Fun With Air Ride – Part 2

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2005 Cadillac CTS with suspension upgrades installed.  (Photo courtesy of Raymond Barrios)

It was a bright beautiful day in Las Vegas, Nevada.  Specifically, it was a Wednesday.  As a parent, being in Las Vegas on a Wednesday, without the kids, feels like the last day of school when you were twelve.  The freedom and possibilities ahead are enough to make you swoon. What will we do?  What won’t we do?! First Stop, Ink Therapy.  Time to stretch my legs after a drive of several hours, maybe shoot some pool.  Time to notice my tires in the parking lot.  I knew the useful life of my tires was coming to an end, and sure enough, as I inspected them, I knew this would be my last trip to Vegas on these tires.  Time to get started on my air ride system.

This is a perfectly reasonable progression, if you think like I do.  Tires are one of the most important features on a car, and completely vital for both performance and safety.  And a four hour drive through the dessert is a very good way to keep performance and safety at the forefront of one’s mind.

This is a performance sedan – sort of. More specifically, my car is the base model of GM’s second most entertaining car, next to the Corvette.  The high-end CTS-V is a 640hp rocket that handles like a slot car.  Agile, nimble, and very sticky, the CTS-V is a legitimate Grand Touring car, and a star on Track Days.  The ride is typical of Cadillac’s latest offerings; soft and smooth, and not terribly well damped.  Until the computer senses that a sporty ride is warranted and notifies the electromagnetic dampers, which spring into action, stiffening the ride, and pushing the sportiness to a world-class level.  It is truly a “best-of-both-worlds” proposition.  Auto enthusiasts have been hearing about mythical “Four-door Corvettes” almost as long as there have been Corvettes.  The CTS-V is the real deal – believe that.  My base model CTS has the same DNA, but is simply not as well-developed.

A good set of budget-priced performance tires would be a huge step in the right direction, and we will cover “How to Choose Performance Tires on a Budget” in a future post. If our mission is to build a better CTS-V, tires are mandatory.  And a good set of tires would likely bump the lateral acceleration from .83g to .87g.  This improvement represents a 4.82% increase in cornering grip, which may not sound like much.  But consider that the current lap record at the Nurburgring is held by a 2009 Radical SR8LM at 6:48.00.  A 4.82% improvement in lap times, approximately 20 seconds, would be enough to make the second through eighth place cars at the ‘Ring produce lap times as good or better than the record holder. This is a significant difference.

As I mentioned in Part 1, our goals for our suspension are as follows:

  1. A more comfortable ride
  2. Reduced understeer, to the point of neutral handling
  3. Improved lateral acceleration
  4. Less body pitch and roll in braking and cornering
  5. No sacrifices in acceleration or braking performance
  6. No excessive maintenance demands or tire wear

 

How are these things accomplished in an automobile suspension system?

  1. Ride comfort is typically accomplished by providing a generous amount of wheel travel with a soft, progressive spring rate. Damping should be on the softer edge of acceptable (as relates to the spring rates) on compression and light on rebound.
  2. Reduced understeer is often accomplished by stiffening the spring rates in the rear and/or decreasing them in the front, increasing the traction in the front relative to the rear.  Adjustments to the front and rear spring rates (especially the anti-roll bars) are the most common ways in which tuners accomplish this, but there are many other factors that come into play here, and a lot can be accomplished by making adjustments to the instant centers, roll center and center of mass of the front and rear suspensions systems.
  3. Improvements in lateral acceleration are about one thing – traction.  Traction under acceleration, coasting and braking.  Traction at turn-in, and traction exiting the corner…traction, traction, traction.  Simple, right? Uh, no.
  4. Body pitch and roll can be reduced by increasing spring and damping rates.  Changes to the suspension geometry can also affect pitch and roll, as well as lowering the center of mass of the vehicle or making changes to the roll centers and/or instant centers.
  5. Acceleration and braking can be affected by the reciprocating mass of the tires, the diameter of the tires, and energy can be absorbed through suspension movement and damping forces instead of being applied to the road.  Vehicular mass as a whole and unsprung weight are also factors.
  6. Relability can be tricky.  Adding components not previously installed, such as air tanks, compressors and lines adds potential trouble in the process.  Yet, 18-wheelers are capable of millions of miles annually on air ride suspensions.  There must be a reliable way to do this, and it starts with DOT-approved components.  Other potential issues arise from the use of wheels with a different offset and poor-quality or poor-fitting components, as well as from abuse or neglect.

In Part 3, we will discuss the solutions we found for these problems, and how they overlap to exacerbate other problems.  We will also discuss compromises that we made, and why.  We will find out if this mission was a success or failure, and perhaps discuss the aftermath…

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