A-Arms: What Makes The Difference?
So what’s the big deal with a-arms? Your car already has a set so why replace them? Let’s look at some very good reasons for upgrading.
It is just like the stuff you did in high school. A suspension system obeys the rules of geometry with respect to how the wheel moves when you hit a bump, or when you take a hot corner. The travel is predictable because it is controlled by where the suspension pivots, how far it can travel and most importantly the angles that start and stop its travel. These angles are established by design. The suspension engineer looks at the vehicle mass, center of gravity, tire size and width then selects a setup that is a compromise between reasonable handling, and the ability to mass produce the parts at a budgeted price. Let’s look at the C2 and C3 Corvette and see what Zora Duntov and the boys chose for the car.
The frame and front suspension parts are identical on the ’63-’82 Vettes. When the Sting Ray came out in ’63 wide tires were not around, and low profile tires were definitely not available. Tires have come a long way since then and they have an affect on suspension since the contact patch, the “foot print”, of the tire on the ground is so much wider than it was back then.
The contact patch is so important because you want it as flat to the road / track as you can get it in all conditions; that’s how you get maximum grip necessary for best braking and cornering. This brings us to the first geometry consideration and that is camber, and camber gain. Camber is the angle of the wheel with respect to vertical as viewed from the front of the car. The picture shows exaggerated negative camber. If the top of the wheels were tipped out it would be positive camber. Positive camber is evil and you don’t want that as we will explain.
When you barrel into a tight corner the car will rotate about the roll center (an imaginary point explained in another not so imaginary article we have written) that is joined, or coupled, to the center of gravity (CG). When this occurs the camber causes the wheels to change their vertical orientation. This movement is called camber gain. What the Shark Bite TM arms do is to raise the roll center so it is closer to the CG; the closer the better (up to a point.) This is because there is less leverage to roll the car. The effect camber gain has is to keep the wheel more vertical in turns. The more vertical the more contact patch is in touch with the ground the more grip the car will have.
As you can see starting with negative camber sets the car up to maximize contact patch in a turn. The opposite is true if for some strange reason you started with positive camber. With positive camber as the suspension is loaded, and the car rolls (leans) the tire would not remain flat and thereby lower available traction.
We have found that the factory design puts the front roll center below ground line! This is not good since there is more leverage (distance from center of gravity to roll center). The result is more body roll and insufficient camber gain to keep the wheels as vertical as possible. The Shark Bite TM aluminum alloy a-arms raise the roll center by 21%, reducing body roll and greatly improving camber gain.
This is another important geometric concern worth considering on your suspension. It’s what should make shopping cart wheels want to track straight (well, maybe that’s not a good example), but it is what makes your steering want to self center after you take a corner. As mentioned earlier, in Zora’s day wide tires didn’t exist so he and his designers settled on 2 degrees positive castor (tipped back from vertical). Wide tires have more contact area and offer more friction and resistance to centering. The affect on your car is that it will not want to re-center. It produces that kind of vague wandering feeling as you drive and a general reluctance to return to center after you turn.
To improve this Shark Bite a-arms have three times the castor over factory a-arms. This figure is typical of most modern steering systems. You will notice the difference in how the car re-centers, as well as a much more stable modern feel as you cruise.
A-arms contribute to what is known as un-sprung weight or mass. It is the mass of the suspension the a-arms, spindles, springs; everything attached to it rather than what it supports like the chassis, engine, body and you. Reducing this un-sprung mass allows the suspension to follow uneven ups and downs of the road or track to keep the contact patch on the ground because there is less mass to push around. The less un-sprung mass the more responsive the suspension can be at following bumps, the more traction you will have.
Shark Bite TM aluminum a-arms are 4.3 lbs lighter that the factory steel a-arm. It reduces the un-sprung weight and improves the response of the front suspension.
Smarter is Better
The easy modification of bolting on new a-arms can greatly improve the geometry and ultimately handling of your classic Corvette. Some of the benefits of modernizing your car with this upgrade are:
• Raising the roll center for less body lean and more traction in turns.
• Improved re-centering with more positive castor.
• Reducing un-sprung weight for better suspension response to bumps.
• They look cool.