How important are Penske front shocks?

[John Nesbitt]

...
I am very intrigued by the relaxed standards of the forum. I really miss the fascinating debates from previous times, and the contributions from the interesting people who got banned for far less than your recent commentary. Perhaps we could have an absolution process and liven things up around here!

You may be on to something there. Bring back Daryl!

[jpetillo]

But if the roll center is at the ground level, the car would not be as reactive to droop limiting as it is.

Steve, I'm not disputing it, but am curious to understand why you believe it is the case that the roll center going from ground level to say beam center makes how you top off the suspension travel have a greater effect. What do you see is happening? John

[sabre1fv]

John,

Thank you for taking the time to expand on the definitions of jacking - very helpful in understanding the concepts; hopefully we can agree with your descriptions. If not, then perhaps additional detailed explanations to help us understand will be presented.

-Jim

[S Lathrop]

Steve, I'm not disputing it, but am curious to understand why you believe it is the case that the roll center going from ground level to say beam center makes how you top off the suspension travel have a greater effect. What do you see is happening? John

This description I am going to give is how I think about what happens when a car corners. This is not a scientific explanation. And it is the product of an old man.

As the car corners, the tire is in contact with the track and is resisting the lateral forces that result from changing the direction of the car. Or the tire is inducing the lateral forces acting on the car as it is cornering. There is a mass that is associated with the front of the car that is being thrown side ways. That force will be transmitted to the tire through the suspension system of the car and through the geometry of the front suspension system. The geometric weight transfer can be though of being like a rod that goes from the roll center to the contact patch. The portion of the mass that is below the roll center is the geometric weight transfer and the portion of the mass that is above the roll center is the portion that will be transferred through the suspension. Think of the geometric weight transfer like a pole vaulter.. Given the angle of the pole, it lifts the vaulter's weight up and over the bar. The lower the angle of the pole (below the optimum for the athlete), the less lift the vaulter gets. With a race car, if the roll center is below ground level, the car actually goes down as the car rounds the corner. The higher the roll center the more lift one gets as the car corners. That should be clear as mud.

In the case of a FV, if the roll center is at ground level, then the tendency for the front to rise as the car corners would not exist, at lease from a geometric lifting.

Previously I was asked for proof about the roll center causing a car to lift as it cornered. The Zink Z10 , as the car was originally setup had a roll center at the front over an inch above the ground. In pictures of the car cornering, you ca easily estimate what the suspension is doing by looking at the inboard ends of the front rocker arms. In those pictures you can see the outboard rocker, inner end, above the front bulkhead, This shows that the front end of the car is rising as the car corners. As we started to run the car lower and lower at the front, the rockers were not showing the car rising at the front in corners. As we lowered the front of the car, the roll center went down. We never droop limited the front of the Z10. We just lowered the ride height and the roll center went down as we lowered the car.

Lastly, at the rear of the car, the car rises to the degree allowed by the droop limiting system and the car goes around the corner hard against the droop limiting system. Droop limiting at the rear has been a feature of every FV I ever saw and I started racing FV in 1968.

[jpetillo]

This description I am going to give is how I think about what happens when a car corners. This is not a scientific explanation. And it is the product of an old man.

As the car corners, the tire is in contact with the track and is resisting the lateral forces that result from changing the direction of the car. Or the tire is inducing the lateral forces acting on the car as it is cornering. There is a mass that is associated with the front of the car that is being thrown side ways. That force will be transmitted to the tire through the suspension system of the car and through the geometry of the front suspension system. The geometric weight transfer can be thought of being like a rod that goes from the roll center to the contact patch. The portion of the mass that is below the roll center is the geometric weight transfer and the portion of the mass that is above the roll center is the portion that will be transferred through the suspension. Think of the geometric weight transfer like a pole vaulter.. Given the angle of the pole, it lifts the vaulter's weight up and over the bar. The lower the angle of the pole (below the optimum for the athlete), the less lift the vaulter gets. With a race car, if the roll center is below ground level, the car actually goes down as the car rounds the corner. The higher the roll center the more lift one gets as the car corners. That should be clear as mud.

That makes perfect sense. Thanks!

