GKTECH REAR SUSPENSION UPRIGHTS

1. INTRODUCTION

The original multilink suspension found in the late 1980s and 1990s Nissans is an advanced design for its time and is more forgiving to ride height changes compared to the MacPherson strut suspension in the front of the S chassis.

When an S-chassis is lowered in the rear, camber and toe settings will change and can be out of adjustment range for the OEM eccentric adjusting pins. The static alignment can be brought within OEM specifications by installing adjustable length suspension arms. The dynamic characteristics of anti-squat bump steer, camber gain and roll-centre height cannot be changed significantly via changing the length of the links (camber, toe and traction arms) enough to have effective results which aren’t compromising trade-offs. For example, bump steer can be reduced by increasing upper front link length at the expensive of reducing anti-squat and vice versa. Camber gain is also affected.

There are “drop knuckles” available on the market to help with geometry improvements. These products help to correct roll centre height, camber gain and bump steer, but fall short by not changing anti-squat and doing nothing more than restoring most of the original kinematics and geometry of a lowered car to OEM ride height specifications.

 The original suspension is designed on the basis of elastokinematics. The lower control arm bushings deflect under different loading scenarios allowing horizontal motion on its pivoting axis to increasing toe-in during acceleration, braking and cornering. This is done to increase stability and traction. If the OEM bushings are replaced with spherical bearings or polyurethane bushings it is no longer the case and bump-steer will also become an issue.  The article about GKtech rear suspension lower arms explains this in detail.

The geometry and kinematics are originally designed for relatively low vertical stiffness and grip R15 and R16 tyres which do not favor high stiffness and grip tyres that are often used to increase performance. In this scenario there is excessive camber gain wearing the tyres out from the inside and the introduction of bump steer causes instability by increasing toe out during bump and roll, reducing available grip. Toe changes throughout wheel travel and can be infuriating when chasing lap times and setting up the chassis.

2. NEW PRODUCT – REAR UPRIGHT

In order to assess the rear suspension geometry and the resulting kinematics accurately, the rear subframe and upright of a Nissan S14 were 3D scanned and points later added in post-processing stage to locate the suspension pivot points. The suspension arms were measured with a coordinate measuring machine (CMM). In order to correctly plot anti-squat it was also important to measure the weight transfer longitudinally, laterally and calculate the location of the centre of gravity. This was done with corner scales and by lifting up one side or axle of the car.

3. GEOMETRY ANALYSIS

After the necessary coordinates were obtained the data was input into 3D suspension modeling software and a virtual suspension model created. This would provide accurate information on the current geometry and kinematics of the suspension and all of the data can be exported to be plotted into charts using spreadsheet software. It is important to analyze the current situation before starting to move the upright coordinates as they are all dependent of each other. Improving one parameter will usually have effect on other parameters. Using computer modeling removes the guesswork from designing good suspension geometry. There is still the question of what tyres are used, modern high performance street tyre characteristics were taken into account when considering what the geometry should be as most of the customers would be using those since the product is mainly intended for track use.

3.1 ANTI-SQUAT

The first parameter that deteriorates after lowering is the anti-squat and this has an impact on the traction. This is true with S14 rear subframes and other versions of the Nissan multilink similar because the anti-squat value changes with ride height. It is caused by side view instant centre point migration due to the positioning of the suspension links diagonally from top view to the chassis and as such the lower arm outer pivot point moving on longitudinal axis. This effect could be removed if the suspension links were mounted parallel to the chassis. The value calculated for anti-squat on a S14 that was lowered 40mm was around -30% meaning the torque from the driven wheels causes the suspension to compress even more and reducing weight transfer to the rear axle, reducing vertical load on the tyre and thus reducing traction. There is debate to the value of anti-squat that should be used. Lower values are more preferable in slippery conditions and on bumpy roads.

