Norm Peterson
corner barstool sitter
I hope you aren't trying to convince people that cars actually roll about a "roll axis" defined by the geo roll centers . . .
Norm
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Norm
Stuntman
Well-Known Member
You seem to be viewing the IC as a plane rather than a single point which migrates with pitch and roll (which is why the RC migrates):
How does a single point intersect the plane of the chassis centerline?
I agree that RCs are typically located many feet to the opposing side of the car, but the LCA is almost never held parallel to the ground dynamically or statically. And how is this a problem?
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thePill
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To be honest Norm, I'm having Bumpsteer issues still. I would really appreciate your help, here, like right now. I would need you here as an extra pair of eyes. I can't discuss it here, super secret Team stuff. It would have to be looked at in person anyway.
We have some other things to check after lunch...
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thePill
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Stuntman,
You seem to be assuming a lot here.
Not rocket science...
Listen to me;
The Instant Center is established with 2 distinct lines but include 2 additional lines as well (MacP is different, using SLA for this). The first two lines are the Upper and Lower control arm lines. They extend from the arms ball joints, through the mounting points and through the Chassis centerline. Where they meet is the Instant Center, beyond the Chassis Centerline. Important to note, the lower link MUST be parallel to the Datum, it's imaginary line is also parallel to the datum.
Next, the tie rod line; needs to be parallel to the lower control arm AND, the mounting point needs to line up with the Upper and Lower Arm mounting points on the Control Arm Attachment Line (Must). The tie rod line extends through the CAAL and the Chassis Centerline to meet with the other two lines at the IC.
Swing Arm Length: This starts at our Outside Scrub Radius, intersects the Steering Axis Inclination, Control Arm Attachment Line and Chassis Centerline before it reaches your IC. The outside scrub radius and Tire Centerline are one in the same however, I say Outside Scrub because that is part of our equation anyway. The Roll Center can be found at the Chassis Centerline where the Swing Arm intersect...
MacP systems are a little different but, the method is the same.
There is very little you posted that sticks to this basic geometry. If the books you read are bringing you to any other conclusion, stop reading it.
You seem to be assuming a lot here.
Not rocket science...
Listen to me;
The Instant Center is established with 2 distinct lines but include 2 additional lines as well (MacP is different, using SLA for this). The first two lines are the Upper and Lower control arm lines. They extend from the arms ball joints, through the mounting points and through the Chassis centerline. Where they meet is the Instant Center, beyond the Chassis Centerline. Important to note, the lower link MUST be parallel to the Datum, it's imaginary line is also parallel to the datum.
Next, the tie rod line; needs to be parallel to the lower control arm AND, the mounting point needs to line up with the Upper and Lower Arm mounting points on the Control Arm Attachment Line (Must). The tie rod line extends through the CAAL and the Chassis Centerline to meet with the other two lines at the IC.
Swing Arm Length: This starts at our Outside Scrub Radius, intersects the Steering Axis Inclination, Control Arm Attachment Line and Chassis Centerline before it reaches your IC. The outside scrub radius and Tire Centerline are one in the same however, I say Outside Scrub because that is part of our equation anyway. The Roll Center can be found at the Chassis Centerline where the Swing Arm intersect...
MacP systems are a little different but, the method is the same.
There is very little you posted that sticks to this basic geometry. If the books you read are bringing you to any other conclusion, stop reading it.
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thePill
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Norm,
We got it, the rack rod wasn't aligned properly. Easy fix, less fabrication...
We got it, the rack rod wasn't aligned properly. Easy fix, less fabrication...
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Stuntman
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Edited with pictures.
I'm not sure what you are having such an issue with. This is all very elementary and i'm surprised if your classes aren't teaching you this stuff. If they just teach you how to make things and expect you to leave engineering to engineers, you should pick up some vehicle dynamics and suspension books for your own self benefit and not try to learn on forums. Although there are some decent engineering forums out there.
As in Satchell's quote, you need to draw a line from the center of the contact patch (in green below) to the IC. Where these two (green) lines intersect is the RC:
While it's easy to assume a car is symmetrical and find the roll center by where that green line intersects the car's centerline, that process does not account for an asymmetrical suspension geometry or to find the RC location in roll:
For simplicity's sake, let's just take the above picture as a viewpoint from behind the car as the car is turning right.
As the load is transferred to the left, the IC for the left side suspension (on the right side of the car) raises while the IC for the right side suspension (on the left) lowers. Drawing the same green lines from the center of the contact patches to the IC shows the new RC location which has migrated laterally to the left (and probably changed in height as well).
Just measuring half of the car the way thePill proposes is only good for a static RC location on a symmetrical car and it does not account for the RC location in roll or an asymmetrical suspension geometry. Which is why its a better practice and more accurate to find the intersection between both (green) lines drawn from the center of the contact patch to each IC.