In the case of a FV, if the roll center is at ground level, then the tendency for the front to rise as the car corners would not exist, at lease from a geometric lifting.

I agree with this if you are not talking about lifting due to something like rising rate springs causing the lifting. Typical definition of roll center location is an action independent of springs and dampers - just bars and links. But I do agree that you can change the apparently roll center location, if you add in something nonlinear, like a rising rate springs.

Previously I was asked for proof about the roll center causing a car to lift as it cornered. The Zink Z10 , as the car was originally setup had a roll center at the front over an inch above the ground. In pictures of the car cornering, you ca easily estimate what the suspension is doing by looking at the inboard ends of the front rocker arms. In those pictures you can see the outboard rocker, inner end, above the front bulkhead, This shows that the front end of the car is rising as the car corners. As we started to run the car lower and lower at the front, the rockers were not showing the car rising at the front in corners. As we lowered the front of the car, the roll center went down. We never droop limited the front of the Z10. We just lowered the ride height and the roll center went down as we lowered the car.

Very good - understood, thanks!

Lastly, at the rear of the car, the car rises to the degree allowed by the droop limiting system and the car goes around the corner hard against the droop limiting system. Droop limiting at the rear has been a feature of every FV I ever saw and I started racing FV in 1968.

Yes, agreed - this is the general rule.

Thanks for the response - John

[jpetillo]

John,

Thank you for taking the time to expand on the definitions of jacking - very helpful in understanding the concepts; hopefully we can agree with your descriptions. If not, then perhaps additional detailed explanations to help us understand will be presented.

-Jim

Jim, I appreciate the comment. I'd be happy to elaborate if anyone is so interested. I'll add a warning that it would be best read immediately before bedtime. John

[jpetillo]

I have also stopped measuring rear droop and use the rear droop limiter to set preload.

Greg, are you saying that you used the droop limiter to limit/set the static ride height?

[sabre1fv]

Steve,

Forgive me for not knowing, but your discussion of the Z10 mentions rockers and I googled you and Z10 and got info on a FF, which would then make sense regarding a rocker. So did you have rockers on your Z10 FV front suspension? A photo would be great if you had one available so I could see the orientation.

Also, unlike John, I do not understand the difference between these two factors: "That force will be transmitted to the tire through the suspension system of the car and through the geometry of the front suspension system. The geometric weight transfer can be thought of being like a rod that goes from the roll center to the contact patch. The portion of the mass that is below the roll center is the geometric weight transfer and the portion of the mass that is above the roll center is the portion that will be transferred through the suspension", can you elaborate on the difference between them? The pole vaulter analogy is a good one which I understand but I would think all forces would be going through the 'pole'.

Thank you.

-Jim

[S Lathrop]

Steve,

Forgive me for not knowing, but your discussion of the Z10 mentions rockers and I googled you and Z10 and got info on a FF, which would then make sense regarding a rocker. So did you have rockers on your Z10 FV front suspension? A photo would be great if you had one available so I could see the orientation.

Also, unlike John, I do not understand the difference between these two factors: "That force will be transmitted to the tire through the suspension system of the car and through the geometry of the front suspension system. The geometric weight transfer can be thought of being like a rod that goes from the roll center to the contact patch. The portion of the mass that is below the roll center is the geometric weight transfer and the portion of the mass that is above the roll center is the portion that will be transferred through the suspension", can you elaborate on the difference between them? The pole vaulter analogy is a good one which I understand but I would think all forces would be going through the 'pole'.

Thank you.

-Jim

Think of the mass of the car as a big brick, sitting on an ice block. And it is supported by a pole from the contact patch to the some point on the brick that is below the center of the mass. As you go around the corner, the brick tips over the end of the pole where it hits the brick. The upper portion of the brick is supported by the suspension and that keeps the brick from flipping over completely. You are going around a corner so the brick wants to fly laterally but the pole and the suspension stop that from happening.

