Rear Roll Resistance Never Needed

[hardingfv32-1]

.... THERE IS ABSOLUTELY NO NEED FOR REAR ROLL RESISTANCE .... Everything you might want to accomplish with car balance can be achieved with modifications to the front end. The rear suspension should be setup for max cornering force and not asked to assume any responsibilities that would detract from this objective. FVs are rear traction limited with the our current tires.

Reasoning for this statement:

I am assume a good std setup on a front running FV with normal/average track conditions.

At max corner force in the center of the turn, the rear suspension has jacked up and put the rear tire at optimum camber (say about 1 deg per side). The rear tire are generating maximum cornering force for a set of std conditions. At the front, for a variety of reasons, we know that the wheels have gone to positive camber. I am assuming no one will state that they are at negative camber in the center of the turn. So the front tires are NOT at the an optimum camber level, yet we have a car that is balanced in the corners. This means to me that we can achieve a balanced cornering setup with the front tires NOT producing maximum cornering force. I can always dial in more static front camber to get more dynamic/cornering camber and thus more front end cornering performance. This is not possible at the rear. All you can do at the rear is degrade the cornering performance, say with a roll resistance requirement.

Challenges to my thoughts?

Brian

[CitationFV21]

Teacher - the computer ate my homework....

I had a reply in drafts and it is gone! I will be back - too POed to rewrite it now....

ChrisZ

[jpetillo]

Brian,

I agree with what you said with regard to mid-corner static conditions. I think we should discuss the dynamic situation that occurs when you enter a corner as you transition to that static condition.

Perhaps the worst condition for the front is when the roll is stopped by the roll bar/spring combination. A that time the outer tire gets an increase in weight transfer and the inner tire gets reduced weight. Does this ever cause the front end to slip, or do drivers compensate by turning in more gently than they would like so that the outer front doesn't get overloaded?

In this case, slaving some of the roll resistance through roll damping at the rear may allow the driver to turn in faster.

What do you think? John

[hardingfv32-1]

Before answering your question, lets settle on an answer for a related question of mine:

How does the force we call weight transfer actually make its way to the tire contact patches?

I ask this because the formula for weight transfer is: Centrifugal force X CG height / Track width. There is no input/variable for degrees or rate of roll. IF we ASSUME that the CG and track are fixed, then weight transfer is a function of the corner force that develops in a corner. Soft springs, hard springs, or no springs (karts), the weight transfer is the same. How do variation in roll, with a sprung system, not effect the forces seen at the contact patch of the tire? The formula says they don't, correct?

Are we dealing with a platform (system) that rests on the contact patches? Some of the components of this platform could be moving/rolling, but the tire contact patches can't sense that. The contact patches can only sense the platform mass as a unit.

Brian

[brian]

Brian, doesn't cg change when the car rolls? It's nice to make certain assumptions to simplify an analysis but in reality everything is changing. I have always thought that two wrongs don't make a right and isn't reducing grip in the rear to regain front grip just that?

[CitationFV21]

Okay here is what I originally wrote - a different take from where the others are going:

If the rear is at optimum capability, then my mantra kicks in:

"Never reduce handling at one end to achieve balance."

But what if adding roll resistance to the back can shift weight so the front handles better without hurting the back?

In FF, you can make a car go from understeer to neutral to over steer. For most FV you can go from understeer to less understeer. If you car is oversteering then you have not optimized the rear relative to the front and you need to work on the rear. But if you get the car balanced, that does not necessarily mean that you can't get more out of the rear, hence ending up with understeer!

If you took 10 cars with equal tires and ran them on a large skid pad, one would be the best in maximum cornering force. So that would be your benchmark for all the others. But could you get there without copying the best car, down to chassis, geometry and weight balance?

On the other hand, if the FV is perfect on the rear, then that makes it even better as a beginner's car as there is less to screw up. With zero roll don't have to worry about corner weights. Air cooled means no complicated liquid cooling system (with all it's belts and pumps) and plain electronics means that anyone can trouble shoot it. Maybe that's how we should be selling the class!

