Words and Images by: Collyn Rivers N8054

There are continuing reports of suspension issues with large campervans: particularly of excess compression of the front springs. Whilst this may seem readily fixed by fitting stronger springs (or an appropriate air bag), doing so at the front alone is likely to result in an undesirable change in the vehicle’s handling.

Such change is rarely discernible to the driver excepting only that the vehicle may appear more directionally stable. That increase in stability however may prejudice the ability to change direction in emergency situations.

For many decades, road vehicles have been designed such that, if cornering too fast for safety, the vehicle automatically adopts a slightly wider radius and thus lessens the cornering forces acting on the tyres. This effect is known as understeer.

The opposite (oversteer) causes the vehicle to tighten the radius. If not corrected, by applying the right amount of opposite steering lock, cornering forces increase until the vehicle may spin.

Minor understeer is desirable but excess understeer may cause the vehicle to  plough virtually straight ahead, regardless of the driver applying more steering correction, if say, attempting to swerve hard to either side, or attempting to corner too fast. These effects and their causes have been known to vehicle suspension engineers since the late 1930s.

Inflated balloons

Causing a tyre to steer is akin to twisting an inflated balloon when it is being pushed down onto a flat surface.

The steering mechanism applies a turning force (torque), via the wheels’ rims, to the tyres’ sidewalls. The sidewalls thus flex, and (primarily) via the air compressed within the tyre, cause the tyre’s footprint on the road to distort in the direction the driver requires.

The footprint, deflected via a springy medium, never totally takes up the angle that the front road wheels’ rims attempt to impose. The tread footprint thus has a less tight curve than that of the steered wheel’s rims. The angular difference between where the wheel rims point, and their footprints lead, is called the slip angle. Meanwhile the vehicle turns in a radius somewhere between the two.

(The term ‘slip angle’ has been used since 1930 or so, and has ever since confused as it relates to tyre grip - not to  sliding action. It relates to the degree of distortion of the tyre’s footprint before it loses all grip and slides out of control.)

Once a vehicle begins to turn, the resultant cornering forces on the vehicle cause the rear tyres to have a slip angle imposed on them. Thus the rear of the vehicle too does not follow the curve otherwise imposed by the rims of the steered front wheels.

Effect of slip angles

When, say, circling a roundabout, if front and rear slip angles are identical the vehicle maintains a constant circle. But, if the front slip angle exceeds the rear, that vehicle turns in a widening circle.

If the rear slip angles were to exceed the front, the vehicle would tighten its turning circle thus further increasing the forces on the tyres until they lose all grip, and the vehicle spins.

Effect of load

The magnitude of the slip angle is also affected by the downward load on the tyre. The greater that load, the greater the slip angle for any given turn - in effect the less the available cornering force.

Cornering force is thus related to vertical load but is less than linear.

When a vehicle corners (or is subject to a strong side gust of wind) it rolls into that corner - or away from the wind. This transfers weight to the outer tyres, thus increasing their slip angles.

This is allowed for by the suspension designer, who attempts to ensure the change in slip angle is maintained equally front and rear by arranging for the springs (and sway bars if used) to transfer that weight by equal amounts front/rear.

If, however, springs at one end only are stiffened, that will cause more of the weight transfer (whilst cornering) to be borne by the related tyres (dual or single).

Increasing spring stiffness will affect vehicle handling whilst cornering (or subject to a strong side wind) because weight is transferred to the outer tyres. If stiffer springs are fitted to the front (only), those stiffer springs resist that roll – and thus transfer a greater load to the associated front tyre. This increases its slip angle, hence increasing understeer, and the vehicle’s resistance to be deflected from a straight line.

Red dotted line - excess understeer: this vehicle runs wide when attempting to corner. Consider the consequences were that to be a left hand turn and a semi-trailer approaching...Green dotted line -  shows the perferred mild understeer. 

Red dotted line - oversteer: this vehicle tightens it turning circle and may finally spin...Green dotted line -  shows the perferred mild understeer. Picture: Courtesty of Wikipedia. 

An unnecessary problem

This problem should not exist. If production load-carrying vehicles made in large numbers improbably needed suspension changes, the makers would address that very fast.

That it affects only some (a few campervans) indicates a possible problem in the manner in which such vehicles are modified, or being used.

That many campervans allegedly affected have Luton peaks (‘cabovers’) is probably no coincidence.

Locating a load area high above the driving cab of a vehicle adds weight in an area not intended to carry it, not least as it introduces a moment arm (a lever) that magnifies the roll forces.

If the front springs are stiffened, some correction is likely to be needed to redress the effect front/rear.

A suspension designer might address this by raising front tyre pressures (to reduce their slip angle), decrease the thickness of a front sway bar, or increase that of a rear sway bar. Using stiffer rear springs increases rear weight transfer, but is better avoided as it unnecessarily adds weight.

In view of the complex interactions of the many factors affecting a vehicle’s dynamic behaviour, resolving over weight caused effects is best addressed by eliminating (or reducing) that weight.

Reduce cabover weight to the absolute minimum, and relocate front weight rearwards such as to attain the vehicle maker’s front/rear intended axle weight limits (or pro rata).

Suspension modification is best not attempted. Doing so requires specialised knowledge to avoid handling issues. With respect, this is an area outside the expertise of most spring makers and vendors.

Those seeking to know more about vehicle behaviour generally may wish to refer to my related article Vehicle Dynamics on my website: www.caravanandmotorhomebooks.com 

Note: for those who understand physics but not that of tyre behaviour, tyres do not exhibit Coulomb friction (i.e. with tyres, friction is neither proportional to load, nor independent of load area).

Category: Technology
Written: Sun 01 Dec 2013
Printed: December, 2013
Published By: