Video at the end is sweeeeeet.
The suspension is very critical in any Time Attack car and the Sierra Sierra car is a mix of very sophisticated and straightforward technology. The most awesome part about the suspension is the Dynamic dampers. The Dynamic dampers have 4-way adjustment. Both the high and low speed compression and rebound damping are independently adjustable for incredible flexibility in platform control and bump absorption. The dampers use what is called through shaft construction. The shock shaft goes all the way through the damper body protruding from the bottom of the damper. This makes it so shaft fluid displacement isn’t an issue in the shock dynamic forces. In a conventional shock, there must be space inside the shock filled with compressible gas to make up for the displacement of the shock shaft as the suspension is stroked. Without that gas volume the shock would simply lock up because the fluid is incompressible. A large gas volume adds some hysteresis to the shocks damping and causes some lag between movement and application of damping force. With a through shaft the gas volume can be quite small and the damping remains constant through the stroke. There is no change in gas reaction force no matter where in the travel the shock is since the shaft is now not displacing and compressing the gas inside the damper body. The gas chamber in the damper only provides pressurization to prevent cavitation caused by high speed oil flow in the valves from causing variation in damping force and is very small in volume. The other advantage of a through shaft design is that there is a widely spaced lower bearing point on the bottom of the shock body to support the shaft with maximum overlap. This makes the damper have less bind and smoother action under side load and longer life.
The front knuckle/upright is a really cool piece, CNC machined from aluminum billet they relocate all of the suspension pick up points to correct roll center, camber curve and bump steer on the lowered car. Check out how the tie rod is held in double shear. Good engineering there. They also have larger wheel bearings and are considerably stiffer to eliminate any chance of brake caliper piston knockback. I hope Sierra Sierra produces these for sale. I want some for Project EVO!
This blade controls the stiffness of the rear sway bar. When it’s straight up and down, it is at maximum stiffness. When it’s turned sideways, it flexes and softens the rear bar. The blades rotation is controlled by a driver activated cable.
The front bar is coupled conventionally via adjustable heim jointed end links. No mushy rubber here.
This cable and linkage controls the rotation of the rear sway bar blade adjusters.
The cable leads to this blue lever where the Emp can control the rear bar adjustment to control chassis balance as fuel loads and track conditions change.
Eibach ERS springs are used all around with the spring rate being known only to the guys at Sierra Sierra. The sway bars are fabricated by Sierra Sierra, using tubular race type torsion bars with blade type adjusters for quick changes in roll stiffness to tune the chassis balance. The stock EVO suspension links are hard to beat as they are forged aluminum and Sierra Sierra modifies them to use spherical bearings instead of rubber bushings for geometry accuracy under forces generated by the sticky tires. The front spindles (uprights) are very interesting. They are machined out of solid aluminum billet and have revised pick up points to correct roll center, camber curve, scrub radius and bump steer. They also have larger wheel bearings to eliminate flex and brake piston knockback. The rear spindles are still factory and do have some bump steer issues at the current ride height, but team manager Richard Raeder plans on addressing the rear spindle next with something similar to the front.
The top mount of the strut is adjustable for camber, caster and steering axis. The lock plate assures that the adjustments will not slip.
The rear mount adapts an eye type mount to the cars unibody. Solid spherical bearings ensure that every bit of travel is controlled by the damper.
The cage on this car is very hard to photograph, it ties the entire car together from shock tower to shock tower to stiffen the chassis as well as to protect the driver. The Recaro seat has head protection and the center net helps keep the drivers upper body in place in a crash.
This rear cradle really stiffens up the back of the chassis.
The rollcage can also be considered an important part of the suspension as a stiff chassis is critical for sensitivity to suspension tuning and for proper action of the suspension. A stiff chassis allows bumps to activate the suspension where oscillations can be controlled by the dampers instead of upsetting the chassis which when flexed acts like an undamped giant spring. Sierra Sierra built the custom welded in and well braced cage from tig welded DOM tubing. The interior uses a Recaro racing seat and Willians harnesses. The car weighs 2770 lbs without the driver but fully loaded with fluids.
Nagisa links adjust toe and camber in the rear of the car. Every single rubber bushing in the suspension has been replaced with spherical bearings.
The Emp’s office features a PI electronic dash display, Momo steering wheel, a dry carbon dash and a race type steering column.
The center console has controls for mechanical and hydraulic brake force proportioning, map selection and other stuff for basic controls.
A halon fire suppression system keeps things safe.
A rear mounted dry cell battery saves weight and helps with weight distribution.
