A Mono Shock rotary damper system...

[jpetillo]

SNIP...
Ultimately, the twin shock z-bar provides the best handling, although with a narrow tuning window....

Greg, do you know why it provides the better handling and that it has a narrow tuning window? Or is this just what most folks believe?

Tell me if I understand the twin shock with Z-bar correctly. The twin shocks are like every other twin shock system. The Z-bar acts like a pro-roll bar as opposed to an anti-roll bar. It allows/helps the car to roll working against what the twin dampers are trying to do in a corner, but causes a vertical spring force to both sides. How do you look at it? Let me know if I'm not thinking about it correctly. John

[CitationFV21]

John,

An anti-roll bar tries to control body roll and hence weight transfer due to cornering forces.

A Z-bar is basically an anti-jacking device. It's purpose is to keep the camber within a certain range. Because it is connected to the chassis, it can imparts a force to the chassis which does affect weight transfer and it also could add spring rates at full travel.

With a twin shock system, each tire is, in theory, able to adjust to bumps with out affecting the other wheel. If a zero roll car hit a bump with both wheels, then the shock is compressed evenly and no problem. If the inside wheel only hits a bump, then the force is transmitted to the outer wheel. So why does zero roll actually work better on a bumpy track than a Z-bar? First, I think the cars are so light that the chassis does not absorb as much force as we think. Second, most Z-bar cars are not set up properly, so they end up being preloaded which means they get upset when going over bumps.

In a proper race suspension, an anti-roll bar is a tool to control weight transfer. In a zero-roll FV the only control we really have is to change the front anti-roll bar. This may also explain the development of the front shock as a tuning tool.

John, I know you like the engineering angle of all this. After all my years of FV, I have found that the most important things with ANY design is the basics - are the tires good. Are the shocks operating properly. Is there no binding in the suspension during travel and is the alignment done properly. If you have these then any FV will handle well.

Now if I was going to design a car from scratch? I like:

1. Decreasing rate rear suspension.
2. Low pivot points for the rear rockers
3. Some way of varying the front roll resistance externally so we can figure out what the front roll stiffness really should be for the track and tires we run.
4. An adjustable roll damper in the rear suspension to take some of the load from front doing all the work

I would love to work with a zero-roll again with all the new shock technology - But right now aerodynamics and simplicity favor the zero roll.

ChrisZ

[hardingfv32-1]

The z-bar does a good portion of the work keeping the car off the ground, vertical force. But as directed by the rules, the car must not settle to the ground IF the z-bar is disconnected, so the vertical force must be provided by the two coil-overs. The roll resistance at the rear could be 50-85%. Typically you use very soft springs with a lot of travel and use the z-bar to pre-load the coil-overs, pulling down the rear suspension. My roll resistance numbers could be off because of this pre-load.

I believe there is really no way to get a small increment of roll resistance, say 10-15% rear roll resistance. Adding roll resistance at the rear will, if starting with a neutral car, cause a loose car condition. Now many drivers will say loose is the fast setup, but we are provided tires that were developed for a zero roll car. A z-bar configuration is just too drastic to develop a car using the current tires.

There could be advantages to a loose car, but there is no advantage to a z-bar car with current tires.

Brian

[hardingfv32-1]

Chris

1) Decreasing rate rear suspension. Why a DECREASING rate?

2) Low pivot points for the rear rockers. What benefit?

3) An adjustable roll damper in the rear suspension to take some of the load from front doing all the work. What makes you think that the rear has any extra cornering power available to help the front with?

Brian

[CitationFV21]

.....Typically you use very soft springs with a lot of travel and use the z-bar to pre-load the coil-overs, pulling down the rear suspension. My roll resistance numbers could be off because of this pre-load.
............
Brian

I used to set my car up with the z-bar unloaded at race ride height (driver and fuel). It was right if it "rattled" while going to the false grid. Droop was adjusted by the length of the arm. In this sense it was only used as a droop stop, not a tuning device. If you needed to offset understeer, you could preload the bar, but I always hate to decrease grip in order to balance the car. Rather the springs, shocks and anti-roll bars should be adjusted.