ChrisZ

[hardingfv32-1]

Brian...
1) For this discussion we must remain focussed on on what the effects of weight transfer are, so CG needs to be ASSUMED to be fixed. Any other variables and people start to drift off point.
2) Why can't I just increase the front camber if the front needs more grip? Front camber is POSITIVE, certainly not negative, at max cornering force. Seem like a lot of room for improvement, IF NEEDED.

Chris...
IF we assume the rear is perfect, then any change will make it less than perfect by definition. Assigning roll resistance to the rear can only lower the maximum corning performance of the rear suspension.

"If your car is over-steering then you have not optimized the rear relative to the front and you need to work on the rear. But if you get the car balanced, that does not necessarily mean that you can't get more out of the rear, hence ending up with understeer!"

First, you MUST ASSUME that the rear is perfect for this discussion. We can not have a question about which end is causing the trouble. We are solving for one variable not two. Without this assumption your views become pointless. I pick the front end to work with, because I am certain that there is room for improvement using more camber. That can not be said about the rear.
If your FV is over-steering, then reduce front end performance by using less front camber. If your FV is under-steering, then increase front camber. Again, this assumes the rear is set perfect.

Brian

[CitationFV21]

Brian...
.....................
First, you MUST ASSUME that the rear is perfect for this discussion. We can not have a question about which end is causing the trouble. We are solving for one variable not two. Without this assumption your views become pointless. I pick the front end to work with, because I am certain that there is room for improvement using more camber. That can not be said about the rear.
If your FV is over-steering, then reduce front end performance by using less front camber. If your FV is under-steering, then increase front camber. Again, this assumes the rear is set perfect.

Brian

"If your FV is over-steering, then reduce front end performance by using less front camber."

You just violated your own rule. If the car is oversteering then either the setup or the weight distribution is wrong - reducing front performance will result in a balanced car, but also less total grip. Now, some people can drive a balanced car better than an undrsteering or oversteering car, but ultimately it will not be fastest.

Remember - I am not disagreeing with you in theory, just that in practice I am not sure we can get there. Assume a go kart with an inflexible frame - the only way to get it to handle is to shift weight around - that is what we do with slot cars - even then, things flex and drive you crazy.....

ChrisZ

[hardingfv32-1]

No argument, we are just hammering on my theory to see if it can work.

"If the car is over-steering then either the setup or the weight distribution is wrong"

By the way I have defined the problem, the rear end setup is perfect and the front is NOT. I will now state the everything else about this car is perfect, including the weight distribution. So we have only the front to adjust. Does it not have a range of adjustment available that can solve any of our balance issues? Our goal is neutral balance. While in real life this might not make for the fastest car/driver combination in all cases, for this discussion we need a simple clear goal... neutral balance. So to balance an over-steering FV with a perfect rear setup, we must reduce the performance of the front end.

I'm trying to keep you boxed in to this very limit type of chassis adjustment. In what cases would it not meet your requirements?

Karts are a completely different ball game because of their solid rear axles. Not a good example for your point.

Brian

[CitationFV21]

If you are going to maximize the front end, two things have to happen:
1. Keep the tire in the best camber to generate cornering force.
2. Maintain the best maximum loading on both tires

(3. you also have to have the best ackerman - but will leave that out for now.

You have already talked about static camber, you can also work with caster to get some more.

Now if you since the back is jacking and by that nature weight is moving forward, then you are loading the outside front tire the most. If you stiffen the roll bar, you get more transfer to the outside but you might have more camber control and if you go to a smaller bar you get more roll and worse camber. Which might be why a softer bar might increase understeer and a stiffer one decrease understeer - just the opposite of what you would expect. The only way to know if your car would do this is to test all sizes.

If you reduce jacking, you reduce weight transfer, I go back to the D13 and think it handles the weight transfer without jacking. They seem to run less static camber which implies almost a de Dion type suspension. Or it could be the roll damper just hits the sweet spot.

