Bobby’s Build: Part 2

When we left Bobby’s build he had created a custom front suspension system, stripped the car back to its bare bones and lowered the car on some new BC Racing coilovers. Now, a few weeks further down the line, Bobby has been busy with the rear suspension system fully redesigned as well as having a few other key areas altered with the car now looking like it is nearly ready for some track testing!

Rear Arms and Geometry

The first area that immediately caught our eye was the rear suspension arms and the incredibly well made rear subframe, which we will touch on shortly. The rear camber arms currently on the car are the second variation of design. The first set that Bobby fabricated offered some adjustment for camber and had the anti-roll bar drop link mounted to the arm via a spherical bearing in a housing. This allowed for small adjustments to be made to the anti-roll bar stiffness by adjusting the installation ratio of the anti-roll bar. However, the adjustment available with the standard anti-roll bar and the drop link mounting position was not sufficient enough to take great effect over the handling.

Therefore, Bobby went back to the drawing board and decided to not only re-design the camber arm, but to re-design the rear subframe to allow the drop link to be moved to a location where it could offer much more adjustment and offer freedom of choice for rear anti-roll bars in the future. Manufactured from CDS tube with laser cut brackets, the camber arm was altered to have a lot of length adjustment and the drop link mount removed. This means that the length of the arm can be altered without moving the drop link mounting point inboard/outboard altering the drop link angle. The drop link mount was relocated to the damper body to a position with the same installation ratio as before. The lower arm now meant that camber and track width, when adjusted alongside the upper camber arm, could both be adjusted with no negative side effects on anti-roll bar performance.

The main arm used for rear camber adjustments is the adjustable rear upper camber arm that has once again been fabricated to a very high standard by Bobby. This arm is mounted to the chassis via brackets that have optional mounting holes. This allows the rear roll centre to be altered and fine tuned by changing the angle from a front view of the rear upper camber arm, adjusting the roll stiffness of the rear suspension. It is also beneficial to adjust camber with the upper camber arm first as it has less effect on other areas such as toe when it is adjusted compared to the rear lower camber arm.

Bobby also re-designed the rear toe arm, giving it length adjustment and inserting spacers between the rose joint and the hub. The adjustment on the toe arm means that bump steer can be fine-tuned at the rear to get the car reacting in corners exactly how Bobby would like for his driving style.

Rear Subframe

One of the most important areas to re-create on Bobby’s list was the rear subframe. This is due to feedback from test days where the car showed a strong imbalance in roll front to rear largely due to the standard locations of the roll centres, worsened further by lowering the car.

“This is one major thing I needed to do as the front to rear roll centre is way out. The front is a lot higher than the rear and I can feel the imbalance, the front reacts so fast, the rear rolls too much.” 

The subframe was pulled off the car, mocked up and a jig created to ensure that all mounting points were square and true. Using some tube and a large amount of expert fabricating skills, the rear subframe was re-manufactured from tube with optional mounting points for the lower camber arm. These options meant that the rear roll centre could be raised to a position where the rear end could feel tighter and take the wallow out of the car. But this isn’t the final article as Bobby will re-fabricate this subframe from much lighter T45 tube once he has tested the adjustments and is happy with the performance of the rear end.

Just before the subframe was put back into the car, solid bushes were put into the subframe sporting vastly increased stiffness over standard as they allow no flex. This allows much more of the traction and lateral forces from the tyres to be transferred more efficiently and quickly into the chassis out on circuit making for a much more responsive car. With the new sleeker subframe installed, it then meant that Bobby has a much wider range of selection for a rear anti roll bar. A roll bar has now been selected with adjustable canti-levers and different main bar options, giving a vast range of adjustment so Bobby can really fine tune the set up out on circuit to gain the competitive edge over his future Time Attack competitors.

Wheels , Tyres and Brakes

The car began life with 17” wheels and Bobby hoped to keep this size for when racing out on circuit due to the reduced diameter from a larger option helping to keep unsprung mass down and keep the rotational inertia low, aiding acceleration and braking performance. However, like most well laid plans this had to change due to the decision to upgrade the brakes on the car to take stopping power to the next level.

The brakes began as Evo 9 Brembo 4 pots 320mm fronts with Evo 9 2 pot 300mm rears. Any petrol head would confirm that these are very good brakes and show great performance when out on circuit. However, Bobby wanted to take the next step and make sure that brake fade would be an item he could totally remove from his list of concerns. Therefore, a rebuild kit was ordered and the calipers were stripped down to their individual components. The standard aluminium pistons were removed and in their place went racing stainless steel pistons. These help to relieve the heat from the fluid due to the material resisting heat far better and creating a strong thermal barrier between the rotor and the fluid. The curved surface also helps to keep the piston cool offering a larger surface area to dissipate heat to the atmosphere faster.

For racing situations the pads are being swapped out for Carbotech XP10F and XP8R racing compounds which require much higher levels of heat to pass through them to get them up to operating temperature. This is borderline dangerous on the street but absolutely necessary when out on circuit to prevent fade as much as possible.

This set up works well with the 17” wheels but, already foreseeing the future demands of racing, Bobby decided that the braking performance could be better still and that the most cost effective way to improve that performance was to opt for larger rotors at the front wheels, so he will be taking a hefty jump up to 360mm rotors. However, this means that the wheel will no longer fit over the brakes, scuppering original thoughts to keep the 17” rim. Therefore, the increase to 18” wheels was decided upon and some Konig Hypergrams in 18×10.5” were installed fitting the car perfectly and adding some style points too.

The tyre choice for this car is twofold because not only is this car being built as a serious contender in the Time Attack UK Championship, it is also a road car that can be driven to and from the circuit. Therefore a street tyre and racing tyre were chosen so that the car could get to the circuit legally. The street option was chosen to be Vredestein Ultrac Vorti 275/35 as this is the furthest Bobby could go without breaking the bank for street tyres. The racing tyre wasn’t so much selected by Bobby due to it being the control tyre of the Time Attack series; the tried and tested, very competitive Pirelli Trofeo R in 295/30. This combination of wheels, brakes and tyres is certainly no longer an area of worry for Bobby.

What Is Next? 

In the words of Bobby….”What’s not next??!” The immediate items next on the list are:

  • Pedal box installation
  • Full new braided brake lines
  • New rear brakes with handbrake delete
  • New fuel tank
  • New alloy coolers and radiator
  • Aerodynamics

Before it is time for any of that though, Bobby is going to be taking his car on a track day to put all of his suspension arms, subframe and anti-roll bars to the test to see how the current set up performs and make some adjustments to see if the system offers enough variation in set up to push the car to its limits!

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