VARIABLE RATIO (VR)
Bishop’s Variable Ratio (BishopVR) steering racks provide remarkable improvements in:

Primary Safety
more relaxing to drive at high speeds ensuring less fatigue and greater safety;
improved control during radical avoidance maneuvers and skid recover;
increased driver confidence in lateral placement of vehicle.

Driver Comfort
reduced need to change position of hands on steering wheel;
reduced turns lock-to-lock ensures easier parking.

BishopVR has a the largest ratio curve flexibility, which can achieve fast ratio changes and multiple ratio changes.

  







PRODUCTS
Y-Rack
Bishop’s Y-Rack design reduces rack roll, thus improving steering stiffness (precision). The inherently wider teeth of a Y-Rack can be up to 110% of the shaft diameter, providing maximum ratio flexibility, even tooth load distribution and superior rack tooth strength. The stronger cross section in the teeth area often enables reduction in shaft size diameter with associated cost and weight reductions.

 

  

 
D-Rack
The recent introduction of Bishop’s D-Rack design makes BishopVR available to all applications and offers a higher level of tooth accuracy, thereby maximizing the VR potential for a given size and configuration. This rack type is ideal for existing applications where VR is required, as the features of the existing rack design can be easily reproduced or improved. Due to the use of the warm forging process, there is still freedom in shape and cross section design.

 

  

 

ActivRakTM
An ActivRakTM VR rack has a unique form of BishopVR, where the rack gain characteristic is tuned to achieve a particular vehicle yaw gain target. This yaw gain target is consistent with what might be achieved using an angle overlay active steering system but at negligible cost and without any additional components or systems. Here typically there will be a significant increase in rack gain, greater than 25%. This increase is achieved in less than 170° pinion rotation from the on-centre position and often in 90° or less.

M-Curve VR
BishopVR offers the possibility for multiple ratio changes - an application with 2 or more ratio changes is referred to as an 'M-Curve' VR. Some existing power assistance systems cannot support the fast ratios required for improved vehicle dynamics at full lock. The use of a second (slower) ratio at lock directly reduces the assistance torque required at lock in column driven assisted steering systems (CEPAS). This same approach reduces the rack speed at lock in rack driven assisted steering systems (REPAS or HPAS), indirectly achieving a similar benefit. The result is increased steering performance without increasing the power assistance requirement.

 

  

 


 

 

PROCESS
Precision Warm Forging Cell
This innovative technology, the Bishop Precision Forging Die and Warm Forming Cell produces variable and constant ratio steering racks at a cycle time of 12 seconds. The racks produced in this process are net shape, free of flash and hence, require no finish machining of the teeth or the rack faces that contact the rack pad. As-forged tooth mesh quality achieved is DIN Class 9 or better and at least as good as all competing process technologies. The process technology has now been developed to the point that virtually all commonly used rack steel grades can be used.

 

  

Y- and D-Forging Dies
The Bishop Y-Forging and D-Forging Dies are the heart of the Warm Forming Cell and precision warm forms a constant or variable ratio Y-form or D-form steering gear rack from a cylindrical bar.


The Bishop Forging Die produces a stress relieved near net shape product of consistent quality with precision gear teeth. However, it avoids the distortion produced by the high residual stresses associated with cold forging and the lower hardness caused by decarburization in hot forging. It is operated with an 800-1,250 tonne screw press and a 150-250kW 3-10Hz induction heater - depending on the rack size.


In variable ratio steering racks the tooth geometry changes from “On Centre” to “Lock” and for this reason the tooth form cannot be produced by conventional means (ie broaching or creep feed grinding). The Bishop Forging Die produces precision warm formed steering gear racks with true involute variable ratio teeth in a one step process at low cost.

Downloads:

ActivRak Flyer
Steering with Feeling Flyer
Racks and Pinions Flyer