Eugene
STICKING TO THE BLACK STUFF
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.

Roll Center
Obviously altering the damper spring seat will lower/raise this roll centre enabling the roll axis to be inclined if wished. Therefore, lowering the front roll centre below the rear, assuming similar centres of gravity, will cause a greater 'chassis' transfer of weight at the rear reducing understeer.
Front and rear Roll Centers
Front roll height 'A' wants to roll a greater amount than rear 'B'. But as body can't twist 'B' is forced to roll more, reducing 'A' roll (B and A roll same amount). Therefore less weight transfer at front, more at rear So rear grip reduced, front improved. Result, less understeer.

Altered Suspension Pickups
One of the effects of Caterham's choice of shorter uprights for newer cars has been to lower the front roll centre, reducing the natural understeer tendency of De-Dion cars. This is one of the common and illegal 'tricks' used in one make racing to gain handling advantages. Note also the effect of castor angle on the turned wheel, increasing negative camber in the direction of load, towards the outside of the radius (although, granted, in a neutral steering car this has no effect as the wheel will not be turned).

Static Corner Weights
If weight transfer affects roadholding it should be obvious that ensuring (as near as possible) perfect static weight distribution will give the best possible start for a car's handling. Looking at the individual corners of the car it should also be apparent that the weight applied to each wheel under static loaded conditions will be different if nominal suspension setups are used, especially if only the driver is the usual occupant. This affects each wheel's ability to supply grip under similar conditions. Adjusting individual spring rates/weights by changing the spring platform height can be used to setup equal corner weights and hence even out the available grip.

Centre of Gravity
In addition to static corner weight, lowering the centre of gravity, as we have seen, will reduce the amount of body roll. In addition to the adjustable platforms of most dampers, which in front suspension geometry will also affect the roll centre, wheel and tyre combinations can be chosen to lower the overall car's height.

Setup For a Live Axle Seven
Therefore, the aim is to set the best possible compromise between ride height and corner weight using the adjustments available.

With a live axle Seven running 185/70/13 tyres the average ride height should be;

Front - 14Omm, measured at the chassis just below the rear wishbone mount.

Rear- 16Omm, measured at the chassis just below the leading edge of the rear wing.

This will set the front roll height slightly below that of the rear, just below the sump pan and should have the lower wishbones set parallel with the ground and set the centre of gravity as low as practical.

A corner weight gauge should then be used to set the correct, balanced weights, preferably with driver in the car. The actual weights measured will vary from car to car, although live axle cars should be notably lighter than De-Dion for a given engine.

Note; the adjustments act in diagonals. For example, increasing the spring platform height at the rear o/s will increase the corner weight at the front n/s as well as the rear o/s. It's a balance between height and weight.

With just the driver installed it is impossible to balance the rear corner weights, so an average compromise must be accepted. The front corners can be balanced well within limits and it is these that have the greatest effect on the cars handling.

Bump Steer
This leaves the steering rack, which should now be raised/lowered so as to have the minimal effect on steering geometry during wheel deflection, i.e. bump steer. There is no magic setup for this, although parallel to the ground with nominal car weight is probably better than nothing. But the only accurate way to set it is to measure the amount of 'pull' on the wheel by the rack as the road wheel is moved up and down. This is easiest if the damper/spring is removed first.

Tyres
Initially I ran NCT tyres and they ran better with reduced camber angles. With Yokohama tyres there is a significant difference and I'm now running 1 degree camber at the front to counter their more rounded profile and provide increased cornering grip.

These tyres have also proven to supply much more grip than the previous NCTS, as would be expected, but their wet weather capabilities are, surprisingly, about the same. I did expect them to be much worse due to less tread pattern, but there you go.

The problem with a subject such as roadholding is that the more you get into it the more you find and so the more you....

Tyres & Slip Angles
The tread pattern on the Yokohama is much more solid than on the NCT. This has the effect of reducing the slip angle the tyre runs at for a given cornering force, increasing the cornering power of the tyre and making turn-in more crisp. To explain, each block of tyre between the tread pattern on an NCT can 'twist'. As the wheel is turned the tread block will continue to follow is original path, not that of the new direction of the wheel, so increasing the effective radius the tyre will follow. This is known as the tyre's slip angle and is the difference between the direction of travel and the direction in which the tyre is pointing.

