GRC (roll centre)

[JohnL]

I think you're right, new thread:

55EXX wrote:
“i see my understanding as being the rc is the point at which the car rotates around”

This is the first and most common misconception about geometric roll centres (GRC, of which there are two, one located in a vertical plane defined by the contact patches at either the front or rear axle lines).

Despite their name, GRCs are NOT the points about which sprung mass ‘rolls’, except in very specific circumstances. These circumstances are:

1) At the infinitely small moment at which weight transfer begins to occur and before the springs / dampers / ARB have begun to deflect.

2) When the GRC and the CG are located at the same height (though in this instance no roll motion will occur).

3) When the springs etc provide zero roll stiffness (e.g. some *Formula Vee ‘zero roll stiffness’ rear suspensions where the springs only have bump stiffness and contribute zero roll stiffness - see footnote).

A GRC is a theoretical point defined by suspension geometry that is involved in dictating what % of the weight that is transferring at that axle line is transferring either geometrically (through the suspension linkages) or ‘elastically’ (via the springs, dampers and ARB). Spring rate etc is also involved in dictating the ratios of geometric and elastic WT, so GRCs aren’t the sole determinant of the %s of transfer occurring through both vectors (i.e. geometric and elastic).

The relative %s of WT occurring geometrically and elastically vary constantly during any corner, and the amount transferring through either vector does matter because WT that occurs geometrically is ‘instant’ and WT that occurs elastically is ‘slow’.

Geometric weight transfer occurs at the rate of the lateral force (acceleration), i.e. if the lateral acceleration increases by say 10% in one second then the geometric WT also increases by 10% in one second. However, whatever % of WT is occurring elastically will occur more slowly, so if the lateral acceleration increases by 10% in one second then the % of WT that is occurring elastically may take say 2 seconds. This may seem trivial, but it has huge implications for the transient handling characteristics of the car, and the car’s reaction to bumps etc.

To make things more interesting, as lateral acceleration increases the % ratio of geometric vs elastic WT will change, with the geometric vector tending to become a lesser % of the total WT and the elastic vector becoming the dominant vector of WT.

I’m only scratching the surface here, this is only a hint of how complex this stuff is, it can do your head in. Even when you have an understanding of some facets of roll theory, it’s hugely difficult to describe it to other people.

“and the cg is the mass centre and determines the length of the moment arm around the rc. the further the cg is away from the rc the longer the moment arm and therefore the greater roll force produce from the same cornering g forces.”

More or less.

“so by creating a environment that the rc comes closer to the cg as beneficial for increasing roll stiffness geometrically by shortening that moment arm other than mechanically by the use of sways and springs rates etc. this would then mean with softer springs and sways a softer ride can be enjoyed and i guess the wheels would follow the undulations in the road better but what other advantage is there? like i said before increased front roll resistance can be increased by bigger sways, and which with soft springs can increase front roll resistance and follow undulations in the road.”

Suspension design is rarely that simple. Advantages of high GRCs are very few, and the downsides are substantial and more numerous. It does tend to be advantageous to bring the GRC and the CG closer by means of lowering the CG, but not by raising the GRC (at least not very often and only in specific instances for specific reasons).

The higher the GRC the greater the % of WT that will occur geometrically and thus the less responsive the car will tend to be to changes in suspension rates, so the handling becomes difficult to tune with spring / damper / ARB rates etc. The lower the GRC the more WT will occur elastically and the car will be more responsive to associated changes to springs etc. This is especially important for racing cars, but also important for ‘performance’ road cars.

Higher GRC is also strongly associated with lateral scrub of the contact patch (with independent suspensions), i.e. as the wheel rises it also moves laterally outward, so when you pass over a bump the chassis tends to get ‘pushed’ laterally (inward when cornering and the outer wheel hits a bump). Force goes both ways (ask Isaac), so this also means that the chassis mass / inertia can ‘push’ abruptly and hard on the contact patch, potentially causing it to lose grip suddenly at / near the cornering limit. Lower GRC results in less lateral scrub with vertical wheel motion.

Then there are the problems with abrupt WT caused by the GRC being high, wheel jacking effects etc…

An instance where changing the GRC height is used as a tuning aid is in V8 Supercars, where the Watt’s Linkage (lateral locating system for the rear axle) can be raised / lowered by means of a lever in the cabin, which raises / lowers the rear GRC. The drivers use this as a means to fine tune turn-in as grip conditions change (yes, rear GRC change affecting front grip at turn in…), but this is also associated with the mandatory use of locked differentials (i.e. I doubt it would be so needed or effective if a more ‘normal’ differential were used).

