V2 ADJUSTABLE REAR LOWER CONTROL ARMS

1. ORIGINAL LOWER CONTROL ARM

The rubber bushings in the OEM arm play a vital part in the multilink suspension’s original elastokinematics that was designed to toe-in during all load cases. This is good for stability but at loads on cars equipped with high grip tyres and stiffer springs the movement causes understeer and toe change is not considered a good thing on competition cars.

Figure 2. Nissan diagonal A-arm suspension arrangement

While this design does present a good case for a road car design, compliance is never wanted for a pure track car as it can be sloppy and suspension tuning is not accurate. It is one of the main reasons why the cars feel more predictable and stable when the lower control arm bushings are replaced with polyurethane ones or spherical bearings. Static toe-in has to be increased for same amount of performance. Compliance toe-in allows for less aggressive toe settings for tyre longevity which is ideal for road cars.

To help combat compliance toe in and increase range of adjustability a new lower control arm was to be developed at GKtech.

2. NEW REAR LOWER CONTROL ARM V1

The first version GKtech lower control arm’s development was outsourced and featured many new solutions that have not been available before.

The chassis side bushings were replaced with spherical bearings with neoprene dust covers for longevity. They were housed inside forged steel tube ends. The middle part is tubular steel and gives a good increase in strength over stamped sheet metal. The anti-roll bar bracket is sheet metal with single mounting point.

A special feature was the ability to change track width with stackable spacer between the arm’s body and outer ball joint housing. This allowed track to be changed ±15mm. Narrower than OEM arm length may be required for tyre clearance from the bodywork, however for racing it is best to use longest available length for less weight transfer giving a more stable platform.

The wheel side ball joint housing was also forged as was the part that was welded to the tubular section. The spherical bearing was fully sealed from environment and longer shank was originally available for roll center correction.

The longer shank was later discontinued because it also quickened camber gain and increased bump-steer.

Figure 3. V1 GKtech rear lower control arm

The design also had a few problems, which is why it was discontinued and redesigned.

The main issue was its weight. The full assembly weighed 5,1 kg per side which is as much as the OEM rear upright. Unsprung weight must be kept to a safe minimum for best damper performance. The wheels will follow the contours of the road much better if they have less inertia to resist movement.

It was also expensive to produce because it had four forged steel pieces. The individual parts count was nearly 40 different items, where in comparison the OEM arm consists of 17 components in total.

As this product was not performing as hoped on the market it was time to redesign it with a smaller weight in mind while still retaining good wheel and tyre clearance.

Figure 4. GKtech V1 rear lower control arm assembly

3. LOWER CONTROL ARM V2

The V2 GKtech rear lower control had many design changes targeted. The whole product was to be lighter, retain the adjustability and clearance from wheel and tyre. This time it would be designed in house to have full control over the design. It also had to retain the feature of being adjustable parallel to the chassis which was one of its main features. Many aftermarket S chassis rear lower control arms move the wheel forwards or backwards when adjusting its length.

The new version only carried over a few parts from the V1 which were the standard length version ball joint shank, high misalignment spherical bearing and its hardware.

The first new version was based on the geometry of the original product. The chassis side spherical bearings were replaced with rod ends that would be used for adjusting the track width, however many other solutions were also considered:

1. Fixed design with different length options
2. Bolt on ball joint housing like many OEM front lower control arms with different mounting positions
3. Outboard rod end adjustment
4. Two link design with independently adjustable lengths

The design that won was the best all things considered. It was of the lightest and simplest construction. The strength was considered to be one of the best of what was possible. The best solution would be a fixed version, but as one size fits for all does not apply here it could not be done. Bolt on ball joint housing added too much complexity and weight.

3.1 V2 FIRST VERSION

The first new RLCA was designed as to utilize the same exact geometry as the V1.

The V2 GKtech rear lower control had many design changes targeted. The whole product was to be lighter, retain the adjustability and clearance from wheel and tyre. This time it would be designed in house to have full control over the design. It also had to retain the feature of being adjustable parallel to the chassis which was one of its main features. Many aftermarket S chassis rear lower control arms move the wheel forwards or backwards when adjusting its length.

The new version only carried over a few parts from the V1 which were the standard length version ball joint shank, high misalignment spherical bearing and its hardware.

