Static Rear Camber Setting

[cendiv37]

From your weight transfer spreadsheet, the total weight transfer number for a 1" CG change is about 12 lb. I assume that includes the change in the roll center. If so, I estimate about 3-4 lb of weight transfer change per degree of rear camber.

I don't think that's correct. One of the assumptions in that spreadsheet is that the weight transfer at the rear is ALL from roll center height and is independent of CG height. Remember my discussions with Robert about this? One effect of a true zero roll rear suspension is to unlink the weight transfer at the rear from the CG height and thus it's effect on weight transfer. It still depends on CG location fore and aft, just not height. Look at the "Rear WT due to RC Height (lb)" calculations in the spreadsheet. These are significant numbers.

I think a normal 100 lb rear wheel system is giving up about 25 lb per deg of rebound. Do we have the numbers to send the system into a over center condition?

I believe we do.

2) Ignoring the possible requirement for a soft land for the droop limiter, what is the benefit of stiffening the spring system as the droop limiter is engaged?

Brian

My point is that the combined "spring rate" of a soft spring pushing into a soft (but non-linear and getting stiffer fast) cushion is a the same as a stiffer spring before it hits a hard limiter: the lift force from the spring falls off fast as the car rises into the cushion just like a stiff springs lift force will fall off faster than a softer spring (before either hits a limiter of any kind).

An advantage of the soft cushion would seem to be that you could run a softer (more compliance) spring but still reduce the spring's impact on jacking BEFORE you actually hit the droop "limit" (where by definition the spring force = the cushion force).

The disadvantage would be that the droop "limit" is less defined, and over bumps and rises, the inertial effect of the car's motion could over-compress the cushion leading to more positive camber than is desirable and thus a possible temporary loss of traction greater than the unavoidable loss just due to the effect of the CG wanting to move away from the track surface in the first place. Of course this is where the "slow" setting of the shock comes into play. Too stiff and the suspension can't follow the pavement at all, too soft and it hits the limiter too hard and overtravels causing loss of camber, leading to loss of traction. Probably oversimplified and incorrect...

[jpetillo]

Question: What is the effect of a soft droop limiter (rubber cushion) used with a soft spring?
Answer: It acts a lot like a stiffer spring once the cushion comes into contact (the lifting force of the spring is opposed by the compression force on the cushion).

Yes, exactly.

[jpetillo]

To be precise, I'm not sure that changing the shock gas pressure actually changes the spring rate of the gas chamber or shock in general. It does add a one time expansion force that will change the ride height. This force is CONSTANT through out the range of travel of the shock. So all you are doing is changing the ride height.

I do not see how gas pressure can be used to tune a shock. The pressure is equal on each side of the piston. There could be some subtle changes in flow characteristics or shock friction, but I doubt you would notice it.

Not quite, but close. The pressure is equal on all parts inside like you said, but the piston rod going through the seal is not balanced off. So, the effective air spring force is the Pressure*RodArea. The pressure changes with the volume inside, nonlinearly. So, depending on how much the volume changes, the added air spring force changes. Adding 10% more pressure will add 10% more air spring force over what it was across it's range. John

[jpetillo]

One of the assumptions ... is that the weight transfer at the rear is ALL from roll center height and is independent of CG height.

Yes, this is the essence of the effect of zero roll resistance.

An advantage of the soft cushion would seem to be that you could run a softer (more compliance) spring but still reduce the spring's impact on jacking BEFORE you actually hit the droop "limit" (where by definition the spring force = the cushion force).

Yes, exactly. (But the "limit" is the cushion force = spring force + the hanging weight of the tires effect, etc. - just being nit picky.)

[lots of stuff] Probably oversimplified and incorrect...

Nope, I think you pretty much summed it up!
John

[hardingfv32-1]

cendiv37 wrote:
"An advantage of the soft cushion would seem to be that you could run a softer (more compliance) spring but still reduce the spring's impact on jacking BEFORE you actually hit the droop "limit" (where by definition the spring force = the cushion force)."

What is meant by "reduce the spring's impact"? What is the benefit of this?

With this setup will you reach definite stop or droop setting in say an average 1.4 G turn?

