The Quest For Neutral Steer

OPTIMIZING A CORVETTE FOR CORNERING 

The ultimate goals in making a good cornering vehicle are:

1.     Increase the size of the tire contact patch and reduce the vehicle weight.

2.     Reduce the amount of weight transfer.

3.     Keep the tire contact patch in full contact with the road surface.

The first item is the easiest to explain. Reduce the weight of the car & buy wider tires. Tires with no grooves (i.e. slicks) have even more contact patch area.

The second item really comes down to lowering the center of gravity of the vehicle. A lower center of gravity will result in lower weight transfer during cornering.

The third item is the most intricate and difficult to achieve. This subject can quickly become very complex and confusing. I have found there are dozens of different approaches and opinions. The following is one approach to the quest for neutral steer. 

First let's start with some basic definitions, my way.

Understeer - The front wheels push through the corner.

Oversteer - The rear wheels come around in the corner.

Neutral Steer - All four wheels reach the limit of traction at the same time.

C4 Corvette Suspension - Both the front and rear suspensions utilize a single transverse fiberglass spring mounted rigid at two locations. The single spring acts as two separate cantilever beams, one for each wheel. The front spring is stiffer since it has less mechanical advantage against the wheel.  Both the front and rear suspensions use antiroll bars.

Springs - A member that converts deflection into stored strain energy, then converts the stored strain energy back into deflection.

Antiroll Bars - (Also sway bar) A torsion bar like spring that connects one side of the suspension to the other.

Effective Spring Rate - The combination of the inside spring and antiroll bar spring rates added together in parallel.

Total Effective Cornering Spring Rate - The combination of the inside spring and antiroll bar spring rates added together in parallel and added to the outside spring rate.

Cornering Wheel Rate - The total effective spring rate translated out to the centerline of the wheel.

Shocks - Dampeners. Shocks only resist velocity. They do not make a car corner better. Springs do the cornering work. Shocks make a vehicle more responsive. Stiffer shocks make a car take a set quicker, or load the suspension quicker.  I went with Bilstiens new sport shock.  I'm not sure I have them valved right.

With all that said, given a car that has the same weight and tire sizes front & back, neutral steer is obtained by matching the cornering wheel rates. Comparing spring rates and antiroll bar sizes from one Corvette to another generates nothing but confusion. Throw in different suspension geometries, weight distributions, and tire sizes from one year to the next and the quandary becomes evident. It really comes down to how it all adds up out at the centerline of the wheel while cornering.

Most of you are familiar with how an antiroll bar (sway bar) works. Let's talk through it anyway. When a vehicle enters a left-hand corner weight is shifted to the outside causing the outside suspension to deflect more than the inside suspension. Let's assume the outside (right) spring deflects 1 inch. The antiroll bar ties the two sides together, but it is also itself a spring. More specifically the antiroll bar is a torsion bar type spring. The lever arm on the right side of the antiroll bar deflects 1 inch. As the antiroll bar twists only a portion of that deflection is transmitted to the inside suspension. A sway bar with a shorter length, shorter lever arm, and larger diameter is a stiffer spring capable of forcing more deflection on the inside left spring. Say for example the sway bar in this situation lifts the inside spring 1/2". The sway bar and the inside spring act together in parallel to form one effective spring. This effective spring is added on to the outside spring to form a total effective spring rate. Translate that total effective spring rate out to the centerline of the wheel and you get the cornering wheel rate. The further the centerline of the wheel is away from the end of the spring the more mechanical advantage it has against the spring and the lower the wheel rate.   Confused?  Well then read this paragraph again because it really says it all.

The cornering wheel rate should be in the same proportion as the static weight distribution of the vehicle. Same for the tire width. Do all of these things and you're on the path to neutral steer.

The spring rates and sway bar sizes in C4 vettes varied drastically from year to year.  Hib Halverson has generated a chart that summarizes all of this.  Hib's Chart

Here is a link to a pretty good articles  Suspension Science ,  The Performance Proffessor

Body Roll

The problem with body roll is that with most suspension geometries body roll pulls the wheels out of alignment. Stiffening the suspension helps to reduce body roll. The tradeoff is that too stiff of a suspension is not able to maintain tire contact on rough surfaces.

