Warning, roll theory inside...
String wrote:
“Exactly how low are you? If you're getting close to the bump stops then I'll bet your problem is the front roll centre location. Increasing the front roll couple relative to the rear will result in worse steering feel, especially on turn-in; a feeling of perpetual understeer and tyre-rollover - sound familiar?”
String’s statement above (and a similar one in another thread a week or so ago) brings up something of some importance to an understanding suspension behaviour, ‘roll theory’. This isn’t only to do with body roll, but is also fundamental to the manner(s) in which weight transfer occurs.
Many of you won’t find this topic of much interest as it’s quite esoteric and (pseudo) theoretical, so please don’t read it and then chime in with something like “blah blah blah’. You were warned!
I don’t pretend that all the following blather is 100% correct or gospel, I’m only putting up some of my personal understanding for discussion, if anyone’s interested…
Before I really begin, we need to be aware that starting to talk about roll centres etc is like opening a big can of very wriggly and slippery worms. ‘Roll theory’ is to do with geometric roll centres (GRC) locations, weight transfer etc. and is very complex and hard to get your head around. If anyone thinks they might have a handle on it by having read what’s written about it in the more commonly available chassis dynamics books, then they’re wrong. The explanations in such books are invariably very simplistic, sometimes wrong and at the very least somewhat misleading. Even professional suspension engineers can struggle with roll theory, and it’s a topic of much discussion and disagreement among them.
I don’t pretend to understand roll theory particularly well, but it is something I’ve read a lot about and given a great deal of (headache inducing) thought to while trying to understand kart chassis behaviour (and yes IMO the fundamental principles are the same, though some will argue…). It’s one of those things where to understand X you need to understand Y and also Z, but to understand Y you need to already have some understanding of Z, but to understand Z you need to understand Y. To understand Y or Z (and thus understand X), you need to incrementally zero in on both, building on the understanding bit by bit (and even then you can’t be sure you’re getting it right). There are a number of simple things going on when weight is transferring, but the interactions between them become complex.
By the way, before I go on (and on and on…), unless I say otherwise, all the following is referring to the suspension at a single axle line, i.e. either the front suspension or the rear suspension.
Oh, and it’s long!
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String,
The lower the GRC the lower the geometric roll stiffness becomes, not higher (as I think you were suggesting?).
As you lower the chassis, the suspension arms (wishbones etc) become angled downward (from the uprights to chassis mounting points). This causes the ‘instant centres’ of the ‘virtual swing arms’ (points in space defined by suspension geometry, of which there is one per corner on an independent suspension) to become lower, and as a result the GRC also lowers. The CG also lowers, but the GRC height above ground relative to the CG height above ground becomes lower more than the CG height does.
When lowering the chassis, if this lowering of the GRC(s) occurs equally at both ends, then the geometric ‘roll couple’ (relative front / rear roll stiffnesses) will remain unaltered (though roll stiffness will tend to decrease at both ends of the chassis). If you lower the front but not the rear then only the front GRC lowers and the geometric roll couple will become biased more toward the rear and geometric roll stiffness will decrease at the front. This will tend to improve turn in and decrease understeer because lateral weight transfer will now be greater and faster at the rear than at the front because front roll stiffness is now less than it was.
Despite what most people have probably read or heard or think, the first thing we need to understand is that the GRC is NOT the point around which the chassis actually ‘rolls’, except at the very instant the chassis begins to ‘see’ a weak lateral acceleration (in theory, but in reality as soon as ANY lateral acceleration is occurring the point around which the chassis is actually rolling has moved away from the GRC to some degree).
The location of the point around which the chassis mass (i.e. all sprung weight) actually rolls IS influenced by the location of the GRC, but once the chassis starts to ‘see’ lateral acceleration we get some immediate ‘geometric’ weight transfer and the chassis also attempts to simultaneously ‘roll’ around another centre located at the outer contact patch (as weight transfer begins to transfer through the springs etc), it attempts in some degree to ‘pole vault’ through the spring (and ARB and damper) over the outer contact patch.
As such the chassis is attempting roll around two points, which it can’t do because it’s a rigid structure (or should be!), so it ends up rolling around a point located in space that is an ‘average’ of the two points, the location of which depends on the relative ‘strengths’ both points may have at any given moment. The point around which the chassis actually rolls will be at a point located somewhere on a line drawn between the GRC and outer contact patch, and the point’s location on this line will move either way along the line depending on the strength of lateral acceleration.
The exact location in space of the actual centre of chassis roll is determined by the location of the GRC, the location of the outer contact patch, and the strength of the lateral acceleration, and changes dramatically with changes in lateral acceleration.