The EVO rides mainly on Advan RS 18x10.0 +25mm offset wheels when on Hankook Ventus Z-214 C91 275/30/18 DOT tires when at Redline Time Attack and Source Interlink Superlap events. For the World Time Attack Superlap event in Australia, the EVO has tested on Volk TE37 and CE28N lightweight forged wheels that are 18x10.5 with a 22mm offset with Hankook VentusTD C70 295/30-18 DOT tires. In 2009, Sierra Sierra used Yokohama 250/650R-18 slicks for Redline Time Attack events on the Advan RS wheels, but has since amazingly developed the car to be faster on DOTs than the slicks.
The tri-plane canards provide front downforce. The diverging angle helps prevent flow detachment and creates downforce even though the angle of attack is greater than the 10 degree point of flow separation. The canards also create side vortexes what help reduce air flow under the sides of the car which helps improve the action of the rear venturi. The blunt front end increases stagnation on top of the carbon splitter enhancing its downforce creation abilities.
The top view shows the splitter and the belly pan. All dry carbon niceness.
What do we have here? The nose section has venturis feeding the low pressure wheelwells. The venturis cause a low pressure depression increasing downforce. Notice the area besides the venturis is also contoured to increase downforce.
Functional ducts in the nose channel air to the front brakes.
Dry carbon duct directs air directly front the front end to the huge oil heat exchanger. Ducts more than double the effectiveness of heat exchangers.
Dry carbon ducts in the nose and chassis direct air through the big C&R radiator and the Cosworth intercooler.
The all important bodywork and aero package is a mix of Kaminari parts modified by Sierra Sierra to be functional to produce plenty of downforce. The front Kaminari nose has a blunt flat face to maximize stagnation of the air flow on top of the ample stiff carbon splitter. The splitter leads to a belly pan with diffusers leading to the low pressure wheel well to maximize front downforce. Dual front canards are integrated into the front splitter; these create some downforce and generate vortexes that keep air from flowing under the sides of the car at speed.
These carbon barge boards are used to improve the effectiveness of the Kaminari side skirts. They prevent the air flowing around the sides of the car from flowing underneath the car which could disrupt the flow of air through the rear diffuser. They are also a surface where a pressure differential can exist to improve downforce.
This small Gurney on the trunk deck lid does several things, it helps activate the flow through the rear diffuser as well as slow the air under the wing increasing its efficiency.
The carbon hood has functional vents where the radiator and intercooler hot air can be vented over the car improving downforce and reducing drag instead of backing up inside the engine compartment.
The car has a flat bottom for excellent undercar air management. The strakes help make sure that the diffuser is fed a laminar air flow even under different angles of attack experienced in cornering. They help keep the amount of downforce that the diffuser can create consistent as the car goes around turns.
The air flowing through the front mount intercooler and the radiator is guided through ducts to the top of the car’s body. This contributes to downforce and reduces drag. It also improves cooling. The car has a flat bottom leading to a rear carbon diffuser which features stakes to generate vortexes to enhance the diffusers effectiveness at different flow aspect angles and angles of chassis pitch. The diffuser has twin double deck elements on the side’s ala current DTM cars to increase the diffusers effectiveness by increasing diffuser area and allowing the use of a deeper included angle without flow detachment.
The Sierra Sierra EVO probably has the most sophisticated undercar aero of any current time attack car. The multi tiered diffuser is designed to get the most possible downforce out of every square inch of the back of the car. The center tier is shorter and shallower to avoid flow separation. It is shorter because it must clear the rear differential. The sides are deeper because they extend further forward. The outermost elements have a twin deck because they are shorter due to the rear wheelwells and the double deck allows a shorter diffuser section to work better. The strakes in the diffuser generate vortexes which causes the flow in the diffuser not to separate increasing its effectiveness.
Because the diffuser area near the rear wheels must be very short due to the wheel wells, a double deck is used. The shortness means that it is hard to maintain the 7-10 degree angle needed to reduce flow separation. The second deck allows another 7-10 degrees of the upper deck over the lower deck so the overall diffuser angle can be steeper with less separation and more effectiveness.
An APR wing on the rear deck connected to the chassis via custom fabricated stays adds an adjustable element to the rear downforce. Kaminari side skirts with Sierra Sierra fabricated carbon barge boards help keep the airflow from curling under the car to disrupted flow to the rear diffuser. The hood, fenders, doors and rear deck are all carbon fiber and save about 100 lbs. APR carbon aero mirrors round out the car’s body package.
The APR rear wing is used as a tunable element to the cars downforce.
Because the wing creates several hundred lbs of downforce and the consequences of a wing failure can be disastrous, sturdy wing mounts are a must.
A race car like the Sierra Sierra EVO is an always evolving piece. This is a snapshot of how the car sits right now, a week after the Redline Time Attack in April 2010. The team is always testing and developing and is in the process of making more changes before the car gets loaded on a plane for the upcoming World Time Attack in Australia where Sierra Sierra will proudly represent the USA and battle the fastest Time Attack cars from Australia and Japan. Stay tuned and we will keep you posted on future developments and results of the Sierra Sierra time machine.
