Roll Center Discussion

[a.moore]

Think of it this way - if roll center height changes weight transfer then a 6" torque wrench and a 6" crowsfoot would be apply a different torque value than a 9" torque wrench and 3" crowsfoot.

[horizenjob]

Think of it this way - if roll center height changes weight transfer

I'm confused here. There is no argument that the relative height of the roll centers front and rear affect the weight transfer. Anti-roll bars, elastic stiffness, or roll center height, geometric stiffness, both add up to produce the roll resistance. The relative roll stiffness, front to rear, determines the weight transfer up until you lift a wheel or wheels off the ground.

People adjust both anti-roll bars and roll center heights on a daily basis in formula cars as they go to different tracks or change tires etc.

[Omaha Vette Graveyard]

If we go to one end of the car and we raise the roll center by 9", that will make a profound difference in how the car feels to the driver. Anyone on this website would notice the difference. If we instead move the roll center to one side or the other by 9 feet, you might notice a difference or you might not.

The roll center 9' to the side would mean the car will 'try' to rotate about that axis (pole vaulting 9' into the air), and it obviously won't do that, so the actual movement of the car will be based on compression of the outside spring and extension of the inside spring. The roll geometry doesn't really matter once the roll axis is that far out.

As a roll center (or roll axis, as is actually the case) moves out of range laterally it becomes irrelevant. The springs/shocks and weight of the car define entirely how the car moves once the geometry its out of range.

It is not that a laterally transient roll center doesn't matter, it's just that it takes roll geometry out of the equation as it moves sideways. I'm not sure that's a good solution, though one can see how it could work just fine in many cases.

Maybe this is the sticking point of force-based vs. geometric roll centers.

-Graveyard

[Driven5]

There are angles in how the suspension reacts to the force caused purely by the height of the roll center or more accurately by the line of force from the tire to the virtual swing arm pivot. Those are the jacking forces.

I don't know how to get my head around the torque wrench example.

You just did. The the lateral component of the normal force to the torque arm is the roll component, and the vertical component is the jacking component. So in this conceptual example, a vertically fixed roll center keeps roll forces constant, while a laterally fixed roll center keeps jacking forces constant. Granted, it's a gross oversimplification that merely illustrates a point, neglecting weight transfer effects and the fact that a car requires two interfaces with the ground to resist roll.

[mjalaly]

Well reduce the weight of your vehicle and there will be less weight transfer

Maybe there should be some weight transfer results toss in to see at what point is there deminishing returns

[Bobber]

Here’s a variation on the force method that seems to work pretty well. Many slow dances with Barbarella in my garage pretty much proved this out. I’m doing it manually now and need to get it more computerized. Comments welcomed. Sam, show me your teeth here; don’t be nancy.


Exhibit 1 shows the suspension geometry to be analyzed.

Exhibit 2 shows the wheels and suspension cycled through their full range of motions. This can be done manually or with an AutoCAD script file.

Exhibit 3 shows a chart of all the chassis and wheel positions with varying degrees of roll and bump/extension. This chart shows all the possibilities of chassis and suspension movement for the suspension geometry shown on Exhibit 1. It gives wheel camber and track in all positions. This chart is good for seeing where the wheels go wonky and what chassis positions you want to avoid. This chart is also good for comparing different suspension geometries since you can overlay one over the other. This chart can also be prepared using a script file.

Exhibit 4 calculates individual wheel lateral tire footprint forces from vehicle lateral g forces. Up to now, we’ve been dealing strictly with chassis geometry, not forces. For the given vehicle criteria, this spreadsheet calculates individual wheel lateral loads for any g-force. This exhibit currently considers lateral force to be proportional to the vertical force on the wheel, including distributed weight. This is where you can adjust your individual wheel lateral forces to reflect nonlinearity in your lateral tire forces relative to their vertical loads. This may take some iteration.

Exhibit 5 shows two things. The upper half shows a deflected chassis view from the chart in Exhibit 3, in this case the view for 5 degrees roll and no bump/extension. On this view, I have applied a unit load to each wheel and used mechanics to find the null point of the loads (fulcrum point) for that side. From this, I find the roll force on the chassis for the unit lateral force at that wheel (16.41 in k for inner wheel, 13.09 in k for outer wheel). This is still all geometric since the force is a unit force.

The lower half of this exhibit shows the modifications to the unit force results to show the results from the actual wheel loads in Exhibit 4 (150 lb for inner wheel, 1838 lb for outer wheel). The roll force from these loads is 2.46 in k for the inner wheel and 24.06 in k for the outer wheel. These are added for a total roll force of 26.52 in k for this configuration and loading. This gives a roll force rate of 442 ft lb per degree. There must be a counter force resisting this roll force to maintain this roll angle. This is supplied by the roll resistance of the suspension springs and ARB, and must equal 442 ft lb per degree. This requires a spring/ARB rate at the wheel of 168 lb per inch as shown on the lower half of Exhibit 5. Any more or less spring/ARB rate will result in a different roll and the appropriate roll chart will have to be used.