-------------------
*Formula Vee cars are forced to use the VW swing axle rear suspension that suffers from the GRC being way too high and thus the rear geometric roll stiffness being way too high. The last thing they want is more roll stiffness from the springs, so the spring (only one rear spring with this type of suspension) bears (via rods and cranks) against the suspension on each side but not against the chassis, i.e. the left suspension bears through the spring against the right suspension, thus giving bump rate but not roll rate.

Note; roll theory is very important for most cars, but tends to become less important the less suspension motion there is and the more the car relies on down-force for grip and handling balance etc. Modern F1 cars have really terrible suspension geometry (front suspension most particularly is truly awful) because the down-force is so great and the suspension motion very small (to keep the under-tray at a stable distance from the track), and the down-force generated grip totally overwhelm any geometric affects. The aero considerations are far more important than the suspension geometry, so the suspension members tend to get placed wherever they will interfere least with the air flow, not with all that much regard for optimum suspension geometry.

It’s interesting to know that our cars have substantially better suspension geometry than current F1 cars…

[nd55]

ughh! This is more than I read the whole of 7th grade....

Nick.

[string]

Engineering is the art of compromise. I think that's all that needs to be said in regard to the above comparisons between modern F1 and 90's Honda suspension.

[JohnL]

ughh! This is more than I read the whole of 7th grade....

Nick.

That's more or less my point. That rave just scratches the surface and doesn't really investigate the physics in any meaningful way, and it gets harder and harder to understand the deeper you go into it.

It's not so much that each aspect of weight transfer and roll motion is hard to understand, but there are always a number of simple things going on at once and it's the interactions between these things that gets complex.

At the level of the amatuer enthusiast it's probably better to try to work with understanding the affects of changes to the elastic vector of WT (ie. springs, dampers, ARBs etc), rather than the geometric vector. There's not all that much we can do with the geometric vector in any case, other than to understand that while the roll geometry created by the designer may not be perfect, it's probably a lot better than we can manage ourselves, and that when we mess with it we will more than likely make it worse...

[Sp00ny]

Nice Writeup John.

Before we know it, you'll be have a sticky up about "Flux Capacitors" haha xD

It would be interesting to hear your view on the roll centre adjusters from aftermarket manufacturers and there advantages(if any)/disadvantages.

[JohnL]

When you substantially lower the ride height, the front wishbones become angled upward from the chassis to the ball joints. This changes the static location of the 'instant centres' of the 'virtual swing arms' and thus changes the static location of the geometeric roll centre (lowering the GRC). This isn't necessarily a bad thing in and of itself, and in fact a somewhat lower static GRC location may actually be a good thing (especially at the front axle line), though it would be advisable to keep it from going below ground level.

However, when the wishbone angles are changed substantially from stock, there is also an increased tendency for the dynamic locations of the VSA ICs to change location with suspension motion (as occurs in roll) in a manner that tends to cause the dynamic location of the GRC to change more than with stock wishbone angles (maybe much more).

This can cause erratic (or at least unintended and adverse) changes in the distribution of dynamic weight transfer in transient cornering, which could make for 'difficult' handling characteristics that could be impossible to sort using spring rates , dampers etc etc.

If 'roll centre adjusters' are used then this reduces the angle of the lower wishbone (lowers the ball joint relative to the chassis mounts), so in some degree will negate some of the adverse geometry change associated with a substantial lowering of ride height.

So yes, I think 'roll centre adjusters' would tend to be a good thing, but since they only affect the lower wishbone they only partially address the problem. A better option (but more expensive) would be to use uprights / knuckles (whatever you want to call them) that have both the lower and upper ball joints located lower relative to the axle axis.

This means we can lower the ride height (i.e. lower the CG!!, which is always a good thing if it can be done without making a hash out of the roll geometry) without excessively affecting either the lower or upper wishbone angles, thus keeping the roll geometry more or less stock, which is more or less a very good thing (most probably, in most cases).

Changing the as designed roll geometry may or may not be a bad thing, but chances are that you won't accidentally improve it, and probably would make it much worse unless you really understand what you are doing...

[Sp00ny]

When you substantially lower the ride height, the front wishbones become angled upward from the chassis to the ball joints. This changes the static location of the 'instant centres' of the 'virtual swing arms' and thus changes the static location of the geometeric roll centre (lowering the GRC). This isn't necessarily a bad thing in and of itself, and in fact a somewhat lower static GRC location may actually be a good thing (especially at the front axle line), though it would be advisable to keep it from going below ground level.

