Detroit Speed & Engineering (DSE) is well known for its performance suspension parts for classic muscle cars and trucks – Chevrolet Camaros, Corvettes, Chevelles, C-10 pickups, Ford Mustangs and more. DSE’s renowned chassis products and high-end project cars are popular in the Pro Touring market among owners who enjoy smooth riding, good handling muscle cars – whether they’re driven on the street, at the dragstrip, on a road course or in autocross competition.
Behind DSE’s product range is the expertise of professionals like Nate Peterman, Senior Design Engineer at the Mooresville, North Carolina location, who has been designing and tuning suspension systems, including his own circle-track chassis, for 16 years. The many technical terms used in suspension and handling development can be confusing for the newcomer, so we asked Nate to explain what they mean and how each one can improve the performance of your classic muscle project.
Suspension tuning for vehicle handling manages the balance of the chassis as it rolls through the corner: whether it oversteers, understeers, or remains neutral. “Oversteer occurs when the front tires have more grip than the rear,” Peterman explains. “When driven to the limit, a car with excessive oversteer has a tendency to spin. Rear-wheel-drive cars with high horsepower typically have throttle-induced oversteer, as the rear tires are handling lateral and acceleration forces.
With understeer, the car doesn’t want to turn, as the front tires have less grip than the rear,” he continues. “The front-end pushes and you’re waiting on it to turn. This is common in front-wheel-drive cars as the front tires are handling both steering and acceleration forces. Neutral balance is when the front and rear tires have near equal grip. Driving through corners becomes very predictable, but it’s not necessarily the fastest way.
“One of the most common situations I see is that a car will understeer on corner entry. Entry is the most important phase of cornering and should be adjusted first. Then work on adjusting mid-corner and exit without compromising entry.” When tuning the suspension to fix a handling problem, Peterman distinguishes between coarse adjustments – using geometry, springs and sway bars to make big adjustments – and then fine-tuning the setup using dampers (shock absorbers) and tire pressures.
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Springs suspend the weight of the car. Spring rate is the amount of force it takes for the spring to deflect. The rate is fundamental to ride comfort because it restricts the wheels’ vertical motions but is also crucial to how the car handles because it affects weight transfer front-to-rear (longitudinal) and side-to-side (lateral).
“The wheel rate is more important than a specific spring rate,” Peterman adds. “Wheel rate is force per unit of displacement at the center of the wheel. The motion ratio determines how the installed spring rate correlates to wheel rate. When commonly asked, ‘What spring rate should I run?’, I compare the vehicle weight to a database to determine a wheel rate range,” he furthers. “This is then used to calculate an installed spring rate. Typically, this yields a good starting point that can be tuned to a customer’s desired feel.”
As weight transfers, the spring rate affects both longitudinal and lateral handling balance. Installing sway bars provides additional lateral control.
Sway bars or anti-roll bars (ARBs) laterally tie the two sides of the suspension together, at either the front, rear or both. Stiffening the bars creates less roll, softening creates more roll. The stiffness can be adjusted using the bar’s diameter, wall thickness or arm length. Longer arms are softer; shorter ones stiffer and quicker to react.
Sway bars often tune the balance of the chassis: if a car experiences understeer, you can use less front bar or more rear bar. If it’s oversteering, less rear bar or more front bar will help to dial in the desired lateral load distribution. Classic muscle cars were originally equipped with undersized sway bars. Installing a larger diameter sway bar greatly reduces the body roll and improves the handling of a vehicle.
“In our market, packaging determines front sway bar geometry,” says Peterman. “We adjust the rate by increasing the diameter and tuning the wall thickness of the bar. Most rear suspensions allow room for an adjustable rear bar, so we’ll determine an appropriate front then provide multiple adjustment holes in the rear. With that, we find a balance where you can go from slight oversteer to slight understeer or neutral, depending on what a driver likes.”
