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I got myself a nice package today from the US of A 
The long awaited return of my ABY rack, back from a specialist over there, freshly rebuilt and looking like new.
"but Alex" I hear you cry, "you must be crazy to send your rack all the way to the US for a rebuild when there are loads of firms over here who offer that service, are you mental?"
Well yes I am perhaps, but this is no ordinary rebuild, by no ordinary steering specialist.
I spent countless hours researching how power racks work and what could be altered to improve the rack on our cars - to reduce the assistance and offer more feedback and road feel. It became something of an obsession for me. Sad I know.
The rotary valve in the rack is what we are interested in here.
In brief the rotary valve consists of 3 parts.
1) The housing with fluid ports from the pump, to either side of the rack and back to the reservoir
2) The rotating spool inside the housing which distributes fluid to the ports in the housing
3) The core shaft
The spool is locked to the output shaft, and the metering core is the input shaft. These input and output shafts are connected internally by a small torsion bar. With the output end restrained, the input end can be twisted a few degrees and the torsion
bar will cause it to spring back to a centered position when released.
Here is a useful image shamelessly stolen from Woodward Power Steering:
The output shaft is restricted by the steering resistance of the wheels. the input shaft is twisted by the steering input. When deflected relative to the housing the spool determines the direction and rate of flow of the pressurised fluid. The stiffness of the torsion bar linking the input and output shafts determines the response curve of the rack - how easily it builds pressure in response to steering input. A weaker bar allows easier valve opening which generates a stronger response - a stronger bar resists valve opening generating a weaker response.
"So what does all this waffle mean?" you ask.
It means change the torsion bar in the rack to a stiffer bar and reduce the response to steering input AND vice versa, input from the other side of the system, force from the the road wheels is translated into steering feel in your hands by virtue of a reduced fluid damping in the rotary valve.
Sounds too good to be true right? Well it almost is.
I am pretty sure I found the only company in the world who have the know-how to retro-fit a stiffer torsion bar into a rack - because of the sensitivity of the spool in relation the housing, if the torsion bar is pinned to the spool or input shaft even 0.001" out of alignment you will have an imbalance in the steering assist. You need a custom made hydraulic alignment jig to do this, and then a computerised test-bed to check the rack after to make sure it offers even response.
So have I actually had done?
Rack stripped, inspected measured and tested.
The original ZF torsion bar measured at 5.38mm. A new bar was machined at 6.15mm
The ports in the rotary valve were modified slightly.
A new seal kit was added
The housing was bead blasted
They even re-blacked the steel parts!
So, will be fitting this soon. I don't think this has been done before, so stay tuned for an update
The long awaited return of my ABY rack, back from a specialist over there, freshly rebuilt and looking like new.
"but Alex" I hear you cry, "you must be crazy to send your rack all the way to the US for a rebuild when there are loads of firms over here who offer that service, are you mental?"
Well yes I am perhaps, but this is no ordinary rebuild, by no ordinary steering specialist.
I spent countless hours researching how power racks work and what could be altered to improve the rack on our cars - to reduce the assistance and offer more feedback and road feel. It became something of an obsession for me. Sad I know.
The rotary valve in the rack is what we are interested in here.
In brief the rotary valve consists of 3 parts.
1) The housing with fluid ports from the pump, to either side of the rack and back to the reservoir
2) The rotating spool inside the housing which distributes fluid to the ports in the housing
3) The core shaft
The spool is locked to the output shaft, and the metering core is the input shaft. These input and output shafts are connected internally by a small torsion bar. With the output end restrained, the input end can be twisted a few degrees and the torsion
bar will cause it to spring back to a centered position when released.
Here is a useful image shamelessly stolen from Woodward Power Steering:
The output shaft is restricted by the steering resistance of the wheels. the input shaft is twisted by the steering input. When deflected relative to the housing the spool determines the direction and rate of flow of the pressurised fluid. The stiffness of the torsion bar linking the input and output shafts determines the response curve of the rack - how easily it builds pressure in response to steering input. A weaker bar allows easier valve opening which generates a stronger response - a stronger bar resists valve opening generating a weaker response.
"So what does all this waffle mean?" you ask.
It means change the torsion bar in the rack to a stiffer bar and reduce the response to steering input AND vice versa, input from the other side of the system, force from the the road wheels is translated into steering feel in your hands by virtue of a reduced fluid damping in the rotary valve.
Sounds too good to be true right? Well it almost is.
I am pretty sure I found the only company in the world who have the know-how to retro-fit a stiffer torsion bar into a rack - because of the sensitivity of the spool in relation the housing, if the torsion bar is pinned to the spool or input shaft even 0.001" out of alignment you will have an imbalance in the steering assist. You need a custom made hydraulic alignment jig to do this, and then a computerised test-bed to check the rack after to make sure it offers even response.
So have I actually had done?
Rack stripped, inspected measured and tested.
The original ZF torsion bar measured at 5.38mm. A new bar was machined at 6.15mm
The ports in the rotary valve were modified slightly.
A new seal kit was added
The housing was bead blasted
They even re-blacked the steel parts!
So, will be fitting this soon. I don't think this has been done before, so stay tuned for an update
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