Weight transfers through two ‘vectors’; ‘geometric’ and ‘elastic’. The elastic vector involves WT through the springs, ARBs, and dampers (though dampers only, but importantly, influence transient weight transfer when the suspension is either deflecting or ‘undeflecting’, not in static state when the suspension has taken a ‘set’, though spring and ARB rates do affect static state WT). The geometric vector involves how much weight transfers through the articulated suspension members themselves (i.e. wishbones etc), instead of through the springs etc.
However, if the value of either the geometric OR elastic vector is zero, i.e. if geometric roll stiffness is zero, OR, no springs etc exist (as with a ‘wobble beam’ tractor front axle, where the beam pivot is also by default the GRC), then regardless of the strength of lateral acceleration all WT will be occurring through the other vector that has a value higher than zero (but this is rarely the case in the real world where both vectors typically have some value above zero).
If the GRC is located at ground level then it’s value will be zero (i.e. there is zero geometric roll stiffness), and all WT will occur ‘elastically’ through springs etc. If the GRC is at the same height as the CG, then the value of the geometric vector will be 100%, and no WT will occur elastically even if the springs etc are quite stiff (i.e. WT will occur entirely via the geometry through the linkages, and body roll will be zero). Most commonly WT occurs through a combination of geometric and elastic vectors.
The value of lateral acceleration affects the ratio of how much weight is transferring elastically and geometrically whenever WT is occurring through both vectors. If the value of either the geometric or elastic vectors is zero then lateral acceleration won’t affect the ratio, no weight will transfer through the vector with a value of zero.
In the real world with WT occurring through both vectors, the ratio of WT through the geometric and elastic vectors varies constantly (except when a steady cornering state is reached, when nothing at all is in a state of change). This is because suspension deflection causes the location of the GRC to change (be changing), and because as lateral acceleration changes the degree to which the chassis is attempting to ‘pole vault’ through the springs over the outer contact patch changes. The chassis attempts to ‘pole vault’ through the vector of the springs etc., which deflect under this load, carrying more WT the more the outer spring compresses (and the inner spring ‘decompresses’).
It’s important to understand that it does matter how much weight is transferring at any moment through either vector, because WT through the geometric vector is ‘instantaneous’ (i.e. it occurs at the same rate that lateral acceleration increases), and WT through the elastic vector is ‘slow’ (i.e. it lags behind the increase in lateral acceleration until a steady state is achieved). It’s important to note that stiffer spring, ARB and damper rates increase the speed at which WT occurs elastically (though unless the springs are 100% rigid WT never becomes ‘instant’ through the elastic vector), and chassis flex (or lack of) will also have an affect.
The different rates at which weight is transferring (at any given moment) through the geometric and elastic vectors, and how this is occurring at either end of the chassis relative to the other end, has very large implications for transient handling characteristics.
If you have say a low GRC location at the front (i.e. low front geometric roll stiffness) relative to the rear (i.e. higher rear GRC location), then you will get a lesser and slower WT at the front than at the rear, particularly in the earlier parts of the corner before lateral acceleration has built to a higher level (because as lateral acceleration increases the influence of GRC location on WT decreases and the influence of the elastic vector increases).
This will tend to give better turn in and early corner behaviour, i.e. reduce transient understeer, and is why front GRC locations are typically designed to be lower than rear GRC locations. However, as the lateral acceleration increases the influence of the geometric roll stiffness tends drop off (less so it’s the rear end of an old Beetle or Corvair etc. which have a very high rear geometric vector value with the majority of WT occurring geometrically most of the time), and the elastic vector becomes more influential (i.e. as acceleration increases, an increasingly greater % of WT occurs elastically and less geometrically), so the relative front / rear spring rates (and ARB rates etc) more strongly governs front vs. rear WT ratio, and thus what % of weight is transferring front vs rear at higher accelerations.
This is also partially affected by the fact that the GRC location typically becomes lower and moves laterally inward (lateral position also affects geometric roll stiffness) during body roll motion, but it’s largely to do with the elastic vector increasing in strength as the outer spring and ARB compresses.
Because of all this, the elastic vector more strongly affects understeer / oversteer at higher lateral accelerations than the geometric vector does, being why most manufacturers tend to fit softer rate springs (etc) on the rear, i.e. to increase near / at the limit understeer. This is also advantageous for RWD cars getting the power to the ground at corner exit because more WT is occurring at the front than at the rear.
So, (typically) production and racing cars tend to have lower front / higher rear geometric roll stiffness (by means of GRC location) to encourage good turn in and limit transient understeer, and most production cars tend to have higher front / lower rear spring rates etc to limit at / near the limit oversteer (i.e. to encourage understeer).
Congratulations for coming this far with me, don’t say I didn’t warn you! There’s a fair bit more to this subject, but I think that’s more than enough for now. This is hard stuff to understand, probably just as hard to explain, and at least some of my understandings may be suspect. I’ve tried to be as clear as I can be, so don’t shoot me if you think I’ve got some of it wrong!
