Top: understeer, bottom: oversteer

Tyres seem so simple: round things full of air that carry loads, make roads feel smoother and enable drivers to steer where they want to go. In reality they are very complex. One authority (Hans Bastiaan Pacejka) wrote a 980 page paper about them.

Tyre behaviour is dictated by a small footprint of canvas, rubber and in some, steel. Vehicle handling and mishandling is ultimately dictated by how those footprints behave and interact.

Imagine an inflated balloon pressed onto a table with its sides pushed almost together. The oval area pressing onto the table is its ‘footprint’. A tyre being steered via its front wheel’s rims acts much like that balloon’s sides being turned slightly sideways. This action does not cause that footprint to change direction, but rather to distort variously across its area. Steered road wheels rims thus exert a torque (turning force) on the sidewalls of the tyre, that turn exerts a turning force across that footprint.

At parking speeds, a vehicle turns much where the front wheel’s rims are pointing. At higher cornering speeds the vehicle turns at an angle part way between where the footprint is pointing and the wheel rims are pointing. That difference (misleadingly) called the slip angle is better perceived as a grip angle: a tyre footprint progressively distorts long before it slides.

For any given tyre its slip angle is a function of its air pressure and load. Its footprint’s friction does not follow the classical model (where grip directly relates to load). With all pneumatic tyres, increasing load increases grip, but at only 0.8 or so of their ability to exert cornering force.

If driven in a continuous circle at constant speed weight transfer causes the load on a vehicle’s outer tyres to increase, and on its inner tyres to decrease. If the front and rear tyres are equally loaded  and have equal air pressure, their slip angles will be the same – and the vehicle will turn at a constant radius. This may seem desirable but, as explained below, it isn’t.

This sketch shows how the footprint’s slip angle causes a vehicle to head at some angle to where it is steered. Pic: courtesy Caravan & Motorhome Books

If loading is increased only at the front, or that tyre’s pressure is reduced at the front, the front slip angle increases. This  causes the vehicle to adopt a wider radius, automatically reducing the cornering forces. This so-called ‘understeer’ causes the vehicle to be stable and to feel stable: the driver is usually unaware it happens. It is undesirable in excess as it can prevent the vehicle swerving hard when most needed.

The opposite (oversteer) is when the rear outer slip angle exceeds the front, causing the rear of the vehicle to swing wide. This causes its nose to turn in, thus tightening the radius and increasing the cornering forces. These in turn further increase the rear slip angle – and so on until rear tyre grip is lost. A driver accustomed to early VWs or vintage sports cars can, by applying opposite steering lock in time, often correct this situation. But get it wrong and the vehicle spins.

The problem with a neutrally steering vehicle is that the slightest change in loading may cause it to under- or oversteer. Rear loading will result in instant oversteer. Today’s road vehicles thus have minor designed-in understeer. Even when towing a nose-heavy trailer most are designed to maintain understeer up to 0.3 G (a force that is uncomfortable for most drivers).

Suspension engineers fine tune the suspension to obtain understeer by mild variation of front/rear spring rates that, for comfort, need to have the front’s at 80 percent of the rear’s. It can also be done by varying the relative thickness of front/rear anti-roll bars, and by minor  adjustment of tyre pressures. The extreme example of the latter is the early VW that had 110 kPa (16 psi) front, and 195 kPa (28 psi) rear – yet still oversteered in a sometimes disastrous manner.

All such suspension tuning is wrecked when someone fits airbags or heavier springs at one end only. One particular Luton cab motorhome is so front-heavy it virtually sits on its bump stops. Owners typically beef up the front end only – thus increasing what (in my opinion) is already excessive understeer. One such  modification resulted in the motorhome to so resist turning that it ploughed into an oncoming vehicle.

Excess understeer and borderline oversteer is unlikely to be detected in normal driving. It shows up when least needed – in emergency conditions, on mud or ice, or when attempting to corner too fast or attempting to swerve at speed. Oversteer is particular dangerous when towing a caravan. It is the prime initiator of irreversible jack-knifing.

Courtesy Bridgestone Tyre

Correct tyre pressure is vital. Since 1980 or so, tyres require inflation to the recommended pressures when cold. This results in the tyres being initially (marginally) underinflated. This allows for the 4 psi (27.5 kPa) increase as they heat up to running pressure. Do not readjust when they are hot or you will be running with underinflated tyres (that have excess slip angle and less cornering force).

This article is copyright Caravan & Motorhome Books. Author Collyn writes and publishes books on many areas of RV solar, electrical and more. Most are available from CMCA NHQ. Many of his articles are atwww.caravanandmotorhomebooks.com 

Category: Technology
Written: Sat 01 Mar 2014
Printed: March, 2014
Published By: