ride height survey
08-29-2012, 08:15 PM
#46
One thing to note as far as bump steer is that Porsche lowered the RS in the front solely by moving the axle center line up 30 mm relative to the lower ball joint on the upright, not by changing the spring's adjustment.
When a non-RS car with stock uprights is lowered to RS height at the front simply by dropping the car on its spring perches the car loses ~ 30 mm of suspension travel and changes the geometry of both bump steer and roll center in ways the RS does not normally deal with.
So, it seems best to lower the car with RS uprights at the front if one wants RS ride height and the best handling (and ride).
When a non-RS car with stock uprights is lowered to RS height at the front simply by dropping the car on its spring perches the car loses ~ 30 mm of suspension travel and changes the geometry of both bump steer and roll center in ways the RS does not normally deal with.
So, it seems best to lower the car with RS uprights at the front if one wants RS ride height and the best handling (and ride).
The axle is in the same location relative the lower shock mounts for both base & RS uprights. The lower ball joint is 30mm lower which increases the height of the RC & all the fun stuff that leads to.
In the end, upward wheel travel is lost by lowering the car regardless of upright. It has to be, in order to avoid the top of the tire contacting the fender.
08-29-2012, 08:17 PM
#47
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Sort of.. using one of Bill's photos for reference:
The axle is in the same location relative the lower shock mounts for both base & RS uprights. The lower ball joint is 30mm lower which increases the height of the RC & all the fun stuff that leads to.
In the end, upward wheel travel is lost by lowering the car regardless of upright. It has to be, in order to avoid the top of the tire contacting the fender.
The axle is in the same location relative the lower shock mounts for both base & RS uprights. The lower ball joint is 30mm lower which increases the height of the RC & all the fun stuff that leads to.
In the end, upward wheel travel is lost by lowering the car regardless of upright. It has to be, in order to avoid the top of the tire contacting the fender.
08-29-2012, 08:40 PM
#48
Rennlist Member
As I recall, measuring from upper strut mounting bolt center to top of strut housing:
Stock Monroe = 347 mm
Stock RS Bilstein = 320 mm
Cup Bilstein = 300 mm
08-30-2012, 12:46 PM
#49
Rennlist Member
I think the best way to look at it is that for the RS Porsche lowered the inner pivot point (at chassis) and outer pivot point (at ball joint) of the front wishbone 30 mm, thus maintaining the angle of the wishbone at ride height.
The RS steering rack thus also maintains relationship with the lower ball joint maintaining design bump steer geometry.
Because the overall height of wheel carrier increased, the RS strut was shortened a similar amount to maintain bump travel at the expense of overall wheel travel
In the diagram below, imagine lowering the CG by either shortening the spring (or lowering spring base on the strut as in usual lowering) or by moving the wheel center up relative to the strut as done in the RS.
The "lever problem line" is the roll couple, the longer it is the more the tendency for the car to roll (note if the CG and the Roll center were at the same height the roll couple = 0 and there is no roll). This affects the instantaneous weight transfer as the car takes a set in a corner as well as overall roll force.
The overall RS effect is decreasing the front roll couple as shown below in the "lever problem line" because the virtual reaction point remains the same the roll center and CG are moving closer to each other. The wishbone angle also stays in a more favorable geometry to develop negative camber at the loaded wheel as the car corners.
If the spring is shortened (as in usual lowering) the virtual reaction point moves as the wishbone angle changes faster than the CG drops and the roll couple increases. The wishbone flattening can actually develop positive camber as the car corners if ball joint = inner pivot height at ride height.
Also when the spring is shortened, the static relationship of the lower ball joint and the steering rack changes so bump steer changes from design as well.
The RS steering rack thus also maintains relationship with the lower ball joint maintaining design bump steer geometry.
