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S197 GR40 System Reasoning of Design

After 3 years of product development, Griggs Racing announces release of its long anticipated GR40 system for S197 Mustangs.  This no compromise chassis performance system is designed for drivers desiring the ultimate precision in handling and cornering performance for both street and track featuring proven National Championship winning geometry

Griggs Racing GR40S197 On Track

Benefits:

  • Increased lateral Acceleration (Gs)
  • Quicker steering response
  • Precise turn in
  • Unprecedented mid corner correction
  • Improved road feel
  • Improved tire wear
  • 30 lbs reduction in front end weight.
  • Stronger Ball joints improve safety.
  • Stronger no-flex spindles improve hard braking performance, pad and rotor life, and safety.
  • Reduced weight increases overall vehicle performance
  • Improved 60 foot times
  • No wheel hop during launch.
  • Very little welding.
  • Easy alignment
Griggs S197 Front Cradle

Reasoning of design

Front:

The S197 chassis is a major departure from all other previous Mustangs from the standpoint of structural rigidity and geometry.  Structurally, the tub, or Unit body frame work is extensively superior to all other Mustangs, to the degree that we deemed it unnecessary to add chassis stiffening devices in the manner of subframe connectors and/or frame kits unnecessary for all but the most serious of competition vehicles.  However, the geometry required a second look.

The new S197 Chassis as delivered from Ford has much improved geometry in the front and the rear; the front being very similar to BMW and other later designed McPherson strut cars.  Modifying it for serious drivers seemed like it would require only simple bolt-on parts.  So we looked at it from this perspective.

At first we thought we could retain the existing K Member, relocate the control arms, reduce compliance in the bushings and apply appropriate rates of anti-roll bar, spring and damping to achieve adequate improvement in performance.   But we found several discrepancies in OEM design that made us re-think our approach, and we decided too great would be the compromises to the performance, safety, and driving pleasure our customers have come to expect from GR40 Mustangs.   

GR40 systems for previous Mustang Chassis were developed on a modular basis, designed around the cars original lower control arm dimensions and adequate steering rack in order to reduce costs born by the end user installing a good chassis system.  With this new car this was not possible for numerous reasons. After considering the 350 lb. to 500 lb. plus increase in mass of the S197, the current trend toward larger tire/wheel combinations and the improvement in grip of DOT track tires in recent years, static load tests were conducted to quantify and identify these design inadequacies.  These were:

  1. The most glaring deficiency is the small lower ball joint which we deemed as dangerous in high performance environment.
  1. The most surprising inadequacy is the flex in the spindle under braking loads. Although the hub with integral bearing is proportionately increased in strength and rigidity to the increased weight of the newer car over the SN95 unit, the spindle is the opposite
  1. The K member is too flexible under braking loads and requires so much reinforcement to contain the flex that a new one needed to be designed.
  1. The front lower control arm design is not triangulated, requiring a heavy replacement that will never be as strong nor as rigid as previous GR40 units.
  1. The front end utilizes two bolt-on cross members; one the K member, and in front of it a 16 pound unit that supports the radiator, and mounts the anti-roll bar. This is the same as mounting a bowling ball under the nose. 

Geometric evaluation identified the following problems:

  1. Anti-roll bar linkage attachment to the struts is offset to the kingpin centerline which results in the ARB inducing a steering load.  We find this unacceptable to precision handling.
  1. Lower control arm and steering rack length insufficient and not conducive to SLA modular add on to design.
  1. Low roll center and lack of camber gain not conducive to the increased mass of the car.  The result would be springs of very high rate to adequately control body roll yielding a poor ride quality and compromised performance on rough surfaces.

Other factors affecting our ultimate design:

  • Availability of service parts. Lack of availability of off the shelf struts with GR40 level of quality and performance suitable for adjustable ride height struts at the time of design encouraged us to produce the SLA system at the start.   
  • Weight.  The entire front OEM suspension weighs 261lbs. including the #1 cross member which supports the radiator and anti-roll bar brakes, spindles and hubs.  Since weight reduction is a priority in performance design.  The GR40 SLA front system with larger Griggs superior performing 4on4 brake system reduces front end weight by 30 lbs.  Most of which is unsprung. 
  • Engine mounts. Mounts are out of the way of available headers and easy to convert to solid mounts.
  • Engine swaps. K member should be adaptable to engine combinations other than modular being installed.
  • Exhaust. Headers, and turbo system installations and accessibility
  • Plumbing. Should be affected as little as possible, other than to improve service reliability, performance and weight.
  • Cost. Commonality of parts for economies of scale.  Ease and low time of installation (low labor cost). No welding should be required if possible.
  • Value:  Performance increase per dollar spent.
  • Experience:  Winning history of our top rated proven SLA design

The Result is a system with proven race winning, customer satisfying geometry altered for the wheel base and relative CG changes of the new car, with commonality of parts such as spindles and control arms that are interchangeable with our current proven GR40 design.  This allows us to keep the GR40 system price down relative to the performance spent while increasing safety and reliability on track.  Customer cost of installation, operation and service will be kept down as well. 

