Static Rear Camber Setting

[hardingfv32-1]

From our discussion in the previous tread, I think we determined that with a std zero roll suspension we jack up to the droop limiter in the mid section of a corner. Starting with a static setting of say 5 deg total, we will always jack into the droop limiter (at 2 deg total) in a normal 1.4 G cornering situation. I am going to assume that starting with less camber will not cause a different droop outcome.

Question: Why not use a much smaller static camber setting to start?

Assumptions:
1) Maximum rear tire performance at 1 deg neg camber or 2 deg total for the rear suspension.
2) Optimum camber (2 deg total) is always acheived at max jacking or when hitting the droop limiter.
3) We always reach the limiter in a 1.4 G cornering situation.
4) That the starting or static camber setting has nothing to do with the rear suspension reaching the droop limiter.

So if our rear suspension creates maximum traction at 2 deg total, why are we starting with say 5 deg? I assume 5 is about average for most cars.

Brian

[smsazzy]

If you are running 2 degrees of droop, which is probably about average, and you are running between 4.5 - 5 degrees rear camber - which I also believe is about average for the Hoosier tire, then you only have 2.5 degrees of suspension travel. If you start with 3.5 of static camber, I would think you would not have enough suspension compliance. It would be like having 100% rebound stiffness.This would cause a severe loose condition on any forward weight transfer, such as trail braking.

Don't tell me you only want to talk about billiard smooth tracks, because those don't exist, and if they did, you could run a solid springless suspension effectively.

[hardingfv32-1]

No, a smooth track was not one of the stated assumptions. I would say compliance is what we are concerned with.

For you and many other FV competitors, it is mandatory that you realize that you are in a rebound, raised, or jacking condition through most of a turn. That said, if you hit a bump the suspension has plenty of compression travel available. Yes, rebound travel is restricted, but it always has been with the droop limiter with no recognized ill affect.

So while starting with very little camber sounds restrictive, in fact it is not... I think

I asked the question because any downside is not jumping out at me.

Brian

[brian]

The only downside I can think of is going over a rise will lift the rear tires off the track if you don't have enough travel. I'm sure there's some drag issues on straightaways to consider as well.

[hardingfv32-1]

Going over a rise is a rebound issue which is now controlled by the the droop limiter. This is generally not a big issue. Regardless, we must use the droop limiter to control our camber in the turns as laid out in the previous thread.

Drag is a possibility, but why wasn't everyone running and RA with 7-8 deg rear camber like the old days to reduce drag? Remember, when we get to the turns the is no reason the suspension will not jack to the proper camber level. This would seem like a win win situation IF drag was really an issue.

I believe there was a trend toward less static camber at the Runoffs this year. Is this another "Flat World" issue where the FV community is working its way close to the edge with small steps?

I assuming we are better off starting and ending a turn with better rear camber.

Brian

[jpetillo]

Brian H, nice topic and I think it was well posed.

Steve S, I agree with you here that the concern with reduced travel is that there may not be enough suspension movement to keep the tire downforce as constant as possible during bumps around corners. I'm not sure that I would agree it would cause the trail braking or forward weight transfer causing looseness. In those cases won't the car want to raise the rear, or Steven are you thinking that the time it takes to rise into the droop limiter will be less if there is more travel and hitting the limiter will cause the looseness? That I'd agree with.

Brian M., is this same issue you're thinking about - the time it takes to top out into the droop limiter when going over a rise will be sooner with less travel. If it takes longer you may not get fully into the bump stop and keep more uniform force down on the tires over that time. This is an excellent point. That's not a hard problem - going over arise - we should be able to figure that out.

If banging into the droop stop too hard or fast under these conditions is the issue, can we just add more rebound damping to increase that time? I realize that adding rebound damping may cause other problems, but let's not consider that for this discussion. John

[hardingfv32-1]

1) I disagree about the concern that there could be reduced suspension travel.

A) If you hit a bump in the turn while jacked you have plenty of compression available, as by definition you are at full rebound.
B) If you go over a dip, yes you have no rebound available, but this is not caused by a low static setting. It is caused by the droop limiter that you are all presently using. Do you notice issues when you go over dips now?
Am I wrong?

