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One of the most common causes of confusion relating to cars and driving is that of handling and roadholding. The two are often discussed as the same thing and terms intermixed. The reality is that an awful handling car can have excellent roadholding and vice versa. The challenge is to get both to be good at the same time!
Weight Transfer Under Roll
The bane of all cars when it comes to handling is their natural tendency to roll under cornering. This roll sets in motion a transfer of weight from one side of the car to the other, usually from the inner to the outer. This reduces the grip provided by the inner tyres as they lose contact with the road surface and loads the outer pushing it towards and eventually over its maximum available grip for a given slip angle. Which end of the car suffers the initial loss or lessening of grip defines the car natural tendency to understeer or oversteer (see later).
The primary requirement therefore is to equalise the available grip at each corner, hopefully making a neutral handling car with the maximum amount of grip available from all 4 corners. It is, after all, the little contact patch between each tyre and the road that provides all the cornering power. Anything else that is adjusted is to maximise the potential from these 4 little patches.
Less Roll?
Ignoring the static weight distribution of the car, the obvious answer is strong springs to resist the roll and stop or minimise weight transfer, helping each tyre contact patch to do the best it can.
However, spring rates on all but race cars can't be set too high as they would give an unacceptable ride on our traditional British roads.
The next option is stiffer anti-roll bars, which have the effect of stiffening the apparent spring rates during roll without affecting compliance on undulating straight roads. The problem here is that the effect of roll can be simulated by only one wheel falling into a pothole or traversing a bump, causing the car body to twist in response. Not nice.
More importantly anti-roll bars 'rob' cornering power or grip from the tyres by adding more weight transfer from the already lightly loaded inner wheel to the heavily loaded outer. (This may help balance the car end to end by robbing cornering power from the end with more, obviously at the expense of overall cornering power.) Exactly the opposite of what we are trying to achieve. This is one area where active suspension can be made to work, by stiffening only the outer end of the anti-roll bar, as with Citroens Xantia suspension.
Increased Spring Rates
With Sevens then, the best compromise seems to be the stiffest acceptable springs with a medium anti-roll bar, the stronger springs cancelling out some of the effect of the roll bar.
Stronger springs, up to a point, bring other benefits. On my car the old spring rates were too soft, relying on the dampers to attempt to reduce roll, which is not the job they are intended to perform. Setting dampers too hard results in a very 'jiggly' ride as they react to a change in state, but do not affect the end result. For example, they resist the initial start of roll or bump, causing a sharp jolt, but then allow the car to roll or settle to the position allowed by the springs. Therefore stronger springs with better damper settings can reduce roll and squat etc. while simultaneously providing a smoother ride.
The front springs on my car have gone from 1051bs to 2601bs at the front and from 751bs to 1151bs at the rear. A massive change you might think but the result has actually been a marked improvement in ride quality due to the dampers now being set at a more realistic level and the car riding more on the springs.
The relative spring rates front to rear also affects the amount of weight transfer at either end, resulting in understeer or oversteer. At the stiffer end of the car more weight is transferred, reducing the available cornering power. Therefore stiffer front springs increase understeer, stiffer rears increase oversteer. However, true spring rates' are as measured when fitted to the car, not their individual values quoted when sold. Unequal length wishbones and angle to applied force vary the ultimate spring value. The result is the front springs on a Seven may not be as stiff relative to the rear as you may think.
Coincident Roll and Gravity Centres
A final method of reducing roll, and one that many race cars come close to, is to arrange it so that the cars centre of gravity and centre of roll at each end are coincident. If this occurs no roll will be apparent and effectively spring rates could be selected for comfort and not used to resist roll. In most cars the height of the engine, petrol tank, occupants etc. makes this ideal impossible, however... this knowledge of roll heights can be used to great benefit.
Roll Transfer via Chassis
The car's chassis can be used to affect the apparent handling by acting as a large torsion bar, transferring weight, or better still roll, from one end of the car to the other. This causes the suspension at one end to work harder than if it were isolated, balancing the total cornering power requested from both ends. For example a car that rolls more at the front, due to a larger difference in roll and gravity centres, will transfer some of that roll to the back because of the stiffness of the chassis. This causes the rear outer suspension to be compresses by a greater amount than its roll rate alone would require, while reducing the front roll angle. This evens out the total amount of cornering available, increasing it at the front, reducing it at the rear. Knowing this, suspension can be modified to incline the car's roll axis to increase roll transfer front to rear during cornering, decreasing the usual understeer. This can be likened to having stronger rear springs during cornering.
Inclined Roll Heights
With all Sevens the rear roll height is set by the location of the 'A' frame mounting bracket, mounted below the axle casing on live axle cars. It is about this point that the rear of the car will attempt to roll during cornering (as an aside this is one reason why these bushes wear out so quickly as they are resisting the natural roll centre(s) of the axle/suspension setup). With this roll centre effectively fixed the adjustable platform dampers will lower/raise the centre of gravity at the rear relative to the roll centre. At the front the roll centre is set by the point of intersection of three invisible lines drawn from a) the upper wishbone, b) the lower wishbone to their point of intersection and c) a line drawn from the contact patch of the tyre to the above intersection point. Where this last line crosses the centre line of the car is the location of the roll centre, as shown below.
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