The single A-arm suspension was designed by Earle S. MacPherson of General Motors in 1947, and is still widely used today. In contrast to a double A-arm suspension, a single A-arm suspension has the shock and coil spring combined into a single unit. This single unit uses less space allowing for more room, which is great for front-wheel-drive cars. The single arm design leaves ample room for the drive axles. Struts must provide support for the suspension with the springs, and the shock portion must also control the rate that weight is transferred during chassis movement. Struts have to do more than their shock absorber counterparts, since all shocks have to do is control the rate that weight is transferred during chassis movement. Some of the more popular single A-arm-equipped muscle cars are the 1979–2011 Ford Mustangs and the 1982–1992 (third generation) GM Camaros and Firebirds.
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The big difference between single A-arm and a double A-arm (single wishbone and double wishbone) front suspensions is that a double A-arm front suspension is adjustable for setting camber and caster with the upper A-arm, while a single A-arm front suspension (also referred to as MacPherson strut suspension) has the camber and caster fixed.
Single A-arm suspension, single-wishbone suspension, or MacPherson strut suspension are all accurate names for the parts discussed in this chapter. I refer to them throughout this book as single A-arm suspensions. Most of them have the A-arm on the bottom with a fixed top-mount for the strut (shock and spring unit). As discussed in Chapter 6, A-arms can be rather flat and triangular shaped. Since they are not part of the frame itself, but attach to the main frame, they are still considered a subframe. The end of the “A” with two legs attaches to the frame and pivots, while the pointed end of the “A” attaches to the steering knuckle and pivots on a ball joint.
With camber and caster being fixed and not adjustable, the only real alignment to be made is toe in or out (see page 44). However, years of running hard into curbs or big potholes or dropping hard from big wheel stands, the camber and caster can also become out of adjustment.
Toe is changed by turning the threaded sleeves connecting the tie rod ends making the tie rod ends farther apart or closer together. If the tie rod ends are on the front side of the steering knuckles (ball joints) then threading the tie rod longer creates toe-out just as threading the tie rod ends shorter creates toe-in. If the tie rod ends are on the rear side of the steering knuckles (ball joints), the opposite actions must be taken.
Camber relates to the wheels being perpendicular or straight up and down. Significant suspension modifications like raising, lowering, or dropped spindles may require camber adjustments.
On drag-race-only cars, take a couple photos of the car from the side: one under full acceleration and the other near the far end of the race-track before the finish line. Then, when the car is on the alignment rack you can reproduce the same stance and set the camber to near 0. Although this can give you a near-optimal camber setting for acceleration, it may not be the best setting for tire wear. Since a strip-only car isn’t going to see a lot of miles, it should be close enough not to wear the front tires unusually.
Caster deals with the spindle location governed by the upper and lower ball joints.
It is important to ensure that the caster is the same on both sides of a drag race car to remove the car’s tendency to pull to one side on a flat surface making it harder to steer in a straight line. This can also cause scrubbing of the front tires resulting in poorer acceleration and in turn increased ETs.
Since camber and caster are fixed in single A-arm suspensions, other suspension changes need to be made to allow movement to correct camber and caster. The replacement struts (which have two lower bolts holding the strut to the spindle) may have the top hole elongated on some models. With the bottom hole in a fixed location, it can be used as a pivot to allow the top part of the strut to be tipped in or out. Tipping the strut in creates negative camber, while tipping the strut out creates positive camber.
If replacement struts are not available, the upper strut-mounting bolt holes can be slotted left to right, again allowing the strut to be tipped in or out. The same hole modifications can be made in different directions or just enlarged to adjust caster. If you take your street car with a single A-arm front suspension to an alignment shop they will probably correct camber and/or caster issues with this method.
Camber and Caster Plates
For racing cars with a different weight engine, a modified chassis stance, or higher horsepower, there is a better solution. Many companies make adjustable camber and caster plates for highly modified cars to be used as the top mount for the struts. Some of these adjustable caster and camber plates are designed to be welded in and some simply bolt into place. Do your homework and get the best parts for your particular application not only now, but for more modifications in the future.