2) "the time it takes to top out into the droop limiter when going over a rise will be sooner with less travel. If it takes longer you may not get fully into the bump stop and keep more uniform force down on the tires over that time."
Are we saying here that it is better to drive around with poor camber so we can have more rebound travel?

3) "If banging into the droop stop too hard or fast under these conditions is the issue"
Sooner, yes, but why any harder or at a faster rate that normal? We are using the same shock valving and experiencing about the same cornering force.

Brian

[jpetillo]

1) I disagree about the concern that there could be reduced suspension travel.

A) If you hit a bump in the turn while jacked you have plenty of compression available, as by definition you are at full rebound.

I'm not sure I agree with this. Perhaps the assumption here should include that you go over a series of roughly similar bumps. I would have guessed that suspension compliance would be dedicated by a combination of spring rate, damping and travel for a given type of bump/bumps and the chassis in question. My point is that you may be right for one bump, but I think for multiple bumps you may need a given minimum amount of travel. If not you could go to no travel, and we know that won't work.

B) If you go over a dip, yes you have no rebound available, but this is not caused by a low static setting. It is caused by the droop limiter that you are all presently using. Do you notice issues when you go over dips now?
Am I wrong?

Yes I do. The rear of the car skitters over when I go over a rise during hard cornering.

2) "the time it takes to top out into the droop limiter when going over a rise will be sooner with less travel. If it takes longer you may not get fully into the bump stop and keep more uniform force down on the tires over that time."
Are we saying here that it is better to drive around with poor camber so we can have more rebound travel?

Perhaps that may be the compromise. But I don't know. That's why I brought up more damping to change when the topping out happens as a possible way to deal with it.

3) "If banging into the droop stop too hard or fast under these conditions is the issue"
Sooner, yes, but why any harder or at a faster rate that normal? We are using the same shock valving and experiencing about the same cornering force.

Good question. But the lower range of camber would come about by some equivalent spring rate increase at the wheels. Let's say we used a solid droop limiter (infinite spring rate), when we hit it that would make the deceleration of the sprung weight spike, and that would happen by pulling up on the sprung weight, reducing vertical force on the tires. So, between a soft spring, and a hard spring and the infinite spring (solid droop stop), the hard spring would cause more upward force than the softer spring with the same damping rate - I say that while providing no proof. John

[CitationFV21]

I will bring up the D-13 again as it seems to run less camber than other zero roll cars.

I have also noticed that cars are running less camber than before. Noble used to recommend 5 - 6 degrees but today that would probably burn out the inside of the tires.

I would also agree if you start with 4 neg instead of 5 neg, then you should reduce your droop to from -2 to -1 as roll and jacking travel seem to be consistent.

I think the tires have changed also, what do the tire manufacturers recommend? To develop a good tire, they need to test and they have to do it on a Vee....so who did their testing? Or was it blind?

All the work on the front, could be to work on the rear. If you make the rear tires less negative, you have to spend more time on the front.

ChrisZ

[hardingfv32-1]

I see more assumptions are required to keep people on the reservation:

Assumption: 5) We are using one std zero roll car and making no changes other than the static or initial camber setting. This car qualified 2nd or 3rd at the Runoffs, so it is handling satisfactory before changing the static camber!

1) So... A) "If you hit a bump in the turn while jacked you have plenty of compression available, as by definition you are at full rebound."

Starting with one bump first, do I have all the compression movement I had before? If not where did it go?

2) The dip example. ANY vehicle hates cresting during hard cornering, but are we sure this change will make it worse? I don't see how. Remember if we are in a turn we have a good chance of being on the limiter or at our redound limit. So we are change nothing our of the ordinary FV cornering situation if we encounter a dip. We have no rebound available!

3) "But the lower range of camber would come about by some equivalent spring rate increase at the wheels." I don't understand how this could be. Please explain further.

Brian

[hardingfv32-1]

Chris

"I would also agree if you start with 4 neg instead of 5 neg, then you should reduce your droop to from -2 to -1 as roll and jacking travel seem to be consistent."