3.2 BUMP STEER

Bump steer is the change of the toe alignment during suspension heave and should always be minimized when designing suspension for use in high performance cars. Mass production cars often have bump steer by design to increase stability under braking (some do this with asymmetric rubber bushings) or to discourage drivers from driving at high speeds on the public roads.

It is caused by suspension arms pivoting offset axes from one another. The most critical parameter when tuning bump steer out of the suspension is the height of the pivot points in relation to each other.

3.3 CAMBER GAIN

Camber gain is the gain in negative wheel camber during suspension compression and is considered a negative effect when the car is traveling straight and a positive effect when the car is cornering. This is because increase in negative camber reduces the area of tyre contact patch with the road during accelerating and braking, but during cornering it increases the contact patch. In the case of independent suspension the wheels roll together with the body of the car in the same direction and without camber gain the camber angle would be positive on the outside wheel (towards direction of roll). This principle does not apply to strut based suspension types as the top pivot point is fixed to the chassis and with dependent suspensions such as live axles as the body motion is independent of the wheel in roll.

Ideally there would be no camber gain during heave and one to one ratio during roll but as that is not possible without complicated linkages if even then, a compromise is to be found. As the suspension was originally intended for low vertical stiffness R15 and R16 road tyres it is likely that the camber gain is excessive for modern R17 and R18 high performance street tyres.

3.4 ROLL CENTRE HEIGHT

The roll centre height is an important parameter for the car’s handling. Its location dictates the amount of geometric roll resistance, geometric weight transfer, speed of weight transfer and how quickly lateral force is applied to the tyres. It also effects weight transfer balance between front and rear axles (RC height is usually different on either axle). Since geometric weight transfer acts directly on the tyres and does not compress the springs, dampers and anti-roll bars (elastic weight transfer) then its response is much faster than elastic weight transfer (elastic suspension components) and reason for keeping roll-centre height above ground level. This has a profound effect on the handling of the car. The higher the roll centre height the faster the weight transfer.  If roll-centre height is too high, the jacking (raising the body – lifting height of centre of gravity – increasing body roll) forces are also higher. It does have the benefit of being able to use softer springs for better mechanical grip. If the roll-centre height is too low, for example below ground level then the geometric roll resistance is negative. This instead increases body roll as the geometric roll resistance value is negative. It is caused by lateral force generating a moment about the instant centre.

The height if the roll centre must be compromised between jacking, geometric roll-resistance. Spring rates must be accounted for along with steady-state cornering weight transfer. The static roll centre height was found at 21mm, which is too low as with 40mm (3 degrees of roll) suspension compression will be -44mm.

4. ANALYSIS SUMMARY AND DESIGN GOALS

4.1 GEOMETRY

After analyzing the kinematics based on the measured geometry of a 40mm lowered S14 it was decided that a regular drop knuckle of 40mm would not be sufficient to achieve the best grip and traction and all of the geometry is to be re-designed. The following parameters need changing:

1.       Increase anti-squat
2.       Minimize bump steer
3.       Reduce camber gain
4.       Raise height of the roll centre
5.       Compatibility with OEM style adjustable upper arms and OEM lower A-arm
6.       Use of spherical bearings to remove elastokinematics and consistency
7.       Fabricated steel construction

4.2 ADDITIONAL FEATURES

1.       Maintain cable operated drum-brake handbrake system in original position
2.       Double caliper mounting for secondary braking system – hydraulic handbrake
3.   Reduce unsprung mass as much as safely possible
4.   Reasonable design for assembled components

5. NEW UPRIGHT GEOMETRY

5.1 ANTI-SQUAT

Anti-squat was increased to around 20% to increase with traction by increasing the height offset between the top suspension links’ (camber and traction arm) pivot points. This helped to raise the side view swing axis and bring anti-squat back to a more suitable static value.