A way to adjust camber with-out slotting the strut mounting holes or using an adjustable plate is to use a camber kit. It consists of smaller-diameter bolts with an eccentric washer installed in place of one of the strut/spindle mount bolts. Since the diameter of the bolt is smaller, you can move the strut in or out due to the extra space created by the smaller bolt. These kits are commonly used on street-based cars since they install easily with no grinding needed. They are not advisable for a high-performance application due to the smaller-diameter bolt being weaker.
On cars equipped with sway bars, it may be possible to remove them and see some gain in dragstrip performance. However, if that sway bar is used to help locate the lower control arm (as it is on some single A-arm suspension designs) it should remain in place. Research your particular application to see if this is the case. Most performance-oriented single A-arm independent suspension cars have sway bars (on the front to give additional handling stability).
Another drag race specific performance improvement is to replace the factory struts. As mentioned earlier, each side of the car utilizes a strut (shock absorber and coil spring kit) to absorb vibrations and bumps. Good-quality struts are very important for today’s higher-horsepower cars. The front suspension in these cars as delivered from the factory works fairly well up to 300 hp. Cars at this horsepower level don’t need a lot of suspension work to make the tires hook. If your car’s horsepower is approaching this level, or you just want to be more consistent and improve the car’s handling, there are several companies that offer front strut performance packages. You can easily start with a pair of single- adjustable struts, race-adjustable struts, or double-adjustable struts, then add the right components and get the right package for your needs.
Once your car’s horsepower level goes beyond 300 I strongly suggest increasing the quality of the knob in a counterclockwise direction creates a stiffer strut setting, allowing a quicker rise in the front of the car. Turning the knob in a clockwise direction creates a softer strut setting and a slower rise in the front of the car.
For a strip-only car, or one that gets very minimal street use, QA1 has an “R” (race) series single-adjustable strut. These have 18 valve settings but are much softer on extension (rebound) for better weight transfer (pitch rotation). These softer settings are great for the strip but not a good idea for street driving. Using these struts on a very rough or wavy road can cause unwanted actions from the cars suspension.
With adjustable struts, it’s best to start near the middle. If your car needs more front-end lift (pitch rotation) then a softer setting may be needed. If your car’s front end is raising too much, a firmer setting is needed.
For drag-race-only application, always run the lightest-weight spring that holds your car at the desired ride height to get the best weight transfer (pitch rotation) possible. Using a lighter-weight spring than factory causes the spring to collapse farther than normal. You therefore need a taller spring to keep the front end at normal ride height. With a lighter-weight spring compressed farther, the spring has stored energy and causes the front of the car to lift quicker and farther during acceleration and weight transfer (pitch rotation).
As explained in Chapter 10, knowing the car’s total weight and the percentages of that weight bias from front to rear is critical. If you need some final chassis adjustments, such as when scaling the car and set-ting chassis preload, the QA1 strut bodies are threaded so the spring base can be adjusted up or down as needed taking the car’s chassis up or down with it.
Another improvement is to install a strut tower brace. In a strut-equipped car, the top of the strut is mounted into the tower. These towers are almost as high as the top of the front fender and are made from welded stamped-steel parts. Due to their height and the stress they are put through, the towers can flex up to 1/4-inch—there goes all the front-end settings you worked so hard to obtain. Steeda makes strut-tower braces. I like Steeda products because they’re fabricated from chrome-moly material and serve to strengthen the front of a car. This helps to eliminate suspension flex and cowl shake while improving steering response and handling. Steeda products have extra clearance for popular aftermarket intakes and superchargers, making them a perfect upgrade. So if your car has a strut-type suspension and doesn’t have a strut-tower brace, I highly recommend getting one.
With race cars no longer having stock suspension and/or different tire diameters than stock, or even different tire diameters front and rear, it is very important to look at proper strut ride-height. The last thing you need in any suspension is to have the strut bottom out in either direction. Similar to the shock absorber measurements outlined in sidebar “Proper Shock Length is Critical” on page 13, it’s wise to remove the strut and measure the compressed and extended dimensions of the front suspension before purchasing aftermarket struts. This way you know that the strut has adequate travel length to allow the suspension to fully compress and extend as it’s designed to. Should a strut bottom out, the suspension’s movement would suddenly ceases and maybe even begins to travel in the opposite direction. The impact this motion has upon the suspension can be very dangerous to the driver. Avoid this by researching the strut’s travel requirements beforehand.