Brian

[CitationFV21]

Still have not done any calculations, but I am still not convinced that the spring is a big player in the jacking.

The first job of a spring is to hold the car off the ground. A car that bottoms loses down force into the tires.

So the softest spring you can use has to hold the car up and keep from bottoming.

Now for regular suspensions, you use a stiff spring to maintain ride height for aerodynamic purposes or to minimize suspension movement.

In one direction we use the droop limiter and the other we make the spring stiff enough to maintain a stable ride height. (matched of course to the proper shock).

The only force involved in jacking should be the lever force (from the tire) and the weight of the car at the transmission.

ChrisZ

[hardingfv32-1]

Chris

John Petillo is doing a spread sheet on the subject that is not ready for publication at this time. A rough estimate is 200 lb of jacking in a 1.4 g turn with the rear weight of the car at say 600 lb. That would mean the spring is responsible for 400 lb of lifting.

Brian

[cendiv37]

Chris,

What we are considering is the change in the lifting force provided by the spring as the car is jacked due to the jacking forces (total rear camber going from negative towards positive = spring getting longer). For any given rear suspension spring/rocker/pushrod geometry on the same car, any spring, soft or stiff must provide the same force at the static ride height. The spring must just hold the car at that height. No matter what spring, the spring force required is fixed by the weight it must hold up and geometry of the rear suspension. The spring force is by definition a lifting force, trying to raise the back of the car. The jacking from cornering adds to this force causing the rear of the car to rise in corners. That's why we corner "on the limiter", at least most of us...

With the spring perch fixed, as the rear camber moves towards positive (the car rises up due to the jacking effect of the side forces), the spring length changes, getting longer. With a stiffer spring (higher spring rate), the spring force will go down faster per degree of camber change (towards positive) than with a softer spring. Therefore with a stiffer spring, the lifting force provided by the spring will fall off faster than with a softer spring as the car rises. Consider the extreme of replacing the spring with a solid block. Assuming no bumps to upset the car, the car would stay at the static setting until the jacking forces could lift the car entirely by themselves (if they ever could). The lifting force from the block goes to zero as soon as the suspension rises at all from the static setting. With an actual spring, the car rises much sooner because the spring is helping to lift the car all the time (the spring force and jacking forces add to determine the height of the rear). Think of a stiff spring as being between a soft spring and a block.

For instance if you set up the car a 4 degrees static and set the droop limit a 2 degrees, the spring force with a stiff spring would be the same as for a soft spring at the static setting, but would it be less at the droop setting.

Hope that makes sense.

[hardingfv32-1]

For those who corner "on the limiter", may I assume there some additional rebound movement left in the droop rubber? Is there enough to help with rebound compliance movement? What would be the source of this extra force?

Brian

[brian]

Aloha! Been on the road, or should say in the air, and am still checking in with this conversation. I appreciate John jumping in confirming shock pressure point I made. I have learned the pressure change process from the top NASCAR shock shop on the west coast. While everyone's experience is different due to spring rates and shim packs, I can clearly feel the difference in pressures on my car. I run the small black Penskes and they're really sensitive but the rear shock can be changed too. Next time you all are fighting a bumpy track, or you get caught out on pregrid when it rains, a pressure reduction will help. A pressure change can act like a high speed bleed.

[hardingfv32-1]

John was not confirming that you can change the characteristics normally controlled by the shock's internal valving. I do not think I have ever seen that documented.

He was stating that there could be a small spring/wheel rate change. Remember, you are only pushing on a area the size of the shaft. A 100 lb pressure change might make a change of 30 lb with a 5/8" shaft. The spring rates of air springs are very non-linear, with little rate change for the majority of the range followed by a rapid rise. This having to do with the compressibility of a gas I think.

This one is easy to check yourself. I just did a front shock @ 70 lb of gas and could not detect any change with the reading on the bathroom scale. I went through at least 3" of shock travel. To me this means no rate change with a normal gas volume as specified by the manufacture.

Brian

[CitationFV21]

Chris,

What we are considering is the change in the lifting force provided by the spring as the car is jacked due to the jacking forces (total rear camber going from negative towards positive = spring getting longer). .....

Hope that makes sense.