On an off topic note, the amount of engineering that the fiero community puts into their cars blows my mind. Here's one guy's calculations:
As you can see, you could easily just find the IC for one side of the car since the car is symmetrical and the RC is at the car's centerline. However in roll:
The RC migrates to the left from the IC migrations. The above is a perfect example how an IC can indeed reach the car's centerline or even cross the centerline to be on the same side of the car as the suspension.
0.02
Agreed. I have already said this in post 743. I also added the IC (and thus RC) migrates with pitch and roll.
I'm not sure what you are having such an issue with. This is all very elementary and i'm surprised if your classes aren't teaching you this stuff. If they just teach you how to make things and expect you to leave engineering to engineers, you should pick up some vehicle dynamics and suspension books for your own self benefit and not try to learn on forums. Although there are some decent engineering forums out there.
As in Satchell's quote, you need to draw a line from the center of the contact patch (in green below) to the IC. Where these two (green) lines intersect is the RC:
While it's easy to assume a car is symmetrical and find the roll center by where that green line intersects the car's centerline, that process does not account for an asymmetrical suspension geometry or to find the RC location in roll:
For simplicity's sake, let's just take the above picture as a viewpoint from behind the car as the car is turning right.
As the load is transferred to the left, the IC for the left side suspension (on the right side of the car) raises while the IC for the right side suspension (on the left) lowers. Drawing the same green lines from the center of the contact patches to the IC shows the new RC location which has migrated laterally to the left (and probably changed in height as well).
Just measuring half of the car the way thePill proposes is only good for a static RC location on a symmetrical car and it does not account for the RC location in roll or an asymmetrical suspension geometry. Which is why its a better practice and more accurate to find the intersection between both (green) lines drawn from the center of the contact patch to each IC.
What exactly are you calling the datum?
What are you defining as the "Swing arm length"? I don't think you are talking about the FVSA length (which most texts say is determined by the length from the wheel centerline to the IC):
On an off topic note, the amount of engineering that the fiero community puts into their cars blows my mind. Here's one guy's calculations:
As you can see, you could easily just find the IC for one side of the car since the car is symmetrical and the RC is at the car's centerline. However in roll:
The RC migrates to the left from the IC migrations. The above is a perfect example how an IC can indeed reach the car's centerline or even cross the centerline to be on the same side of the car as the suspension.
0.02
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thePill
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That looks correct.
Edit: The rack rod was bent somehow and looked straightened!!! Never thought I'd see that on a steering component. That could have been bad...
Edit: The rack rod was bent somehow and looked straightened!!! Never thought I'd see that on a steering component. That could have been bad...
w3rkn
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Glad you all have that hashed out now... & btw, since gravity is the applied force and nothing loads/sways the suspension while stationary, the RC is indeed a plane.. not a point.
As perpendicular motion (forward motion) must be applied for suspension to function, as a stationary car would be pointless. A single point through space is a vector, bisecting that, is your RC plane.
As perpendicular motion (forward motion) must be applied for suspension to function, as a stationary car would be pointless. A single point through space is a vector, bisecting that, is your RC plane.
Norm Peterson
corner barstool sitter
Sorry, but I can't even begin to make any sense out of that.
You're bisecting a line of infinite length? Perhaps you meant to say "intersecting"?
Conventionally, a geo roll center is taken as a point somewhere within the vertical-transverse plane containing the (transverse) axle line.
Forward motion by itself need not involve suspension motion or elastic component function.
Norm
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thePill
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As I said before, consider two RC's... Static or at rest for initial geometry setup and, the RC under load. The RC usually moves in an arc.
How does lowering your CG affect our RC, it's stabilizes or limits it's movement under load.
How does lowering your CG affect our RC, it's stabilizes or limits it's movement under load.
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Stuntman
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The RC does not move in an arc on track (or stay in a constant position) because the car is constantly rolling, pitching, and heaving, causing RC to migrate all over the place and not on a nice arc.
I agree a lower CG reduces and stabilizes RC migration.
Trackaholic
Well-Known Member
Just to clarify this for my own education:
The kinematic RC motion is not changed at all by changing the CG, as it is based on the suspension geometry.
However, a lower CG will reduce body roll AT THE SAME CORNERING LOADS, which will in turn reduce the movement of the RC due to the reduced body roll.
Does that summarize things correctly?
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Stuntman
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^Yes to the second paragraph. For the first: CG affects the kinematics of the RC (which you said in the second paragraph). However the change in CG height does not change the geometry of the RC.
Norm Peterson
corner barstool sitter
I think what you're trying to say is that (all else held constant) CG height affects the magnitude of RC migration but not the path it follows.
Norm
w3rkn
Well-Known Member
It is only a single point in theory... in actuality when in motion, things change. No suspension works in static environment.
Taking a seat axis and lowering it, will have an effect on the contact point in the frame. Because at 100mph, the force applied to an upright seat VS reclined is different.
A single point in time, will plot a arc through it (ie: plane).
Gravity pulls one direction..
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