Because the overall height of wheel carrier increased, the RS strut was shortened a similar amount to maintain bump travel at the expense of overall wheel travel
In the diagram below, imagine lowering the CG by either shortening the spring (or lowering spring base on the strut as in usual lowering) or by moving the wheel center up relative to the strut as done in the RS.
The "lever problem line" is the roll couple, the longer it is the more the tendency for the car to roll (note if the CG and the Roll center were at the same height the roll couple = 0 and there is no roll). This affects the instantaneous weight transfer as the car takes a set in a corner as well as overall roll force.
The overall RS effect is decreasing the front roll couple as shown below in the "lever problem line" because the virtual reaction point remains the same the roll center and CG are moving closer to each other. The wishbone angle also stays in a more favorable geometry to develop negative camber at the loaded wheel as the car corners.
If the spring is shortened (as in usual lowering) the virtual reaction point moves as the wishbone angle changes faster than the CG drops and the roll couple increases. The wishbone flattening can actually develop positive camber as the car corners if ball joint = inner pivot height at ride height.
Also when the spring is shortened, the static relationship of the lower ball joint and the steering rack changes so bump steer changes from design as well.
08-30-2012, 01:17 PM
#50
Yep, the total bump travel is still reduced though - ignore the damper & just picture the wheel vs. strut mount.
'Bump steer' is due to the difference in the arc the lower ball joint travel vs. the tie rod end. The links are diff't lengths so the relative difference in motion is exaggerated as they move away from horizontal. The diff't tie rod mounting orientation makes it difficult to compare but the RS upright moves the tie rod end down along w/ the lower ball joint.
The diagram also shows why its better to set camber using the lower strut mount vs. tilting the top of the strut inboard - the instant center (labeled virtual reaction point) moves down & takes the RC along with it when the top of the strut is moved toward the center of the car.
'Bump steer' is due to the difference in the arc the lower ball joint travel vs. the tie rod end. The links are diff't lengths so the relative difference in motion is exaggerated as they move away from horizontal. The diff't tie rod mounting orientation makes it difficult to compare but the RS upright moves the tie rod end down along w/ the lower ball joint.
The diagram also shows why its better to set camber using the lower strut mount vs. tilting the top of the strut inboard - the instant center (labeled virtual reaction point) moves down & takes the RC along with it when the top of the strut is moved toward the center of the car.
08-30-2012, 01:41 PM
#51
Rennlist Member
Yep, the total bump travel is still reduced though - ignore the damper & just picture the wheel vs. strut mount.
'Bump steer' is due to the difference in the arc the lower ball joint travel vs. the tie rod end. The links are diff't lengths so the relative difference in motion is exaggerated as they move away from horizontal. The diff't tie rod mounting orientation makes it difficult to compare but the RS upright moves the tie rod end down along w/ the lower ball joint.
The diagram also shows why its better to set camber using the lower strut mount vs. tilting the top of the strut inboard - the instant center (labelled virtual reaction point) moves down & takes the RC along with it when the top of the strut is moved toward the center of the car.
'Bump steer' is due to the difference in the arc the lower ball joint travel vs. the tie rod end. The links are diff't lengths so the relative difference in motion is exaggerated as they move away from horizontal. The diff't tie rod mounting orientation makes it difficult to compare but the RS upright moves the tie rod end down along w/ the lower ball joint.
The diagram also shows why its better to set camber using the lower strut mount vs. tilting the top of the strut inboard - the instant center (labelled virtual reaction point) moves down & takes the RC along with it when the top of the strut is moved toward the center of the car.
Of note is the 964/993 RSR uses a pair of locking plates at the standard adjustment point to set the ballpark "basic setting" then the final trim is done at the top monoball slots. This precludes slippage at the stock adjustment point.
I think if you define total bump travel as between tire top and the strut mount or body work, it is reduced there by lowering and nothing short of body work or moving the mount will regain it.
If you define it as the distance between top of damper body and bump stop, that can be regained by shortening the damper body but at the expense of total wheel travel excursion since the strut rod must be shortened as well.
