the car anymore! Yep, that’s a problem. The original length, offset, and ball-joint mounting angle is likely way off. You can see that well-designed aftermarket A-arms are vital to using the geometry modifications you’ve made. They are also necessary to achieve a matching performance alignment. Now that you know how they fit into the grand suspension scheme, let’s examine what makes them tick, and what makes one better than another.
Most tubular A-arms are made from mild steel and have an outside diameter of 1.125 to 1.75 inches with a wall thickness of .85 to .125 inch. Lower arms are often more beefy because they’re the weight-bearing arms on all but a few muscle cars (those have the springs mounted to the upper A-arms). Design and quality are more important than tube size.
Some arms are also available in 4130 chrome-moly steel. There’s quite a bit of misinformation floating around about 4130. Chrome-moly alloy has a percentage of chromium and molybdenum in it. This makes it fairly tough. Chrome-moly is not lighter than steel because it is steel. Compared to low-alloy steels (like 1020/1026, which many aftermarket suspension parts are made of), it is simply a stronger steel alloy. The fact that it’s stronger makes it possible to use thinner-wall tubing, which reduces overall weight. If used properly, it is a very good choice for suspension parts.
To get the strength advantages of 4130, it must be heat treated. Annealed 4130, for our purposes, is no stronger than common 1026. Properly heat treated, it is nearly twice as strong. Welding 4130 requires special procedures. All welding should be done with heat treatable steel rods or wire, the joints should all be stress relieved, and the whole part then heat treated to a suitable Rockwell C Scale hardness. If these procedures are not followed, then the steel in the area around the welded joints take on a coarse martensitic grain structure. This is a hard, brittle crystalline matrix within the steel. If an otherwise strong component with brittle joints sounds like a bad thing to have on your car, you’re catching on.
Professional-grade chrome-moly parts are expensive; most cheap ones are, at best, equal to cheaper mild steel parts.
Speaking of cheaper parts, inexpensive foreign knock-offs of popular A-arms are available. They are typically very poor quality, made from cut-rate materials, and have questionable welding. I don’t have to tell you that this junk has no place on your car, right? Enough said.
An early development for improving the range of adjustability is the offset cross shaft. Here, the centerline of the mounting portion of the shaft is offset from the centerline of the bushings (upper, inner pickup points). If that offset is turned inboard, toward the engine, you gain some camber, usually equal to three 1/8-inch shims. That’s often a big help in attaining a good performance alignment. These offset cross shafts were originally designed for cars that had been hit and couldn’t be aligned properly. Since impacts tend to bend the frame inboard, the shafts are assumed by the manufacturer to be used to move the pickup points outboard. In a performance application, you want just the opposite. So if the cross shafts say “This side toward engine,” ignore it and turn that side away from the engine.
Another development that’s come from the racing community is the use of interchangeable offset inserts (or slugs), which fit into machined pockets in the cross shafts. These let you adjust caster without using any shims. Anytime you reduce your dependence on shims, it’s generally a good thing. The only down-side to using slugs is that you have to disassemble the cross shafts to change them, so they can’t be adjusted with weight on the suspension.
Yet another racing innovation that has worked its way into newer cars and the aftermarket are adjustable arms. In contrast to the more common welded steel A-arms, these use turnbuckles and jam nuts to adjust the actual length and offset of the arms. Their modular construction makes them very versatile because the length, offset, and even type of ball joint being used can easily be altered by interchanging a few parts. Because modular construction is very cost effective, the retail price is usually modest too. Of course they’re not all sunshine and light either; retrofitting this type of arm to older platforms can present some fitment issues.
Originally, muscle cars had arched, very organic looking A-arms that were contoured in a way to allow for frame and component clearance. Naturally, a turn-buckle-style arm must be more-or-less flat in cross section to allow the adjustment sleeves to turn, so there can be frame clearance issues regarding droop. Since this type of arm is designed to take nearly all of its loads in a horizontal plane, banging them vertically on the frame rails is less than ideal. These issues are easy to deal with once you know they exist. (I cover which cars fall under this heading in Chapters 8, 9, and 10.)
Down travel has been improved on some models with the use of vertically offset bushings. These retain the same pickup points as a non-offset bushing, but raise the horizontal components of the arms in relation to the frame for more clearance. Taller-than-stock ball joints or taller-than-stock spindles used to improve the suspension geometry also raise the outer end of the arms for more clearance. Ball-joint spacer kits can also be used, but they offer no change in geometry because the pickup points are unchanged. Many of these configurations actually have more available down travel than stock.
The lack of down-travel bump stops is seldom an issue on performance-handling-oriented cars because the arms have plenty of clearance, relatively short springs, and firm rebound dampening. The only time the arms may contact the frame is when the car is on a chassis lift or being raised with a floor jack, and even then there is very little remaining spring tension and no induced shock.
