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2000 Honda S2000 - Adjustable Geometry, Optimum Handling - Project S2K

We build custom front upper control arm mounts with Spoon gussets.

By Andrew Wojteczko

The ability to adjust the vertical position of the upper control arm pickup points in a ULCA (upper/lower control arm) suspension allows for adjustable roll-center height and anti-dive. These terms get thrown around pretty loosely, so let’s clear up what they actually mean and why we want to be able to adjust them.

Roll-center height is simply the vertical location of the point which the car rolls. The higher the roll center, the greater percentage of roll is resisted by the control arms (geometric load transfer) versus the spring and damper (elastic load transfer). If you move the roll center up to the vehicle’s center of gravity, you’ll achieve 100 percent geometric load transfer, and the suspension won’t move when cornering (though the total vehicle will still roll). More importantly, however, is the roll inertia and how this affects the vehicle’s response. Raising the roll-center height reduces roll inertia and, consequently, affords faster response during turn-in, which is great for tight circuits. Lowering the roll-center height increases roll inertia, causing a slower response at turn-in, which is better suited to high-speed tracks. One more thing that must be considered is that a high roll center will cause a vertical force component on the chassis that will raise ride height during cornering (though total load at the tires will remain the same). This can be an issue in long sweepers on cars like our S2000 that are ride-height sensitive for downforce production. A very low roll center (below ground) will cause the opposite, resulting in the ride height decreasing during cornering. I’d like to go more in depth on this, but space is limited.

Anti-dive is pretty much the sideview equivalent of roll-center height, and the same principle of transferring load through the control arms (geometric), as opposed to the spring/damper (elastic) to resist motion applies. The difference is it’s acting in pitch, instead of roll, allowing for manufacturers to limit nose-dive in braking while keeping relatively soft ride rates for comfortable travel on rough roads. This is most commonly achieved by installing the front upper control arm at an angle (approximately 5 degrees on an S2000) that also causes the wheel to regress in bump, once again reducing ride harshness. However, this non-vertical wheel path causes caster to increase in bump and will change the weight of the steering feel as the suspension compresses. Considering change in steering weight is a critical piece of information for a driver to know when the tire is at its peak lateral grip, removing this effect on a race car will certainly help improve lap times and consistency. This increase in caster will also cause a change in wheel loads to occur, reducing load on the outer front/inner rear and increasing load on the inner front/outer rear changing the balance of the car to oversteer through bumpy sections. Finally, using the control arms to resist pitch increases friction at the bushings, further hindering suspension performance.

By Andrew Wojteczko
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