Even in summertime, how heat affects the replaceable chassis components is not that obvious, but Rob Marshall learns that it is very much relevant.
Every day is a school day, as the cliché goes, but the more interesting motorcar facts tend to be those that are not noticed. While hardly any attention is given to it, heat, for instance, is very much relevant to springs and shock absorbers. Yet, Meyle is the first specialist to pull us up on our vocabulary. The company highlights that ‘shock absorber’ is inaccurate terminology, it should be referred to as a vibration damper. With our knuckles rapped justifiably, the company is correct and, hence, why we recommend that garages also ditch the term ‘shock absorber’ from their vocabulary.
To explain why, energy that is introduced to the vehicle by a pothole, for instance, has to go somewhere; it cannot just disappear. Dampers achieve this by damping the vibration using friction, which creates heat. While this is not obvious on everyday road-legal vehicles, it explains why dampers in some competition vehicles possess heat exchangers. Even so, the sealed nature of dampers means that their inner workings remain mysterious to many technicians. We know that the dampers work with springs to keep the wheels in contact with the road and, therefore, maintain driver control, but how this is achieved is not so clear. The damper is a precision component, which utilises a piston that forces oil through fine internal chambers. The resultant bottlenecks slow piston movement and the friction warms the fluid. This heat energy is then dissipated within the damper. Meyle also says that dampers tend to be warmer after a journey – not that anybody checks – but it is still an interesting fact.
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, mono- tube 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.
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.
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.
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.
LKQ Euro Car Parts reasons that, because debris is the most common cause of fluid overheating, regular fluid changes are the solution. A problem with this occurs when the customer is unprepared to bear the cost of this preventative maintenance, especially if the manufacturer service schedule omits any such requirement, or states that the system is ‘sealed for life.’
Does going electric pose a solution?
Unfortunately, pure electric power steering (EPS) also suffers from heat-related failures, due mainly to its high current requirements. Thankfully, an ECU-controlled EPS system will possess heat monitoring sensors that will not just create a fault code and illuminate a warming light but the system may operate in a reduced-current limp-home mode.
LKQ Euro Car Parts finds that an overheating power steering motor can be caused by a component failure, such as failed windings. Where brushes have worn excessively, or there is an excessive build of conductive carbon dust within the motor housing, the unit can warm to such an extent that it could pose a fire risk. There may also be a problem with the controlling ECU. For instance, the solder is the softest item on the printed circuit board (PCB), often with the lowest melting point. When higher than expected temperatures are experienced, this tends to be the first item to melt, resulting in dry joints on the PCB, resulting in EPS failure.
The harm of hot air
Like conventional spring and dampers, air suspension systems utilise conventional dampers. Arnott reports, therefore, that the oil will not cause overheating within the damper in normal use. While inflatable bags replace conventional metal springs, they also do not produce much heat at all, although a poorly- positioned exhaust pipe is known to cause the rubber bladder to degrade prematurely.
Logically, the air compressor is the only component that can be damaged by overheating. This explains why the unit is designed not to be run continually and how its temperature is controlled either by monitoring its running time, or by a real- time temperature sensor. Arnott explains that a sticking relay (which it advises should be renewed whenever a compressor is replaced), or leaky air springs, are the most common reasons why compressors have to work harder and hotter.
While LKQ Euro Car Parts agrees that leaks are the main cause of overworked and overheated compressors, technicians should not forget other factors. These include driver abuse, such as towing excessive loads, or carrying too much weight. Insufficient airflow, caused perhaps by aftermarket modifications that include body kits, also impedes compressor cooling. Apart from faulty relays, an irregular power supply, loose connections and damaged wiring can also result in overheating.
If a compressor has been exposed to temperatures greater than 100 degrees Celsius, suspect damage. Visual signs of overheating include discoloured labels on the compressor body and melted plastic air lines and/or connectors. You may also detect a burning smell either near, or when dismounting/ dismantling the compressor. As failure tends to be caused by an air leak, it is vital that you address the underlying cause, otherwise, a replacement compressor will fail very soon afterwards.