I’ve recently had my meagre attention diverted by an interesting discussion about replacing torsion bars with coil springs (completely, not “helpers”). It’s here, for those who have somehow managed to maintain a more balanced life in the past couple of weeks Anyone install coil overs instead of using torsion bars?
A good discussion which left me thinking it’s feasible, with minor reinforcement to the damper mount area of the frame, although it could be a challenge to balance the required spring stiffness and stress in the available space. Discussion continues.
Meanwhile, I promised an update on ARB bushings.
I moulded a set of bushings in a tasteful shade of PU, to be a close fit to the front ARB and match the shape of an original clamp with a 1mm preload (bushing 1mm taller than clamp). That setup required 3.8Nm (2.8lbf.ft) to turn the bar – lower than typical configurations from recognised suppliers. Raising the clamp with a 1mm spacer, to cancel the vertical preload, reduced the torque to zero. Intermediate values will be available to the obsessed, you know who you are.
No big surprise there, it just confirms that a bushing that fits the bar and clamp will allow the bar to rotate freely, which is desirable for smooth ride and turning. Does it matter? That will depend on the setup of the car and sensitivity of the driver, but I think anyone with a feel for engineering quality would accept that matching size and shape of such parts is desirable.
While doing this work I searched through some old notebooks and found a similar, but more detailed study from the dawn of time. This confirmed that the ARB bushing can be a valuable component of a well-tuned chassis, starting with a bushing that fits the bar well – a slightly larger hole if unavoidable, never smaller. Stiffness (durometer) is a useful tuning parameter, the late lamented PTFE liner valuable too. Those with deep curiosity and time on their hands can find this and more in an SAE paper published a mere 30 years ago, at the March 1994 conference.