[Marty Nygard]

Without haven't having driven an FV, I am struck with the low-and-flat early argument. As I have frequently quoted, "any suspension will work if you don't let it"!
Love to watch you guys!
M

[sabre1fv]

Steve,

I'll try one more time to gain an understanding of how you are dividing up the masses of a given car. What makes sense to me is the static weight of the car on its tires, and when cornering, the overturning moment of the lateral G forces acting on the CG of the car. Granted that any acceleration, braking or steering input can affect the given load on any tire while cornering given a smooth track; and of course I'm ignoring aero effects. How does what I have written differ from your mention of "The portion of the mass that is below the roll center is the geometric weight transfer and the portion of the mass that is above the roll center is the portion that will be transferred through the suspension"? I will also include cornering on a banked corner adds vertical G loads...

Thank you for trying to enlighten me.

-Jim

[Hardingfv32]

Steve,
I'll try one more time to gain an understanding of how you are dividing up the masses of a given car. .
-Jim

A car's lateral load transfer (LLT) is the sum of three systems.

1) A portion of LLT is created by forces acting on/at the roll axes. There is no movement of the chassis for this to develop. A good example would be a kart. No suspension, but you know there is load transfer.

2) Another portion of LLT is generated by the movement of the chassis 'around' the roll axis. This is the actual chassis/body movement that you can observe.

3) The last portion, is the LLT that is generated by the unsprung masses.

I have a spread sheet that actually calculates these numbers for an FV. A few of us developed it years ago and the spreadsheet was written by J Petillo.

Brian

[jpetillo]

Steve,

I'll try one more time to gain an understanding of how you are dividing up the masses of a given car. What makes sense to me is the static weight of the car on its tires, and when cornering, the overturning moment of the lateral G forces acting on the CG of the car. Granted that any acceleration, braking or steering input can affect the given load on any tire while cornering given a smooth track; and of course I'm ignoring aero effects. How does what I have written differ from your mention of "The portion of the mass that is below the roll center is the geometric weight transfer and the portion of the mass that is above the roll center is the portion that will be transferred through the suspension"? I will also include cornering on a banked corner adds vertical G loads...

Thank you for trying to enlighten me.

-Jim

Let me try to further Steve's analogy.

If we start off with Steve's brick. There is a roll axis that let's say is a pole mounted along the bottom of the brick Steve mentioned. (This roll axis represents the geometric instantaneous roll axis that connects the instantaneous roll center for the rear with that of the front. The roll axis is the axis the car wants to roll about when external forces are applied, and is mechanically connected to the tires with suspension bars and such.) If you and a buddy hold that pole from either end and pick it up, the brick will want to flop over as it rotates around the pole if you do not perfectly balance it, because, in case of a Vee and most cars, the Center of Mass (COM) is above the roll axis. What keeps the brick from flopping over if it is not in perfect balance is the spring on each side, like we have in the front of our cars.

When you go around a corner, the turning force is a lateral force created by the tire contact patch on the ground and pushes horizontally on the tires, and the tires push on the roll axis through those suspension bars (not including springs). Since the car starts to turn in, let's say, a circle, there is an effective centripetal force and that can be thought of as acting on the COM, but in the opposite direction to the force from the tires. Since the COM is above the roll axis it tries to push the top of the brick over that you and your friend were careful balancing. Now, the springs are there to resist that motion and so it rolls over only so far until the springs are providing a balanced restoring force. In this case the car has now set.

I hope that helped.
All that being said, the weight transfer you get is purely dependent of the height of the COM and the mass of the car, for a given cornering speed and radius of the turn. But weight transfer that is after the car stops rolling and has set. It does not matter whether the springs are holding the mass from flopping over, or the roll axis is though the COM where the car would have no tendency to roll, the total weight transfer is always the same.

Yes, cornering on a banked corner adds vertical G loads which push the car down the slope towards the inside of the corner. This is one reason you can corner better on a banked curve. John

[Hardingfv32]

Yes, cornering on a banked corner adds vertical G loads which push the car down the slope towards the inside of the corner. This is one reason you can corner better on a banked curve. John

An FV in a banked corner can be unsettling. The added vertical load can move the suspension from the droop limit (ideal camber) to a setting of less camber reducing rear corning force. The front has no such issue with the added load. This result is a loose condition.

You see that in Buttonwillow's banked sweeper. Gets exciting when doing it flat, 6400 in 4th, in a FV.

Brian

[provamo]

certainly you mean load transfer?