You are maintaining a range between static and droop because everyone thinks that moving the two settings close to each other is bad. The question is exactly what would happen that would be considered bad? I not saying that it is not necessary, but that I don't know why it would be.

Brian

[CitationFV21]

Chris

"I would also agree if you start with 4 neg instead of 5 neg, then you should reduce your droop to from -2 to -1 as roll and jacking travel seem to be consistent."

You are maintaining a range between static and droop because everyone thinks that moving the two settings close to each other is bad. The question is exactly what would happen that would be considered bad? I not saying that it is not necessary, but that I don't know why it would be.

Brian

I need to do a study, but forgetting about what camber does to the tire, I maintain that the more the negative camber, the less the force of jacking. It seems that the maximum jacking force would be if the axle was parallel to the ground as the torque would be vertical. Now I can't quantify this, so I can't give lbs of jacking force per degree camber....so the less the camber, the more the effective jacking.

Now for a practical application. Had the car set up and running fine with -5 and -2. Steve P was checking shock pressure and mine was low. Pump up from about 50 to 120 and go out - car almost undrivable. Get back to paddock and measure static at about -4. Extra pressure increased effective spring rate. Adjust spring perch so I have -5 again and car handles fine. Now I also could have decreased droop to -1 and see what happened, but tough to test under racing conditions. Go with what you know.

Moral of story - when you change anything - see what else it affects.....

I think if you get into droop stop before car is at full roll, you transfer weight from inside to outside and hence loose cornering force. I don't want to get close to droop stop until car is at full suspension load. I don't use droop stop to tune car, I use camber and shock setting - others may disagree.

ChrisZ

[hardingfv32-1]

Chris

I'm starting to get a sinking feeling that we can not separate the static setting from the droop setting under normal circumstance.

Brian

[smsazzy]

You would have to test it on a chassis dyno, but I believe the drag you would lose (in the form of rolling resistance) running 8 degrees+ of rear camber would be lost by the frictional losses of the axles going into the tranny at such a high angle.

[hardingfv32-1]

Very good point on the axle friction, a definite compromise.

Chris

"Had the car set up and running fine with -5 and -2. Steve P was checking shock pressure and mine was low. Pump up from about 50 to 120 and go out - car almost undrivable. Get back to paddock and measure static at about -4."

Something strikes me as odd about your example. I accept your statement completely, as I have been in a similar situation. That said, why would the handling be worse if the static camber is at a better point, 4 deg in your example? I assume that "undrivable" means loose and that the droop was about 2 deg.

Brian

[CitationFV21]

>>Chris

"Had the car set up and running fine with -5 and -2. Steve P was checking shock pressure and mine was low. Pump up from about 50 to 120 and go out - car almost undrivable. Get back to paddock and measure static at about -4."<<<

Something strikes me as odd about your example. I accept your statement completely, as I have been in a similar situation. That said, why would the handling be worse if the static camber is at a better point, 4 deg in your example? I assume that "undrivable" means loose and that the droop was about 2 deg.

Brian

Brian,

Sorry if I was not clear. Instead of 3 degrees of change before I got into the droop stop I had 2. Now it isn't necessarily the camber that made the car undrivable, it is the fact that the droop did not allow the suspension to move in that direction over bumps or during transitions. This resulted in uneven loading of the tires (I assume the inside more that the outside). I would like to do a test day and crank the droop to zero and run a video backwards and watch what the suspension is doing - maybe that will be the cheap man's data acquisition...

You would have to test it on a chassis dyno, but I believe the drag you would lose (in the form of rolling resistance) running 8 degrees+ of rear camber would be lost by the frictional losses of the axles going into the tranny at such a high angle.

I forgot about this - although it is why I try to run less than recommended at Lime Rock. Lots of high speed stuff and you don't need any excess drag.

ChrisZ

[hardingfv32-1]

Chris

1) With your droop at 2, do you assume that you were hitting the limiter? Would that be the case starting at either 4 or 5 deg static?