Rear   

The OEM three-link is a substantial improvement over the old splayed 4 link design.  However the length and location of the links allow considerable geometric change to occur when the car is lowered, and forward bite is therefore diminished along with rear roll understeer induced. Further design detriment is the migrating instant center described by the linkage which induced more instability in chassis balance with changes in throttle applications than with a TorqueArm.  And, as with most any OEM derived street car, the compliance in the rubber bushings of the linkage induce wheel hop and unpredictable rear axle steer.

Griggs S197 TorqueArm
GR40 Heavy Duty Torque Arm

The rear roll center located by the OEM Panhard bar (track bar) is correct with the car lowered; however it is not adjustable in length or height. So we provide an aluminum adjustable length Panhard bar unit to provide rear track adjustment.  Optional is an adjustable length steel unit with a noise reducing urethane bushing in one end for quiet street rides.  This is similar to our long proven Panhard bar of previous years GR40 designs.

Due to the increase in vehicle mass and CG height, we have added an Adjustable Rear Anti-Roll Bar to aid in chassis balancing with the Panhard bar installed.  This Anti-Roll Bar has a patent pending on its bind free mounting design.  Further, we have developed a bolt on Watts-link utilizing all but one of the components from our previous Watts Link designs.  This unit features all of the design elements of our proven SN95 watts link, but requires no welding or cutting of the tub to install.  It is suitable for street or track.


GR40 Adjustable Rear Anti-Roll Bar


GR40 Adjustable Front Anti-Roll bar

The shock mounts on the axle housing as delivered are inadequate and flex when submitted to coil-over loads from such things as bottoming, so we provide a gusset to be welded into the existing bracket to correct this.  We also provide a bolt-on bracket for this purpose which allows the car do be driven for a time until the gussets can be welded in place.  We have used both for ultimate rigidity.

To correct the anti-squat geometry when the car is lowered to track height, we provide bolt on brackets that lower the control arms on the rear axle, and also serves as the mounting location for the rear anti-roll bar.  Rear lower control arms provided eliminate wheel hop and uncontrolled axle steer.  They are available as adjustable length aluminum units with Teflon lined rod ends each end, or in light weight steel with Urethane Bushing one end for noise cancellation.


MCA 4500 5

The car will still wheel hop some with the OEM third link during a launch, so we can provide an adjustable low compliance unit.  However for the ultimate in traction, and driver confidence for control at the limit, removal of the 3rd link and installation of the TorqueArm is the best option.  The TorqueArm is the only other part that requires welding, (very little), and a simple modification to the exhaust, which improves the cars sound and performance.. 

Track testing results exceeded even our expectation. Track testing was performed on 275/35 18 Hoosier RS6 tires. Launch was improved and wheel hop was eliminated. Consistency was easy to attain.  Handling far exceeded the performance level attainable with the three link and strut system, especially over track irregularities and when cresting hills at speed while turning. The system has spent some serious time on track at the hands of a variety of highly skilled drivers as well as many miles of street driving.  It truly is a joy to drive fast, and feels as solid and accurate as any car from anywhere, yet it has a surprisingly light feel,  most drivers saying it feels like it weighs about 2000 lbs. 

With testing completed and parts on the shelf ready for shipment, we can now announce the GR40 SN197 system it ready to improve your car. Further, complete turn key track toys can be ordered though selected dealers.

These new GR40 equipped SN197 cars are a major improvement in Mustang quality and performance.  We believe that over time they will prove to be an incredible performance value.  Performance numbers from independent testers will be published as soon as such is available.

Buildup of Rear Suspension In Order of Priority:

Stage 1:

  1. Geometry Brackets and Rear Lower Control Arms
  1. Rear Coil Over Shocks w/ bracket support kit (Adjustable damping and ride height).

Stage 2

  1. Panhard bar, aluminum adjustable
  1. Rear Anti-roll bar

Stage 3 Optional, but the very best.

  1. Watts Link (delete Panhard)
  1. TorqueArm (welding required).
  1. Rear Adustable Ball–mount Anti-roll Bar optional.

Buildup of Front Suspension in Order of Priority:

  • Unfortunately Ford left us no choice.  The entire front end must be done at once to achieve the ultimate in handling.

Options when ordering are:

  1. Aluminum Double Adjustable Digressive Shocks (add $700.00)
  1. Solid motor mount system (no extra charge)
  1. High sensitivity Steering Rack

high resolution photos of the S197 GR40 System

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