STEERING & SUSPENSION

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As power steering fluid reaches high temperatures and deteriorates, it is surprising that routine replacement is not recommended by so many OEM service schedules. Such preventative maintenance is worth upselling as an extra operation, especially on cars with pricey racks and pumps.

The importance of heat dissipation

As with most fluids, heat affects damper oil – notably, its viscosity. While quality damper manufacturers select and tune these properties carefully, a hotter fluid will result in a reduction in damping ability. Varying the damper construction offers a partial alternative.

KYB reveals that the internal differences between its mono and twin-tube dampers allow them to shed heat energy at different rates. It says that, thanks to their single-cylinder design, monotube dampers possess larger pistons and increased valving, so that they can dissipate heat more quickly. The benefit for the driver is mainly superior road handling. To prove the point, KYB’s Gas-a-Just mono-tube range provides up to 30% higher damping forces than an equivalent oil twin-tube alternative. Generally, you will find that oil dampers are specified for vehicles, where ride comfort is more of a priority, compared with dampers that have had pressurised nitrogen gas added, which are optimised more for sporting driving styles.

Springing

Meyle reminds us that springing reduces the shock load on both the car and its occupants. In addition, manufacturers select spring and damping combinations very carefully to cushion incoming shock loads before they reach the bodyshell. Naturally, without dampers, the spring would continue to oscillate, causing the car to bounce, with the tyres relinquishing road contact, causing the driver to lose control. As it moves, a road spring generates a small quantity of heat, although never enough to damage either itself, or surrounding components. Invariably, most technicians will agree with LKQ Euro Car Parts’ findings that most spring problems result from corrosion, where the protective coating has been compromised.

Passive overheating

While conventional springs and dampers do not produce excessive quantities of heat, related components can be damaged by other parts malfunctioning. First Line highlights that brake dragging, or binding, can cause thermal damage to the wheel bearing. It elaborates that the bearing housing should not exceed 82 ºC in normal use, with the outer ring sustaining no more than ten degrees more than that. Yet, we should consider that a brake disc can attain temperatures up to 600 degrees Celsius and, as Meyle attests, up to 50% of this heat energy can transfer to the hub.

Schaeffler and LUK Euro Car Parts elaborate that a well-adjusted and undamaged wheel bearing tends not to be affected directly by these higher temperatures. Instead, its grease lubricant degrades and it is this loss of lubrication that leads to bearing failure. Most bearing problems are caused also by the seal. When in poor condition, it allows water and grit to wash out and degrade the grease. While wheel bearing temperatures do not deviate far from ambient in normal conditions, the bearing housing acting as a heat sink from the binding brakes can both damage the seal and cause the grease structure to collapse. Yet, Schaeffer reminds technicians that not setting the correct taper preload can also cause bearing overheating.

Meyle highlights that, while less likely to be damaged by thermal transfer, suspension joints are still vulnerable. While their grease is unlikely to degrade, heat radiation can cause rubber sleeves to become brittle and reduce the strength of any plastic seat. Both conditions cause premature wear and, possibly, noise as well.

Suffering steering

The power-assisted steering (PAS) system tends to suffer the most from overheating. As hydraulic and electro-hydraulic fluids rely on pressurised fluid to deliver power assistance, heat is a by-product of the system’s inbuilt inefficiency. LKQ Euro Car Parts reassures technicians that PAS fluid is designed with a very high boiling point. Yet, low fluid levels, a blocked, or obstructed heat exchanger, air leaks and even electrical faults can cause the fluid to overheat. This is one reason why LKQ Euro Car Parts recommends that the fluid is not just changed but the whole system is flushed, whenever a new component is replaced.

Meyle explains that PAS fluid does not last forever, because the heat changes its property and, over time, its lubrication ability is impaired. One reason for this is that the traces of air and oxygen within the fluid cause it to oxidise, making it darken, or even form a black sludge that undermines the fluid’s lubrication properties even more. The net result of this is increased pump and rack wear.