ChrisZ

[CitationFV21]

Chris

1) Decreasing rate rear suspension. Why a DECREASING rate?

2) Low pivot points for the rear rockers. What benefit?

3) An adjustable roll damper in the rear suspension to take some of the load from front doing all the work. What makes you think that the rear has any extra cornering power available to help the front with?

Brian

1. Steady rate would be best, but unless you have a reason for keeping the car off the ground (ground effects) then decreasing makes it easier to drive. Now with FV I don't think we have spring rates that are too sensitive to this. The BRD used to have very small rockers and cars using a pull rod rather than push rod might be more sensitive to setup. Most others you can't change what you have.

2. Even with zero-roll, you do feed some loads into the chassis. I think the D13 guys had a reason for feeding the loads in under the pivot point of the axle. It might help counteract jacking.

3. I would want it adjustable because depending on the car set up the front may run out before the rear does. Could be because of weight distribution for example. Ideally, you should be able to set a car up on a skid pad and that will get you the maximum grip, but only at that speed. So any adjustment will be helpful, even if sometimes that adjustment ends up at zero. I wonder if anyone with a D13 has played much with the roll damper?

(You might get the idea that I am a D13 fan, but I have never driven one - I have seen many drivers get in them and go very fast and it seems to be the best beginner's car, except for the poor visibility...... )

ChrisZ

[problemchild]

Chris said "I used to set my car up with the z-bar unloaded at race ride height (driver and fuel). It was right if it "rattled" while going to the false grid. Droop was adjusted by the length of the arm. In this sense it was only used as a droop stop, not a tuning device".

Say what? Sorry Chris, but this statement is just so impossible. The Z-bar is never a droop stop and is constantly active and influential. The only time it has no effect is the milli-second long periods that it becomes unloaded through travel in the rear suspension..

[CitationFV21]

Chris said "I used to set my car up with the z-bar unloaded at race ride height (driver and fuel). It was right if it "rattled" while going to the false grid. Droop was adjusted by the length of the arm. In this sense it was only used as a droop stop, not a tuning device".

Say what? Sorry Chris, but this statement is just so impossible. The Z-bar is never a droop stop and is constantly active and influential. The only time it has no effect is the milli-second long periods that it becomes unloaded through travel in the rear suspension..

Greg,

I think I gave away my set up information when I sold my Lynx, I think I got it from either John Mills, Adams or Fred Clark. Maybe Fred or someone can jump in with the exact wording.

When the car was rolling down the pit lane there was no tension on the z-bar. Of course with heim joints the minute you hit a bump that changed, but the idea was not to have the bar have too much effect on the spring rate. It did result in a rising rate as the wheels together went up as the z-bar was added to the spring rate. But when the wheels went into droop together, the z-bar fought against the spring rate. Now Brian is right, if you took the shocks and springs off, the car could "sit" on the z-bar but would not be strong enough to be able to drive the car. [Edit - the rule he correctly quoted is that if you take the Z-bar off, the car needs to be supported by the coil springs - this implies that the Z-bar has some spring fuction.] If you jacked the car up at that point it would look like a zero roll in full droop and picking up on wheel would cause the other wheel to go down, but keeping the total camber the same without changing the chassis roll.

Now I did not put on softer springs like Brian suggested. The springs (think around 110 - 120) were adjusted so I got the right camber with the z-bar disconnected. I then loaded the car with driver and fuel and set the bar with no preload. Jacked it up and if the droop was wrong, slid the adjusters along the bar till I got the right tension in droop to give me my camber.

This setting was critical and if you changed the weight or as the fuel load changed, you got a either preload or whatever you want to call the other way (unload, post load?) Of course this other loading tended to push the wheels into droop which meant more jacking - not good.