5.2 BUMP STEER

Minimizing bump steer proved troublesome as ideally the toe arm would have to be much shorter than OEM upright compatible adjustable arm. The centre area of the bump steer curve was improved by reducing the height of the pivot point on the upright. It was relocated more towards the rear to further reduce bump steer. The result is a significant improvement within ±40 mm, especially on the bump side as the cars would most likely be set up lower than suggested. Toe-out characteristics were replaced by toe-in characteristics to help with traction and stability. 

5.3 CAMBER GAIN

Camber gain was optimized to maximize tyre contact patch by adjusting the length of the front view swing axis by adjusting the height of the top rear link (camber arm) and lower A-arm.

5.4 ROLL CENTRE

Roll centre height was raised by 30mm approximately to increase geometric roll resistance and improve handling by having the tyres generate lateral load more quickly. Weight jacking increased slightly. The roll centre is now high enough for good handling and not too low to become negative during cornering (3 degrees of roll). This was achieved by lowering the lower A-arm pivot point and adjusting top rear link (camber arm) height accordingly to keep camber gain in check.

5.5 SUSPENSION ARM PLACEMENT

All the newly relocated suspension arms’ pivot points required 3D modeling of the upright concurrently with geometry optimization to avoid contact between components and binding throughout the suspension heave. This also made sure the handbrake assembly would not get in the way of the suspension mountings and vice versa. As the drum brake mechanism is operated by a cable that also had to be 3D modeled and placed in the assembly to check for clearance and model the upright around it. The resulting suspension geometry was compromised very little by design constraints. Mainly the bump steer curve as the area for minimal bump steer was reduced by use of OEM upright compatible toe arm.

6. UPRIGHT DESIGN

The initial stage of the design was done during suspension geometry optimization stage by placing the pivot points with suspension arms and spherical bearing housing tubes, wheel bearing assembly, braking systems, drivetrain components and the coilover assembly. All these systems had to clear the upright and surrounding components without contact. The clearances have been improved every design iteration.

6.1 HANDBRAKE CABLE CLEARANCE

Handbrake cable clearance was confirmed by 3D modeling the cable in CAD and then 3D printing a section of the upright to test fit with a drum-brake mounting plate and the cable.

While some other products on the market reverse the drum brake assembly to help with clearance around the suspension arms, GKtech upright does not rotate it as that would hamper performance and the suspension arms were positioned around it as having a fully functional 3D model of the suspension removed guesswork of what the outcome of link re-arrangement would be.

6.2 DOUBLE CALIPERS

The knuckle supports a secondary brake caliper to support independent hydraulic handbrake systems. Both standard S-chassis single piston steel calipers and twin piston aluminum calipers fit from other Nissans. A problem that had to be to overcome with fitting calipers on both side of the upright was that they needed to be rotated accurately to have enough clearance from the suspension links and upright itself which has been the main cause of updates for the design of the upright.

6.3 WEIGHT REDUCTION

6.3.1 CALCULATIONS

In order to find forces that act on the upright a calculation module was built in spreadsheet format. Lateral, horizontal and vertical forces were calculated using input parameters such roll centre height, centre of gravity height, track width, wheelbase, spring and anti-roll bar rates, corner weights, unsprung mass in each corner plus tyre unloaded radius and its spring rate. Reasonable accelerations were assumed such as 1G for braking and accelerating, 1,2G for cornering. These values will be later used in topology optimization.

6.4 FINITE ELEMENT ANALYSIS

Finite element method was used to visualize where material use can be optimized by removing it from unnecessary areas to make the product as lightweight as possible without sacrificing strength or stiffness. Strength was improved in areas that were critically loaded and had stress concentrators. Some were removed by changing the design and some by welding assembly strategy. This is necessary as the upright is loaded dynamically and high stress risers will cause fatigue in the material. The warmer colors represent areas with more stress (Figure 22). While some are unstressed in specific load cases, they become much more stressed in others. This stage was the most time consuming as any modification to the shape of the upright due to clearance issues or other problems meant analyzing and optimizing the design again. In the latest update Wilwood 4 piston caliper mounting option was added to the front caliper mounting tabs.