Having gotten the chassis into the ballpark with springs and sway bars, it’s time for fine tuning. Damping controls the oscillations of the springs, keeping the wheel planted to the ground and body movements in check. To achieve this, dampers – also known as shock absorbers or shocks – handle both compression and rebound forces, as the spring compresses and extends. The shocks are typically tuned to handle more rebound force than compression force, because the spring and bar help to stem the latter.
“You can use rebound to dial in your front-to-rear weight transfer under acceleration and braking,” Peterman explains. “Increased front rebound or rear compression will slow down weight transfer under acceleration; The opposite is also true. During braking, if a car is getting on the nose too quickly, you can increase front compression or rear rebound to settle the car. In the middle of the corner, your shocks should be steady state. The springs and sway bars are the tuning device at that point.”
DSE offers a range of dampers that are non-adjustable, single-adjustable (rebound only) or double-adjustable (high- and low-speed rebound).
Says Peterman, “All of Detroit Speed’s test vehicles use double-adjustable dampers. Once we get a car dialed in, we’ll build our non-adjustable shocks to those specs. That takes all the guesswork out of it for customers who buy non-adjustables. Then, for customers who like to dial a car in at the track, it’s nice to have a small amount of tuning, because every driver likes a different feel.”
Further tuning comes from tweaking the suspension geometry. Perhaps the most obvious geometry measurement to the naked eye, is camber. Camber is the difference from vertical, taking a front view of the tire. A tire that leans in at the top has negative camber. Leaning out at the top is positive, but very few cars run positive camber nowadays. “The goal with camber is to keep the tire contact patch as flat as possible as the car rolls through the middle of the corner,” says Peterman. “For a street car, we typically use negative 0.5-degrees camber on the front. For more performance we run additional negative, improving turn-in and creating additional front grip.”
He adds that the typical solid rear axles of classic muscle cars aren't made for camber adjustment, but rear camber adjustment can be built into them for use on race cars.
Viewed from the top, toe is the angle of the tires relative to the longitudinal axis of the vehicle. It can be measured in degrees, millimeters or inches. Toe-out means the tire is pointing slightly outward when viewed from above; toe-in means it’s pointing slightly inward.
“There are a lot of tuning aspects with toe,” Peterman tells us. “Toe-out on the front helps corner entry. The car steers in quicker, it’s more responsive, but it’s less stable at high speed. For an autocross car, we’ll run a little bit of toe-out to help increase front grip. But on a road course at over 100mph, you don’t want a lot of toe-out because it starts getting darty, with an unstable feeling for the driver. In that case, we’ll run a little bit of toe-in. Most classic muscle cars have a solid-stick rear axle, so you don’t have the adjustment for rear toe-in or toe-out. For independent rear suspension (IRS), you do have toe adjustment. A RWD car benefits from toe-in at the rear through greater stability. For FWD cars, you run toe-out at the rear to help them turn because they tend to understeer.”
Castor (or caster) is perhaps the hardest angle to visualize. Looking at the car from the side, it’s the angle between vertical and the axis on which the steering ball joint pivots. Castor adds stability to the front tires by providing self-aligning torque.
“Classic muscle cars had anywhere from one to three degrees of positive castor, to work with the bias-ply tire construction of the era,” Peterman notes. “When modern radial tires are installed with stock alignments, there’s not a lot of self-aligning torque and they tend to wander down the road. When we build control arms, we build in anywhere from six to nine degrees depending on the application. It greatly improves that self-aligning torque so the car tracks well in a straight line and the wheels straighten naturally on corner exit.”
Another benefit of castor is that as you turn, the inside tire will gain a little positive camber and the outside tire a little negative camber, which improves tire wear by reducing the need to run as much static camber.
Whatever your ultimate goals are for your vehicle, there are a range of suspension tuning variables that can combine to make Pro Touring cars ride and handle as well as much newer, outwardly more sophisticated vehicles. As Nate Peterman concludes, “A well-handling suspension is a compromise of many tuning variables. Choosing a well-developed package will enable you to really enjoy your classic car on twisty backroads.”
