gixxerdrew
DSM Wiseman
- 664
- 21
- Oct 5, 2007
-
Yokohama, Japan,
Asia
As many of you may have noticed, our cars have a split front lower control arm rather than a traditional "double wishbone". Some of you may have wondered why.
There are two big advantages of this system
1) Packaging, there was simply not enough space to fit the engine and a full double wishbone suspension in our cars without widening the whole thing. There is no question this is a superior layout to another compact option, McPherson strut as seen on the EVO. The lack of control over camber and roll center is a very significant flaw in that system.
2) Steering geometry. With an AWD/FWD platform one key issue is the kingpin inclination or KPI.
The KPI represents the line in space at which the suspension rotates when steering. It is called a Kingpin because old suspension designs actually were a kingpin. You can normally determine its location by drawing a line between the upper and lower ball joints (where the control arms meet the spindle) through to where it intersects the tire contact patch. In a RWD car you design this to be optimal for a host of reasons that we have to throw away when the front wheels are drive wheels (AWD or FWD). The reason for this is that drive forces will make the wheel want to rotate around the kingpin and the control for those forces is the steering rack. The location of the kingpin and where it crosses the tire contact patch determines the moment (leverage) that the acceleration forces have against the steering rack and thus into the drivers hands. In extreme scenarios, this means torque steer can rip the steering wheel out of your hands.
The result of all this is a design requirement that the kingpin cross the contact patch outboard of the tire center line. This is so that the steering rack has leverage over the tire. In the real world this is tough to achieve since with wide tires and big brakes, the lower ball joint would be outboard of the rotor and the control arms would go right through it. So the solution is to split the lower control arm into two parts. This creates a virtual ball joint further outboard. You can determine this virtual ball joint by drawing two lines. From the inboard lower control arms pickups to their outboard pickups and continue until they intersect. Where those lines cross is the virtual lower ball joint location. The steering will act like the pivot point is actually much further out than it is in reality. This is a simplified explanation.
Neat little system, it is used on several platforms including Audi, Ford and I can think of a few other FWD/AWD cars.
As we began to widen the front track, the steering axis remained stationary but the contact patch center moved outboard. By the first race of the year we realized how serious the problem was when the steering rack exploded and the steering wheel physically bent and needed two guys to turn when pushing in the pits. The exploding racks continued, one after another. It was realized a rack wouldn't even last a single race.
Obviously, this was not going to work out. We couldn't change the steering geometry without changing suspension pickup points.. That was illegal in the rules. The highest forces needed are when the kingpin is roughly at the contact patch center which is exactly where we were, and while this completely eliminated torque steer it overloaded the steering system. We needed a way to control pressure at the rack and a way to have power steering around the pits even if the car had to be off or had no running engine. The solution was electro-hydraulic power steering. I was already not too fond of the DSM power steering system, the brackets are way to flexible and it is always in the way for other maintenance work. After some research, I found the perfect power steering pump. It came from the Renault Clio, had its own BIG integrated reservoir, built in pressure regulator and used substantially less power than the MR2 pump. We installed the new pump forward of the left front wheelhouse and put the fill cap through the hole in the factory body so that it could be filled from the engine bay.
Using a temporary adjustable restrictor and gauge in the pressure side line I was able to determine the optimal restriction size and monitor pressure. The concept was to give good response time for the driver (keep pressure built in the system) and minimize the steering rack forces. You have to balance out driver feedback and fatigue. Once we got that sorted out the heavy adjuster was replaced by a fixed size orifice built into one of the lines.
The result was good and we have not had a problem since and the driver never noticed the system existed (perfect!)
There are two big advantages of this system
1) Packaging, there was simply not enough space to fit the engine and a full double wishbone suspension in our cars without widening the whole thing. There is no question this is a superior layout to another compact option, McPherson strut as seen on the EVO. The lack of control over camber and roll center is a very significant flaw in that system.
2) Steering geometry. With an AWD/FWD platform one key issue is the kingpin inclination or KPI.
The KPI represents the line in space at which the suspension rotates when steering. It is called a Kingpin because old suspension designs actually were a kingpin. You can normally determine its location by drawing a line between the upper and lower ball joints (where the control arms meet the spindle) through to where it intersects the tire contact patch. In a RWD car you design this to be optimal for a host of reasons that we have to throw away when the front wheels are drive wheels (AWD or FWD). The reason for this is that drive forces will make the wheel want to rotate around the kingpin and the control for those forces is the steering rack. The location of the kingpin and where it crosses the tire contact patch determines the moment (leverage) that the acceleration forces have against the steering rack and thus into the drivers hands. In extreme scenarios, this means torque steer can rip the steering wheel out of your hands.
The result of all this is a design requirement that the kingpin cross the contact patch outboard of the tire center line. This is so that the steering rack has leverage over the tire. In the real world this is tough to achieve since with wide tires and big brakes, the lower ball joint would be outboard of the rotor and the control arms would go right through it. So the solution is to split the lower control arm into two parts. This creates a virtual ball joint further outboard. You can determine this virtual ball joint by drawing two lines. From the inboard lower control arms pickups to their outboard pickups and continue until they intersect. Where those lines cross is the virtual lower ball joint location. The steering will act like the pivot point is actually much further out than it is in reality. This is a simplified explanation.
Neat little system, it is used on several platforms including Audi, Ford and I can think of a few other FWD/AWD cars.
As we began to widen the front track, the steering axis remained stationary but the contact patch center moved outboard. By the first race of the year we realized how serious the problem was when the steering rack exploded and the steering wheel physically bent and needed two guys to turn when pushing in the pits. The exploding racks continued, one after another. It was realized a rack wouldn't even last a single race.
Obviously, this was not going to work out. We couldn't change the steering geometry without changing suspension pickup points.. That was illegal in the rules. The highest forces needed are when the kingpin is roughly at the contact patch center which is exactly where we were, and while this completely eliminated torque steer it overloaded the steering system. We needed a way to control pressure at the rack and a way to have power steering around the pits even if the car had to be off or had no running engine. The solution was electro-hydraulic power steering. I was already not too fond of the DSM power steering system, the brackets are way to flexible and it is always in the way for other maintenance work. After some research, I found the perfect power steering pump. It came from the Renault Clio, had its own BIG integrated reservoir, built in pressure regulator and used substantially less power than the MR2 pump. We installed the new pump forward of the left front wheelhouse and put the fill cap through the hole in the factory body so that it could be filled from the engine bay.
Using a temporary adjustable restrictor and gauge in the pressure side line I was able to determine the optimal restriction size and monitor pressure. The concept was to give good response time for the driver (keep pressure built in the system) and minimize the steering rack forces. You have to balance out driver feedback and fatigue. Once we got that sorted out the heavy adjuster was replaced by a fixed size orifice built into one of the lines.
The result was good and we have not had a problem since and the driver never noticed the system existed (perfect!)
