Doctor Diesel

Back to basics – Ride Quality and Comfort

Web01Ride comfort can be a somewhat contentious subject, being a distinctly subjective evaluation that’s dependent on personal preferences, and significantly affected by the quality of road surfaces. It’s best described by the use of two terms primary ride and secondary ride.

Primary ride is the result of general suspension and body movement over significant undulations and larger bumps, both in compression and rebound, and is mostly controlled by the springs, shock absorbers, and anti-roll bars. Secondary ride is a term for the harshness and noise that a car occupant experiences over smaller surface imperfections, like coarse chippings, ridges, broken tarmac and concrete, and depends mostly on tyre properties and suspension and sub-frame bush isolation. In more recent times, suspension bushes, the mildly elastic rubber or plastic components that locate the various suspension arm, anti-roll bar, and sub-frame mountings, have become more influential on ride comfort; in the context of multi-link and trailing arm rear suspension systems what are called “voided” bushes are engineered with hollow spaces in the rubber to allow different degrees of elasticity and movement in different directions.

Fundamentally, primary ride will depend on the degree of vertical suspension movement offered, which depends on the length and stiffness of the springing medium, usually a coil spring, sometimes a torsion bar, or a torsion beam. It will depend very much on the amount of damping facilitated, in both compression and rebound, and the finer characteristics and quality of the shock absorber. It will often depend upon the sophistication, and therefore cost, of the system (usually here we are thinking of rear suspension) and a multi-link system will generally offer better ride comfort and roadholding than a simpler torsion beam rear suspension system. However, the subjective ride comfort of a well damped, softly sprung, long suspension travel car may still generate passenger discomfort on account of a poor secondary ride, where a less than smooth road surface may translate into annoying body vibration, high frequency body movement, and significant road noise. This tendency will often be provoked by low-profile tyres or (some) low-rolling resistance tyres with inflexible walls.

It has been a general presumption that cars which handle better, will be firmly suspended and therefore cannot offer the best of ride comfort, but this presumption is often confounded when clever engineers, such as traditionally those at Lotus, design suspension systems that offer systems with precise steering and great roadholding along with good subjective ride comfort. Having said this, enthusiastic drivers will often prefer a firmer suspension set-up, and sporting model variants will usually be offered with reduced suspension travel, firmer damper settings, firm anti-roll bars, and low-profile performance tyres on large diameter wide alloy wheels.

Primary ride comfort has generally improved with the abolition of, for example, solid rear axles, whilst secondary ride has possibly deteriorated with the popularity of low-profile and wider tyres with less wall flexibility and shock absorbing properties, although this has been countered by improved suspension bush technology and other methods of chassis vibration insulation. Ride comfort is usually tested during the design stage by fixing accelerometers and other instruments to a life-size dummy and recording the speed and degree of movement in three dimensions, along with internal noise, steering wheel vibration and other properties, over specially designed test circuits. For all this, suspension tuning is as much an art as a science, and hundreds of hours are spent tuning ride and handling characteristics on any new model to ensure that it offers the blend of comfort and driving characteristics that the engineers feel their market demands. Sadly, they somehow do not always quite succeed in pleasing everyone, but adaptive and adjustable suspension systems that automatically respond to road surface changes and offer personal preference settings for ride quality are increasingly available.

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