Drag cars, however, typically use every bit of their droop travel to enhance weight transfer, and use shocks with very little rebound dampening. So it is critical to use some kind of travel limiter. The most common are straps or cables going from the frame to the lower A-arms. These should limit the travel of the suspension to a point just before the upper A-arms can contact the frame. It’s already common to use such limiters on drag cars to tune weight transfer and to prevent damaging the front shocks by top-ping out the piston in the shock body.
Tubular Lower A-Arms
Another question I hear often is, “Do I need tubular lower A-arms?” No, you don’t, unless you do. Let’s review what a lower A-arm is and what it does so you can determine the right answer for yourself.
The lower A-arms on most muscle cars are weight bearing; they hold the car up. Some, like early Mustangs, mount the springs to the upper arms. All of this technology applies to those cars, too; just substitute the word upper for lower. At any rate, this is a component heavily loaded with cornering forces and shock from road imperfections. The spindle pin that mounts the wheels is usually located very close to the lower A-arms, so much of the load incurred by the wheels passes directly into the lower A-arms. The spindles are usually much taller above the spindle pins, so they act as a lever, reducing these loads on the upper A-arms.
All of the factory arms on these cars are stamped from thin sheetmetal, on huge hydraulic presses, with hundreds of tons of force. This is a very cost-effective way to make complex parts by the millions, but it also highly stresses and stretches the steel. Most GM cars have press-in ball joints, which add additional stress. These arms were designed by the factory engineers for a purpose and a certain fatigue life. Generally, this purpose included driving around to the grocery store, the office, or dropping the kids off at school, probably on small-bias-ply tires. They were not designed to last 40 years and then meet the dynamic demands of modern-level performance on a muscle car.
When they go, they generally go suddenly and without much warning. Typical modes of failure include splitting at the ball-joint socket, which can release the ball joint and cause the suspension to collapse. Or the end of the arm simply fatigues to the point that it snaps off and the suspension collapses. The typical chain of events goes something like this: fun—snap—crash! Thankfully, flagrant failures are relatively rare.
If you plan to run stock lower A-arms, do everyone a favor and inspect them well for cracks, stress risers, signs of impact, etc. If the paint is too thick to get a good look at the arms, then glass bead or sandblast them first. I’ve seen arms on cars that I know were beat hard but were fine, and others from a little old lady’s car that had more than one bad crack in them. Don’t just assume they are okay! This fatigue and age issue is among the best reasons to consider new tubular lower A-arms.
What else do they do? Usually, nothing. Well, perhaps “nothing” is too strong a word. The majority of tubular lower A-arms are simply tubular copies of stock stamped A-arms. If you do what you’ve always done, you’ll get what you always got. They sometimes have better bushings than stock and they also look cool. If nothing else, they’re new and have no history, which means no negative history of impacts or corrosion. That alone may justify their purchase if you’re looking at serious track time.
We all know what looks cool, but what makes one bushing better than another? Like most suspension topics, that’s a bit of a loaded question. It depends on what you’re planning to do with the car. For our purposes you basically have four popular choices: rubber, polyurethane, Delrin (DuPont’s trade name for a hard acetyl plastic), and metallic.
Rubber is typically the choice of factory engineers. It’s durable, and does a good job of absorbing NVH. These bushings are also cheap and easy to make.
Rubber bushings don’t pivot on their mounting bolts like many people think. The rubber is vulcanized (melted) to both the inner sleeve and the outer shell. The center sleeve is slightly longer than the outer shell and is usually serrated on the ends. When the mounting bolts are tightened, the inner sleeve becomes stationary. When the suspension moves, the rubber actually deflects, or twists. They actually become elastomer springs and contribute a bit of progressive rate to the suspension.
They also tend to center themselves automatically, which returns the car to a fairly consistent ride height. That’s why it’s critical to tighten rubber bushings only at ride height. Tightening them when the suspension is at full droop can bind up the suspension, raise the ride height, and make the car ride like a 1-ton truck.
The downside to rubber bushings is how this same ability to deflect easily and absorb shock and vibration, works against it in a performance application. Under heavy cornering, the deflection translates into movement, which affects the alignment settings in a negative way. On milder cars this is not a big deal, but it certainly can become a major issue as you raise performance thresholds. Modern, factory high-performance cars attenuate this problem by using harder rubber compounds. This hardness is measured as a durometer reading. The higher the number, the harder the bushing, so high-performance rubber bushings are often referred to as high-durometer rubber. There’s a limit to how hard you can make the rubber before it won’t deflect enough to allow proper suspension travel, and modern engineers know exactly where this threshold is.
Polyurethane bushings work differently than rubber. They do actually pivot on the inner sleeve. That also means that the ends, or thrust surfaces, pivot against the frame or mounts. Polyurethane varies in hardness (like rubber) and it is generally slightly harder for performance applications. Also like rubber, polyurethane exhibits a property called stiction (yes, that’s really a word). It’s short for static friction, which is a property that tends to prevent relative motion between two movable parts. Because rubber bushings deflect (not pivot), stiction isn’t an issue with them. There’s nothing rubbing together; it’s just twisting rubber.