Originally the toe arm on the upright was larger for strength, but had to be scaled down for rear caliper clearance. While it remained strong enough for normal load cases, there were problems with it being strong enough to withstand impacts to the wheel and as such the design was strengthened by simulating stronger impacts than initially and reinforced accordingly. The updated arm is 3x stronger compared to the original design that had increased brake caliper clearance.

6.5 COMPONENT DESIGN

As a single solid design of the upright was complete it was necessary to divide it into components for manufacturing purposes. The upright was split into several sheet metal parts plus machined parts for lower A-arm tapered ball joint and spherical bearings. The sheet metal pieces are CNC laser cut and CNC formed. Machined parts are produced on a CNC lathe as this provides great accuracy, repeatability, automation and accuracy for interference fit parts such as the spherical bearings and transition fit for the tapered ball joint.

The spherical bearings required offset spacers which reduce the bore size to use OEM suspension link mounting hardware. The handbrake carrier plate bolt hole is much thinner than on the original upright and also required a spacer for the nut to have it correctly threaded to the bolt and not rub against the sheet metal when fully tightened.

GKtech products follow Kaizen model to constantly improve products. Updates are mare as soon as faults are found or better solutions and designs envisioned. Main reasons for updates have been caliper clearance issues and toe arm mounting strength.

1.       Prototype – rear caliper clearance issue, strut mounting being too low for some coilovers
2.       First production version
3.       Improved component fit for welding assembly
4.       Improved traction arm bolt clearance from brake caliper
5.       Improved drum handbrake component clearances for easier install
6.       Strengthened toe arm mounting
7.       Fix assembly problem with bottom ball joint from previous update, improve welding clearances from lightening holes
8.       Add clearance between upper arms and brake calipers, also between front caliper and upright
9.       Improve toe arm mounting arm strength by 2x,  further increase clearance for brake calipers, improve welding clearances, remove some excess material to save weight - 75% of sheet metal components updated
10.   Improve manufacturability and ease of assembly by reducing number of components and improve material stress flow in design
11.   Add provision for 4 piston Wilwood calipers, update caliper mounting sheet metal, add provisional clearance for Wilwood Dynapro Single handbrake caliper mounting bracket, simplify toe arm parts, update sheet metal parts for more even stress flow

7. TESTING

During test fitting of the prototypes a few design faults were discovered that were to be removed for the production version. Aluminium calipers from Z32/R32 did not clear the toe arm mounting area and there was very little clearance between the front caliper and traction arm bolt. The coilover mounting stud that was placed below the toe arm on the knuckle was also too low for some coilovers and had to be raised for production version.

The driver feedback was all positive. The car was more stable than with OEM rear uprights, the handling was sharper and there was more traction. Tyre wear was even and made them last longer. As the camber gain is now more suitable for high grip tyres, static negative camber can be used for better corner entry grip and still have even tyre wear.

Once the initial non-interfering design was complete, the shape of the upright was optimized using Finite element method to reduce weight and increase strength with a safety factory of 4 at 1.2 G lateral load. The finalized designed of the knuckle is hollow and machined parts thinned out (image 10). The final version weighs 3,6kg and with spherical bearings installed it is 3,8kg.  

8. SUMMARY

All the design goals have been completed that were originally set in place and additional features added with every update. The latest version is the strongest, has best clearance for brake calipers and drum brake system and certain Wilwood calipers can be used (handbrake one requires a mounting bracket). Over several updates the difficulty of assembling the uprights on the welding jig has been reduced by simplifying design of parts and reducing their number. Weight has increased by 0,1kg since the original production version, but strength has been improved significantly on the upright’s toe arm.


Author: Siim Õisma
Bsc Automotive engineering, Msc (in training) Product development
GKtech engineer since 2013
Published by Zac Bognar.