[a.moore]

Think of it this way - if roll center height changes weight transfer

I'm confused here. There is no argument that the relative height of the roll centers front and rear affect the weight transfer.

Both Staniforth (and whoever wrote that section in his book) and Sam feel that it does.

[horizenjob]

Hi Andrew, we agree, I just phrased that poorly. I mean to say that the height of the roll centers affecting weight transfer cannot be argued against. Clear as mud now?

[Sam_68]

Sam, show me your teeth here; don’t be nancy.

Why should I even waste my time explaining to a troll, for the umpteenth time, why it matters that both his diagrams and his dolly are two-dimensional, whilst a car even in the simplest (steady state) analysis, works in three dimensions?

You just don't get it, and never will, it seems. I'm done trying to explain it to you.

[KB58]

The fact that Lotus/Caterham/Locost cars handle well seems to indicate that either:
1. The designers fully understand roll centers.
2. It just doesn't matter that much.

[mjalaly]

The fact that Lotus/Caterham/Locost cars handle well seems to indicate that either:
1. The designers fully understand roll centers.
2. It just doesn't matter that much.

What does the modern caterham suspension look like geometry wise??

[Bobber]

Two things going on at once, here.
Remember those words, "There's a lot of things wrong with the car but a few that are right. And those things that are right overrule those that are wrong". And "Any suspension will work, etc.

Now back to the anal engineering: To tie the two ends together, proceed as shown.

[horizenjob]

So in this conceptual example, a vertically fixed roll center keeps roll forces constant, while a laterally fixed roll center keeps jacking forces constant.

OK, but in real life the biggest influence on jacking forces is the vertical motion of the roll center and motion one lateral direction has a very different affect than the other direction. So on the Vsusp for Car9 at one end the lateral movement indicates a small amount of jacking force change. At the other end the lateral change is showing a very small change in jacking force, but a larger change in anti-jacking on the inside wheel.

As the racecar engineering article points out, these changes in jacking force do not represent changes in the vertical force on the wheel which is weight and weight transfer.

Why should I even waste my time explaining to a troll, for the umpteenth time, why it matters that both his diagrams and his dolly are two-dimensional, whilst a car even in the simplest (steady state) analysis, works in three dimensions?

You just don't get it, and never will, it seems. I'm done trying to explain it to you.

Bobber is not a troll, that is a silly comment. I don't actually recollect any explaining from you in this thread. What Bobber is doing is explaining, the occasional sense of humour is just extra benefit, unlike my humour which is more like collateral damage.

[Driven5]

...troll...

Why should I even waste my time explaining to a troll, for the umpteenth time, why it matters that both his diagrams and his dolly are two-dimensional, whilst a car even in the simplest (steady state) analysis, works in three dimensions?

You just don't get it, and never will, it seems. I'm done trying to explain it to you.

I can guarantee that you learned how to solve problems in 2D before you learned how to solve them in 3D...So why would you deride others for doing likewise? Arrogance and pride are nothing more than alternate forms of ignorance.

As the racecar engineering article points out, these changes in jacking force do not represent changes in the vertical force on the wheel which is weight and weight transfer.

During steady state, after the body has taken a set, this is true. However, the springs and dampers cannot react forces from a transitioning sprung mass in isolation from the tires.

[Sam_68]

I know what the word 'troll' means, and I use it advisedly. Boober [sic] isn't anything like as dumb as he's making out, but neither is he quite as clever (knowledgeable) as he'd like to pretend to be. He clearly hasn't understood my previous explanations (there's enough info there for anyone to think it out and understand it for themselves, if they are ever going to do so), so he's posting what he thinks are cleverly crafted but deliberately provocative arguments and comments to try to entice me into giving a fuller explanation,

For that reason alone, I'm not going to.

He's using the psychology of the elementary school yard. I find that it's bad policy to humour mischievous children... it only reinforces their behaviour.

I can guarantee that you learned how to solve problems in 2D before you learned how to solve them in 3D...So why would you deride others for doing likewise?

You're right, I did. But when I realised (myself) that there was a fatal flaw in that model, I was bright enough to accept that there is no 2D solution (and no valid solution that doesn't account for spring compliance) and moved on. You can flog the dead horse of two-dimensional. 'springless' analysis as long as you like, with ever more complex mathematics, but it's still never going to work.

Once somebody has circumnavigated the earth, you don't carry on sailing round looking for the edge...

But perhaps I'm simply expecting to much of a nation where large parts of the populace aren't willing to accept Darwin's Theory of Evolution, that global warming isn't really happening, or that it isn't a really dumb idea to invite Donald Trump to run their country? Maybe Flat Earth Theory and 2-dimensional suspension analysis merely add to the suite of national mass idiocy?