Polyurethane is a different story. Because polyurethane does indeed pivot, stiction causes the bushings to bind up to some degree, unless there is a low-friction barrier (grease) between the surfaces. That’s why rubber bushings don’t need lubrication, but poly certainly does. An effort has been made to reduce this tendency by adding dry graphite flakes to the polyurethane, resulting in poly-graphite. Depending on the percentage of graphite used, this can either have a noticeable effect or it can just be a placebo for marketing. Generally speaking, if you rub the bushing with your finger and it comes off gray, there’s a good amount of graphite in the bushing.
Reducing stiction is important for several reasons. First, stiction is binding and you know that binding results in a non-linear suspension action and more erratic handling. In this case it also squeaks. That annoying noise is the sound of lost performance. The type of grease you use to lubricate the poly bushings is critical. Do not use regular petroleum grease on polyurethane bushings; it makes the bushings soft and grippy, and they then stick and squeak even worse.
Polyurethane can also cold flow. If you stress it beyond its design limits, it stretches and stays there. In bushings, this is usually exhibited by egg-shaped holes. Since most bushings are free to rotate 360 degrees, this binding usually occurs in torsion. (I discuss more details in relation to rear trailing arms in Chapters 8, 9, and 10.)
If you must use polyurethane bushings, get the graphite-impregnated ones if you can. Use poly-specific bushing grease or at least synthetic grease on them, and keep them well lubed even if you have to occasionally take them apart to lubricate them. If they’re squeaking, they’re not working.
Delrin has a high degree of natural lubricity; it’s slippery. It’s also fairly hard, like oak or maple wood. Rather than injection molding it like you would with polyurethane, Delrin is usually sold in rods, sheets, or blocks, and then cut and machined to fit a specific application. Functionally, it has more in common with aluminum than rubber. It’s a very tough material and, as long as it’s not exposed to a lot of hard road grit (which abrades it), bushings made from this material should last a lifetime.
To achieve such durability, Delrin bushings need to be greasable. The grease acts not only as a lubricant but also as a barrier to dirt and moisture. Moisture won’t affect the Delrin, but it affects steel bolts or sleeves. A rusty inner sleeve abrades the Delrin and accelerates wear. The grease should be introduced at the center of the bushings and pushed out-ward so any grit that gets into the bushing is pushed out when you grease it. In this way the grease also cleans the bushings.
Again, there is a compromise. You pay a small penalty in NVH isolation due to the hardness of the material. This varies with the construction of the bushings, but is generally equal to the feel of an additional 5 to 10 psi of tire pressure. If the bushing uses a thick cross section of Delrin, there is more material to absorb NVH than one with a thick aluminum or steel shell and only a thin Delrin liner. These might be more appropriately labeled Delrin-lined metallic bushings. Because these bushings pivot very freely, they allow the suspension to soak up large bumps very well. Any additional NVH is medium and high frequency. In other words, you feel the texture of the road a bit more.
The biggest downfall of Delrin is cost. It’s a fairly expensive material and machining the bushings is much more time consuming and costly than injection molding them. Generally, though, it’s well worth the extra money.
The last major group of bushing materials is the metallics. These can be steel, brass, bronze, or aluminum. Many people’s knee-jerk reaction to metallic bushings is that they probably create a very harsh ride. On the contrary, properly constructed metallic bushings can provide a very smooth ride. Many of the cars associated with a luxurious ride (like the big Chrysler Town and Country sedans and finned Cadillacs of the late 1940s and 1950s) used all steel bushings in their suspensions. They were the standard of the auto industry for many, many years. So what happened? The manufacturers discovered that rubber bushings were much faster and cheaper to make. The compliance of the rubber bushings also meant that suspension components didn’t have to be made as precisely for fit and function.
Metallic bushings have never lost their popularity with race car builders, though. In fact, they are more popular than ever, either as factory-style bushings or spherical metallic bearings like Heim joints. (I discuss spherical joints in reference to trailing arms in Chapters 8, 9, and 10.)
The harsh reputation of metallic bushings surfaced because some of the racing parts are sloppily built. They have lash between the components that creates NVH, they can buzz and rattle, and they often have a short lifespan.
Precision is the key here. Precision takes time, and that costs money, so if you see some A-arms with metallic bushings, they’re usually just steel tubing that’s part of the arm itself. If they’re really, really cheap they’re probably designed to be disposable, and you should avoid them for your street car. The more precise arms yield almost unbeatable performance because they’re super smooth and have basically zero deflection. They deliver a smooth ride and a long service life with ride quality similar to Delrin. Steel lower A-arm bushings are usually race-only pieces made to fairly loose tolerances. The one exception might be Howe Racing’s Precision Series bushings, which are constructed more like a ball joint than a normal cylindrical bushing. Their lash is adjustable and they can be lubricated through a channeled grease bolt. They’re only made to fit stock A-arms, though, so all of the issues with using stock arms in performance applications still apply.
Written by Mark Savitske and Posted with Permission of CarTechBooks