The suspension can be stiffened by either adding a stiffer spring, a stiffer sway bar, or both. Generally it is desirable to have more relative sway bar on the non powered wheels and more spring on the powered wheels. Too much sway bar on the powered wheels will cause the inside wheel to spin and lose traction.

A nice product for the rear suspension is the Smart Camber Rods. These change the suspension geometry in order to maintain the same rear wheel camber throughout suspension travel. In effect the smart camber rods eliminate the undesirable effects of body roll on the rear suspension. It is my opinion that with these installed you can leave the stock rear sway bar in place and focus on stiffening the rear spring.

 Alignment

Alignment is critical for auto-crossing.  Everyone should have their car aligned before the season begins.  It’s amazing the difference it will make.

Here is a very brief description of the alignment parameters

Caster –  This is the amount of tilt in the vertical axis that the front wheels steer about.  Positive caster is when the top of the steering axis is rotate back.  When turning the steering wheel positive caster will result in more negative camber.  Imagine that the turning axis is rotated from vertical all the way back to horizontal.  Any steering will result in a change in camber.  Positive caster will improve cornering but make the car harder to turn.

Camber – This is how far the top of the wheel is tilted in, or out.  More negative camber can increase cornering, to a point.

Toe -  Toe is a measure of how far the front of the wheel is tilted in or out.  Out is very unstable but will make the car more darty.

Rake Angle – This is a measure of how level the car sits front to back.

 What alignment should you use?  That’s the big question.  Here is what I have found best for my 1994 Corvette.

Front

Camber – As much negative as possible, usually around –1.5 degrees

Toe – Zero

Caster – As much positive as possible without sacrificing negative camber, about 6 degrees.

Rear

Camber – Negative ½ to ¾.

Toe – 1/16 in

Rake - Zero 

This link is to a page with more alignment specs and also how to on home alignments. Home Alignments

Tire Temperatures

This is the single best way to determine if the alignment settings and tire pressures are correct.  Generally the temperatures from outside to inside should be the same, assuming racetrack does the same amount of cornering in each direction.

In short, if the tires are hot on the outside then not enough negative camber.  Hot in the middle means too much air pressure.  Hot on the inside means to much negative camber.  If the track has more cornering to the left, then the right side of all four tires will be hotter or vise versa for the other direction.

Take the tire temperatures immediately after the run.  You can use an infrared temperature gun or a pyrometer.

From what I have said above it should be evident that just changing one spring component alone is not a magic fix. Changing a single component, such as the front sway bar diameter, can throw the vehicle balance completely off. You must consider all components together.

Sport Suspension System by Vette Brakes & Products

After going through the above calculations I went with new springs front and back, a 32mm front sway bar, rear smart camber rods, and a full polybushing kit.  I had the springs custom made to specific rates  by Vette Brakes & Products.  Changing the springs is not an easy job.  I recommend you have it done at a GM garage.

      Vette Brake & Products

Adjustable Rear Spring

Instead of using the rear spring in the sport kit I went with the adjustable rear spring.  This is a great way to dial in the car.  Adjustments are made as follows.  To make the spring stiffer move the pins outward. Too much oversteer then reduce the rear spring rate.  Too much understeer then increase the rear spring rate.  The idea is to move the load away from the tires that are losing traction.  I've found the the rear spring can be set stiffer when racing on concrete.  Beware, you will need to glue the plastic spacers to the pin block and use loctite on all of the bolts.  Otherwise these parts WILL fall out.

Polyurethane Bushings

The main advantage of going with polybushings is that they squish less and thereby help to keep the wheels in alignment.  They also help to increase responsiveness.  The upper and lower control arm bushings as well as the trailing arm bushings are the most difficult to change.  You will need to use a press to get the old ones out.

Rear Smart Struts

I also installed rear smart struts.  These help to maintain the same negative camber throughout wheel travel by changing the connection point geometry of the lower camber rods.  These help to reduce any undesirable affects of body roll.  Changing suspension connection points is only legal in SCCA prepared and modified classes so check the rules first.

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