Still confused.

Take 2 springs 1x 100 lbs per in and 1x 200 lbs per inch.

Put them on a car with a motion ratio that compresses the first one inch. The 200 lbs spring will only compress 1/2 and inch. The force being exerted to hold the car up is the same - 100 lbs. Now the car jacks 1/4" of motion. The 100 lbs spring has lost 25 lbs of force for 75lbs and the 200 has lost 50 lbs so in theory it is down to 50 lbs. (100 - 50). I think this is what you are implying. But if the spring rate contributes to the jacking as you say, maybe the 100 lbs spring goes 1/4 inch and the 200 lbs spring only jacks 1/8" which results in the same force? So in the end the spring equals itself out?

And I doubled the spring rate - for most people that would make the car undrivable the first time you hit a bump. You forget that for 1" of BUMP travel, the softer spring gives 100 lbs of force but the 200 lbs spring gives 200 lbs of force.

With normal springs I just don't see the spring having that much of an affect - I look forward to John's calculations, but the only way I see this working is to size the spring so you free load the spring in jacking. - Then your force drops to zero. But now you have a rattling spring which may not seat right when it comes down. And if you attach the spring to both ends of the shock, now you have created a droop stop with a changing rate. You could make this work by changing the rocker geometry and the pull/push rod length to set ride height, but we are getting way complicated.

Maybe that is why Brian H is saying you need a rising rate in bump, so you can have a decreasing rate in droop. I still wish this was summer so we could put some video cameras on cars and watch what is really happening.

ChrisZ

[hardingfv32-1]

Chris,

You are correct, the rear spring does not contribute to the jacking force. The confusion is from using the word "jacking" to represent the elevated rear ride height which the jacking force helps create. Put simply, if the rear of a FV weighs 600 lb, that weight will not change no matter what the ride height while the car is in a "smooth steady state" turn. If the jacking force being generate is 200 lb, then the spring system must ONLY provide 400 lb to get the rear suspension back to the 600 lb total. The rear spring will expand and the ride height change until it reaches 400 lb.

When comparing two springs with different rates, in the same spring system (rocker, pushrods, etc.), you will end up with two different ride height or camber settings after adding in the same jacking force. Just for this example I am ASSUMING that the jacking force can be the same at different camber angles.

So in your example, the fully jacked ride height/camber for the 100 lb spring could be 2 deg and the 200 lb spring 3 deg., assuming all the same starting conditions.

Fact: Doubling or tripling the rear spring rate does not a make FV un-drivable for anyone.

"You forget that for 1" of BUMP travel, the softer spring gives 100 lbs of force but the 200 lbs spring gives 200 lbs of force." That sentence could be re-stated: A bump that causes 1" of movement with a 100 lb spring only makes a 1/2" of movement with a 200 lb spring.

Brian

[CitationFV21]

.................
Fact: Doubling or tripling the rear spring rate does not a make FV un-drivable for anyone.

"You forget that for 1" of BUMP travel, the softer spring gives 100 lbs of force but the 200 lbs spring gives 200 lbs of force." That sentence could be re-stated: A bump that causes 1" of movement with a 100 lb spring only makes a 1/2" of movement with a 200 lb spring.

Brian

Brian,

We are close to agreeing. Your right, a bump of 1" would not necessarily be a movement of 1", but the force has to go somewhere. If the spring is too stiff to absorb the shock, then the force gets transmitted to the chassis. Don't you remember all those F500's bouncing all around as they passed us. Now that they have some suspension at least they are predictable.

Now if you don't live in the land of frost heaves, then you can get away with stiffer suspensions. The CA FF guys always had a fit at Pocono or Lime Rock when they came east. We will see how long Lime Rock stays smooth. I think in some places they went down 3 to 4 feet to get the base below the frost line!

ChrisZ

[hardingfv32-1]

Chris,

So, is the jacking issue any clearer?

I'll try and run the Nelson Ledges Nat this summer. That should be the definitive test for a stiff suspension!

F500 spring rates must be 500 lb or greater. They are not relevant. FF is closer to what I use, but still much stiffer. You do not see FF bouncing off the track in rough turns even in the East.

Brian