I believe the Z-bar was the third answer to a cable or strap camber limiter or a camber compensator leaf spring (used on the 61-64 Corvair http://www.corvaircorsa.com/handling02.html). You are right in that it is very rarely in the sweet zone of zero preload, but its much more gentle than the cable, and easier to adjust that the leaf spring. Being persnickety, it is why zero roll has become the standard today (and weight, and aerodynamics..)

ChrisZ

[jpetillo]

Chris, Thanks for you posts. What you have been describing about the Z-bar is what I expected. Thanks for the posts.

Greg, I thought what Chris has been describing was how it worked, or certainly can work that way. How do you set it up?

John

[problemchild]

The Z-bar is just a simplified 3rd-spring like is seen on all kinds of expensive hi-tech race cars. Setting it at zero pre-load at ? ride height is a tuning adjustment, just as setting the lever lengths, and the spec itself are tuning adjustments. If it is connected, it is a tuning tool. It is never a droop stop although it acts as a droop limiter. IMO, its function as a 3rd spring in compression as much more useful than its function in controlling droop (which could be done with a droop stop if that was all that was desired).

Chris's assertion that he is using the Z-bar only as a droop stop is impossible. Pulleys and a cable (or mechanical stop) would provide a droop stop and would provide stunningly different handling characteristics than a Z-bar with the same springs/shocks.

[hardingfv32-1]

A z-bar is the alternative to the typical zero roll rocker and mono spring setup. I BELIEVE what prevents it current use in a current front running car is the requirement of the rules to be able to disconnect the z-bar and maintain ground clearance. Is this true or can the added rear roll resistance be negated by pre-loading the z-bar and springs?

I don't see anything that would make this system overly sensitive, unless we are talking about a step where in fact you could never dial the loose condition out of the car because of all the rear roll resistance. Did a front running z-bar cars ever push?

Brian

[CitationFV21]

I think we are getting mixed up in nomenclature.

When I say droop, I mean both wheels together, not one or the other.

A Z-Bar is an anti-jacking device, not a springing medium, as the rules require.

The fact that you can preload it does not change it's function.

If you preload it to bring the rear camber negative you "lock the suspension" to that setting.

If you preload it to provide assistance to lighter springs you then encourage jacking.

Most Z-bar cars oversteered which on tight, smooth tracks may have been an advantage.

On bumpy tracks the increased spring rate in bump was a disadvantage.

Also, Z-bar cars needed to be tuned for corner weights, while a zero roll does (can't?) not. Which takes on tool out a tuner's hands -didn't Mike say the Speedsport could tune in some weight transfer in the rear?

ChrisZ

[hardingfv32-1]

Unequivocally, the z-bar provides vertical spring force with no roll resistance. Its effect on jacking is no different than the single spring in a modern zero roll setup.

If z-bar cars were generally loose, I would say that it comes from the basic problem that such a system is providing too much rear roll resistance. It is asking too much from the rear tires.

Varacins uses bump rubbers to create roll resistance. The roll resistance is not over the complete range of chassis roll, but focused on the outer range/limits of the roll movement.

Brian

[P-2 Mark]

Brian,

Where does Mike Varacins place the bump rubbers to create the roll resistance?

Thanks!

Mark

[hardingfv32-1]

Remember he is using a mono shock rocker/shuttle. It slides sideways (perpendicular to the chassis axis) on a shaft to get the roll action required for a FV. The bump rubbers are on this shaft and restrict the sideways travel of the rocker/shuttle. In a normal formula car using a mono shock system the shuttle does not move as much and ALL of its movement is controlled with belleville washers, etc. Chassis roll is controlled in this manner. On the FV the situation is completely different when it comes to roll control.

Now that said, you can get the same effect by having your FV rear rocker's movement restricted at some point that you find useful. The design issue would be how to mount the rubber stops.

The purpose of all this is to provide the rear suspension with a level of roll resistance at SOME point in the chassis roll curve. The car will get loose (if starting from neutral) as it approaches maximum roll or maximum cornering force. Certainly a driver preference that I'm not sold on it at this point. While it would be nice to get loose in a slow tight turn, I question having it happen in a flat-out sweeper. Am I interpreting the pros and cons correctly?