2) "the droop did not allow the suspension to move in that direction over bumps or during transitions"

Are you implying that the car handles VERY poorly when it is on the limiter in a BUMPY turn? Provide a little more droop clearance and everything is better?

Brian

[jpetillo]

I need to do a study, but forgetting about what camber does to the tire, I maintain that the more the negative camber, the less the force of jacking. It seems that the maximum jacking force would be if the axle was parallel to the ground as the torque would be vertical. Now I can't quantify this, so I can't give lbs of jacking force per degree camber....so the less the camber, the more the effective jacking. ChrisZ

Chris you are right that more negative camber gives less jacking. It's on the order of about 10-25 lbs per degree. But, using the center of the tire is the contact patch location, the jacking just keeps on rising as the car rises. There is no magical angle where things change, like zero degrees. For the camber angles we're working around, it's a fairly linear rise. John

[jpetillo]

I'm starting to get a sinking feeling that we can not separate the static setting from the droop setting under normal circumstance. Brian

Brian, sometimes you can and sometimes you can't. (Don't quote me on that.)

It depends on your setup. Some D-13's (maybe all?) for example start with the droop limiter squished very much at static camber. With this setup, you cannot separate the static setting from the droop setting. They are tightly coupled.

The other example is my car, which is not on the droop limiter as static settings, and will rise into it as the rear rises. In that case as long as my static settings are not touching the limiter, I can change static setting without affecting max droop much. There is always some small affect, but in this case they're almost independent. The softer the spring the less effect.

John

[DanRemmers]

What is the maximum angle the rear axles can move up or down without causing drag or damage?

That question is to help answer the next question: In a typical zero-roll car, can the chassis roll enough to hit those limits? Or does some other geometry prevent that?

My guess is that the front beam doesn't allow this except in an extreme case like hitting a bump mid corner. If that did happen, wouldn't the whole back of the car unload since the force of the bump would be transmitted directly to the gearbox?

My car is a solo car, so it's stiffer all around, and I keep the rear camber small since the tires are 10" wide. Still, I believe the fastest solo vees have z-bar rears.

[hardingfv32-1]

The axles can move 10-15 deg either direction if free to do so by the rocker system.

Rear axle camber is changes about 1/4 deg for every 1 deg of chassis roll. You need an off track excursion or accident to see big rear axle movement.

Brian

[brian]

Chris, one thing you mentioned in your senario, but didn't take into consideration, was the increase in pressure in your shock. Some shock folks believe in running as little pressure as possible. Without knowing the spring rate and shim packs in your shock, I cant' say for sure pressure change was a factor but it can effect the handling of the car. I've often used gas pressure to alter the high speed characteristics of the shock.

[cendiv37]

Brain H.

I made a CAD "stick" model of the rear swing axles of a Vee. Chassis roll only changes camber about 0.1 degree of camber per degree of roll. As expected, the outside goes more negative and the inside more positive (supporting Chris's assumption). However, the change is probably small enough to ignore.

Bottom line, whatever the total camber of the car, it is split pretty closely from side to side so if you are at zero degrees total, you will be near zero on both inner and outer tires. If you are at 4 total, you will be at near 2 negative on the outer and 2 positive on the inner.

The effect of roll center height change as total camber changes is a big effect (as John is suggesting). The roll center height will significantly change the weight transfer at the rear and the increased weight transfer combines with the higher roll center to increase the jacking force (which is what is already lifting the rear of the car towards more positive total camber). Again, this is what I mean by the system being capable of going "over center" if the camber is let go uncontrolled without a droop limiter. I agree with John's statements about the spring rate having an effect since the spring(s) are also trying to lift the rear of the car. The faster the spring force falls off with chassis rise (towards positive camber), the less it can contribute to lifting of the rear of the car.

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).

I'll post a table of camber angle vs. roll center height vs. roll angle numbers this weekend if I get some time... Been busy.

[hardingfv32-1]

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.

Brian

[hardingfv32-1]

Bruce, thanks for the input.

1) While I understand your approach and would find it very helpful in explaining what is happening with the rear suspension, I do not see the numbers adding up. 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 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?

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