Why wouldn't you put the Roll Center below ground?

[Caito]

Hi guys, before an endless discussions begins. Where people start arguing there's no ideal roll center position, nor height nor anything. I'm not aiming there

In general lines, so as to reduce jacking forces (produced due to having roll center above ground) I ask myself, why wouldn't I put it below ground?

What would be the pros and cons of having a roll center below ground.


Thank you very much

Bye bye

[Jersey Tom]

Jacking (and anti-jacking) forces aren't inherently bad.

[Belatti]

First you have to understand what does the roll center height causes to know where do you want it.

To make it simple lets quote good ol Millikens:





Now be may have 4 basic positions:

1) Roll center height (from now on RCH) = center of gravity (CoG)
No pivoting here, it means that there is no roll. Its like trying to spin a door applying force in the hinge. The car is turning, lateral force is applied, but there is no roll. Hence, all the force is "catched" by the wishbones. This makes the car as hard as a rock, as spring/dampers doesnt work here. Its good for nothing.

2) RCH between CoG and the ground. Depending the percentage of that height you distribute how much force goes through the wishbones and how much through the spring/dampers. The range between 15% and 30% of RCH compared to CoG is the most common place to locate it in many racing cars.

3) RCH = ground height. All the lateral forces passes from the chasis to the wheels throught dampers/springs, so virtually the wishbones makes no force under pure lateral load condition.

4) RCH below ground. More force than whats actually transferred passes through the spring/dampers, so that the wishbones is loaded unders "a negative" force. This means outer top wishbone for example is not under compresion, but under traction.
This is the case of tourism racing cars that have to maintain the suspension geometry from the original street car when you reduce their ride height, there you have to find the best compromise between what you gain from aero and reduced CoG height and what you loose for poor suspension geometry. Here you dont have jacking, but the contrary. Also it is the case of heavily "tuned" street cars... puaj!!!... .

I think I gave you the elements for you to figure out why you wouldnt locate it there or near the center of gravity...

[mep]

4) RCH below ground. More force than whats actually transferred passes through the spring/dampers, so that the wishbones is loaded unders "a negative" force. This means outer top wishbone for example is not under compresion, but under traction.

Yea but what is bad about it?
Just use strongers prings/dampers.

The disadvantage is that you have more roll due to your increased leaverage arm between RC and CG. On the other side its easier to achieve a low RC than a high one.

[RH1300S]

It's an interesting idea, one that I have pondered before. TBH I am having trouble getting my head around the locked up suspension idea with the RCH & CG being in the same place - if someone could make it easy for a layman to understand it would be great!

With RCH below ground surely you would get the jacking force trying to push the chassis into the ground (which sounds good) - but simultaneously would it not also try and lift the loaded wheel?

[machin]

Yea but what is bad about it?
Just use strongers prings/dampers.

If you use stronger springs/dampers then yes you gain by having less body roll (although the car's total LOAD transfer is still the same as total load is determined by cornernig G forces, CG height and average track width, not suspension geometry), but you ruin the bump control.. i.e. you get a harsh suspension... and that means less traction over bumps...

You also need to watch out as increasing the roll resistance at one end of the car compared to the other will change the roll resistance distribution and could then lead to over or understeer (the stiffer end will take more of the load transfer and will therefore loose grip first (all other things being equal))...

[mep]

As far as I know its not uncommon to have RC below groung leven on F1 cars.
Especially on the front of the cars.

[Caito]

Thank you very much for the answers!!

Jersey Tom: Would you please extend your answer?

Belatti: Nice answer. Got away with 1, 2 and 3. Still have to re-think the 4th one.



RH1300S, I think I know the reason, will extend when I get home.



Bye bye!

Caito.-

[Jersey Tom]

Well Caito, what specifically do you want to know?

You'll have some movement of the sprung mass up or down with applied lateral force. Why would that inherently be bad?

In my mind, could potentially be great if you manage it well.

[Caito]

Ok Jersey Tom. I just don't see in which case an upward jacking force would produce a net benefit. Maybe you could just cite an example for reference purposes. I thought I would like the car as stuck to the ground as possible.

If it's not too much to ask, another case in which the downward jacking force would be benefical, so I can continue the thinking on my own, cause at the moment I believe I'm a bit stuck.


Thank you very much, bye bye

Caito.-

[Jersey Tom]

"Stuck to the ground" has nothing to do with it. The total load of the tires is the same, you're just moving the sprung mass up and down.

Exception being ground-effect sensitive aero devices, in which case, again... anti-jacking forces would not inherently be bad.

[Strucnjak]

you can also change your response charateristics with roll centre position, you are really tuning your elastic to geometric weight transfer ratio by where you position the rc.

A car with the rc closer to the cog will have higher % of geometric weight transfer that occurs, geometric weight transfer is instantaneous so you can have a car that points well but doesnt have as much mechanical grip compared to a car with a lower roll centre which will have better mechanical grip (using your springs more) but poorer response(but is really dependant on axle stiffness, tyre side wall stiffness and spring and damper rates)

Write a spreadsheet that calculates your wheel loadings, elastic weight transfer, geometric weight transfer, jacking force, roll angle, wheel displacment and you will get a good idea of what changing the rc does to your car in steady state cornering, its the simplest way to understand the pro's and con's...

[Caito]

The total load of the tires is the same, you're just moving the sprung mass up and down.

Oops, I was clearly missing this point.

One axis car just for simplification. If I have a negative jacking force pulling my car down, hence, compressing the springs. How can it be that the load of the tires is not increasing?I'm still missing a part.

Wouldn't a negative jacking force pull the car down enough for it to have a significant effect in CoG height? Which would be an improvement, but I think the downsides are much bigger.

[mep]

Write a spreadsheet that calculates your wheel loadings, elastic weight transfer, geometric weight transfer, jacking force, roll angle, wheel displacment and you will get a good idea of what changing the rc does to your car in steady state cornering, its the simplest way to understand the pro's and con's...

Do you have such a sheet?
I was thinking about doing such a thing with excel but didn't had the time to do so until now.
If you have such a similar thing it would be very kind to provide it.
[-o<

[Jersey Tom]

The total load of the tires is the same, you're just moving the sprung mass up and down.

Oops, I was clearly missing this point.

One axis car just for simplification. If I have a negative jacking force pulling my car down, hence, compressing the springs. How can it be that the load of the tires is not increasing?I'm still missing a part.

Wouldn't a negative jacking force pull the car down enough for it to have a significant effect in CoG height? Which would be an improvement, but I think the downsides are much bigger.

The total steady-state load on the car can only come from gravity and aero. That's just basic physics.

Normally, the sprung mass is just supported by the springs. When the tires generate lateral force, the sprung mass is either pulled or pushed through the links. In order to reach equilibrium again, the springs have to either take up more or less load, when then moves the sprung mass.

With "reasonable" RC heights, the net jacking forces wind up being pretty small anyway.