Gene Grimes used to run rear tires all around. The extra grip in the front handled great, but it didn't go down the straight. Some people have run short tires in the back with problems but this might point out that we have less tire in the front that we should have. I usually run the wide fronts to balance my car, but give up a little on the straight.

I need to go through my pictures to see what the front end is doing.

ChrisZ

[hardingfv32-1]

We are not trying to maximize the front. We are trying to balance the car by adjusting the performance of the front. This takes very little extra camber. 1 deg additional negative camber is about equal to using the wider fronts. So that is the range we are talking about.

Brian

[jpetillo]

Before answering your question, lets settle on an answer for a related question of mine:

How does the force we call weight transfer actually make its way to the tire contact patches?

I ask this because the formula for weight transfer is: Centrifugal force X CG height / Track width. There is no input/variable for degrees or rate of roll. IF we ASSUME that the CG and track are fixed, then weight transfer is a function of the corner force that develops in a corner. Soft springs, hard springs, or no springs (karts), the weight transfer is the same. How do variation in roll, with a sprung system, not effect the forces seen at the contact patch of the tire? The formula says they don't, correct?

Are we dealing with a platform (system) that rests on the contact patches? Some of the components of this platform could be moving/rolling, but the tire contact patches can't sense that. The contact patches can only sense the platform mass as a unit.

Brian

Brian, the equations for weight transfer can be based on different frames of reference, but all give the same final answer. What you said is correct, that given a CG location the weight transfer between inner and outer is determined, but it's not completely determined between front and rear. What keeps the car from rolling (springs and anti-roll bars) has some affect on the front to rear biases. If the CG was aligned with the car's instantaneous roll axis, then what you said is exactly right. But, when the CG is above the roll axis, to stop the roll you have to apply a torque to resist the roll. For Vees with zero roll resistance in the rear, that torque is provided only by the front two wheels. The extra force added to the front outer to stop the roll is taken from the rear outer, and the force reduction from the front inner is added to the rear inner.

Since the cars only roll a few degrees, the movement of the CG is pretty negligible in its affect on the weight transfer.

So, different front roll bars should have little affect on the mid-corner static weight transfer, but they will affect the weight transfer in a dynamic condition where you are transitioning from initial turn in to mid corner.

Chris Z, although it's against your cardinal rule (cardinal rules are great things), I think I agree that if the rear is assumed to be at its optimum you can't make it better, so to balance an oversteering condition you have to reduce front grip. This is not considering that you can make up for some of the oversteering by driving differently, but I'm guessing that you'd perhaps do things like just steer faster to stop the oversteer and the fronts would have a too-great a slip angle and lose traction - pretty much the same result as reducing front grip from changing camber settings.

I think what I said is correct, and I left out a lot of nit-picky details that would put me to sleep so take these as generalities. Let me know what you think and if you agree or not. John

[jpetillo]

Now since the back is jacking and by that nature weight is moving forward, then you are loading the outside front tire the most. ChrisZ

Chris, I'm just going to comment on this one point. I thought about what you said and started to agree and disagree - ended up somewhere in between! The amount of forward weight transfer due to jacking alone is negligible. I'm talking about the increased weight to the front just from raising the rear end that same amount. (We can calculate that.) I also think that the roll moment that the front anti-roll bar is countering also doesn't change much as the rear raises because the roll axis raises with the jacking so the moment doesn't change. However, the rear weight transfer is definitely affected from the height change of the CG that is mostly in the back. My guess is that the rear jacking increases weight transfer mostly at the rear and not much at the front. John

[hardingfv32-1]

So, if we restrict our discussion to weight transfer only.... springs and sway bars have no effect on weight transfer ONCE we reach a steady state, mid turn condition?

Now to model the dynamic part of this weight transfer situation, getting to and leaving the middle of the turn.