Brian

[Speedsport]

While it would be nice to get loose in a slow tight turn, I question having it happen in a flat-out sweeper.

But there are tracks that have more tight turns in one direction, and sweepers in the opposite. That's why it works great to be able to vary the resistance for right and left corners independantly.

[hardingfv32-1]

Give the man a cigar! Thanks for the explanation.

Brian

[P-2 Mark]

Such as Mid-Ohio with a fast turn one sweeper to the left and two tight slower right hand turns
in the carousel and key-hole etc.....

Mark

[Speedsport]

It's a bit more complicated then that, but thats the basic idea. The other interesting thing that happens is the rate at which the roll changes is different for fast and slow corners. It took a lot of data recording, but with travel sensors on the carriage, we learned some intereting things. By recording the travel and comparing it to the corner location and amount of compression of the rubber packers, some fine tuning is possible.

[CitationFV21]

...........................

The purpose of all this is to provide the rear suspension with a level of roll resistance at SOME point in the chassis roll curve. The car will get loose (if starting from neutral) as it approaches maximum roll or maximum cornering force. Certainly a driver preference that I'm not sold on it at this point. While it would be nice to get loose in a slow tight turn, I question having it happen in a flat-out sweeper. Am I interpreting the pros and cons correctly?

Brian

Mike has given some good explanations of how he uses his adjustments, now how much $$$ to get the data to figure out where to use them!

Mario Andretti was the first that I know of, in road racing, to use the technique of setting his car up for specific turns in order to maximize his speed. This was a trick from oval racing where you gave up one turn to maximize another.

Brian, your right, the question is balance. With no horsepower to kick the back around, the old hairpin at Pocono must have been painful in a Vee. On the other hand, I can't think of any turn at the Glen where I would want oversteer. RA either. (but the blue guardrail is not there....)

The challenge is to make a predictable car at all speeds and at all turns, and since that is hard, I think some drivers are better at adapting their driving to suit the car. Any time you can get more predictable tuning, that is a great advantage.

I just wish I had the time and money to really test. These days test days are more than race weekends.

ChrisZ

[Veefan]

Although a front one, it looks interesting.


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[P-2 Mark]

How do the downrods apply pressure to the monoshock since there doesn't appear to be a pivot
or bell-crank for them to work?


Mark

[hardingfv32-1]

The pivot or shaft is below the rod end on the spring side of the coil-over. In this case the distance from the shaft to the rod end in about 2" and represents one leg of the fulcrum or rocker. The other leg is the large gray triangle section that the push-rods are attached to. Again looks like it is about 2" long. Both side of the shaft are made and assembled the same. Note the stack of Belleville washers just inside the lock nut. This is your roll control. The rocker or shuttle can slide sideways with its movement restricted by the washers. I think you would characterize the bearings as a linear type, probably housed in the large aluminum section next to the washers.

Brian

[P-2 Mark]

Brian,

You mentioned the stack of "Belleville washers" just inside the lock nut. Is it possible to use these
on a standard Zero-roll Vee set-up to control the roll of the rear, and if so where?

Thaks!

Mark

[hardingfv32-1]

These washers generally will not collapse more than .060", so it would take a very big stack to get the movement you are use to. You would have to come up with a design with a lot less movement, possibly like Mike's.

Remember with Mike's system, he is only introducing roll resistance at the ends of his roll curves. At a point approaching max cornering force his system starts to kick in and make the car progressively loose. Is this is a very delicate adjustment. When you hit the bump rubbers depends on how well your are cornering and/or jacking in THAT turn at THAT moment. Since the amount of jacking you get is proportional to the grip your tires are providing, poor track conditions (oil spill!) would effect how much roll resistance you get.... I THINK. Are you up for all this variability? Most FV drivers are not.

I will start a new thread on rear roll resistance.

Brian