|An Explanation of Camber
see also Dieter Könnecke's technical site
The term tracking incorporates camber, caster and toe. Toe and camber (by the adjustment of ride height) are the only two components of tracking that can be adjusted on the MGF.What is camber?
Looking from the front of the car looking at the wheels. Camber is the angle that the vertical axis of the wheel makes with the road surface. Negative camber is when the top of the wheel leans into the centre of the car. Positive camber is when the top of the wheel leans out. On an MGF camber should be ±0.5 degree (±30 minutes).Basic design theory
|Figure1: a simple parallelogram-
in bump the tyre remains perpendicular to the road
Thanks to Dieter Könnecke for animated figure
|The basic suspension design of the MGF, in
common with other sports cars, is by unequal length double wishbones at all four wheels.
Consider the scenario: the car is driving along, and the wheel hits a bump. The wheel is deflected upward.
What wheel orientation will provide the most grip upon the road surface of that deflected wheel? The wheel should remain vertical, with the tyre tread parallel to the road surface to ensure optimum grip upon the road surface.
In this simplest model, equal length wishbones making up a parallelogram will provide the ideal solution.
|However, in a corner the body of the car rolls, or leans over. With a parallelogram, the wheel would remain parallel with the vertical axis of the car. Therefore as the car rolls as it goes through a corner the inner edge of the outermost tyres would loose contact with the road. Effectively there would be too much positive camber relative to the road surface, reducing the tyre contact area with the road. The car would easily lose grip especially if a wheel rode over a bump exacerbating the geometry change.||
Figure 2: In cornering the simple parallelogram causes the top of the wheel to lean outwards- Positive camber.
Thanks to Dieter Könnecke for animated figure
|Figure3: The Unequal length wishbone
set-up- allows compensation for roll induced positive camber on the outside
|To compensate for this roll induced reduction in contact area, engineers derived unequal length wishbone suspension systems. These enable an arc of suspension movement, introducing a degree of compensatory negative camber. This ensures the tyre orientation remains essentially the same with respect to the road surface with maximum tyre contact area with the road inspite of the roll angle of the vehicle. These changes in wheel camber angle occur over a predetermined range of suspension movement.|
This essential design has been used to great effect over the years in a number of classic sports cars, and remains a firm favourite with engineers today.What happens when you lower an MGFs ride height
The problem occurs when you attempt to lower a vehicle equipped with unequal length wishbones.
|Figure4: Lowered suspension with
Unequal length wishbones- suspension acts as though in a roll and produces
undesired Negative camber.
||By lowering the car, the effect is similar to causing
a deflection in the suspension movement- the camber is made more negative. Go round a
corner, or over a bump, the camber is made more negative still, putting all the load on
the tyre at its inner edge as opposed to the whole tread pattern.
Therefore it is no surprise to find that the inner edge of the tyre wears rapidly away!
Camber thrust occurs when a wheel is lent over from the perpendicular - it immediately wants to turn. You may have noticed this when riding a bicycle: if you lean over, the bicycle will change direction with very little steering effort required from movement of the handle bars. So it is with cars too.
A relatively modest camber angle relative to the road will generate camber thrust on the front wheels - usually around 0°30' is optimal on road car suspension (which is, co-incidently, the MGF's static front camber setting). The camber thrust can be used to reduce the tyre slip angle of the front wheels (the camber thrust opposes the direction of the lateral thrust, see slip angle page for more explanation on this) - so effectively increasing the resistance to understeer by augmenting tyre grip.
Increasing static negative camber may, or may not help the cause: this will depend upon the extent of suspension travel secondary to roll (where roll, as we have seen promotes positive camber, relative to the road surface) and the way that the suspension geometry has been designed to alter camber with this movement (compensatory negative camber). Remember, it is the camber relative to the ground that is crucial, not the camber relative to the chassis (as we've seen above).
However, with no doubt, excess negative camber is certainly a 'bad thing' causing tyres to overheat, wear alarmingly, and potentially causing unpleasant 'bump-steer' characteristics.
What is the solution to excess negative camber?
|The only way to overcome the problem of
excess negative camber with very low suspension heights is a camber adjustment kit- one
that would effectively modify the length of one of the suspension wishbones. At rest the
wheel could then be set to near ideal camber at what ever static ride height selected.
Such a solution would ideally have similar deflection characteristics to the standard set
up- but as the ride height is dropped lower and lower this becomes impossible to achieve,
without re-locating the suspension pick up points (i.e. where the wishbones pivot from the
body work of the car).
At present, only Mike Satur retails a camber compensation kit - and this kit only applies to the rear suspension (the wheels which have the greatest negative camber at the outset, before lowering). However, no camber compensator is currently available for the front suspension. Therefore, if at the choosen ride height excessive tyre wear is occuring (I don't find it a problem on my car set to a ride height of 320mm), one will have to accept it as a consequence of lowering ones car for the potential benefits of better handling (from a lowered centre of gravity) and appearance.
Above, Mike Satur's rear camber compensator: replaces rear tie-bar link. See his web site for further details.