I'm going use the mental example of the weight transfer being represented by a liquid filling two fixed volume containers, one at the front and one at the rear. As the car enters the turn, liquid flows into these containers at different rates based on the spring, sway bar and shock(?) combination. No matter what the combination the containers always end up filling front and rear fluid container to their same limits. Can one container fill-up before the other? Or if they must finish up at the same time, if one end starts slow it must end faster relative to the other.

Brian

[CitationFV21]

........

Chris Z, although it's against your cardinal rule (cardinal rules are great things), I think I agree that if the rear is assumed to be at its optimum you can't make it better, so to balance an oversteering condition you have to reduce front grip. This is not considering that you can make up for some of the oversteering by driving differently, but I'm guessing that you'd perhaps do things like just steer faster to stop the oversteer and the fronts would have a too-great a slip angle and lose traction - pretty much the same result as reducing front grip from changing camber settings.

I think what I said is correct, and I left out a lot of nit-picky details that would put me to sleep so take these as generalities. Let me know what you think and if you agree or not. John

John

If we are playing in the theoretical then if the back is perfect and the front is not, then the car cannot be oversteering. Lets look at an exagerated example:

Take a FST and put FV tires on the front. In a steady state condition the front will always break away before the back, the car will have terrible understeer. To get it to be balanced, you would have to find a way to decrease traction in the rear. It may be balanced but it will handle worse. This was the problem with front wheel driver cars - they had to understeer. Then drivers did all things to trick weight distribution like power braking, throwing the car into a drift etc.

If a car has optimized handling at one end and not the other, and assuming the suspension alignment is best it can be, then the only thing you can do is move weight around, play with tire width, or change suspension geometry.

I think we are getting into the The Kobayashi Maru test - we either do not have enough information or the parameters are skewed so it cannot be solved.

ChrisZ

[hardingfv32-1]

No... there is all the info you need for the problem I stated. It is very restricted to control the focus of our discussion. Your FST example could be correct, but it has no bearing on this thread as we must assume that we are using the tires that are in current production.

"and assuming the suspension alignment is best it can be" I have NOT assumed that the front camber is correct or anything about the front suspension for that matter.

Brian

[problemchild]

You guys should get a room

So many presumptions going on ...

Starting with ..... all FVs are naturally nuetral balanced? All FVs understeer. If yours does not .... then there is something wrong with your rear suspension. Change the droop or rebound settings and we'll have your FV pushing like a pig in 10 laps. Most of us need help getting the front end to work better ..... hence the theory that we can use rear roll resistance to help the front while sacrificing minimal rear grip which we have surplus. I'm not promoting that theory ..... I don't need to .... but that would be the theory that people using rear roll resistance would argue.

Also .... put decent front FV tires on a FST ..... it will be loose as can be. For those not aware, FST tires are hard and last for 25-30 sessions. Whether FV or FST, we're only using a couple inches worth of rubber. New FV tires generate much more grip than new FST tires. Six session FV tires generate similiar grip to new FST tires. Eight session FV tires generate similiar grip to 20 session FST tires.

Carry on .....

[CitationFV21]

Also .... put decent front FV tires on a FST ..... it will be loose as can be. For those not aware, FST tires are hard and last for 25-30 sessions. Whether FV or FST, we're only using a couple inches worth of rubber. New FV tires generate much more grip than new FST tires. Six session FV tires generate similiar grip to new FST tires. Eight session FV tires generate similiar grip to 20 session FST tires.

Carry on .....

Greg,

I stand corrected - I assumed that the width would exceed the harder compound. What I was trying to do is to "over tire" the rear compared to the front.

Not to hijack the thread, but since you drive both, does a FST not handle better than a FV given new tires?

ChrisZ

[tiagosantos]

What is it exactly that causes the rear end jacking? And is there anything that can be done to limit the jacking effect? I don't understand enough about how our typical zero roll setup works mechanically to figure this out just yet.