The first new version was based on the geometry of the original product. The chassis side spherical bearings were replaced with rod ends that would be used for adjusting the track width, however many other solutions were also considered:

1. Fixed design with different length options
2. Bolt on ball joint housing like many OEM front lower control arms with different mounting positions
3. Outboard rod end adjustment
4. Two link design with independently adjustable lengths

The design that won was the best all things considered. It was of the lightest and simplest construction. The strength was considered to be one of the best of what was possible. The best solution would be a fixed version, but as one size fits for all does not apply here it could not be done. Bolt on ball joint housing added too much complexity and weight.

3.1 V2 FIRST VERSION

The first new RLCA was designed as to utilize the same exact geometry as the V1.

The first concept (Figure 5) was drawn up on the idea of keeping it as simple as possible.

Figure 6. 16” wheel clearance check

As design work progressed the front tube had to be bent to clear 16” wheels and larger wheels with high offset (high positive values). High misalignment spacers were also designed to make sure there is enough range for motion.

3.2 FIRST PROTOTYPE

Figure 7. GKtech rear lower arm first version to be prototyped

The first prototype version worked well with the first generation Nissan multilink rear suspension where the lower arm mounting is at a relatively high angle such as the S13, R32 and Z32. The next generation featured drastically lowered anti-squat which meant that the along with other pivot points, the front lower arm point was lowered a lot since it has the most effect on anti-squat.

This meant that the anti-roll bar mounting was quite high up relative to the suspension arm leaving clearance from the bracket on the arm itself.

The first one also featured fully enclosed bearings including the threads on the rod ends

Figure 8. First manufactured assemblies

3.3 FIRST TEST FITTING

The first test fitting was successful, but the ride height had to be relatively low for the anti-roll bar to clear the bracket on the arm as the anti-roll bar was not 3D modeled to check for clearance. There was also a problem that track width could not be adjusted without the anti-roll bar end link making contact with the bracket on the arm. This required the first redesign. The part was also track tested and there were no visible deformations or other damage.

Figure 9. First test fitting

Figure 10. Check rotor and dust shield clearance

3.4 SECOND PROTOTYPE

The second prototype had a redesign on the bracket that mounts the anti-roll bar end link. The end link position was lowered so the part could also be lighter and stronger as the bends were closer together. It also featured extra holes for mounting the end link in correct upright position when the arm length is adjusted. The holes can also be used for tuning motion ratio of the anti-roll bar. Mounting the end link closer to the wheel increases the motion ratio which effectively hardens the anti-roll bar rate because the wheel has less leverage to bend the bar.

The anti-roll bar bracket also had a bend added close to the front tube end to triangulate it for more strength.

Figure 11. Second prototype finite element analysis

The second prototype was fitted and fitment was greatly improved for the anti-roll bar end link. A feature that was removed is the bottom cap for the ball joint housing as it reduced clearance from the brake rotor.

The front rod end thread enclosing caps were removed during the second prototype stage as they interfered with wide high offset larger diameter wheels and tyres. The tube end was also cut shorter in length. This also removed the idea of adding double adjuster mechanism so the arm would not have be removed from the subframe as it would have reduced tyre clearance.

Figure 13. Removal of the end cap and some of the tube length for tyre clearance

3.5 V2 FINAL VERSION

The final version of the V2 GKtech rear lower control was further optimized for less weight and more strength and since them the design has not been updated as there has not been any negative feedback. The V2 lower control arm assembly weighs 2kg less than V1 whilst still retaining strength, good tyre clearance and plenty of adjustability. The adjustable motion ratio was originally added for end link clearance and fitment but is also a beneficial tool for tuning chassis balance.

Figure 14. Final optimization of the lower arm

Originally the S14 version of the arm featured wider rod end spacers than the s13 but latest production feature uses S13 spacers as standard and custom thickness and diameter washers added to be used on S14 as it is the only difference.

Figure 15. Final assembly

3.6 V2 PRODUCTION

All the components in the V2 arm’s body are from high strength CrMo steel that are CNC machined for accuracy and repeatability. They are hand TIG welded together on a custom jig that makes sure the rod end housings/ tube ends are parallel to each other as if they were not, the arm could not be installed into the subframe when the rod end offset has been adjusted.

The jig features the ability to turn the arm upside down without removing it from the jig to reduce risk of tube ends not being parallel after welding.

After welding the welded assembly is covered in black electrophoresis coating for resistance against corrosion. Then the bearings are added along with the rest of the hardware. The parts also quality checked and test assembled before shipping.

Figure 16. Manufacturing jig