If there is one component that has contributed the most benefit for the lowest cost in the design and development of suspension systems over the past 20 years, it is the humble microcellular polyurethane (PU) bump stop or ‘spring aid’. The more we know how to tune these ultra-simple and durable components, the more the chassis development engineer can take advantage of their beneficial influence.
One of the biggest advantages of PU is that it has progressive (rather than linear) compressibility characteristics, which permit increased ride travel for a given damper stroke, and therefore offers the potential for efficient and minimal use of vertical packaging space for struts and dampers. Compared with rubber, PU components should always give a better compromise between suspension deflection, rear axle gross-weight body control, and ultimate roll control.
Thirty years ago rubber was the only material available for this kind of work. It worked well enough, but its linear load-deflection characteristics and liquid-like behavior caused it to expand laterally under high compression and spread way outside the striker plate on the damper body unless contained by a metal shroud. While the metal shroud provided progressive load-deflection characteristics, it also resulted in a much longer block height, which in turn limited the available wheel travel.
Above: Ford engineers testing the Focus at the Lommel proving ground
Temperature sensitivity was also an issue. By contrast the structure of PU causes it to deform internally to a high degree, usually making the need for containment at high loads unnecessary. PU also generates useful hysteresis or internal damping, whereas rubber acts more like a spring that has to be damped. By the mid-1980s the advantages of PU meant it gained real traction as a bump-stop material, and also for damper top mounts in the case of Jaguar. Soon the longer rear bump-stops typically specified to provide support at higher vehicle loads became known as ‘spring aids’ in which linear springs combined with the long corrugated polyurethane parts work together to give progressive springing – a much cheaper and infinitely more tuneable alternative to variable-rate coils.
So PU’s compressibility characteristics are a dynamicist’s dream, permitting the same wheel travel for less metal-to-metal damper stroke. Other big advantages are simplicity and cleanliness of component production, which does not require heat or vulcanization. Changes in profile, length and stiffness can be achieved quickly and cheaply, which are important considerations because of the increasing sensitivity of car chassis systems to the increased grip that modern ultra-low-profile tires offer. Prototype parts can even be modified or turned from raw PU bar on a workshop lathe.
Of all the major handling modes, cornering roll is probably the least liked by customers and road testers. Despite improved anti-roll bar systems (mainly due to strut-mounted links), ARBs can never do much more than, say, halve the roll without badly affecting ride comfort, unless actively controlled (as is the case with some BMW and Range Rover models). Ultimately, bump stops and spring aids are among the most influential components in limiting body lean, and therefore in how the outside tire loadings are shared between the front and rear axles toward the limit of adhesion. Changing bump stop/spring aid stiffnesses and cross-sectional profiles adds to the effective wheel rates at any given moment, thus affecting the understeer/oversteer balance in the same way that springs do, especially during a transitional maneuver.
One of the most important considerations during suspension setup and design is optimizing the ‘contact point’ relative to wheel position (taking into account suspension velocity ratios). The design should always provide for the front bump-stop to make contact with the damper body before the rear one, and for the relative wheel rates to rise faster at the front axle, thus ensuring an understeer tendency from start to finish.
The fun and games usually start when a vehicle has a weight bias to the rear and marginal rear-tire section widths. This is when the desire to eliminate pitching pushes the development toward rear ride frequencies that are stiffer than ideal in order to help ensure that rear tire grip is maintained at the limit of adhesion. The resulting tendency toward oversteer can usually be mitigated with efforts to push as much mass forward as possible, combined with considerable damping and bump-stop development. Proof is to be found in some rear-engined Porsche models, which show that if all rear suspension frequencies have to be reduced beyond the theoretically ideal, then clever damping and bump-stop tuning can still result in minimal pitch sensitivity and acceptable handling.
The importance of rear bump-stop design during high-speed transient handling becomes clear from the driving seat when one discovers that very small changes in stiffness progression near the limit of adhesion can materially affect the breakaway characteristics, as can so many other details, down to and including elements such as exhaust system hanger stiffnesses.
Nowadays there is an acceptance that sports cars can just about get away with a nominal 120mm design ride height. Sporty sedans need 140mm, while 160-180mm is necessary for family cars. Low ride heights can present quite a challenge for the development engineer in preventing the car from grounding, especially when allowing for the heavily cambered roads and punishing surfaces found in the UK.
Nowadays, it seems that with PU parts, wonders can be achieved with as little as 60mm metal-to-metal wheel bump travel at the front axle (approximately 50mm to bump-stop hard on) and 70mm at the rear. This means that achieving an acceptable level of sportiness and comfort with big bump absorption does not require as much compression travel as before, something that results from careful tuning of the compression damping, bump stops and damper top mounts.
More interesting is that on the 30 or so ‘sporty’ car projects that have used PU bump stops, all made do with around 15mm of wheel travel to the bump-stop contact point on the front axle, and 20mm at the rear. Given the abuse to which the cars were subjected during chassis tuning and durability testing, much of the energy absorption capabilities could only have been achieved through the bump stops and spring aids.