Coil-Overs, Struts, Springs, K-Members
As with all drag race cars a single A-arm or strut car benefits from narrower tires in the front. If you have a race-only car you may choose to run Moroso front tires (not DOT-approved tires). They are lighter in weight than a DOT-approved street tire and are for race-only usage. If you have a dual-purpose car that is street driven you need a tire that is DOT approved for street use, such as Mickey Thompson front tires. Either of these tires have what is referred to as a crowned tread. Having less tread on the ground with the crowned tread creates less rolling resistance and faster ETs but at a great sacrifice to cornering capabilities.
As with double A-arm cars, strut cars need taller and lighter weight front springs (trick springs), which contain “stored energy” from being lighter in weight (smaller wire diameter) and yet still maintain ride height (by being taller). As the car begins to raise in the front with the lighter weight spring, the car lifts farther and quicker as weight is transferred than the stock factory springs.
Moroso makes a line of “trick springs” that bolt in with no modifications necessary (sometimes you may need to trim them for a more desired ride height but only after the car has set for a few days). These springs (PN 47240) fit the 1993 to 2002 Camaro and Firebird. They are rated for 1,500 to 1,700 pounds of front end weight. This is a 275-pound-per-inch spring. Moroso also makes “trick springs” for the 1979 to 2004 Ford Mustang. The Mustang version (PN 47220) is a 250-pounds-per-inch spring and rated for 1,750 to 1,900 pounds of front end weight.
Moroso makes front struts for the 1982 to 1992 GM F-Bodies (Camaros and Firebirds) as well as the 1979 to 1993 Mustang. In a 50/50 standard-ratio strut there is as much resistance to the compression of the strut as to the extension of the strut. With these Moroso 90/10 struts there is only 10-percent resistance on the extension side to allow the car to raise quicker and farther for maximum weight transfer. The compression side has 90-percent resistance to help the front of the car from coming down too quick and too hard. Coming down too hard can cause a bouncing effect in the front of the car lifting the front wheels off the ground again and again. This unwanted repeated movement unloads the rear tires and then loads them again, as many times as the car bounces. For improved top end handling the chassis needs to come down slowly and return to the correct ride height.
At the far end of the spectrum, toward the race-only part, lie the K-frame swaps. These upgrades are readily available for all the popular single A-arm performance cars like third- and fourth-gen GM F-Bodies (Camaro/Firebird), Fox-Bodied Fords, and all SN95 and newer Mustangs.
The upgrade consists of removing the factory K-member that ties the left and right front subframe together and replacing it with a tubular after-market unit. These are typically a bit stronger than the factory parts they replace, but they are notoriously lighter as well. The installations vary as widely as the manufacturers that make the parts.
For example, the Spohn Performance replacement K-member for third-gen GM F-Bodies (1993–1997 Camaro/Firebird) is smaller, lighter, and significantly stronger than the stamped steel factory part it replaces. This particular K-member is recommended by the manufacturer for use in road racing and drag racing vehicles. Spohn also claims that vehicles equipped with their K-member upgrade are perfectly safe for street driving. The smaller dimensions of the overall tubular K-member package offer greater access and clearance under the car for things like exhaust headers, turbocharger system plumbing, aftermarket oil pans, etc.
This particular K-member is designed to be compatible with all other factory parts, like the motor mounts, steering rack, and front suspension arms. While you’d expect the front suspension components to have been upgraded by the time an enthusiast is ready to upgrade something as serious as the K-member, it’s good to know all the factory stuff still bolts up and functions just fine with this aftermarket K-member in place. Additionally, Spohn offers an upgrade steering rack for this K-member as well. It’s a popular Pinto rack, and is a manual-steering unit. This sheds even more precious weight off the front end of the car, once installed.