However, when the wishbone angles are changed substantially from stock, there is also an increased tendency for the dynamic locations of the VSA ICs to change location with suspension motion (as occurs in roll) in a manner that tends to cause the dynamic location of the GRC to change more than with stock wishbone angles (maybe much more).

This can cause erratic (or at least unintended and adverse) changes in the distribution of dynamic weight transfer in transient cornering, which could make for 'difficult' handling characteristics that could be impossible to sort using spring rates , dampers etc etc.

If 'roll centre adjusters' are used then this reduces the angle of the lower wishbone (lowers the ball joint relative to the chassis mounts), so in some degree will negate some of the adverse geometry change associated with a substantial lowering of ride height.

So yes, I think 'roll centre adjusters' would tend to be a good thing, but since they only affect the lower wishbone they only partially address the problem. A better option (but more expensive) would be to use uprights / knuckles (whatever you want to call them) that have both the lower and upper ball joints located lower relative to the axle axis.

This means we can lower the ride height (i.e. lower the CG!!, which is always a good thing if it can be done without making a hash out of the roll geometry) without excessively affecting either the lower or upper wishbone angles, thus keeping the roll geometry more or less stock, which is more or less a very good thing (most probably, in most cases).

Changing the as designed roll geometry may or may not be a bad thing, but chances are that you won't accidentally improve it, and probably would make it much worse unless you really understand what you are doing...

Ah, you see this write up in technical terms should be basically changing the idea of "lowest handles best" impression, within reason of course.

All in all, I don't think 99% of people on this forum including myself would have enough knowledge to improve upon the stock GRC or even improve upon it once the car's ride height is dramatically changed.

Most Tyre/Alignment places wouldn't have a clue either. Expessially the K-Mart tyre and auto type places.

If we get a RC Adjuster then where would you suggest to get it "fine tuned" and adjusted at? A suspension specialist?

[nd55]

> If we get a RC Adjuster then where would you suggest to get it "fine tuned" and adjusted at? A suspension specialist?

?????

The ones I've seen aren't adjustable. You either bolt them on or you don't.


Nick.

[JohnL]

^ That's my understanding.

Lowering the CG is always a good thing (in itself) because it reduces weight transfer, so more of the car's weight is distributed over more area of rubber when cornering (i.e. it increases grip).

The problem is that substantially lowering the CG nearly always creates substantial concomitant geometry changes which can create problems that may outweigh the positives of the lower CG, which then have to be dealt with properly in order to realise the full benefit of the lower CG (or just to make the car handle as well as it did before, let alone better).

A moderate lowering isn't likely to make a pig's ear out of the roll geometry (though some suspensions are bound to be more sensitive than others), and would usually be a worthwhile modification (as is commonly found). The trouble arises when people start thinking that if if a little bit is good then a whole lot must be much better, which is only the case if you know where the pitfalls might be and how to deal with them.

Suspension isn't simple, and shouldn't be modified without due care and consideration because it is primarily important to the car's safety. It should never be ignorantly changed just for bling appeal...

[55EXX]

so in affect the lesson is if you want better handling a bit lower not dumped is the way to go. like the whiteline flat out and control springs. the flatout spring are dumped and whiteline doesnt recommend them for best handling whereas the control springs have been designed with the suspension geometry in mind to create the best compromise for better handling tho they aren't as low as the flatout ones.

i understood all that you said. one line that stands out to me.

roll theory is very important for most cars, but tends to become less important the less suspension motion there is

that would be me in the bracket of the less suspension motion the less important. rc adjusters would ony be a thing i would look into if i were to seriously dump my car. if i were a track guru and done everything and wanted to squeeze out everylast ms then i would look further into it. yeah i'm sweet to stop here with roll theory while i'm on top. i don't wanna catch the bug and want to know it all then end up more confused. quit while i'm ahead.

thanks john L wish i didn't need to spread rep points around.

[EfiOz]

Nice write up John on a complex subject. I had to write an article similar to this for a mag and it was one of the hardest subjects to explain in lay mans terms I've come across.

A couple of addons.

We don't let drivers touch the RRC in V8 Supercar. They get front and rear ARB adjusters and they stuff that up enough as it is. The rear roll centre is used to induce a jacking moment to unload the inside rear so the car will turn in. This coupled to a fairly low FRC gives the car a mild "dog-at-the-lamp-post" attitude.

We used to use spacers on the lower ball joint on Dallara F3 cars to alter the FRC. Quick and easy!

[nd55]

> We used to use spacers on the lower ball joint on Dallara F3 cars to alter the FRC.

more info, please?

What spacers, what knuckles etc
What was the criteria to raise and lower the RC?
That's a highly stressed point. DId they ever break?
What material?

Nick.