Slip Angle
Slip angle increases with load and cornering force, up to a 'stall point' (about 1 0 degrees) at which point more steering lock will have no further effect.

Sidewall compliance also affects the slip angle for any given tyre, distorting the whole tyre carcass under side loads. This can be felt on a road with a large camber towards the edge, or in large side winds, as the side forces distort the tyre and 'pull' the steering, requiring steering input from the driver to maintain a straight line.

Understeer & Oversteer
You can see from this if the front tyres run a greater slip angle or reach their limit first the car is understeering; oversteer is when the rear tyres run the larger slip angle or reaches their limit first. Good balance is therefore essential for maximum cornering capability and has been the aim of most of the previous drivel.

Circle of Forces
As there is a fixed, although non-linear, ratio between the work a tyre can do and its slip angle, asking the tyre to perform more work, as with transmitting power or breaking forces, reduces the available grip. One reason why breaking or accelerating in a corner is not usually a good thing.

This is often described as a circle of forces and shows the percentage of grip available under power or breaking.

Circle of Forces

In example A, around 75% power application has reduced total cornering power to around 80% of maximum.

In example B, around 50% breaking has reduced total cornering power to around 90%.

If the tyres were close to their limits before the application of power or brakes this may well push them over and result in a sudden loss of cornering power.

Not Quite a Circle!
As with most things to do with handling there is a but. The circle of forces is not truly round and acceleration and/or breaking can setup weight transfers front to rear with beneficial results(trail braking deep into corners springs to mind). Just don't practise it on the roads.

Checking the Results
If a car has been setup to its maximum potential each tyre should be performing roughly the same amount of work. The amount of work performed can be measured by the heat build-up after a good thrash. If all tyres are of the heat same then, bingo, there's not a lot more to do. In reality the always present weight transfers will make this impossible, but it can be used to see if an alteration is for the good or not.

The temperature across a tyre can also be used to check for correct camber setup with even temperatures across a tyre showing this to be so. If you have the dosh a pyrometer enables tyre tread temperatures to be measured accurately, or alternatively try a few of those temperature strips that change colour.

Note; tyre pressure will affect these results by changing the tyre contact patch size and shape.

And those things that have been totally ignored
There are may aspects of car handling that have been totally ignored here such as rear axle roll steering, which even a live axle Seven is prone to; weight distribution front to rear and how this can affect handling in conjunction with rear or front wheel drive (slightly nose heavy rear wheel drive is often said to be best ). Tyre pressure also has an effect as a higher pressure increases cornering force for a given slip angle and load, up to a limit, as do stiffer side walls. (Wider tyres run smaller slip angles for a given load increasing cornering power but at the cost of increased rolling resistance which is often worse). The wheels toe value also has an effect with toe in increasing understeer and toe out increasing oversteer, at any end of the car. And then there is the oft quoted unspruing weight, where less is more...

Summary
So what does all this mean at the end of the day?

As far as I can tell, and within the limits I (Sarah) set myself (budget!), my Seven now has a good power delivery from the Vegantune engine, it will corner predictably with good balance (better handling) and with high lateral grip (better roadholding). Handling appears to boarder on neutral with slight understeer, with the ability to provoke oversteer with mild power application or stamping on the brakes. The 70 profile tyres adequately isolate road irregularities without the need for softer main springing and the natural ability of the live axle setup to report back on the happenings down at road level has not been reduced.

I hope this information has been entertaining, although I must admit most of it is next to useless for every day Seven enjoyment. All I can say is it's kept me out of the pub for a few nights

As always, with things to do with suspension, talk to those who know before attempting any major adjustments.

If you disagree with anything said here, have any other ideas or simply thinks it's a load of rot then say so. If I don't get any new ideas soon its back to the pub!



 :

Eugene - fun

NOTE:
It's not what you've go it's what you do with it!