It’s also important to know that not all aftermarket K-member kits are completely comfortable in a street environment. For instance, Racecraft Inc. also makes a K-member for 1982–1992 GM F-Bodies. This one, however, is specifically designed for drag racing cars. It offers the kind of clearance you hope for in a tubular K-member, so things like big-tube headers have plenty of room. It weighs only 21.7 pounds (when equipped with Pinto rack-and-pinion steering mounts) compared to the factory unit it replaces, which weighs 44.8 pounds. That’s a savings of 23.1 pounds off the nose of the car! But, when you equip it with Racecraft’s own 2-inch-drop spindle, you must commit to using Racecraft’s Pinto rack-and-pinion kit and tapered-pin bumpsteer kit; and you really should upgrade to their Grade 8 hardware kit too.
It’s a pretty major investment of both time and money to upgrade to an aftermarket K-member like these, but when you get to this level of performance, it’s all justifiable.
Naturally, the Ford Fox body and Mustang components require similar effort for similar benefits. Spohn also has them, and other manufacturers (like UPR Products) have a complete line of replacement K-members for Mustangs dating back to 1979. These are also touted as being tough enough for daily driver use on the street.
AJE Suspension has brought a cutting-edge design to the market that has changed the way we look at front suspensions with K-members that are lighter, stronger, and that have more header clearance than the factory K-member provides. The AJE K-member’s unique design provides engine mounts that mount any engine. This mounting system is used on various vehicles in addition to the 1979 to 2004 Mustang for which it was originally designed. This K-member was also designed to use a coil-over setup; however, it can still be used for the factory suspension. The straight tube and triangular design provides strength and openness for more header room.
Ford has had at least six different K-members for the 1979 to 2004 Mustang to be able to mount different engines, different rack locations, and different anti-dive angles. AJE has taken all of these into account when designing their latest K-member and front suspension package. The new struts are adjustable for installed height, ride height, and camber angle. They have different camber angles, different spindles, different spindle heights, different steering arm locations, different ball joint attachments (Boss thickness), and different strut mounting. The AJE new strut assemblies can be used on any 1979 to 2004 Mustang.
AJE’s K-member lets you mount any engine including a twin turbocharged big-block that can produce ETs in the low-6-second range in the quarter-mile, plus speeds of more than 230 mph. When mounting different engines, oil pan–to-rack clearance becomes an issue. So being able to mount the rack in a factory location, 1 inch lower, or even changing to a 1971 Pinto-style rack can be a big adventure for the oil pan clearance! This K-member lets you do that but it also allows you to mount a factory engine, factory springs, factory A-arms, and factory racks!
AJE has designed a factory replacement tubular A-arm that lets you retain the coil springs. However, the coil-over kit, tubular A-arms, and K-member can remove up to 70 pounds from the front end. The coil-over kit also allows ride height adjustments, making it one of the best options for your Mustang.
Coil-overs let you select the best spring rate for your intended use. When going from factory springs to coil-overs the spring rate needs to change. The factory rates are higher. Looking at the spring location in the factory A-arms, you need to factor in the leverage ratio of about 2:1 and the coil-over is closer to a 1:1 ratio. This means that a coil-over setup used for road racing needs to be in the 300- to 400-pound range. A daily driver needs 150- to 200-pound rates. A drag race car with around 500 hp and weighing 3,000 pounds or less could use 125- to 150-pound springs. A driver wanting a good ride and good handling also could use a variable-rate spring in the 175- to 350-pound range.
AJE’s new strut assemblies provide new options. Some Mustang owners want to lower the ride height of their car. With a stock setup you have to trim coils off the springs or go to a softer rate making it more likely to bottom-out the strut. With the new AJE struts you can lower the body of the strut in 1/2-inch increments to keep this from happening.
Another modification Mustang owners like is to use the later SN95 spindles on a Fox Mustang. Bigger and better brakes along with more drop in the spindle are the pluses. The SN95 spindle has more camber angle and the A-arm is longer. Using SN95 A-arms moves the wheels out about 1 inch.
When using Fox A-arms with SN95 spindles, the strut doesn’t match up to the attachment point in the strut tower, making the camber wrong. AJE’s new struts have an adjustable-camber key that can be used in a factory location, or add 3 of camber angle or decrease 3 of camber angle. For drag racing the Fox spindles can provide lighter brakes and are able to pull the tires under the car for a lower ride height. The AJE struts come with adjustable valving so whether you are drag racing, road racing, or just taking a drive, AJE has the front suspension you need.
Written by Dick Miller and Posted with Permission of CarTechBooks