(by the way, what is it then? Are we grip limited in the front or rear, or does that depend on who you ask? My car oversteers, I wouldn't say badly, wouldn't call it balanced either. But seeing as I haven't touched the alignment at all since I bought it from a much heavier driver, I'd be very happy to find out that there is more grip to be had in the rear. Would not mind an understeering car at all, if the overall grip was higher than it is right now).

[jpetillo]

So, if we restrict our discussion to weight transfer only.... springs and sway bars have no effect on weight transfer ONCE we reach a steady state, mid turn condition?

No.

Now to model the dynamic part of this weight transfer situation, getting to and leaving the middle of the turn.

I'm going use the mental example of the weight transfer being represented by a liquid filling two fixed volume containers, one at the front and one at the rear. As the car enters the turn, liquid flows into these containers at different rates based on the spring, sway bar and shock(?) combination. No matter what the combination the containers always end up filling front and rear fluid container to their same limits. Can one container fill-up before the other? Or if they must finish up at the same time, if one end starts slow it must end faster relative to the other.
Brian

Yes, No.

[CitationFV21]

........But seeing as I haven't touched the alignment at all since I bought it from a much heavier driver,...........

Very critical that you check your alignment with you in the car. This has happened to many of us before.

With you in the car, set your rear camber. (if you are lighter, then you will have to make it more negative)

Then check your droop stop.

And maybe soften your shock.

I think one of the biggest problems with zero roll vee suspensions, is that people get into the stop either too soon or too hard.

I would like to see more work done with progressive stops that cushion the transition.

Maybe we should break this off to a separate discussion.

ChrisZ

[jpetillo]

John
If we are playing in the theoretical then if the back is perfect and the front is not, then the car cannot be oversteering.

No, the back being perfect means that we are getting the maximum traction from the rear tires that we can. It says nothing about the front. It can oversteer if the front gets even more traction.

Lets look at an exaggerated example:
Take a FST and put FV tires on the front. In a steady state condition the front will always break away before the back, the car will have terrible understeer. To get it to be balanced, you would have to find a way to decrease traction in the rear.

Yes, under the assumption that you can't do more to get more traction in the front. I believe that this was Brian's premise about the rear.

It may be balanced but it will handle worse.

Perhaps. I expect that you can drive an unbalanced car that has ultimately more traction faster than a balanced car with less. I may not, however.

If a car has optimized handling at one end and not the other, and assuming the suspension alignment is best it can be, then the only thing you can do is move weight around, play with tire width, or change suspension geometry.

Agreed!

I think we are getting into the The Kobayashi Maru test - we either do not have enough information or the parameters are skewed so it cannot be solved.
ChrisZ

I think we have enough information for the level of the discussion at this time, but perhaps the premise is not yet agreed on by everyone. John

[jpetillo]

What is it exactly that causes the rear end jacking? And is there anything that can be done to limit the jacking effect? I don't understand enough about how our typical zero roll setup works mechanically to figure this out just yet.

What causes jacking has nothing to do with zero roll, although your suspension (springs and droop limiters) determines how much it jacks. Jacking comes from the inner and outer tires having different amounts of traction resulting from weight transfer in a corner, and they are effectively getting shoved together. If you shove them together, the rear end jacks up. John

[hardingfv32-1]

.... ALL FV UNDERSTEER ....

And as shown by any photo of a FV cornering at above 1G, good hard corner, the front outside tire camber is POSITIVE. Is this not a FACT?

Why can't something simple be done to correct the front camber situation, like starting with even more negative camber?

WHY IS THIS NOT THE SOLUTION TO THE FAMOUS FV UNDER-STEER?

Brian

[hardingfv32-1]

Chris

Start a thread on droop limiters, but be warned, that no one has any idea what is going on with the droop limiters.

In general there is no better handling car than mine (not saying the best) west of the Rockies. I do not have a droop limiter. The shock is used to keep the camber from going to positive camber as a safety precaution. If you want, you can say the droop is set at 0 deg. I use a normal rocker type zero roll rear suspension.

The use of droop control is a complete FV myth.

Brian